Winning articles in 2015

Methods of optical physics offer unique opportunities for the investigation of brain and higher nervous activity. The integration of cutting-edge laser technologies and advanced neurobiology opens a new cross-disciplinary area of natural sciences — neurophotonics — focusing on the development of a vast arsenal of tools for functional brain diagnostics, stimulation of individual neurons and neural networks, and the molecular engineering of brain cells aimed at the diagnosis and therapy of neurodegenerative and psychic diseases. Optical fibers help to confront the most challenging problems in brain research, including the analysis of molecular-cellular mechanisms of the formation of memory and behavior. New generation optical fibers provide new solutions for the development of fundamentally new, unique tools for neurophotonics and laser neuroengineering — fiber-optic neuroendoscopes and neurointerfaces. These instruments broaden research horizons when investigating the most complex brain functions, enabling a long-term multiplex detection of fluorescent protein markers, as well as photostimulation of neuronal activity in deep brain areas in living, freely moving animals with an unprecedented spatial resolution and minimal invasiveness. This emerging technology opens new horizons for understanding learning and long-term memory through experiments with living, freely moving mammals. Here, we present a brief review of this rapidly growing field of research.

We review the status of the theory of ionization of atoms and ions by intense laser radiation (Keldysh's theory). We discuss the applicability of the theory, its relation to the Landau–Dykhne method, and its application to the ionization of atoms by ultrashort nonmonochromatic laser pulses of an arbitrary shape. The semiclassical imaginary time method is applied to describe electron sub-barrier motion using classical equations of motion with an imaginary time ${t}\to {{\text{i}}}{t}$ for an electron in the field of an electromagnetic wave. We also discuss tunneling interference of transition amplitudes, a phenomenon occurring due to the existence of several saddle points in the complex time plane and leading to fast oscillations in the momentum distribution of photoelectrons. Nonperturbatively taking the Coulomb interaction between an outgoing electron and the atomic residual into account causes significant changes in the photoelectron momentum distribution and in the level ionization rates, the latter usually increasing by orders of magnitude for both tunneling and multiquantum ionization. The effect of a static magnetic field on the ionization rate and the magnetic cumulation process is examined. The theory of relativistic tunneling is discussed, relativistic and spin corrections to the ionization rate are calculated, and the applicability limits of the nonrelativistic Keldysh theory are determined. Finally, the application of the Fock method to the covariant description of nonlinear ionization in the relativistic regime is discussed.

We draw attention to a similarity between mutually related kinetic material phenomena that are odd in the magnetic field and produce an electric current or heat flow perpendicular (1) to the magnetic field, (2) to the electric field strength or to the temperature gradient. These phenomena include the Hall effect, the Righi–Leduc effect in nonmagnetic metals, the anomalous Hall effect in magnets, the odd Senftleben–Beenakker effect in molecular gases, and the phonon Hall effect in dielectrics. While these phenomena have much in common in terms of geometry, their formation mechanisms—dynamic and dissipative—are different. However, in all cases, the flow perpendicular to the magnetic field arises from the spin–orbit interaction of carriers with magnetic moments.

This paper, written to mark the twentieth anniversary of the discovery of the top quark, offers some insight into how the understanding of this heaviest known particle has developed from prediction through search to discovery to the current knowledge of its production mechanisms and properties. The central role of the top quark in the Standard Model is considered, and the window of opportunity it opens for seeking new physics beyond the Standard Model is discussed.

We review the succession of ideas underlying the emergence of the nonequilibrium diagram technique (Keldysh diagram technique). Simple examples are used to illustrate the implementation of the technique and to demonstrate possible difficulties and the ways to overcome them. Together with well-known facts, some lesser-discussed details are considered, in particular, whether the so-called three-component technique is necessary. Several applications of the nonequilibrium diagram technique are discussed including, notably, tunneling systems and linear response problems. We hope that some parts of the review can be useful even for the reader familiar with the nonequilibrium diagram technique.

While ubiquitous at all levels of organization in nature, including in nanotechnology, low-frequency 1/f noise is not yet understood. A possible reason is the unjustified application of probability theory concepts, primarily that of independence, to random physical phenomena. We show that in the framework of statistical mechanics, no medium can impart a definite diffusivity and mobility to a particle that performs random walk through it, which gives rise to flicker fluctuations in these properties. A universal source of 1/f noise in many-particle systems in this example is a dependence of the time behavior of any particular relaxation or transport process on the details of the initial microstate of the system as a whole.

We review the physical processes that occur at the center of the Galaxy and that are related to the supermassive black hole ${\text{SgrA}}^*$ residing there. The discovery of high-velocity S0 stars orbiting ${\text{SgrA}}^*$ for the first time allowed measuring the mass of this supermassive black hole, the closest one to us, with a 10% accuracy, with the result ${\boldsymbol{M}}_{\bf h}=({{\bf 4.1}\pm 0.4)\times 10^6}{\boldsymbol{M}}_{\odot}$. Further monitoring can potentially discover the Newtonian precession of the S0 star orbits in the gravitational field of the black hole due to invisible distributed matter. This will yield the 'weight' of the elusive dark matter concentrated there and provide new information for the identification of dark matter particles. The weak accretion activity of the 'dormant quasar' at the galactic center occasionally shows up as quasiperiodic X-ray and near-IR oscillations with mean periods of 11 and $19\ {\text{min}}$. These oscillations can possibly be interpreted as related to the rotation frequency of the ${\text{SgrA}}^*$ event horizon and to the latitude oscillations of hot plasma spots in the accretion disk. Both these frequencies depend only on the black hole gravitational field and not on the accretion model. Using this interpretation yields quite accurate values for both the mass ${\boldsymbol{M}}_{\bf h}$ and the spin ${\boldsymbol{a}}$ (Kerr rotation parameter) of ${\text{SgrA}}^*$: ${\boldsymbol{M}}_{\bf h}=({{\bf 4.2}\pm 0.2)\times 10^6}{\boldsymbol{M}}_{\odot}$ and ${\boldsymbol{a}}={\bf 0.65}\pm 0.05$.