Extracts from the Internet


Soliton collision in Bose-Einstein condensate

J.H.V. Nguyen (Rice University, USA) with colleagues showed that solitons moving in a Bose-Einstein condensate can pass through each other preserving their size and shape. The 7Li atom condensate obtained through evaporative cooling was kept in a cylindrical potential trap. Laser radiation across the trap generated an additional potential barrier, and the cloud fell into two. Upon transition through the Feshbach resonance the scattering length was decreased by variation of the magnetic field. Then, in two parts of the trap solitons consisting of ≈ 28000 atoms each appeared at a distance of 26µm from each other. The barrier having been switched off, the solitons began moving towards each other passing through the trap over and back several times. The real-time observation of the solitons was conducted using radiation scattering. As was predicted theoretically from the solution of Gross-Pitaevskii equation, for thezero phase difference Δφ = 0 of the soliton wave functions, the solitons strengthened each other at the collision point, and in the counter phase Δφ = π the gas density lowered because of destructive interference. In some cases a spatial gap was left between colliding solitons. However, it has been proved that solitons do not spring back elastically but pass through the gap and through each other. To this end, solitons of unequal size were created, for example, with 2:1 ratio of the number of atoms which could be distinguished both before and after the passage. When colliding, the sufficiently dense solitons which could not already be thought of as one-dimensional objects bjects collapsed, i.e. were destroyed. Similar soliton collapses had already been observed before in nonlinear optics. Source: Nature Physics 10 918 (2014)

FFLO phase in a superconductor

V.F. Mitrovic (Brown University, USA) and her colleagues have confirmed experimentally the theoretical prediction made by P. Fulde, R. Ferrell, A.I. Larkin, and Y.N. Ovchinnikov in 1964. According to their calculations, near the upper critical magnetic field a superconductor can be divided into discrete layers between which normal-conductivity layers reside at the order-parameter nodes. This state is called the FFLO phase after the names of the authors. This effect is possible provided that a superconductor contains unequal number of electrons with opposite spin directions. Then unpaired electrons induce the formation of quasi-particles in Andreev bound states accompanied by the occurrence of non-superconducting layers. The experiment was performed at the National High Magnetic Field Laboratory (LNCMI, Grenoble, France). The organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 was examined by nuclear magnetic resonance on 13C atoms, and the superconductor had been specially fabricated using this isotope. The sample was exposed to a sequence of pulses, and the spin relaxation time was found from the spin echo. The maxima of the magnetic-field dependence of the relaxation time testified to the presence of Andreev states and the FFLO phase. Earlier, the latter phase was only observed by an indirect method from the phase diagram of the superconductor. Possibly, the FFLO phase may occur in the substance of neutron stars in which superconductivity and strong magnetic fields supposedly exist. Moreover, the FFLO phase can find practical application in spintronics. Source: Nature Physics 10 928 (2014)

Gas of dipolar molecules

T. Takekoshi (the University of Innsbruck, Austria) with colleagues obtained and studied the ultracold gas of 87Rb133Cs molecules in the lower state of hyperfine splitting of theirenergy levels. The 87Rb133Cs molecules were obtained through magnetic association in an ultracold mixture of 87Rb and 133Cs gases in the magnetic field. Transitions of molecules between levels were induced by lasing at specially selected frequencies with the use of stimulated adiabatic Raman passage. With efficiency of 90 % the gas was set to the lowest of the hyperfine splitting states, the state being controlled using the magnetic field. The dipolar nature of 87Rb133Cs molecules was shown by the characteristic quadratic shift of transition frequencies depending on the electric field. Investigated and confirmed in the experiment was also good stability of dipolar 87Rb133Cs molecules in the lowest energy state under pair collisions in a gas. Source: Phys. Rev. Lett. 113 205301 (2014)

Magnetic mirror for the IR range

Radiation reflection from a mirror is as a rule due to the interaction between the vector of the electric field of an incident electromagnetic wave and the electric charges in the substance. S. Liu (Sandia National Laboratories, USA) with colleagues designed a nonmetallic mirror for the IR range in which the magnetic rather than the electric field of the waveunderwent interaction with the mirror substance.In the latter case, the reflection induces phase reversal, while in the magnetic interaction the phase of the reflected wave coincides with the phase of the incident wave, which was demonstrated in experiment for the first time. Magnetic mirrors on the basis of metamaterials had already been made of metallic and silicon elements, but they showed great reflection loss, and the phase variation was not directly verified in experiments. S. Liu with colleagues fabricated a new metamaterial consisting of a two-dimensional array of approximately cubic subwave microresonators made of a low-loss dielectric (tellurium) on a BaF2 substrate. The measurements were taken using the time-domain spectroscopy based on the laser radiation reflection from a sample and the comparison of the reflected light with the reference beam upon their mixing in a GaSe crystal. A part of the surface of the same sample was covered with a layer of gold, which allowed a direct comparison of the reflectivities of a magnetic and a usual mirror and find the distinction in the reflected wave phases. The phase remained unchanged under reflection from the metamaterial, which testified to a magnetic character of the reflection. Such magnetic mirrors are promising for the creation of sensitive chemical sensors and temperature-sensitive elements. Source: Optica 1 250 (2014)

A stellar-mass black hole in an ultraluminousX-ray source

A probable explanation of the nature of ultraluminous X-ray sources observed in galaxiesis thought to be matter accretion onto intermediate-mass black holes (≥102M) or super-Eddingtonaccretion onto stellar-mass black holes. Part of the ultraluminous sources emit according to the first mechanism (e.g., source X-1 in galaxy M82) and some sources according to the second mechanism, as was shown in the new analysis of source P13 in galaxy NGC 779311 accomplished by C. Motch (University of Strasburg, France) with colleagues. They used the data of X-ray and optical telescopes obtained within several years. Object P13 is a binary system with orbital period of 64 days consisting of a supergiant star with mass of (18-23)M and a black hole. The high luminosity and the shape of P13 are typical of ultraluminous X-ray sources. By modeling optical and UV emission modulations due to X-ray star heating the black hole mass was found not to exceed 15M. Thus, this ultraluminous X-ray source belongs to the class of sources with a stellar-mass black hole, although its X-ray luminosity is about two times higher than the Eddington luminosity. Source: Nature 514 198 (2014)

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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.

Permanent editor is Yu.N. Eroshenko.

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