Physics-Uspekhi. The Extracts from the Internet

Quantum model of “Zitterbewegung”

Sun, 31 Jan 2010 21:00:01 GMT

С. Roos and his group at the Institute for Quantum Optics and Quantum Information (Innsbruck, Austria) carried out an experiment with ⁴⁰Ca⁺ ions in which they recorded the “quivering motion” effect (Zitterbewegung) predicted for electrons by E. Schrodinger in 1930 using the Dirac equation. Rapid spatial oscillations of a particle arise owing to a superposition of states with positive and negative energies. The experiment with ions is a quantum model of the “Zitterbewegung” of electrons; in other words, the joint evolution of the vibrational degrees of freedom and the internal spin state of the ion is described by the same equation as the spatial trajectory of free electrons. Laser pulses transferred ions into a prescribed initial state and a short time later their fluorescent radiation was observed. The properties of this radiation yielded the characteristics of the “Zitterbewegung”. The “Zitterbewegung” can be experimentally observed with a quantum model in which it is linked to the spin state of the ion. At the moment it is quite unfeasible to detect this effect with real electrons since its amplitude is only ≈ 10^-10 cm and its frequency is ≈ 10²¹ Hz. The idea of quantum models was suggested by R. Feynman in 1982 for those cases in which the complex behavior is beyond any hope of direct or numerical computer experiments. Source: Nature 463 68 (2010)

The E₈ group in crystals

Sun, 31 Jan 2010 21:00:02 GMT

R. Coldea (Oxford University) and his colleagues in Germany and Great Britain discovered in neutron scattering experiments that the distribution of spins in quasi-one-dimensional ferromagnetic crystal of cobalt niobate CoNb₂O₆ at low temperature and in high magnetic field obeys the symmetry group E₈. In 1989 A.B. Zamolodchikov (The L.D. Landau Institute of Theoretical Physics) showed that in certain cases this Lie group describes the spectrum of excitations for the Ising model. In recent years the E₈ group was also discussed in elementary particles theory but has never been observed in real physical systems. The phase transition in quasi-one-dimensional Ising model occurs when the magnetic field directed along the atomic chains increases above a certain critical value which separates the magnetically ordered and the paramagnetic phases. The experiment studied the spin excitations in the two above phases and also investigated the properties of crystals in the immediate vicinity of the phase transition point. The measured ratio of the two lower resonance frequencies (two meson states) of atomic chains close to the critical magnetic field of ≈ 5 T was found to lie close to the golden ratio 1.618, in agreement with the prediction of A.B. Zamolodchikov's theory. Source: Science 327 177 (2010)

Goos – Hanchen effect for neutrons

Sun, 31 Jan 2010 21:00:03 GMT

Researchers at the Delft University of Technology (The Netherlands) and the Rutherford Appleton Laboratory (Great Britain) were able for the first time to detect the Goos – Hanchen effect in the reflection of spin-polarized neutrons by a potential barrier. This effect which was already predicted by Isaac Newton for light, produces a gap between the points of incidence and reflection of the beam. In a manner of speech, we can say that the beam penetrates into the sample and is reflected at a certain depth under the reflecting surface. The effect was first measured for light by F. Goos and H. Hanchen in 1947. In the case of neutrons, reflection imparts a certain phase shift to the wave function of a particle, which may be represented by a spatial translation along the reflecting surface. The experiment above made it possible to measure the Goos – Hanchen effect owing to the fact that the height of the relecting potential barrier in magnetic field depends on neutron spin direction (and on the orienttion of the neutron magnetic moment) so that neutrons of different degree of spin polarization are reflected at a different efficiency. The neutron beam was reflected by a layer of permalloy Fe_0.2Ni_0.8, 3µm thick. The choice of this material with high magnetic permeability was predicated on the need to create magnetic field under the reflecting surface and thus produce phase-shifted wave function. Reflected neutrons were recorded by a neutron reflectometer OffSpec. A small change in the polarization of the reflected beam was an indication of nonzero Goos – Hanchen effect. The Goos – Hanchen effect may be used for practical applications, such as generation of coherent neutron beams, and for creating neutron “waveguides”. Source: Phys. Rev. Lett. 104 010401 (2010)

Electron “liquid crystal” in iron-based superconductor

Sun, 31 Jan 2010 21:00:04 GMT

J.C. Davis (Brookhaven National Laboratory), Р.С. Canfield (Ames Laboratory) and co-workers conducted a scanning tunneling spectroscopy study of the compound Ca(Fe_1-xCo_x)₂As₂ (x is the fraction of dopant atoms) and discovered a static spatially ordered state of electron wave functions whose structure resembles that of liquid crystals. This compound is a parent of one class of superconductors — layered iron-based high-temperature superconductors (see review papers in Phys. Usp. 51 1201, 1229, 1261 (2008)). Electron wave functions of elongated shape were observed on the specimen surface, stretching along one of the crystal axes. Their longitudianl size was approximately eight time the distance between two neighboring iron atoms, and they were in spatial correlation with the dopant atoms. It was conjectured that the “liquid crystal” state of electrons is critically important for the high-temperature superconductivity to emerge. Similar ordered electron states have already been reported in cuprate superconductors. This may be an indication that the superconductivity mechanism in iron-based superconductors is closer to the currently unknown superconductivity mechanism in cuprates than to the Bardeen – Cooper – Schrieffer mechanism in low-temperature superconductors. Source: Science 327 181 (2010)

Gamma-radiation background at intermediate galactic latitudes

Sun, 31 Jan 2010 21:00:05 GMT

The results have been published of observing cosmic gamma radiation at intermediate galactic latitudes 10° ≤ |b| ≤ 20° in the energy range 100 MeV to 10 GeV, obtained by the Large Area Telescope (LAT) on board the cosmic Fermi gamma observatory during the first five months after launch. New background measurements do not confirm some earlier observations conducted by the EGRET gamma telescope on the orbital Compton Observatory in 1991 – 2000. According to the standard model of the origin of the diffuse galactic gamma radiation, it is generated when charged particles of cosmic rays interact with interstellar gas and radiation. This model is calibrated using cosmic rays data and provides sufficiently specific predictions for the gamma radiation background. The EGRET has earlier detected an excess of gamma radiation flux in comparison with the standard model outlined above. This information gave rise to conjectures on additional contributions to gamma background from particles of dark matter and other hypothetical sources. The LAT is more sensitive than the EGRET by approximately an order of magnitude but the LAT data reveal no excess in the gamma background and thus the measured spectrum supports with high accuracy the predictions of the standard model of background generation. Source: Phys. Rev. Lett. 103 251101 (2009)

Parity violation in nuclear collisions

Thu, 31 Dec 2009 21:00:01 GMT

The “chiral magnetic effect”, predicted in 1998 by D. Kharzeev, R.D. Pisarski and M.H.G. Tytgat was first detected at the RHIC accelerator (Relativistic Heavy Ion Collider) in the US using the STAR detector of the Brookhaven National Laboratory. The effect consists in parity violation (violation of the symmetry under mirror reflection) which manifests itself in a partial spatial separation of positively and negatively charged quarks along the direction of the orbital angular momentum of the colliding nuclei. The separation of charges is caused by the difference between the numbers of quarks of different chirality in the new metastable domains and by the strong magnetic field of intensity up to ≈ 10¹⁵ T which arises in nonzero impact parameter collisions of nuclei. The experimenters collided nuclei of Au + Au and Cu + Cu with center-of-mass energy of 200 GeV. The distribution of charges on the two sides of the plane of reaction (i.e. along the direction of the angular momentum) was measured using correlation analysis. Possible sources of background noise and experimental uncertainty such as the contribution of scattering of three or more particles and of resonance decays were studied and eliminated. This resulted in identification of statistically significant correlations which could correspond to parity violation. As predicted by theoretical calculations, the measured effect was found to be stronger in the case of copper nuclei than with gold nuclei. Source: Phys. Rev. Lett. 103 251601 (2009)

Measurement of ultralow temperatures

Thu, 31 Dec 2009 21:00:02 GMT

D.M. Weld and coworkers at the Massachusetts Institute of Technology developed a sufficiently universal technique of thermometry of ultracold atoms in optical lattices. The method of spin-gradient thermometry was tested successfully in nonuniform magnetic field on the gas of ⁸⁷Rb atoms trapped in a three-dimensional laser beam lattice. The gas was maintained in the Mott insulator regime in which conductivity is zero due to strong repulsion between particles. The width of the transition zone between atom clouds in different spin states (and having different magnetic moments) depends on temperature; these clouds separated in the gradient of the external magnetic field. As temperature decreases, the transition becomes sharper; incomplete separation at finite temperatures is caused by spin excitations. Therefore, the temperature of the gas can be found from observations of the transition zone. In this particular experiment it was possible to measure temperatures down to 1 nK and the researchers predict that the new technique will allow measuring temperatures down to ≈ 50 pK. The study of atoms in optical lattices at ultralow temperatures is important for implementing quantum spin Hamiltonians which model materials with unique properties, such as high-temperature superconductors. Source: Phys. Rev. Lett. 103 245301 (2009)

Relativistic plasma “mirror”

Thu, 31 Dec 2009 21:00:03 GMT

М. Kando and coworkers at the Advanced Photon Research Center (Kioto, Japan) and the P.N. Lebedev Physical Institute of the RAS (Moskow, Russia) studied the reflection of laser pulses from relativistically moving “mirror”. The “mirror” was the plasma wave (modulation of electron density) generated by the high-power laser pulse. When laser light propagates toward the “mirror”, the double Doppler effect increases the frequency of the reflected light by a factor of 37 to 66. The frequency jump of this magnitude may prove useful in a number of practical applications, for instance, for laser acceleration of ions or for generation of ultrashort pulses. The reflection coefficient in this experiment was 1.3 × 10^-4 to 0.6 × 10^-3 which is close to the quantity predicted by the theory for this experiment. The experiment with reflection by a relativistic plasma “mirror” was first carried out in 2007. In this new experiment it proved possible to achieve considerably higher reflectivity and to investigate in detail the characteristics of the reflected light. Source: Phys. Rev. Lett. 103 235003 (2009)

A close pair of compact stars

Thu, 31 Dec 2009 21:00:04 GMT

С. Badenes and his coworkers in the US and Israel discovered a binary system at a distance of about 50 pc from the Earth in which one object is a white dwarf with a mass of about 0.9M_☉ and the second object is a neutron star or a black hole with a mass higher than 1.6M_☉. This system therefore contains the nearest to the Earth known compact remnant of supernova explosion. The binary system SDSS 1257 + 5428 with the orbital period of 4.6 hours was found by studying the spectral features of objects of the Sloan Digital Sky Survey catalog and was then additionally investigated by the DIS spectrograph on the 3.5-meter ARC telescope in New Mexico. The components of the pair gradually move closer because of the emission of gravitational waves. Estimates of the orbital parameters make it possible to conclude that these compact stars will collide not later than about 500 million years. Т.A. Thompson and his colleagues at the Ohio State University used this data to evaluate the total number and rate of mergers of binary systems of this type. By their calculations, the Galaxy contains about 10⁶ such systems and the rate of mergers in the Galaxy is estimated as ≈ 5 × 10^-4 yr^-1. It cannot be excluded that a collision of a white dwarf and a compact object generate gamma bursts, powerful neutrino flashes and also ultrahigh-energy cosmic rays. Binary systems with white dwarfs might constitute the principal source of background gravitational wave signal in the currently planned space interferometer LISA. Sources: arXiv:0910.2709v1 [astro-ph.SR], arXiv:0912.0009v1 [astro-ph.HE]

Waves on a cell membrane

Thu, 31 Dec 2009 21:00:05 GMT

The propagation of wave excitations along the outer shell of live cells depends on many physical and chemical factors, some of them nonlinear, so that the observation and theoretical explanation of the properties of waves on cell membranes is an interesting and complex problem of biophysics. Surface waves are directly linked to the ability of cells to move. Researchers at the Institute of Biophotonics Engineering and Research Center for Applied sciences (Taipei, Taiwan) were able to clarify how waves propagate on the surface of fibroblasts, i.e. cells of connective tissue. The method of observation they used is known as noninterferometric wide-field optical profilometry (NIWOP). It is based on using the interval of linear dependence of light intensity on the amount of spatial shifting out of focal point of the microscope. This technique made it possible to achieve the required resolution in the three-dimensional picture of wave propagation. The experimenters measured wave profiles and their speed (≈ 100 nm s^-1), their dispersion and other characteristics. Observations confirm the detailed theory of wave propagation suggested by R. Shlomovitz and N.S. Gov in 2007. Their model was based on the interaction between the force of repulsion that arises when the protein actin polymerizes, and the compressive force produced by the protein myosin which forms links between actin fibers. The researchers discovered that waves disappeared when they added reagents blocking myosin and polymerization of actin, which confirms the decisive role of these proteins in the wave process. The outlined results may lead to important applications in medical sciences and biotechnologies. Source: Phys. Rev. Lett. 103 238101 (2009)

ЕМС effect in light nuclei

Mon, 30 Nov 2009 21:00:01 GMT

In 1983 the European Muon Collaboration (ЕМС) discovered an effect which reflected how the maximum momentum of quarks in a nucleon depends on the characteristics of the nucleus to which the nucleon belongs. J. Seely et al. at the Thomas Jefferson Laboratory carried out an experiment which revealed new features of the ЕМС effect. A conclusion was made in a number of theoretical papers attempting to interpret the ЕМС effect that it is linked to the mean density or mass of the nucleus. The new experiment demonstrated, however, that these parameters give an ambiguous description of the effect and may be of only secondary importance. This experiment studied the scattering of 5.8 GeV electrons by targets of ²H, ³Не, ⁴Не, ⁹Ве and ¹²С nuclei. It was found that the magnitude of the ЕМС effect of the nucleus ⁹Ве is close to that of ¹²С even though the mean density of the nucleus ⁹Ве is considerably lower. The conclusion is that the mean nuclear density is not a decisive factor for the ЕМС effect. It was also established that the light nuclei ³Не and ⁴Не differ greatly in the magnitude of the ЕМС effect. The key to the explanation of the obtained results may lie in the cluster structure of nuclei. E. g. the nucleus of beryllium may be regarded as two bound nuclei of ⁴Не plus an additional neutron revolving around them, the average density of the ⁹Ве nucleus being considerably lower than the density of each of the ⁴Не nuclei. In view of this, the ЕМС effect in such nuclei may be a function of not average but local density so that the ЕМС effect of the nucleus ⁹Ве is analogous to the effect of individual nuclei ⁴Не. In other words, the properties of nucleons in nuclei are determined not by the mass or mean density of the nucleus as a whole but by the local environment of the nucleon, such as the characteristics of the clusters contained in this nucleon. Source: Phys. Rev. Lett. 103 202301 (2009)

Emission cone of Vavilov – Cherenkov radiation in “left-handed” matter

Mon, 30 Nov 2009 21:00:02 GMT

Researchers at the Zhejiang University, Hangzhou (China) and Massachusetts Institute of Technology (USA) observed for the first time the reversed Vavilov – Cherenkov radiation generated in a “left-handed” medium (a medium whose dielectric permittivity and magnetic permeability are simultaneously negative). As V.G. Veselago predicted in 1967 (see Phys. Usp. 10 509 (1968)), the cone of emission of the Cherenkov radiation and the energy flow in “left-handed” materials are directed backward relative to the motion of the particle. The experiment described here used microwave radiation propagating through a metamaterial, that is, an array of conductors. A charged particle was imitated by a sequence of dipoles with the phase changing in a certain manner; the dipoles were excited in a waveguide consisting of 14 gaps. The speed at which this “particle” was moving was v = 1,9c/n where n is the refractive index of the metamaterial. This set of dipoles is completely equivalent from the standpoint of emission of radio waves to a real charged particle; however, the imitation made it possible to achieve considerably higher (and measurable) intensity of the Vavilov – Cherenkov radiation in the frequency range 8.1-9.5 GHz. The Vavilov – Cherenkov radiation could not propagate in metamaterials studied earlier (owing to the nature of their anisotropy) so for observing the Vavilov – Cherenkov radiation a metamaterial with a special configuration of unit cells was fabricated. The reversed Vavilov – Cherenkov radiation may prove useful in fast particle detectors, e.g. in accelerator experiments. For details on media with negative refractive index see papers by V.G. Veselago in Phys. Usp. 46 764 (2003), Phys. Usp. 52 649 (2009) . Source: Phys. Rev. Lett. 103 194801 (2009)

Bose – Einstein condensate of strontium atoms

Mon, 30 Nov 2009 21:00:03 GMT

Two independent groups of researchers from the Rice University in US and the Institute for Quantum Optics and Quantum Information (IQOQI) in Austria prepared the Bose – Einstein condensate of atoms of strontium isotope ⁸⁴Sr whose natural abundance is only 0,56%. Even though the abundances of the isotopes ⁸⁶Sr and ⁸⁸Sr are much higher, they cannot be cooled evaporatively owing to the excessively large (in the case of ⁸⁶Sr) or too small (⁸⁸Sr) atomic scattering length. Contrary to these two, the rare isotope ⁸⁴Sr has the scattering length of 123 Bohr radii which suits cooling ideally; in the experiments of both groups, evaporative cooling was the concluding stage after laser cooling in magnetooptic trap. The transition to condensate state was identified by monitoring the optical profile of the cloud of gas and from the value of the chemical potential calculated from the dynamics of expansion of the cloud. It is suggested that the condensate of ⁸⁴Sr atoms be used in ultraprecise experiments, in new systems for quantum computation and as a buffer gas in cooling other isotopes, e. g. the fermion isotope ⁸⁷Sr, to degenerate state. Sources: Phys. Rev. Lett. 103 200402 (2009) ; Phys. Rev. Lett. 103 200401 (2009)

Laser acceleration of neutral atoms

Mon, 30 Nov 2009 21:00:04 GMT

U. Eichmann and his colleagues at the Institute for Optics and Atomic Physics (Berlin) and the Max Planck Institute discovered the effect of acceleration of neutral atoms by pondermotive force in the field of nonuniform laser radiation. Typically, one considers the effect of the pondermotive force on charged particles but in fact a similar force may arise in the case of neutral atoms in view of the dynamical polarization of atoms after they were excited to Rydberg states. The electron on a distant orbit may then be accelerated as a free particle by the pondermotive force (the acceleration of the nucleus is much weaker because of its large mass). If the electron after acceleration remains bonded to the atomic nucleus, the momentum of the accelerated electron is transferred to the atom as a whole. In the experiment of the German scientists a beam of neutral helium atoms was illuminated by short focused pulses of laser light, and roughly one per cent of atoms felt acceleration. In some cases the acceleration of an atom was greater than the acceleration of free fall g by a factor of 10¹⁴ — record-high for accelerations of neutral atoms in external fields ever observed. Source: Nature 461 1261 (2009)

Polarization of microwave background and cosmological parameters

Mon, 30 Nov 2009 21:00:05 GMT

The observation of polarization of cosmic microwave background is one of the most efficient methods of studying physical processes in the early Universe and of improving the cosmological parameters. Measurements of polarization of microwave background became technically possible in 2002 and have been conducted since then with gradually better precision by a number of instruments. From 2005 to 2007 observations were conducted at the South Pole with a 2.6 m QUaD radio telescope equipped with 31 pairs of orthogonal bolometers sensitive to the polarization of electromagnetic waves and functioning at two frequencies: 100 and 150 GHz. By now the data gathered during this period has been processed and more accurate values of cosmological parameters were computed. The accuracy of results is the highest if datasets of several detectors are used simultaneously (WMAP, ACBAR, QUaD etc.). For example, according to the latest data, the most probable value of the Hubble constant is H₀ = 70.6 km s^-1Mpc^-1, the index of the density perturbation spectrum n_s = 0.960, and a scenario is possible in which the index depends on scale (on the running index). It was also possible to improve the constraint on the tensor mode of perturbations (gravitational waves) in comparison with scalar perturbations (density perturbations); now the ratio of these components is estimated as r<0,33 at the confidence level 95%. Source: Astrophysical Journal 705 978 (2009)

Testing of Bell's inequalities in a system of Josephson qubits

Sat, 31 Oct 2009 21:00:01 GMT

Researchers at the University of California at Santa Barbara experimentally confirmed that Bell's inequalities are violated in a macroscopic system composed of two Josephson qubits (quantum bits) implemented using superconducting Josephson contacts. Violation of Bell's inequalities has already been tested in a number of quantum processes which excluded the possibility of the “hidden-variables” interpretation. In the new experiment two Josephson qubits were transferred to entangled quantum state (Phys. Usp. 49 1111 (2006)) using an electromagnetic resonator after which quantum correlations between the states of qubits were measured. For this particular experiment, Bell's inequality can be written in the form S<2 where S depends on the state of qubits. Measurements showed that S = 2.0732 ± 0.0003 i.e. Bell's inequality is violated at the level of 244 standard deviations and therefore the state of the qubits in this experiment cannot be classically described. Source: Nature 461 504 (2009)

Persistent current in a ring

Sat, 31 Oct 2009 21:00:02 GMT

J. Harris (Yale University, USA) and his co-workers measured for the first time the persistent electric current in metallic (non-superconducting) rings. The persistent ring current predicted theoretically by М. Buttiker, Y. Imry and R.B. Landauer in 1983 is an element of the equilibrium quantum state of electrons in the ring. It was predicted that the persistent current in micron-size rings at temperatures T<1 К may reach 1 nА. The external magnetic field breaks the time reversal symmetry which forces one of the possible current directions to be selected, plus this current is a periodic function I(Φ) of the magnetic flux Φ across the ring (due to the Aharonov – Bohm effect). Attempts were made earlier to use superconducting quantum interferometers (SQUIDs) to measure the magnetic field generated by the persistent current but the sensitivity of this method proved insufficient owing to, among other factors, the feedback to the ring current from oscillations of the superconducting current in SQUIDs. J. Harris and his team measured the effect induced by the magnetic field of the current in the micromechanical silicon cantilever probe. Aluminum rings were attached to the end of the probe and the system measured the shift of the resonance frequency of mechanical vibrations of the cantilever due to the interaction between the magnetic moments of ring currents and the external magnetic field. The oscillations of the cantilever were caused by a piezo-mechanical vibrator and were observed using a laser interferometer. This technique has the sensitivity (≈20 pА Hz^-1/2) approximately an order of magnitude better than the approach based on SQUIDs. The results of measurements both with a single aluminum ring and with a large set of rings are in good agreement with theoretical predictions. It was found that current oscillations I(Φ) in different rings of the set had uncorrelated phases both in the first and the second harmonics. Measuring persistent currents in microscopic rings can help in studying quantum phase transitions and quantum coherence at low temperatures. Source: Science 326 272 (2009)

Splitting of Cooper pairs

Sat, 31 Oct 2009 21:00:03 GMT

Two independent groups of researchers created efficient sources of electrons entangled in spin states (EPR-pairs); they act by splitting Cooper pairs of electrons tunneling through a superconductor. The difficulty that faced earlier attempts to generate EPR pairs stemmed from the fact that electrons in metals sit below the Fermi surface so that releasing them immediately destroys entanglement. The ground state in superconductors is the condensate of Cooper pairs in spin-singlet state so these pairs can be separated from the superconductor by way of tunneling. L. Hofstetter and his colleagues in Switzerland, Hungary and Denmark solved the remaining problem of splitting Cooper pairs into individual electrons by using the Coulomb repulsion of the electrons that went through tunneling into two quantum dots. Quantum dots would appear in the region where the nanowire made of indium arsenide intersected with the central superconductor and two metal contacts in normal (non-superconducting) state. Control electrodes make it possible to vary the depth of the potential well of quantum dots in such a way that only one electron passed through each quantum dot at any one moment of time. The electrons of a Cooper pair are inherently quantum-correlated (entangled) along the direction of spin and this entanglement of electrons survived after the pair was split. The experiment of L.G. Herrmann and colleagues in France, Spain and Germany generally resembles the one described above but it used carbon nanotubes instead of nanowire. New sources of EPR pairs of electrons may find important applications in fundamental research, such as the study of the Einstein – Podolsky – Rosen paradox. Sources: Nature 461 960 (2009) ; arXiv:0909.3243v1 [cond-mat.mes-hall]

Bose – Einstein condensate of calcium atoms

Sat, 31 Oct 2009 21:00:04 GMT

S. Kraft (Federal Physico-Technical Institute, Germany) and his co-workers were able to produce for the first time the Bose – Einstein condensate of atoms of the alkali earth metal, ⁴⁰Ca. At the initial stage of the experiment atoms were laser-cooled in a magneto-optical trap using the transitions ¹S₀-¹P₁, ¹S₀-³P₁ etc. The main process that restricted the effectiveness of cooling was losses via three-particle atomic interactions. At the last stage the atomic cloud was loaded into an optical dipole trap and cooled evaporatively. The transition of about 2×10⁴ atoms to the Bose – Einstein condensate state at 170 nK was identified using the characteristic Gaussian density profile. The formation of the condensate was also confirmed by the large chemical potential of the gas, which was calculated on the basis of the measured velocities of anisotropic expansion of the cloud. Quantum intercombination transitions with very narrow spectral lines ¹S₀-³P₁ (only 370 Hz) are possible in atoms of ⁴⁰Ca. Consequently, Bose – Einstein condensates of ⁴⁰Ca atoms are very promising for highest-precision measurements of the gravitational field. Furthermore, ⁴⁰Ca atoms in the non-degenerate ground state have no magnetic moment, which additionally increases the accuracy of measurements in view of the absence of interaction with external magnetic fields. Source: Phys. Rev. Lett. 103 130401 (2009)

Magnetism of carbon

Sat, 31 Oct 2009 21:00:05 GMT

Researchers in Czechia and the Netherlands J. Cervenka, M.I. Katsnelson and C.F.J. Flipse have clarified the mechanism of formation of ferromagnetic properties of specimens of polycrystalline graphite at room temperatures. A number of experiments established that various forms of carbon manifest ferromagnetism (see e.g. Phys. Usp. 47 102 (2004)) but its nature remained unclear. Hypotheses were advanced that carbon owes the observed magnetism either to metal impurities or to defects of crystal structure. The new experiment used a magnetic force microscope, a SQUID magnetometer and an atomic force microscope for measurements; this made it possible to study simultaneously and at high spatial resolution both magnetic and electronic properties of samples. The obtained microscopic images provide direct evidence supporting the second hypothesis: the graphite magnetism arises owing to defects in the structure of atomic layers and that impurities do not play a significant role. A two-dimensional network of defects (only 2 nm thick each) is formed along atomic planes of carbon in graphite; these networks delimit homogeneous regions, i.e. grains of the polycrystal. Ferromagnetism originates with unpaired electrons localized on defects at grain boundaries. Magnetic carbon may find applications in spintronic devices and in medical fields for designing biological sensors. Source: Nature Physics 5 840 (2009) ; arXiv:0910.2130v1 [cond-mat.mtrl-sci]

Testing the isotropy of the the speed of light

Wed, 30 Sep 2009 20:00:01 GMT

Ch. Eisele, A. Yu. Nevsky and S. Schiller (Institut fur Experimentalphysik, Heinrich-Heine-University at Dusseldorf, Germany) conducted a test of independence of the speed of light of the direction of propagation, with record accuracy so far. The setting of the experiment resembles that of the classical Michelson – Morley experiment. The setup includes two mutually perpendicular optical waveguides with slightly different resonance frequencies in which standing electromagnetic waves are excited by a laser. The difference between frequencies in the two waveguides is measured by observing beats of the sum signal as a function of spatial orientation of the apparatus. In the 13 months that the experiment was run the apparatus was turned by 90 degrees approximately 175,000 times. Considerable effort was made to exclude external factors of different types that produce systematic errors. No anisotropy of the speed of light was detected at accuracy of ≈ 10^-17 which confirms the local Lorentz invariance at this level. The measurements conducted are important for testing the suggested approaches to constructing the Unified field theory: indeed, the Lorentz invariance holds only approximately in some models. Source: Phys. Rev. Lett. 103 090401 (2009)

Ferromagnetism in Fermi gas

Wed, 30 Sep 2009 20:00:02 GMT

W. Ketterle (Massachusetts Institute of Technology) and his colleagues succeeded for the first time in detecting ferromagnetic properties of ultracold gas consisting of ⁶Li atoms. Ferromagnetism was previously observed in gases only in the state of Bose – Einstein condensate. The possibility of transition of Fermi gases to ferromagnetic state when the interparticle interaction is repulsive has been discussed in a number of theoretical papers but no definitive conclusions have been made on the feasibility of such transitions. In the new experiment, a cloud of ultracold gas which is a mixture of atoms with oppositely oriented spins was held in an optical trap. The transition to ferromagnetic state at a temperature of less than 1 µK was identified by an indirect method of detecting a drop in the rate of inelastic three-particle collisions that result in the formation of molecules, by detecting the point when the minimal kinetic energy of particles is reached, and by recognizing certain characteristics of expansion of the gas cloud. These properties were exactly those predicted for the ferromagnetic gas. The gas field contained about 100 magnetic domains of volumes of about 5 µm³ with about 50 atoms of the gas in each. This experiment demonstrated that ferromagnetism may arise in a system of Fermi particles possessing no crystal lattice. Source: Science 325 1521 (2009)

Buffer-gas cooling

Wed, 30 Sep 2009 20:00:03 GMT

S.C. Doret (Massachusetts Institute of Technology) and his colleagues developed a new method of cooling a gas to the state of Bose – Einstein condensate — the gas was cooled via collisions of its atoms with atoms of auxiliary buffer gas. The Bose – Einstein condensate is typically produced by a preliminary laser cooling but this method works with only a few gases. In the experiment of S.C. Doret and his colleagues, ⁴He atoms were evaporated by light pulses from the inner walls of the vessel and a small fraction of them (≈ 10^-5) were raised by microwave pulses to an excited state ⁴He^*. Atoms of ⁴He in the ground state acted as buffer gas; collisions with them cooled ⁴He^* atoms to a temperature of ≈ 500 µK. Then in a short time ⁴He atoms were again absorbed by vessel walls so what was left in the vessel was a cooled gas of ⁴He^* atoms. Further cooling for the transition to the Bose – Einstein condensate used the conventional evaporation technique. The buffer-gas method can cool many atomic and molecular gases for which other methods of cooling are inapplicable. Source: Phys. Rev. Lett. 103 103005 (2009)

Improved atomic force microscope

Wed, 30 Sep 2009 20:00:04 GMT

Researchers at the IBM research laboratory in Zurich (Switzerland) and the Institute for Nanomaterials Science (Utrecht, the Netherlands) improved the resolution of the atomic force microscope (AFM) to a level at which individual atoms composing a molecule can be observed. This became feasible by using a single molecule of carbon monoxide CO as the tip of the needle of the AFM. The factor that limited the resolution of AFM with a metal tip was the distortion of the structure of the specimen by van der Waals forces when the tip approached the specimen surface, or atoms jumped to the tip off the specimen surface, adsorbed on the needle, and also destroyed the observed features. The advantages of a CO tip used as the needle tip lie in the considerable stability of this molecule against outside influences and van der Waals forces. As an illustration, L. Gross and his colleagues carried out observations of a carefully scrutinized molecule C₂₂H₁₄. The improved microscope made it possible to resolve all five carbon rings as well as all constituent carbon and hydrogen atoms in the molecule. It was possible to measure interatomic distances of only 0.14 nm, which is a record resolution of an atomic force microscope. Source: Science 325 1110 (2009)

Search for gravitational waves

Wed, 30 Sep 2009 20:00:05 GMT

It is assumed that cosmic gamma bursts of short duration (less than two seconds) occur because of a merger of two neutron stars, or a neutron star and a black hole in a binary system, while long-duration gamma bursts with a softer spectrum are generated in supernova explosions. In both cases gamma radiation must be accompanied with a powerful burst of gravitational waves. The data collected by the detectors LIGO (in the USA) and VIRGO (in Italy) were compared with the catalog of 137 gamma bursts most of which were observed by the Swift satellite. The study found no statistically significant correlation of signals which puts a bound on the total energy of gravitational radiation, or (if energy is given) on the distance to the sources of bursts. Typical gamma bursts occur in other galaxies, at cosmologically large distances. A gamma burst can be recorded by LIGO or VIRGO only if the burst source happens to be relatively near. Preliminary evaluations showed that such events cannot be dismissed as impossible but that their probability is very low. An earlier search by the LIGO detector of a gravitational signal from a relatively near and extremely bright gamma burst GRB 030329 also failed to find a signal. According to calculations, gravitational waves from gamma bursts should be confidently recorded by future improved detectors with sensitivity raised by approximately an order of magnitude compared to today's LIGO and VIRGO. Source: arXiv:0908.3824v1 [astro-ph.HE]

Parity violation in ytterbium atoms

Mon, 31 Aug 2009 20:00:01 GMT

Parity violation (the invariance of the properties of systems under mirror reflections) was first detected in 1957 in the asymmetry of the ejection of electrons from decaying cobalt-60 atoms in magnetic field. The record amplitude of this effect was observed in cesium atoms. D. Budker and coworkers at the Berkeley National Laboratory (USA) measured the degree of parity violation in atoms of ¹⁷⁴Yb, which proved to be approximately 100 times stronger than in cesium atoms. Transitions were observed between ytterbium atomic levels 6s² ¹S₀ → 5d6s ³D₁ that corresponded to the wavelength of the absorbed photons of 408 nm. These transitions are forbidden by selection rules; in other words, they occur with very low probability. Transitions to upper levels in a beam of ytterbium atoms were excited by laser pulses in crossed electric and magnetic fields. The time rate of these excitation transitions of atoms was measured by observing the fluorescent emission from spontaneous transitions of atoms to lower levels. The time rate of atomic transitions in the left-hand-oriented configuration of fields was higher than in the case of right-handed orientation. Parity violation arises as a result of mixing of levels of different parity due to the weak interaction in the external electric field. So far the accuracy achieved in the experiment is only 14% but it proved sufficient for detecting the record-high parity in ¹⁷⁴Yb. Future experimental measurement of parity violation in various isotopes of nuclei and in transitions between levels of hyperfine splitting could provide new data on the distribution of neutrons in nuclei and perhaps could find effects go ing beyond the Standard Model of elementary particles; they will also lead to measurements of the anapole moment of nuclei (see Phys. Usp. 40 1161 (1997)). Source: Phys. Rev. Lett. 103 071601 (2009)

Mechanism of growth of carbon nanotubes

Mon, 31 Aug 2009 20:00:02 GMT

Researchers at University of Lyon (France) and the Rice University (USA) established that as new atoms join a carbon nanotube in the course of its growth, the nanotube revolves around is axis. This mechanism of nanotube growth was predicted in a theoretical paper by B. Yakobson. Field emission microscope was used to observe nanotubes in the process of catalyzed growth, with the FEM tip moving along the growth zone of the nanotube. The image of the nanotube was projected onto a phosphorus-covered screen and the processes taking place were video-recorded. In one of the experiments a nanotube made 180 rotations around its axis during 11 min of observation. Another interesting result was the observation that the nanotube revolved stepwise, not smoothly, doing one complete turn in approximately 24 steps. Atoms join the rotating nanotube in pairs and form a helical structure. Unique mechanical and electronic properties of carbon nanotubes hold great promise for the future in designing new superstrong materials and elements of microelectronics. Clarifying the mechanism of nanotube growth may help control this process in industrial-scale production. Source: Nano Lett. 9 2961 (2009)

Transparent aluminum

Mon, 31 Aug 2009 20:00:03 GMT

J. Wark (Oxford University, Great Britain) and coworkers observed a transition in aluminum to a phase transparent to UV radiation in response to irradiation by laser pulses. Aluminum foil was exposed to pulses of the most powerful in the world x-ray laser FLASH operating in Hamburg. The radiation flux density for 92 eV photons over a small area of the foil reached 10¹⁶ W cm^-2. The light of the laser produced single ionization of practically all aluminum atoms by knocking out electrons off the L shells, without destroying the crystal structure; this kept the specimen transparent to UV photons for 40 fs. Source: Nature Physics 5 693 (2009)

Nanolaser went over the diffraction limit

Mon, 31 Aug 2009 20:00:04 GMT

М.Т. Hill (Technical University of Eindhoven, The Netherlands) and his colleagues in The Netherlands and the USA designed a microscopic laser whose transverse dimensions are below the diffraction limit of the radiation it emits. The device consists of alternating semiconducting InP/InGaAs/InP structures of square cross section, 90 to 350 nm thick, bounded on both sides by dielectric layers of SiN 20 nm in thickness; the entire structure was coated with an outer layer of silver. Epitaxy, electron-beam lithography and some other methods were used to create the laser. Electric current was passed through special contacts connected to the semiconductor, and electrons and hole were injected into the structure. The layered structure forms a waveguide through which gap plasmon modes can propagate, undergoing reflections from silver layers at waveguide boundaries — like light does in the Fabry – Perot interferometer. The device can generate laser radiation with wavelength of about 1500 nm while being less that a quarter of wavelength thick. The diffraction limit is overcome both because the wavelength in a dielectric is shorter than that in vacuum and also because photons are transformed into surface plasmons in metal layers. The generation of laser light was recorded even at room temperature despite the fact that the device is more efficient if it is cooled considerably. Nanolasers may find applications in, for example, computers to transfer signals between components of microelectronic circuits and thus greatly speed up their work. Source: Optics Express 17 11107 (2009)

Massive compact galaxy in early Universe

Mon, 31 Aug 2009 20:00:05 GMT

P. van Dokkum (Yale University, USA) and his colleagues measured on the 8-meter Gemini telescope the dispersion of stellar velocities in the 1255-0 galaxy. The galaxy is observed at redshift z=2,186, i.e. in the epoch when the age of the Universe was a mere 3 billion years. However, with the mass of the galaxy ≈ 10¹¹ mass of the Sun, stars in it move with velocity dispersion of 510⁺¹⁶⁵_-95 km s^-1, which is approximately 2.5 times greater than the velocity dispersion in typical present-day galaxies. The size of the galaxy 1255-0 is however, six times smaller than that of present-day elliptical galaxies of the same mass. It is not clear so far what mechanism formed such dense galaxies and what was their evolution path. It is conceivable that in the time since such galaxies were formed, they merged with other surrounding galaxies and acted as seeds of very dense central regions of today's giant galaxies. It is also possible that central supermassive black holes formed very early in such galaxies. Source: Nature 460 717 (2009)

Mass of the Ω^-_b-baryon

Fri, 31 Jul 2009 20:00:01 GMT

The CDF collaboration at the Tevatron accelerator has measured the mass of the Ω^-_b-baryon consisting of a b-quark and two s-quarks. The existence of a doubly strange particle Ω^-_b is predicted by the Standard model of elementary particles; it has first been observed by the D0 collaboration and announced in August 2008. These particles was created in the CDF experiment in p-anti-p collisions at the center-of-mass energy of 1.96 TeV and were identified at the confidence level of 5.5σ through the chain of decays to lighter particles: Ω^-_b→J/ΨΩ^-, J/Ψ→μ⁺μ^-, Ω^-→ΛK^^-, Λ→pπ^-. On the whole, approximately 5×10¹¹ p-anti-p collisions were studied and 16⁺⁶_-4 production events of the Ω^-_b-baryon were recorded. The resulting mass of 6054.4±6.8(stat.)±0.9(syst.) MeV differs from the value 6165±10(stat.)±13(syst.) MeV reported by the D0. The results of the D0 and CDF are statistically incompatible and neither experiment found any special features near the mass measured in the other experiment. Such features could be an evidence that in fact the two experiments observed different particles. Furthermore, the rate of creation of Ω^-_b-baryons in the CDF experiment was found to be lower than in the D0. The causes of these discrepancies have not yet been identified. Russian scientists from JINR and ITEP take part in the CDF Collaboration. Source: arXiv:0905.3123v1 [hep-ex]

Novel magnetic effect

Fri, 31 Jul 2009 20:00:02 GMT

Researchers at the National Institute of Standards and Technologies (NIST, USA) and the Institute of Solid State Physics (Chernogolovka, Russia) have discovered that the magnetic ordering in ferromagnets longer range under certain conditions than was earlier expected. They studied heterostructures consisting of a thin ferromagnetic film coated with a grid of antiferromagnetic compound FeMn. They used the technique of magnetooptical visualization which makes it possible to follow in real time the formation, growth and disappearance of magnetic domains. The antiferromagnetic grid creates the pinning effect — the ferromagnetic magnetization gets pinned in certain directions. It was assumed that the magnetic interaction of this type should penetrate into ferromagnetic films to a depth of at most several tens of nanometers beneath the FeMn film. In fact the structure of domain walls (domain boundaries) in the ferromagnetic film was sensitive to this influence even at a distance of 50 µm from the nearest FeMn strip, which is greater by three orders of magnitude than the expected penetration range. One possible explanation of this effect is the topological stability of the domains in the upper grid of the antiferromagnetic; however, further investigation is required to fully clarify the phenomenon discovered. The results of the experiment are important for designing magnetic storage devices with high density of data recording. Source: Phys. Rev. B 79 144435 (2009)

Quantum walks

Fri, 31 Jul 2009 20:00:03 GMT

Researchers at the Bonn University were able to implement the algorithm of quantum random walks suggested by R. Feynman, for neutral cesium atoms in the potential field of two overlapping optical lattices. Standing waves formed by two laser beams created a one-dimensional periodic potential barrier of height k_B×80 µK where k_B is the Boltzmann constant. Cesium atoms possessed thermal energy k_B×10 µK and could exist in two states of hyperfine splitting of levels each of which was quantum-correlated with one of the directions of atomic displacement to the left or to the right; the permeability of the barrier depended on the internal state of the atom owing to a certain polarization of laser beams. The “step” of an atom in the lattice was initiated by switching the polarization, after which the position of the atom could be determined using its fluorescent emission. In contrast to the classical random walk that takes place, for example, in diffusion, in the quantum case momentum imparted to the atom transferred it to a state of quantum superposition of two possible directions of motion. The next pulse created a new configuration of the atomic wave function which included a superposition of the states of the preceding steps. The experiment implemented up to N=24 steps. The observation of the final positions of trapped atoms showed that their displacements could indeed be regrarded as quantum walking in which the summary displacement proportional to N. In the case of decoherence at each step of quantum walking the characteristics of walking tended to the classical law N^½. The quantum walk algorithm has already been implemented earlier in a number of systems but the experiment of М. Karski and his coworkers outlined above is the closest to the original one suggested by R. Feynman. Source: Science 325 174 (2009) ; arXiv:0907.1565v1 [quant-ph]

A single-molecule optical transistor

Fri, 31 Jul 2009 20:00:04 GMT

J. Hwang and his colleagues at the Swiss Federal Institute of Technology (ETH Zurich) designed a transistor using a single molecule of dibenzanthanthrene dye impregnated into a crystalline matrix of an organic compound tetradecane. When the molecule was cooled by liquid helium to a temperature of 1.4 K, its effective cross section of interaction with a photon increased to the value of the cross section of focused laser beams used in the experiment. One of the beams served as the “gate” of the transistor, controlling the quantum state of the molecule. Depending on what energy level the molecule was on, it scattered the second, more powerful beam differently and this allowed the control over the passage of the beam through the molecule; this is an analogy to how current is controlled in a conventional electron transistor. In the future such photonic devices may become a viable alternative to electronic systems owing to high speed and low heat production, for instance for building optical computers. Source: Nature 460 76 (2009)

Generation of gamma radiation very close to a black hole

Fri, 31 Jul 2009 20:00:05 GMT

Joint observations using gamma and radio telescopes allowed astronomers to establish that the high-energy gamma radiation of the M87 galaxy is generated in the immediate vicinity of a supermassive black hole. The giant elliptic galaxy M87 is the central galaxy of the Virgo galaxy cluster and is located at a distance of 50 million light years from the Earth. A black hole with a mass of about 3×10⁹ solar masses is found at the center of the M87 galaxy. Matter falls onto the black hole from the accretion disk and produces high-power electromagnetic flares in various frequency ranges and ejecting relativistic plasma jets thrown to distances of thousands of light years. Currently gamma telescopes have low resolving power and are incapable of pinpointing the exact area within the galaxy from which the highest energy radiation is emitted. It proved possible to overcome this difficulty since gamma flares are accompanied by simultaneous radio flares. When a gamma flare ends, charged particles continue to move along the jet and therefore the radio emission intensity continues to rise for considerably longer but gamma and radio flares start practically at the same time and therefore must be produced in the same source which was identifiable in view of very high resolution of the radiotelescopes. Gamma flares from the nucleus of the M87 galaxy were recorded for two years using the atmospheric Cerenkov detectors VERITAS, H.E.S.S. and MAGIC, and radio flares were observed in parallel, using an array of ten VEBA radiotelescopes of the National Radio Observatory (NRAO). It was established by radio observation that flares are generated at a distance from the central black hole not bigger than 50 times the radius of its event horizon. These observational data can help improve the theoretical models of jet formation and clarify the mechanisms of generation of emission. Source: Science 325 444 (2009)

Landau levels in graphene

Tue, 30 Jun 2009 20:00:01 GMT

Researchers at the National Institute of Standards and Technology (NIST) and Georgia Institute of Technology carried out for the first time direct measurements of Landau levels in a graphene specimen using a scanning tunneling microscope. Landau levels are defined as discrete quantum energy levels of electrons moving on cyclotron orbits in magnetic field. The velocity of free electrons in graphene (i.e. in a one atom thick carbon layer) is almost independent of their energy; in other words, the law of dispersion resembles that of massless particles. As shown in theoretical papers, this implies that energy spacings between consecutive Landau levels are not all equal as they are in ordinary metals and in two-dimensional electron gas. It was also predicted that at the zero level the energy of electrons is zero regardless of the strength of the applied magnetic field. Graphene films were studied on a silicon carbide SiC substrate in high vacuum at ultralow temperature. Conductivity was measured as a function of applied voltage using a microscope needle. About 20 Landau levels were detected by recording conductivity peaks. The experiment showed that in agreement with theoretical expectations, Landau levels in graphene are not equidistant and energy at the zero level is permanently zero. Source: Science 324 924 (2009)

Collective quantum tunneling in nanowires

Tue, 30 Jun 2009 20:00:02 GMT

A. Bezryadin and his colleagues at the University of Illinois discovered the effect of quantum tunneling of an entire bunch consisting of about ≈10⁵ electrons. Since the number of simultaneously tunneling electrons was so considerable, this process can be called the macroscopic quantum tunneling. Tunneling took place owing to a phase slip in a superconducting wire (of a diameter of about one nanometer) from a state with higher electric current to a state with lower electric current. Excess energy is released in this quantum transition as heat, the wire warms up and transfers from superconducting to normal state. It is likely that the discovered effect of macroscopic tunneling will find applications for quantum calculations. Sources: Nature Physics 5 503 (2009)

Acoustic lasers

Tue, 30 Jun 2009 20:00:03 GMT

P.M. Walker and his coworkers at the Nottingham University (UK) and the V.Ye. Loshkarev Institute of Semiconductor Physics in Kiev (Ukraine) designed a coherent source of sound waves working in the teraherz frequency range. A collimated beam of phonons with wavelength of about one nanometer is generated in layered semiconductor structure (superlattice) with specially selected spacings between fifty alternating layers of gallium arsenide and aluminum arsenide. Each layer is only several atoms thick. The new device is given a name saser (sound laser). The generation mechanism is based on strong electron-phonon coupling in semiconducting layers. Electrons in the upper layer are excited by nanosecond pulses of a conventional laser and emit phonons. Phonons are reflected at interfaces between layers and returned to the upper layer where they again interact with electrons; this leads to synchronization and coherence of the emitted sound pulses. Sound lasers have already been created before but they used other principles for sound generation and worked at much lower frequencies — in the GHz range. The acoustic emission of the new saser at frequencies on the order of one THz may find many useful applications. Sources: Phys. Rev. B 79 245313 (2009)

Gamma-ray burst during the reionization epoch

Tue, 30 Jun 2009 20:00:04 GMT

The BAT gamma telescope of the Swift space observatory detected a remotest cosmic gamma-ray burst GRB 090423. Immediately after detection of the gamma burst, x-ray and optical afterglow was observed by several telescopes and certain features of its spectrum led to the determination of the red shift of the burst: z ≈ 8,2. The host galaxy of the burst GRB 090423 has not yet been identified. Judging by the shape of the burst, it belongs to the class of long gamma-ray bursts produced by explosions of a special class of supernovas known as hypernovas. The burst GRB 090423 flared up at the epoch when the age of the Universe was only about 600 million years. However, its properties (total energy output, peak luminosity and shape of the light curve) do not differ from those of typical gamma bursts that arrive from intermediate and small red-shift sources, which points to similarity in the processes involved in explosions of early and contemporary stars. The burst GRB 090423 took place at the reionization stage which ended only at z ≈ 6; hence, observing gamma-ray bursts at high red shifts can help in understanding the physics of reionization processes and star formation in the early Universe. In particular, the conjecture that the rate of gamma burst generation follows that of star formation is found to be wrong because if it were true, the probability of observing gamma-ray bursts with redshifts z > 8 would be very low. A possible hypothesis is that a relatively higher number of very massive stars were formed during early cosmological epochs than are now, and that the UV radiation of these stars could be the source of reionization of the Universe. Sources: arXiv:0906.1578v2 [astro-ph.CO]; arXiv:0906.1577v2 [astro-ph.CO]

Exoplanet in the M31 galaxy

Tue, 30 Jun 2009 20:00:05 GMT

More than 300 planets outside the Solar system have been found by now from the periodic fluctuations of brightness or trajectories of stars. Eight more planets were found by observing microlensing at lensing stars which gravitationally focus radiation of more remote source stars on the line of sight. Planets gravitationally affect the light curve in the course of the passing of the lens across the source star as background, warping this curve in a certain manner. A number of papers analyzed the likelihood of this type of detection of planets in the nearest neighbor galaxies. G. Ingrosso (University of Salento, Italy), A.F. Zakharov (ITEP and JINR, Russia) and their coworkers carried out a new theoretical study of the strategy of searching for new planets in other galaxies. They predicted the characteristics of corrections to the light curve and found other parameters of lensing events involving planets, and also gave Monte Carlo predictions of the probabilities of specific observations with telescopes working with mirrors of different diameters. Thus this lensing technique can detect planets in other galaxies only 20 times more massive than the Earth. The authors of the paper analyzed earlier observations of lensing of stars in the galaxy M31 (Andromeda galaxy) and noticed that one of the stars of the M31 may have already displayed the signs of a planet. It was assumed earlier that a binary lensing star consisting of a pair of ordinary stars may have been responsible for the microlensing event РА-99-N2 recorded in 2004. However, according to G. Ingrosso, A.F. Zakharov et al the satellite of the main star is only 6.34 times more massive than Jupiter. This mass is approximately one half of the mass of the lightest stars — brown dwarfs — so that the satellite can be treated as a planet. Source: arXiv:0906.1050v1 [astro-ph.SR]

Rydberg atom in a molecule

Sun, 31 May 2009 20:00:01 GMT

V. Bendkowsky and her coworkers at the Universities of Stuttgart and Oklahoma prepared and then investigated Rb₂ molecules in which one of the rubidium atoms is in a Rydberg state with the outer electron in the s-state and n = 34-40 while the second atom remains in the ground state. The bonding between these atoms is caused by the scattering of the Rydberg electron on the electrons of the second atom. As Enrico Fermi established in 1934 the interaction potential in this sort of scattering of low-energy electrons may be attractive (negative scattering length). Since the frequency of the electron in the Rydberg atom is much higher than the characteristic frequency of interaction, the wave function is practically undistorted by scattering. The molecules obtained have a lifetime before decay of about 18 µs and are about 100 nm in size, which is 1900 times the Bohr radius. The molecules were created by laser-excitation of rubidium atoms in a Ioffe – Pritchard magnetic trap at a temperature of 3.5 µK. Oscillation spectra of molecules were measured in the ground and first excited states. The results obtained are in good agreement with the results of theoretical calculations. The researchers expressed hope that similar molecules may soon be created with the Rydberg electron in p-state, as well as a three-atom molecule and a molecule belonging to a class of what is known as trylobite molecules in which the Rydberg electron has a high angular momentum. Source: Nature 458 1005 (2009)

Superconductivity of europium

Sun, 31 May 2009 20:00:02 GMT

Atoms of bivalent rare-earth element europium at normal atmospheric pressure possess high magnetic moments; this is an obstacle to europium's transition to superconducting state. However, researchers of the Washington University in St. Louis J.S. Schilling and М. Debessai established that europium becomes weak Van Vleck paramagnetic under pressure of 80 GPa and acquires superconducting properties. Its superconducting transition temperature is T_c = 1,8 K, and T_c grows linearly with increasing pressure, reaching T_c = 2,75 K at 1142 GPa. Eu specimens were studied in a diamond anvil cell. The superconducting transition was identified by measuring electric conductivity and electric susceptibility in ac field. Electron levels of europium get distorted under high pressure so it is transformed from a bivalent to a trivalent element. A similar effect has already been observed in Americium that also becomes Van Vleck paramagnetic and superconducting under pressure (T_c = 0,79 K). Europium is the 53rd element which manifested superconducting properties in elemental state, and the 23rd element in which superconductivity arises only on application of high pressure. Sources: Phys. Rev. Lett. 102 197002 (2009) ; arXiv:0903.1808v1 [cond-mat.supr-con]

“Incandescent lamp” made of a nanotube

Sun, 31 May 2009 20:00:03 GMT

B.C. Regan and colleagues at the California NanoSystems Institute at UCLA (Los-Angeles) studied the incandescence of current-heated carbon nanotube. This experiment allowed them to test Planck's law of radiation almost to the limit of applicability of thermodynamics. Two gold contacts were lithographically attached to the ends of a nanotube placed over a hole in the silicon substrate in vacuum. The multilayer carbon nanotube was about 100 atoms thick. When electric current was passed through it, it heated up and began to glow. The emission wavelength was on the order of the nanotube length and much larger than the tube thickness. The radiation spectrum of nanotube emission was studied at different temperatures (different currents) by using a microscope and a set of optical filters. The classical thermodynamics used together with Max Planck's quantum hypothesis to derive the spectrum of blackbody radiation is applicable only to macroscopic systems consisting of a very large number of particles. The spectrum of thermal emission of a nanotube was nevertheless found after corrections for geometric factors to be in good agreement with the Planck formula. A nanotube has microscopic dimensions but is still large enough for a statistical description. At the same time, quantum properties of a microscopic system already manifest themselves sufficiently well. This experiment therefore dealt with the properties at the borderline between the thermodynamic and quantum regimes. Source: Phys. Rev. Lett. 102 187402 (2009)

Topological Hall effect in MnSi

Sun, 31 May 2009 20:00:04 GMT

A number of neutron scattering experiments provided indirect evidence that the distribution of electron spins in the compound MnSi contains a nontrivial topological structure composed of quasiparticles known as skyrmions. Skyrmions are described by chiral soliton models suggested by Tony Skyrme in 1961. It is assumed that topological excitations we call skyrmions arise in MnSi due to the spin-orbital interaction and form a lattice composed of three helices. Two independent groups of experimenters carried out new experiments that confirm this picture. М. Lee and his colleagues in the USA and Japan studied the Hall effect in a specimen of MnSi under pressure of 6-12 kbar. They found an additional contribution to conductivity detected at the magnetic field in the interval 0.1-0.45 T, which is not typical of the conventional Hall effect. In the model with skyrmions this contribution is known as the topological Hall effect. In another experiment A. Neubauer and his colleagues in Germany also studied the Hall effect in MnSi, but in a different segment of the phase diagram of this compound. Similar features of the Hall effect were discovered and could be explained in terms of skyrmion distribution. Source: Phys. Rev. Lett. 102 186601 (2009)

Refined Hubble constant

Sun, 31 May 2009 20:00:05 GMT

240 Cepheid variables and several supernova explosions of class Ia in remote galaxies were observed using the Hubble Space Telescope (HST) which made it possible to improve the accuracy of the Hubble constant by a factor of more than two, reaching the level of ≈5%. Some of the observed Cepheids (variable stars with a known period vs. luminosity curve) were located in the same galaxies as six of supernovas of class Ia; this made it possible to carry out direct joint calibration of various distance units. The team also recorded Cepheids in the galaxy NGC 4258 which contains cosmic masers. Using the geometry of the maser emission, the distance to the galaxy was measured with sufficient accuracy. Furthermore, the Hubble telescope measured the parallax of ten Cepheids in our Galaxy. This led to further refinement of the accuracy of calibration of distance measurement. The importance of calibration is illustrated by the fact that the relative accuracy of the Hubble diagram for supernovas of class Ia is less than 1% but the presence of systematic errors does not yet permit reaching the same accuracy in measuring the absolute value of the Hubble constant. Likewise the observation of baryonic oscillations in the spectrum of microwave background radiation does not yet reach better accuracy without the assumption that the Universe is flat plus the hypothesis that the parameter of the equation of state of dark energy is w = p/(ρ c²) = –1 (the cosmological constant). According to the new HST data, the refined value of the Hubble constant is H₀ = 74,2 ± 3,6 km s^-1Mpc^-1. Together with the WMAP data over the last five years this gives w = –1,12 ± 0,12. Source: arXiv:0905.0695v1 [astro-ph.CO]

Unusual elementary particle

Thu, 30 Apr 2009 20:00:01 GMT

The CDF collaboration detected an unusual particle in an experiment on the Tevatron accelerator; so far this particle could not be classified in terms of the accepted quark recipe of building mesons and baryons. The researchers selected B⁺ → J/ψφK⁺ decays. It was found that the intermediate state of the decays are new particles Y(4140) with a mass of about 4140 MeV. The experimenters recorded 14±5 such events at statistical significance of 3.8 standard deviations. The assumption that Y(4140) is one of the states of the system of c-anti-c quarks met with serious difficulties in describing the observed characteristics of the decays; the structure of the new particle is still unclear. There is a possibility that Y(4140) is a hadronic molecule, a hybrid particle that includes gluons as its components, or that Y(4140) is a new four-quark state. The Y(4140) particle is another in a series of exotic particles discovered in recent years (see Phys. Usp. 50 1289 (2007)). Source: arXiv:0903.2229v1 [hep-ex]

The Efimov effect for four particles

Thu, 30 Apr 2009 20:00:02 GMT

In 2006 H.-C. Nagerl and coworkers at the University of Insbruck in Austria observed for the first time bound quantum states of three cesium atoms (see Phys. Usp. 49 438 (2006)). The object of study was the Bose – Einstein condensate in which the intensity of interaction between atoms was controlled by the magnetic field and the bound states were identified by determining recombination losses. The effect of formation of trimers, predicted by V.I. Efimov in a theoretical paper in 1970, can arise even in the absence of bound paired states of atoms. After this discovery, in 2007-2008 two groups of researchers made a theoretical prediction that a similar effect is possible for four particles too; furthermore, some observational evidence of formation of bound quartets was found among the data of the first experiment. H.-C. Nagerl and his colleagues carried out a new experiment using a technique completely reproducing that of the earlier experiment. The presence of bound quantum states of four particles was demonstrated unambiguously. The range of applicability of the Efimov effect was thus extended to quartets of atoms. In contrast to trimers, bound quartets form only a pair of universal states and not an infinite sequence. The data obtained in this work will be used to test principally important aspects of quantum mechanics. Source: Phys. Rev. Lett. 102 140401 (2009)

Doubly magic nucleus ²⁴О

Thu, 30 Apr 2009 20:00:03 GMT

Nuclei of the unstable oxygen isotope ²⁴О having a large excess of neutrons were studied at the GSI laboratory (Darmschtadt, Germany). It was found that these nuclei are doubly magic, i.e. that their proton and neutron shells are completely filled, despite a strong distorsion of level structure at the nucleus stability boundary. A beam of ²⁴О nuclei was generated in the accelerator by sending ⁴⁸Ca nuclei into a target. This process created three ²⁴О nuclei per second which then collided with a carbon target, losing one neutron each. The form of the momentum distribution of the resulting ²³О nuclei made it possible to determine the structure of nucleon shells in ²⁴О nuclei. It was confirmed that ²⁴О are indeed doubly magic nuclei with spherically symmetric shells. The study of ²⁴О isotopes is important for nuclear astrophysics because they may be born in supernova explosions; these nuclei may also be present in the crust of neutron stars. Source: Phys. Rev. Lett. 102 152501 (2009)

Optical Maxwell's demon

Thu, 30 Apr 2009 20:00:04 GMT

J.J. Thorn and his colleagues at the Oregon University created an optical barrier that lets through atoms of ⁸⁷Rb in one direction only. Two almost parallel laser beams separating a dipole atomic trap into two parts were focused near its center. The frequency of one of the beams differed from the resonance frequency of transition between sublevels of hyperfine splitting in ⁸⁷Rb. This beam created a potential barrier whose permeability for an atom depends on its state: atoms on the lower subsevel pass through the beam unobstracted while the potential created by the barrier for the excited atoms is repulsive. The frequency of the second laser equals the frequency of transition between the sublevels. If an atom in the ground state crosses the second beam and passes from one side of the trap to the other, it absorbs a photon from the second beam and after this is unable to pass through the barrier in the opposite direction. As a result, atoms accumulate on only one side of the barrier. This system of beams which operates as an atomic diode is an analog of Maxwell's demon. By analogy to Maxwell's thought experiment, it is possible to experimentally check how entropy is transferred between the parts of the system in the case of the optical barrier. The reduction in entropy caused by the displacement of atoms to one side of the trap is compensated for by the production of entropy when photons are scattered. Furthermore, this technique is of interest for developing new ways of laser cooling of atoms. Source: arXiv:0903.3635v1 [physics.atom-ph]

Superconductivity in SrFe₂As₂

Thu, 30 Apr 2009 20:00:05 GMT

Scientists at the Tokyo Institute of Technology discovered that after the compound SrFe₂As₂ was treated with steam, it become a superconductor with the temperature of superconducting transition T_c ≈ 25 К. The compound SrFe₂As₂ belongs to a class of new layered iron-based superconductors. Its intense study began in 2007 (see Phys. Usp. 51 1201 (2008)). However, superconductivity arises in these compounds only when they are doped with certain impurities or under high pressures. H. Hosono and his coworkers exposed a film of pure (undoped) SrFe₂As₂ to moist atmospheric air which resulted in the samples becoming superconducting. Checking demonstrated that exposure to individual components of air (nitrogen, oxygen or carbon dioxide) without H₂O did not produce such changes. So far the exact mechanism of the effect of H₂O remains unclear. It is suggested that either oxygen atoms of water molecules penetrate the structure of the crystal SrFe₂As₂, or that strontium atoms bind to the OH group and form atomic vacancies. Superconductivity arises in response to application of water in some other (not iron-based) layered compounds too but neutron diffraction studies established that interlayer spacings in SrFe₂As₂ do not grow in response to water treatment but are reduced. It was also found that the superconductivity of SrFe₂As₂ caused by water is strongly anisoropic in magnetic field (T_c depends on the field orientation) — in contrast to the almost isotropic superconductivity of SrFe₂As₂ doped with cobalt atoms. Source: arXiv:0903.3710v1 [cond-mat.supr-con]

Search for the Higgs boson

Tue, 31 Mar 2009 20:00:01 GMT

New constraints on the possible mass of the Higgs boson have been established by combining the data of two experiments CDF and D0 conducted at the Tevatron accelerator of the Enrico Fermi National Accelerator Laboratory. It was obtained earlier at the CERN electron-positron collider that the Higgs boson mass is greater than 114 GeV. On the other hand, theoretical calculations for processes involving the Higgs boson produced the upper bound on its mass as 185 GeV. Systematic searches are conducted nowadays for the Higgs boson in the possible mass range of 114-185 GeV and certain intervals within this range have already been excluded (see Phys. Usp. 51 979 (2008) ). According to the data of CDF and D0, the Higgs boson cannot have the mass in the range between 160 and 170 at a probability of 95%. In addition, the mass of the W boson has been measured in the D0 experiment with record accuracy: 80,401 ± 0,044 GeV. Improved accuracy of measuring the W boson mass may help in Higgs searches by improving the knowledge of boundaries of possible mass ranges and the accuracy in calculations of reactions involving the Higgs boson. Source: http://arxiv.org/abs/0903.4001

Increasing the luminescence efficiency of carbon nanotubes

Tue, 31 Mar 2009 20:00:02 GMT

F. Papadimitrakopoulos and his coworkers at the University of Connecticut found a way to enhance the luminescence efficiency of single-wall carbon nanotubes up to a record-high level of 20%. The luminance of nanotubes is limited by defects on their surface, such as absorbed oxygen molecules. Earlier attempts of suspending nanotubes in solutions produced luminescence efficiency of at most 0.5%. In the new experiment nanotubes were coated with a layer of a compound FC12 — an analog of flavin mononucleotide (its composition is not very different from that of vitamin B). As this coating was added, FC12 molecules self-organized themselves into a tube coaxial with the carbon nanotube; this process automatically removed extraneous molecules from the nanotube surface. The luminescent glow of nanotubes is caused by irradiating them with IR light or by electric excitation. Carbon nanotubes with high-efficiency luminescence may lead to numerous useful applications, e.g. in nanoscale photodetectors and in biological sensors. Source: Science 323 1319 (2009)

Cooling of nanotubes

Tue, 31 Mar 2009 20:00:03 GMT

P. Avouris and colleagues at the IBM Research Center and researchers at Duke University (USA) discovered that heat can be dissipated from carbon nanotubes into the substrate in contact with it even if no chemical bonding exists between the two. The experiment was conducted with nanotubes on silicon oxide substrate in a configuration resembling that of the field transistor. Thermal vibrations were recorded by Raman spectroscopy techniques. Heat transfer from nanotubes is caused by electric interaction between charges: electrons in a nanotube interact with charges induced by electric fields of the substrate, energy is transferred to substrate charges close to its surface, and thermal vibrations then travel deeper into the specimen. This effect is important for solving the problem of cooling of microelectronic devices based on using carbon nanotubes. Source: physicsworld.com

Stochastic resonance in digital electronics

Tue, 31 Mar 2009 20:00:04 GMT

W. Ditto (Arizona University) and his colleagues have discovered that the work of a logic gate can be stabilized by a certain level of stochastic noise. Noise typically constitutes a disruptive factor for the functioning of electronic devices, for instance, it may cause unpredictable switching of the state in logic elements. In fact, stochastic resonance emerges in some nonlinear systems so that it becomes possible to separate the useful signal by increasing the level of broadband noise because then the sum of signal and noise exceeds a certain threshold value (on stochastic resonance see Phys. Usp. 42 37 (1999) and Uspekhi. fiz. nauk 179 266 (2009) (in Russian) ). Stochastic resonance manifests itself, for example, in the case of alternating climate cycles and in neuron systems. Arizona State University researchers mathematically modeled a logic gate with two rectangular signals plus a noise signal fed in as input; they found that as noise level increases the logic gate begins to function predictably and this stability survives in a sufficiently broad range of noise amplitudes. The theoretical predictions were supported by the study of an electronic analog of the nonlinear system in question. Another useful property of the designed logic gate was the possibility of reversing its logic by sending a control signal. Source: Phys. Rev. Lett. 102 104101 (2009)

X-ray observations of the pulsar PSR J0108-1431

Tue, 31 Mar 2009 20:00:05 GMT

The space X-ray observatory Chandra detected the oldest of the currently known isolated pulsars (those not in binary systems) emitting in the X-ray range. Radio observations established that the pulsar PSR J0108-1431 is about 170 million years old. The unexpected result was the very high luminosity of the pulsar despite its old age and slow rotation rate (at a period of about 0.8 s). Approximately 0.4% of energy connected with the slowdown is transformed into X-ray radiation. The pulsar PSR J0108-1431 lies at a diatance of 770 light years and moves at a speed of about 200 km s^-1. What continues to remain unclear is the mechanism of X-ray emission. It is possible that two components are present in the radiation, one generated in the magnetosphere of the pulsar and the other close to the pulsar poles. Sources: http://arxiv.org/abs/0803.0761, http://chandra.harvard.edu/press/09_releases/press_022609.html

Dark matter in dwarf galaxies

Tue, 31 Mar 2009 20:00:06 GMT

The Hubble Space telescope was used to study 29 dwarf galaxies in the core of the Perseus galaxy cluster lying at a distance of 250 million light years from the Earth. In contrast to the neightbouring spiral galaxies, dwarf galaxies have smooth regular shape without visible signs of decay caused by tidal gravitational forces exerted by the cluster core and other galaxies. This is an indication that the dwarf galaxy masses are sufficiently large to resist tidal destruction. This invisible mass is that of dark matter (hidden mass). The mass-to-luminance ratio of the investigated dwarf galaxies reaches ≈120 solar units. It has thus been established that dwarf galaxies of elliptic galaxies in cluster cores as well as dwarf spheroidals of the Local group of galaxies contain a relatively high amount of dark matter. Sources: http://arxiv.org/abs/0811.3197 , http://hubblesite.org/newscenter/archive/releases/2009/11/full/

Conductivity of graphene

Sat, 28 Feb 2009 21:00:01 GMT

In the experiments carried out so far the conductivity of graphene (one-atom thick carbon layer) was measured on a substrate and no data was available on graphene conductivity in vacuum, that is, when the structure of graphene is not disturbed by the influence of substrate charges (see e.g. Phys. Usp. 51 744 (2008) ). Theoretical investigation of this characteristic is not easy since the effective electrodynamic coupling constant α_g in graphene is approximately 300 times greater than the fine structure constant α = 1/137, so perturbations theory cannot be used. J.E. Drut and Т.А. Lahde developed a new theoretical approach for calculating electron properties of graphene. They applied a numerical “lattice” computation technique resembling methods used in quantum chromodynamics where the coupling constant is also large. Computations revealed how the energy gap in the graphene electron spectrum changes depending on where graphene is placed. It was shown that conductivity is affected by the coupling constant α_g. For graphene on SiO₂ substrate the authors obtained α_g ≈ 0.79 so graphene is conducting. As α_g increases beyond the critical value α_g^cr = 1.11 ± 0.06, graphene turns into insulator. For instance, in vacuum α_g ≈ 2.16 and graphene is insulator. The validity of the formulated theoretical predictions can be tested in future experiments. Source: Phys. Rev. Lett. 102 026802 (2009)

Type-1.5 superconductors

Sat, 28 Feb 2009 21:00:02 GMT

V.V. Moshchalkov and his colleagues in Belgium and Switzerland confirmed experimentally that the two-component superconductor MgB₂ possesses superconducting properties of both type-1 and type-2 and consequently proposed to refer to it as superconductor of intermediate type-1.5. They studied the spatial distribution of superconducting vortices in a single crystal of MgB₂ as it was cooled and went into superconducting state in external magnetic field perpendicular to the specimen surface. It was found that vortices are not uniformly spread but form denser patterns of spider web or ribbon-like shape. This implies that at small distances vortices repel one another like they do in type-2 superconductors but attract one another at larger distances like they do in type-1 superconductors. This phenomenon is theoretically explained in terms of two weakly coupled order parameters in superconducting MgB₂ so that MgB₂ manifests a combination of the properties of two superconductors with different ratios ξ/λ (coherence length to penetration length) typical of type-1 and type-2 superconductors. Measurement results are in good agreement with the predictions of numerical modeling of vortex dynamics and calculations in terms of the Ginzburg – Landau two-component theory. Source: http://arxiv.org/abs/0902.0997

Entangled state of mechanical oscillators

Sat, 28 Feb 2009 21:00:03 GMT

J.D. Jost and his colleagues at the US National Institute of Standards and Technology (NIST) for the first time entangled two mechanical systems — two oscillators consisting of pairs of ions ⁹Be⁺—²⁴Mg⁺ in a potential well. First all four ions were trapped into a potential well in which the internal degrees of freedom of two ⁹Be⁺ ions got entangled. Then two laser beams were used to modify the shape of the potential well and the ions were separated into two pairs of ⁹Be⁺—²⁴Mg⁺ of size ≈4 µm the separation between the pairs was 0.24 mm. These pairs of ions resemble macroscopic springs with weights at their ends. Laser pulses forced the internal state of entanglement in each pair to transfer to the mechanical oscillations of ions in the pair relative to one another. This resulted in the formation of two pairs of ions entangled over mechanical oscillational degrees of freedom. Source: http://arxiv.org/abs/0901.4779v1

Powerful gamma-ray burst

Sat, 28 Feb 2009 21:00:04 GMT

NASA's Fermi Gamma-ray Space Telescope recorded a space gamma-ray burst of extraordinary power, designated GRB 080916С. The optical afterglow of the burst was observed with the GROND detector of the 2.2 m telescope, as well as with other telescopes. The source of the burst was probably a supernova explosion in a remote galaxy with red shift of z = 4.35 ± 0.15; consequently, GRB 080916С belongs to the 5% of the remotest recorded gamma-ray bursts. It is assumed that violent energy releases resulting in gamma-ray bursts take the form of narrow jets. It was established from the variability of gamma emission that the Lorentz factor of a burst exceeds 1090. If we assume for the sake of comparison that the emission is isotropic, the energy released in the explosion comes to 6.5 × 10⁵⁴ erg, or roughly 9000 times the energy of ordinary supernovas. This allows classifying the gamma-ray burst GRB 080916С to have record-high energy. As observed with some other bursts, high energy photons arrive with certain time delay relative to photons from the low energy part of the spectrum. This may indicate that emission from different parts of the spectrum is generated in different conditions at different stages of the explosion, or in different regions of the jet-carried ejecta. Source: http://arxiv.org/abs/0902.0761

Coherence of electrons in photosynthesis

Sat, 28 Feb 2009 21:00:05 GMT

It was earlier assumed that the energy transfer between electrons of different molecules in protein complexes responsible for photosynthesis proceeds classically (via Coulomb collisions). However, data were obtained in 2007 that electrons in protein molecules are quantum-coherent, so energy is transferred by a wave mechanism. The 2007 experiment required prolonged irradiation of molecules by laser light which resulted in their degradation and destruction; furthermore, the spectra could be obtained only at select points of protein complexes. I. Mercer (of University College Dublin, Ireland) and his colleagues in Great Britain worked out a new and perfected technique which made it possible to clarify details of energy transfer in photosynthesis. By using irradiation with a train of short laser pulses with different wavelengths Mercer et al obtained a two-dimensional spectrum of protein complexes and were able to build a spatio-temporal picture of the processes. Thus the new experiment has confirmed that electrons transfer energy in a coherent manner. Sources: Phys. Rev. Lett. 102 057402 (2009) ; http://focus.aps.org/story/v23/st5

Casimir – Lifshitz force in repulsion mode

Sat, 31 Jan 2009 21:00:01 GMT

In 1961 E.M. Lifshitz, I.Ye. Dzyaloshinskii and L.P. Pitaevskii, Adv. Phys. 10 165 (1961) (see also Uspekhi. fiz. nauk 73 381 (1961) (in russian) ) formulated the conditions at which the Casimir – Lifshitz force between two plates becomes repulsive: it is necessary that the dielectric permittivity of the intermediate dielectric layer be lower than that of one of the plates but higher than that of the other. The Casimir – Lifshitz force in repulsion mode was measured in a number of experiments but only at distances not larger than several nm where the Casimir effect works in the Van der Waals mode and the contribution of intermolecular forces is large. F. Capasso, J. Munday and A. Parsegian were the first to conduct detailed measurements of the Casimir – Lifshitz force at distances from 20 to 300 nm in which the effect of repulsion stemming from the distortion of the spectrum of zero quantum oscillations is directly measurable. They observed the interaction between a sphere 40 µm in diameter coated with a thing gold layer and a quatz plate in liquid bromobenzene. Using a small sphere instead of the second plate proved to be very convenient as this removed the need in the complicated procedure of setting the plates parallel to one another at a small separating distance. Moreover, this method allows determining the force from the velocity of the sphere, and determining this velocity by measuring the displacement of the beam of light reflected from the sphere. Preliminary calibration was carried out far from the quartz plate (where the Casimir effect is very low): various velocities of motion through the liquid were put in correspondence with hydrodynamic forces. The measured force of repulsion of the sphere from the plate was in good agreement with calculations in terms of the Lifshitz – Dzyaloshinskii – Pitaevskii theory. The quartz plate was replaced in the control experiment with a gold plate; as was expected, in this case the sphere was attracted to the plate. Source: Nature 457 170 (2009)

Symmetry of energy gap in Ba_0.6K_0.4Fe₂As₂

Sat, 31 Jan 2009 21:00:02 GMT

Experiments show that the energy gap in high-temperature cuprate superconductors (the bonding energy of a Cooper pair) has different signs on different regions of the Fermi surface. Angle-resolved photoelectron spectroscopy has not confirmed the existence of a similar asymmetry of the energy gap in iron-based superconductors such as Ba_0.6K_0.4Fe₂As₂. In fact, however, these superconductors cannot be described in terms of the Bardeen – Cooper – Schrieffer theory in which the gap is assumed to be symmetrical. A hypothesis was advanced that the result is negative because the spectroscopic techniques used to study these materials are not sensitive to the phase of the wave function of electrons that carries the symmetry information. A.D. Christianson and his colleagues conducted new studies of the energy gap in Ba_0.6K_0.4Fe₂As₂ by using inelastic neutron scattering which makes it possible to measure the phase. The characteristic effect on the magnetic moments of neutrons established that the symmetry of the gap in Ba_0.6K_0.4Fe₂As₂ differs from the d-symmetry in cuprate superconductors and is most likely of the type s± in which case electrons split into groups with the opposite phase of the wave function. In this case the pairing of electrons is created through antiferromagnetic fluctuations. Source: Nature 456 930 (2008)

Stability of coherence of laser light

Sat, 31 Jan 2009 21:00:03 GMT

М. Bellini (University of Florence, Italy) and his coworkers confirmed a theoretical prediction of R. Glauber (1963) stating that removal of individual photons from a laser beam leaves the beam in coherent quantum state. A laser beam was passed through two optical splitters. The first splitter split the beam in two and their interference gave the measure of the degree of coherence. One of these beams was sent through the second splitter with very little splitting efficiency which allowed the authors to single out individual photons from this beam. These photons were recorded by a detector that could be triggered by single photons. In accord with R. Glauber's theoretical prediction it was found that extraction of individual photons did not destroy the coherence of the laser beam. Furthermore, М. Bellini and his coworkers developed a technique for adding single photons to a beam; in this manner they confirmed that the operations of extracting and injecting of photons are noncommutive. Source: http://physicsworld.com/cws/article/news/37106

Quantum cascade laser

Sat, 31 Jan 2009 21:00:04 GMT

K.J. Franz (Princeton University) and his colleagues were the first to discover in 2007 that the quantum cascade laser they built generated, in addition to the "normal" beam, a laser beam at the second frequency but of lower power (see Appl. Phys. Lett. 90, 091104 (2007)). The lasing area of the laser in question consists of tens of layers of various semiconductors, each only a few atomic layers thick. Later the same team of researchers obtained further interesting results. It was found that the radiations at the two frequencies are in anticorrelation which is caused by a "competition" for charge carriers that can take part in the radiation at both the principal and the second frequency: as temperature increases, the output power of the second beam grows while that of the first one decreases. A possible explanation of the discovered effect was also suggested. It is assumed that the second beam is generated by electrons with momenta k = p / $\hbar$ ≈ 3.6 × 10⁸ m^-1 in nonequilibrium states while quasi-nonequilibrium electrons with zero momenta are responsible for the first beam. Lasers emitting via the new mechanism may find useful practical applications. The nearest task facing the researchers is to suppress the emission of the laser beam at the principal frequency so as to achieve emission of the second type only. Sources: Nature Photonics 3 930 (2008); http://engineering.princeton.edu/news/laser_08

Fast-moving stars

Sat, 31 Jan 2009 21:00:05 GMT

A group of astronomers led by R. Sahai using NASA's Hubble Space Telescope found 14 young stars moving at huge velocities through the interstellar gas and creating a bow shock ≈10¹¹-10¹² km in size, leaving behind a trace of glowing gas. The shock wave is produced by the collision of the powerful stellar wind with the surrounding gas. The stars were found to be fairly young — not older than a million years or so — and have masses not more than eight time the solar mass. Hubble's observations yielded the shape and structure of shock waves. The stars move at about 180000 km h^-1, which is roughly five times higher than the characteristic velocities of ordinary young stars. It is assumed that these stars get such high velocities from slingshot ejection out of a stellar cluster in a close flyby, or from gravitational interaction between two binary stars or a binary and a single star, or when the second component of a pair exploded as a supernova. Such fast-moving stars were first detected by the IRAS telescope at the end of the 1980s; however, the stars observed then by the IRAS were considerably more massive. Source: http://hubblesite.org/newscenter/archive/releases/2009/03/full/

New measurements of the fine structure constant

Wed, 31 Dec 2008 21:00:01 GMT

Using a combined method based on the Bloch oscillations effect and atom interferometry, М. Cadoret and his coworkers in France were able to measure the fine structure constant with a relative accuracy of 4,6×10^-9. Other recent superhigh-precision experiments for α directly measured only the the anomalous magnetic moment of the electron after which α was calculated using the formulas of quantum electrodynamics. In the experiment of М. Cadoret and his colleagues, measurements of α were more direct (did not need the assumption of validity of QED formulas) as they used recoil pulses from atoms in periodic potential. Rubidium atoms were illuminated by two oppositely directed laser beams with slightly different frequencies; atoms absorbed photons from one beam and then re-emitted them into the other beam. The frequency difference was compensated for by the Doppler effect on moving atoms, and its measurement yielded the value of α. The agreement with the results obtained in other experiments and with theoretical QED achieved the test of this theory at currently the highest accuracy. Source: Phys. Rev. Lett. 101 230801 (2008)

Light pulse in optical filament

Wed, 31 Dec 2008 21:00:02 GMT

The controversy concerning the momentum of light in a transparent medium remains the object of debate for nearly a hundred years. The problem of choosing between the expressions given by H. Minkowski and M. Abraham lies in the ambiguity of dividing the total momentum into that of the field and that of the medium, and in the need to take into account the action exerted by the electromagnetic field on the medium when light is emitted or absorbed (see Uspekhi Fiz. Nauk 118 175 (1976) (in Russian)). Chinese researchers W. She, J. Yu и R. Feng carried out a new experiment which confirmed Abraham's expression. Optical filament 1.5 mm in length and half a micron in diameter was suspended vertically in a hermetically sealed vessel. Light from two lasers was sent downward through the filament. The first laser, at wavelength 650 nm and power output 0.5 mW served to illuminate the fiber and facilitate observing its motion which was photographed 10 times/min through a lens installed in the wall of the vessel. When a light pulse from the second laser at wavelength 980 nm and variable power output of 0 to 79 mW emerged from the lower end of the filament, it imparted to it a momentum and the upward-directed recoil caused filament bending. This behavior confirmed Abraham's expression for momentum: if Minkowski's approach were correct, there would be a downward stretching load on the filament. The experiment was successful owing to the small weight of the filament: the recoil momentum compensated for the weight of the free end segment of the filament. The experiment confirmed the theoretical evaluation which predicted this compensation to occur at laser power output of about 4 mW. A not very different result was observed when the second laser worked in continuous, not pulsed, mode. Source: Phys. Rev. Lett. 101 243601 (2008)

The Magnus effect for light

Wed, 31 Dec 2008 21:00:03 GMT

Е. Hasman and his colleagues at the Technion-Israel Institute of Technology have been the first to observe in the adiabatic mode the spin Hall effect for photons, also known as the optical Magnus effect. This effect was observed earlier but only in the nonadiabatic case of strong nonuniformity when a particle's trajectory is stopped abruptly. The spin Hall effect for photons consists in the interaction between the spin of a particle and the curvature of its trajectory, resulting in an additional force affecting the trajectory of motion. Hasman et al studied propagation of laser light along a glass cylinder. The beam went through total internal reflections and its trajectory was twisted into a helix along the surface. Measured at the exit face of the cylinder were the beam direction and the Stokes parameters. The experiment carried out in the Technion confirmed the detailed theory of the optical Magnus effect, based on the dynamic effect of the geometric Berry phase. Source: Nature Photonics 2 748 (2008)

The Lamb shift in solids

Wed, 31 Dec 2008 21:00:04 GMT

The Lamb shift of atomic energy levels stems from the interaction between electrons and virtual electron-positron pairs created in the vacuum. Typically it is not possible to observe the Lamb shift in solids since energy levels in them form broad bands. However, A. Wallraff and his coworkers from Switzerland and Canada were able to measure the Lamb shift of the microscopic quantum bit (qubit) in a resonator. The qubit consisted of two tiny pieces of superconductor connected by two tunnel junctions. This system is known as transmon. The energy levels of the transmon are dictated by the distribution of Cooper pairs in superconductors. A transmon was placed in a microwave resonator where it could absorb and emit photons of certain frequencies. By virtue of its shape, the transmon possessed a large dipole moment; also, a special resonator configuration was chosen so as to enhance the effect of interaction with virtual photons. The Stark effect contributed only negligible corrections because it it was felt only outside the area of resonance with virtual photons. As a result, the observed Lamb shift of transmon's energy levels was approximately 1.4% of the energy difference between the neighboring levels, which is 10,000 times greater than the Lamb shift in the hydrogen atom outside the resonator. The Lamb shift results in decoherence of the qubit state. The experiment conducted by A. Wallraff and his colleagues provides a recipe for avoiding undesirable decoherence in future quantum computers: choose device configurations that are not in resonance with virtual photons. Source: Science 322 1357 (2008)

Stimulated emission of surface plasmon polaritons

Wed, 31 Dec 2008 21:00:05 GMT

Surface plasmons and plasmon polaritons constitute electromagnetic pulses in the electron gas, localized or moving along the metal-dielectric interface, respectively. These quasiparticles are strongly absorbed in the range of optical frequencies and have short propagation length, which create problems for possible practical applications. It was suggested that the problem may be solved by using optically active impurities. М.А. Noginov (Norfolk University, USA) and his coworkers were able for the first time to use this technique and achieve both the compensation of losses of surface plasmon polaritons and the observation of their stimulated emission which is similar to the stimulated emission of photons in lasers. A 32 to 82 nm thick silver layer was deposited onto a face of a grass prism. The silver layer was coated with a polymer film doped with dye molecules. Excitation of surface plasmon polaritons was produced by light pulses first on the side of the prism (for the sake of calibration needed to measure the reflection profile R(θ)) and then on the side of the polymer film. The dye molecules absorbed photons and emitted surface plasmon polaritons. The threshold for polariton emission and the spectrum of polaritons agreed with theoretical predictions for a laser-like radiation. This experimental study may lead to useful applications in creating novel metamaterials and plasmon nanodevices. Source: Phys. Rev. Lett. 101 226806 (2008)

A very hot white dwarf

Wed, 31 Dec 2008 21:00:06 GMT

The space telescope FUSE detected a white dwarf KPD 0005+5106 with record-high surface temperature of 200,000°С. At this temperature, and object is visible in the UV range of spectrum. White dwarfs (their internal pressure is sustained by the degenerate electron gas) evolve from massive stars after the thermonuclear fuel inside them is exhausted. High temperatures can be produced only immediately after the white dwarf is formed, before it starts to cool down, so the observation of a white dwarf with a temperature of 200,000°С is a very rare event. Source: http://www.space.com/scienceastronomy/081212-hot-star.html