Extracts from the Internet

Experimental realization of the discrete time crystal

M.D. Lukin (Harvard University, USA) and his colleagues were the first to demonstrate experimentally a discrete time crystal whose characteristics recur in equal time intervals much like the properties of ordinary crystals are periodic in space. Time crystals had been predicted theoretically by F. Wilczek, although the original idea referring to thermally equilibrium systems turned out to be physically unrealizable. But later it was shown that a discrete time crystal can be formed in a nonequilibrium pumped system. It was precisely such a system that M.D. Lukin with colleagues examined in their experiment. A discrete time crystal was realized in a system of nitrogen-substituted vacancies in a room-temperature diamond crystal. Observed were collective vacancy spin oscillations with periods equal to integer multiples (2 and 3) of the repetition interval of microwave pumping pulses. A discrete time crystal was obtained independently by another research group — J. Zhang (the University of Maryland and the National Institute of Standards and Technology, USA) with colleagues — in a chain of trapped ions. Sources: Nature 543 217 (2017), Nature 543 221 (2017)

Bose – Einstein condensate with a negative effective mass

The effective mass of a quasi-particle or another subsystem is expressed in terms of the second momentum derivative of its energy meff=(d2E/dp2)-1, and in some parts of the dispersion relation E(p) the effective mass meff can be negative, whereas the substance density remains positive. The negative meff has already been realized in some systems with spin-orbit coupling of atoms. The experiment of M.A. Khamehchi (Washington State University, USA) et al. showed that some regions of Bose – Einstein condensate of 87Rb atoms can also have meff<0. The spin-orbit interaction was generated by lasing that bound coherently the atomic levels |F,mF⟩=|1,-1⟩ and |1,0⟩. In the range of negative meff, the Galilean covariance was violated, i.e., the acceleration direction of part of the condensate was opposite to the action of force created by the trapping potential. Such an asymmetric dispersion induced self-trapping, i.e., the compression prevailed over the general expansion. The experimental data are well described by the Gross – Pitaevskii equation whose solution was helpful in clarification of the role of negative meff in the observed phenomena. In particular, it is responsible for the shock waves and the soliton trains carrying away the energy through the condensate boundary. Source: Phys. Rev. Lett. 118 155301 (2017)

Homogeneous Fermi-gas in an optical trap

Gas inhomogeneity in atomic traps obstructs observation of a number of subtle effects that must take place in the homogeneous case. B. Mukherjee (Massachusetts Institute of Technology, USA) with colleagues obtained a homogeneous Fermi gas of ultracold 6Li atoms using a special type of the trapping potential. The trap walls were created by a laser beam in the form of a hollow tube bound by two transverse cross sections done by additional beams. The gravitational force was compensated by the magnetic field so that the atoms might freely levitate. In such hybrid potential the trapped gas was to high accuracy homogeneous. This made it possible to observe phenomena inaccessible to examination in inhomogeneous systems. In the momentum distribution of spin-polarized atoms, a saturation of occupation numbers of particles in the momentum space, i.e., Pauli blocking was observed. The measurement of compressibility revealed a superfluid transition in a spin-balanced Fermi gas and strong attraction in the polaronic regime. Source: Phys. Rev. Lett. 118 123401 (2017)

Nano-optical antenna in an X-ray detector

To design small-size X-ray scintillation detectors is difficult because of the low photon yield. In the new device demonstrated by T. Grosjean (the University of Bourgogne-Franche-Comte, France) and his colleagues, this difficulty is got over because the scintillator is coupled to the optical fiber and to the photodetector not directly but through a nanoantenna operating in the optical range. A horn dielectric antenna was fastened to the end of single-mode optical fiber through photopolymerization. The antenna mouthpiece faced the optical fiber 125µm in diameter and on the other side of the antenna a piece of scintillator was fixed. From the outside, the device is covered by a thin aluminum layer which transmits X-rays but reflects well photons of the optical range. Owing to such a construction the majority of optical photons X-ray generated in the scintillator get into the optical fiber and then are registered by a photodetector. The minimum X-ray flux that could be registered made up ≈103 photons s-1 µm-2. The new detector may have important practical applications. For example, it can be assembled on an endoscope and may play the role of an ultracompact X-ray dosimeter in radio-therapy. Optical nanoantennas are described in the review of A.E. Krasnok et al. Phys. Usp. 56 539 (2013). Source: Optics Letters 42 1361 (2017)

Excess of gamma rays from the Galactic center

In some works, the gamma-ray flux from the center of our Galaxy measured by the cosmic gamma-ray telescope Fermi-LAT at energies of several GeV was noticed to exceed the flux expected in conventional models of gamma-ray generation by cosmic rays interacting with the interstellar gas and radiation. One of the explanations of the excess of gamma-ray emission is the dark-matter particle annihilation. Fermi-LAT continues observation of the Galactic center, and the new analysis of the data accumulated over 6.5 years takes into account the uncertainty in the background radiation intensity, in the cosmic ray fluxes and in the models of their propagation. Moreover, a possible contribution of Fermi bubbles, i.e., giant diffusive gamma-ray sources located on both sides of the Galactic disc was taken into account. Even with allowance for these uncertainties the excess of gamma-ray emission from the center of the Galaxy remains statistically significant. The signal from control regions along the galactic disc in which the dark-matter density is low was found also to show the excess of gamma-ray emission, and therefore the annihilation may be not the only source of the excess. Using the available data as the upper limit of the possible annihilation signal, the Fermi-LAT collaboration obtained new restrictions on the dark-matter particle annihilation cross section. Source: arXiv:1704.03910 [astro-ph.HE]

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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.

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