Nonlinear phenomena in degenerate quantum gases

In this introductory survey, we give an overview of the main physical problems and corresponding themes of research addressed in this Special Issue. We also briefly discuss some avenues of potential interest for future research in degenerate quantum gases.     

Double incoherent Raman scattering induced by molecular interactions in gases

A new Raman band is observed in two gases, CF4 and CH4. It is attributable to a double incoherent Raman scattering that takes place in pairs of interacting molecules. Spectral scattering intensities are given on an absolute scale. Zero order spectral moments are measured and are found higher than their long-range interaction value.     

Nonlinear electromagnetic effects induced by torsion

Nonlinear electromagnetic effects induced by massive electrodynamics with torsion are discussed. The charge is shown to be conserved in the case the torsion vector is divergence-free. A torsion-noise effect is suggested.     

Nonlinear effects in gases in the Couette problem

The nonlinear processes of heat and mass transfer in a rarefied gas confined between two infinite parallel plates maintained at different temperatures and moving at a relative velocity are considered. The profiles of the gas macroscopic flow velocity, density, temperature, heat fluxes, and shear stress were calculated on the basis of kinetic equations by the discrete velocity method in a wide range of Knudsen numbers at different values of temperature difference between the plates and plate velocities. It was shown that under certain conditions, the direction of gas flow near the “hot” plate can change to the opposite. It was discovered that the longitudinal and normal components of heat flux at a certain temperature difference between the plates change their orientation to the opposite in transition and nearly free molecular regimes.     

Nonlinear wave interactions in porous media filled with a quantum fluid

The problem of the nonlinear interaction between the fourth sound and an acoustic wave propagating in a porous medium filled with superfluid helium is solved. Based on the Landau equations of quantum fluid dynamics and on the Biot theory of mechanical waves in a porous medium, nonlinear wave equations are derived for studying the aforementioned interaction. An expression is obtained for the vertex that determines the excitation of an acoustic wave by two waves of the fourth sound. The possibility of an experimental observation of this process is estimated.     

Nonmonotonicity in the quantum-classical transition: chaos induced by quantum effects

The classical-quantum transition for chaotic systems is understood to be accompanied by the suppression of chaotic effects as the relative variant Planck's over 2pi is increased. We show evidence to the contrary in the behavior of the quantum trajectory dynamics of a dissipative quantum chaotic system, the double-well Duffing oscillator. The classical limit in the case considered has regular behavior, but as the effective variant Planck's over 2pi is increased we see chaotic behavior. This chaos then disappears deeper into the quantum regime, which means that the quantum-classical transition in this case is nonmonotonic in variant Planck's over 2pi.     

Influence of induced interactions on the superfluid transition in dilute fermi gases

We calculate the effects of induced interactions on the transition temperature to the BCS state in dilute Fermi gases. For a pure Fermi system with two species having equal densities, the transition temperature is suppressed by a factor (4e)(1/3) approximately 2.2, and for nu fermion species, the transition temperature is increased by a factor (4e)(nu/3-1) approximately 2.2(nu-3). For mixtures of fermions and bosons the exchange of boson density fluctuations gives rise to an attractive interaction, and we estimate the increase of the transition temperature due to this effect.     

Nonlinear interactions and quantum entanglement for collective fields in near-resonantly driven systems

This paper proposes a novel form of multimode nonlinear interactions by using a near-resonantly dressed atomic ensemble in an optical cavity.Due to quantum interference,a pair of collective fields come into the bilinear interactions,whose strengths are proportional to the population difference between dressed states which are coupled to the collective fields.By such an interaction,it is possible to obtain perfect multimode squeezing and collective Einstein-Podolsky-Rosen(EPR) entanglement in the cavity output.     

Nonlinear interactions between electromagnetic waves and electron plasma oscillations in quantum plasmas

We consider nonlinear interactions between intense circularly polarized electromagnetic (CPEM) waves and electron plasma oscillations (EPOs) in a dense quantum plasma, taking into account the electron density response in the presence of the relativistic ponderomotive force and mass increase in the CPEM wave fields. The dynamics of the CPEM waves and EPOs is governed by the two coupled nonlinear Schr?dinger equations and Poisson's equation. The nonlinear equations admit the modulational instability of an intense CPEM pump wave against EPOs, leading to the formation and trapping of localized CPEM wave pipes in the electron density hole that is associated with a positive potential distribution in our dense plasma. The relevance of our investigation to the next generation intense laser-solid density plasma interaction experiments is discussed.     

Novel quantum phases and mesoscopic physics in quantum gases

Among many notable jubilees brought by the year 2012, the one of a special importance for the community of statistical physicists was the 140th birth anniversary of Marian Smoluchowski (Maryan Ritter von Smolan Smoluchowski, 28.05.1872 - 5.09.1917), who was one of the pioneers of statistical physics and, on a larger scale, one of those who shaped modern physical science as a whole. The present issue of EPJ ST entitled From Brownian motion to self-avoiding walks and Lévy flights aims to reflect the evolution of Smoluchowski’s ideas in the field of statistics of interacting random and self-avoiding walks, stochastic equations for many-particle systems, physics of glass-forming and noise driven systems. Majority of papers in this issue were presented at the international conference in statistical physics that took place in Lviv (Ukraine) on July 3-6, 2012.     

量子气体中的输运行为

输运测量是了解物质性质的一个重要手段.本文简单介绍最近在量子气体中实现的输运实验及其主要结论,包括在类似于介观物理器件中的Landauer输运和强相互作用费米气体中的自旋输运行为.我们着重讨论自旋动力学的特殊性以及其由于全同粒子相互作用所导致的特殊自旋扩散流的形式.     

Spin-charge separation in ultracold quantum gases

We investigate the physical properties of quasi-1D quantum gases of fermionic atoms confined in harmonic traps. Using the fact that for a homogeneous gas the low-energy properties are exactly described by a Luttinger model, we analyze the nature and manifestations of spin-charge separation, where in the case of atoms "spin" and "charge" refer to two internal atomic states and the atomic mass density, respectively. We discuss the necessary physical conditions and experimental limitations confronting possible experimental implementations.     

Nonlinear dipole-dipole interactions and spectroscopic properties of van der waals complexes in the gas phase

We suggest a semiempirical approach to describing the influence of local nonlinear dipole-dipole interactions on the formation of van der Waals complexes of 1: 1 composition in the gas phase. Based on this approach, we quantitatively interpret the experimental data on the patterns of the shift in the electronic (complexes of a 3-aminophthalimide molecule with water and methanol molecules) and vibrational (complexes of a HCl molecule with acetone and acetonitrile molecules) absorption spectra attributable to the processes of complex formation. We confirm the conclusion that a nonlinear dipole-dipole interaction should be considered as one of the most important physical mechanisms that result in the association of molecules both in the gas phase and, under certain conditions, in the condensed state.     

Nonlinear screening in two-dimensional electron gases

We have performed self-consistent calculations of the nonlinear screening of a point charge Z in a two-dimensional electron gas using a density functional theory method. We find that the screened potential for a Z=1 charge supports a bound state even in the high-density limit where one might expect perturbation theory to apply. To explain this behavior, we prove a theorem to show that the results of linear response theory are in fact correct even though bound states exist.     

Spin gases: quantum entanglement driven by classical kinematics

A spin gas is a natural extension of a classical gas. It consists of a large number of particles whose (random) motion is described classically, but, in addition, have internal (quantum mechanical) degrees of freedom that interact during collisions. For specific types of quantum interactions we determine the entanglement that occurs naturally in such systems. We analyze how the evolution of the quantum state is determined by the underlying classical kinematics of the gas. For the Boltzmann gas, we calculate the rate at which entanglement is produced and characterize the entanglement properties of the equilibrium state.