Extracts from the Internet


Spin echo method for neutron studies

P. Schmidt-Wellenburg (Paul Scherrer Institute, Switzerland) and his colleagues developed a new method of neutron spectroscopy based on measurement of spin echo in magnetic and gravitational fields. The method resembles magnetic resonance tomography typically applied to atomic nuclei. Cold neutrons were placed in a chamber and were reflected from its walls showing a certain kinetic energy distribution and, correspondingly, the distribution in the maximum trajectory height relative to the chamber bottom. The magnetic field in the chamber had a vertical gradient, and therefore neutrons with different energies were on the average in fields of different heights and experienced spin precession with different rates, which led to dephasing. The experiment began with an electromagnetic pulse that made spins co-directed, and several tens of second after the exposition to another two auxiliary pulses the polarization was measured. This method made it possible to reconstruct the neutron energy spectrum with record accuracy and determine the magnetic field gradient with an error of only 1.1 pT cm-1, which is eight times smaller than that in the previous experiments. The new method is planned to be used, in particular, for measuring the neutron electric dipole moment which is predicted to be associated with a CP invariance violation, but has not yet been revealed with the now attained accuracy. Source: Phys. Rev. Lett. 115 162502 (2015)

Thermodynamic time arrow in quantum systems

T.B. Batalhao (the Federal University of ABC, Brazil) with colleagues experimentally measured the entropy production in a microscopic quantum system. The investigated nuclei of 13C atoms in the composition of CHCl3 chloroform molecules were in an oscillating magnetic field. When the oscillation frequency was low, the nuclear spins had time enough to change direction, but upon frequency heightening the “breakdown” occurred and the spin flip was chaotic, which introduced disorder and, accordingly, raised the entropy. The measurements were based on the effect of interaction between 13C and H nuclei in the same molecules which were quantum q-bits to which the quantum tomography method was applied. Although the experiment was performed with a macroscopic liquid chloroform sample, the ensemble of carbon nuclei could be regarded as different realizations of states of single nuclei. The magnetic pulses had an asymmetric time profile, and chloroform was exposed to first direct and then time-reversed pulses. The experimental results confirmed the Kullback-Leibler relation describing the difference of the change in the system entropy upon direct and reverse variation of its properties. Before, the Kullback-Leibler relation had been checked for the classical systems only, and the quantum regime was examined for the first time in the work described here. The results of the experiment can provide insight into time irreversibility from the point of view of quantum phenomena. Source: Phys. Rev. Lett. 115 190601 (2015)

Phase transition with a change in the symmetry and topology

Most phase transitions are caused by a change in the symmetry in the framework of the Landau theory. However, topological phase transitions exist that cannot be described by the order parameter but are characterized by topological invariants. E.H. Rezayi and F.D.M. Haldane predicted in 2000 that a hybrid transition is possible in some cases when a change in the topology is accompanied by symmetry breakdown, but no unambiguous evidence of such processes had ever been observed. A group of research workers from USA and Netherland revealed such a rare transition in a two-dimensional electron gas in a superpure GaAs/AlGaAs crystal. The measurements were carried out at a temperature near 0.012 K and a pressure of 104 atm. The original goal was observation of a topological transition associated with a fractional (the occupation factor ν=5/2) quantum Hall effect with rising pressure. But it turned out that the topological transition was preceded by a transition with a change in symmetry according to the Landau theory. This effect is possibly explained by a non-Coulomb electron interaction and mixing of the Landau levels. Topological phase transitions are a promising subject of research and can find useful applications in nanoelectronics. Source: Nature Physics, online publication of October 26, 2015

Microscopic heat engine

I.A. Martinez (the Barcelona Institute of Science and Technology, Spain) with colleagues realized the Carnot cycle with a unit optically trapped Brownian microparticle as a working substance of a heat engine. A 1 µm polystyrene microsphere immersed in water was held by laser radiation and was observed in a microscope. The same as large heat engines, the microscopic engine operation was supported by the temperature difference. The electric field fluctuating with the white noise spectrum induced jitter and Brownian motion of the particle and thus imitated the conditions from room temperature to thousands of Kelvin, and the displacement of the particle from the trap center corresponded to a change in in the working substance volume. By changing two parameters (intensity of electric field fluctuations and trap rigidity) the Carnot cycle was realized including two isothermal regions joined by two adiabatic ones. The engine efficiency was η=0.25 ± 0.05. The experiment illustrates thermodynamics on small scales and characterizes the sources of irreversibility. Its results may appear to be useful in nanomotor design. Source: Nature Physics, online publication of October 26, 2015

Hidden baryons around central galaxies

The analysis of the data of Planck cosmic telescope carried out by C. Herandez-Monteagudo (Centro de Estudios de Física del Cosmos de Aragón) (CEFCA), (the Center of Cosmos Physics Research of Aragon) Spain) with colleagues evidenced that a considerable portion of baryons in the Universe reside not inside galaxies but in the gas clouds around the central galaxies in the galactic groups and clusters observed in the Sloan Digital Sky Survey. The kinematic Sunyaev-Zel’dovich effect was investigated which consists in a Doppler shift of relic radiation frequency upon scattering by moving gas clouds. This leads to the additional temperature fluctuations in relic radiation which were observed by Planck. On the basis of this effect the specificities of gas motion around central galaxies were disclosed and it was suggested that about half of all the baryons should reside around central galaxies. Baryons from the galaxies could have been ejected in different processes such as radiation of active galactic nuclei. Source: Phys. Rev. Lett. 115 191301 (2015)

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