The theoretical description of the nonlinear photoionization of atoms and ions exposed to high-intensity laser radiation is underlain by the Keldysh theory proposed in 1964. The paper reviews this theory and its further development. The discussion is concerned with the energy and angular photoelectron distributions for the cases of linearly, circularly, and elliptically polarized laser radiation, with the ionization rate of atomic states exposed to a monochromatic electromagnetic wave and to ultrashort laser pulses of various shape, and with momentum and angular photoelectron spectra in these cases. The limiting cases of tunnel (y much less than 1) and multiphoton (y much greater than 1) ionization are discussed, where y is the adiabaticity parameter, or the Keldysh parameter. The probability of above-barrier ionization is calculated for hydrogen atoms in a low-frequency laser field. The effect of a strong magnetic field on the ionization probability is discussed. The process of Lorentz ionization occurring in the motion of atoms and ions in a constant magnetic field is considered. The properties of an exactly solvable model - the ionization of an s-level bound by zero-range forces in the field of a circularly polarized electromagnetic wave - are described. In connection with this example, the Zel'dovich regularization method in the theory of quasistationary states is discussed. Results of the Keldysh theory are compared with experiment. A brief discussion is made of the relativistic ionization theory applicable when the binding energy of the atomic level is comparable with the electron rest mass (multiply charged ions) and the sub-barrier electron motion can no longer be considered to be nonrelativistic. A similar process of electron-positron pair production from a vacuum by the field of high-power optical or X-ray lasers (the Schwinger effect) is considered. The calculations invoke the method of imaginary time, which provides a convenient and physically clear way of calculating the probability of particle tunneling through time-varying barriers. Discussed in the Appendices are the properties of the asymptotic coefficients of the atomic wave function, the expansions for the Keldysh function, and the so-called 'ADK theory'.
We discuss methods and approaches for describing molecular states in the spectrum of heavy quarks and investigate various properties of the exotic charmonium-like state X(3872) in detail in the framework of the mesonic molecule model.
Theoretical studies on more than three spatial dimensions are currently showing a distinct shift toward the 'brane world' picture, in which ordinary matter (with the possible exceptions of gravitons and hypothetical particles interacting very weakly with matter) is within a three-dimensional submanifold - brane - embedded in a multi-dimensional space. The extra dimensions may be large and indeed infinite and may show up directly in current or future experiments. In the present paper the basic ideas of the brane theory are presented in an accessible way using simple field-theoretical models.
The goal of this review is to outline some unconventional ideas behind new paradigms in the modern theory of turbulence. Application of nonstandard, topological methods to describe the structural properties of the turbulent state is considered and the transition to kinetic equations in fractional derivatives for describing the microscopic behavior of a medium is examined. Central to the discussion is the concept of the percolation constant C approximate to 1.327..., a universal parameter describing the topology of nonequilibrium (quasi)stationary states in complex nonlinear dynamical systems allowing self-organized critical behavior. Much attention is given to the formation of power-law energy density spectra in turbulent media. A number of topical problems in modern cosmic electrodynamics, including the self-consistent fractal model of a turbulent current sheet, substorm dynamics, and the formation and dynamical evolution of large-scale magnetic fields in the solar photosphere and interplanetary space, are also discussed.
Experimental data on impurity states in narrow-gap lead telluride based semiconductors are summarized. Theoretical models describing the nontrivial properties of such states are presented. Applications to the design of highly sensitive far-infrared detectors are considered.
The mutual influence of superconductivity and magnetism in F/S systems, i.e. systems of alternating ferromagnetic (F) and superconducting (S) layers, is comprehensively reviewed. For systems with ferromagnetic metal (FM) layers, a theory of the proximity effect in the dirty limit is constructed based on the Usadel equations. For an FM/S bilayer and an FM/S superlattice, a boundary-value problem involving finite FM/S boundary transparency and the diffusion and wave modes of quasi-particle motion is formulated; and the critical temperature T, is calculated as a function of FM- and S-layer thicknesses. A detailed analysis of a large amount of experimental data amply confirms the proposed theory. It is shown that the superconducting state of an FM/S system is a super-position of two pairing mechanisms, Bardin - Cooper - Schrieffer's in S layers and Larkin - Ovchinnikov - Fulde - Ferrell's in FM ones. The competition between ferromagnetic and antiferromagnetic spontaneous moment orientations in FM layers is explored for the 0- and pi-phase superconductivity in FM/S systems. For FI/S structures, where FI is a ferromagnetic insulator, a model for exchange interactions is proposed, which, along with direct exchange inside FI layers, includes indirect Ruderman - Kittel - Kasuya - Yosida exchange between localized spins via S-layer conduction electrons. Within this framework, possible mutual accommodation scenarios for superconducting and magnetic order parameters are found, the corresponding phase diagrams are plotted, and experimental results are explained. The results of the theory of the Josephson effect for S/F/S junctions are presented and the application of the theory of spin-dependent transport to F/S/F junctions is discussed. Application aspects of the subject are examined.
Advances in nonequilibrium pattern formation in reaction-diffusion systems are reviewed. Special emphasis is placed on patterns found in the spatially extended Belousov Zhabotinsky reaction dispersed in aerosol OT water-in-oil microemulsions (BZ-AOT system): Turing patterns, packet and standing waves, antispirals and segmented spirals, and accelerating waves and oscillons. All experimental results are explained theoretically and reproduced in computer simulations.
Microstructure fibers have opened a new phase in nonlinear optics. Due to their unique properties, fibers of this type radically enhance all the basic nonlinear-optical phenomena, offering new strategies for frequency conversion, spectral transformation, and control of ultrashort laser pulses. These fibers allow supercontinuum radiation to be efficiently generated using nano- and subnanojoule femtosecond pulses. Here, we analyze the physical mechanisms behind the enhancement of nonlinear-optical interactions of ultrashort pulses in microstructure and hollow photonic-crystal fibers and discuss applications of microstructure fibers for highly efficient supercontinuum generation and frequency conversion of femtosecond laser pulses.