On the type of a phase transition in the system of exciton polaritons in an optical microcavity

The type of a phase transition in the quasi-equilibrium system of exciton polaritons in a two-dimensional optical microcavity has been analyzed. It has been shown that, although the system contains two types of bosons undergoing mutual transformations into each other, only one phase transition to the superfluid state with the quasilong-range order occurs in the two-dimensional system. This phase transition is a Kosterlitz-Thouless phase transition. A new physical implementation—excitons in a photon crystal—has been proposed for the Bose condensation of exciton polaritons. The superfluid properties of the ordered phase are discussed, and the superfluid density and Kosterlitz-Thouless transition temperature have been calculated in the low-density approximation.     

玻色气体的磁性

物质磁性一直是凝聚态物理研究的重要课题.以往对磁性的探索主要是以费米子(局域或巡游的电子)为研究对象.由于传统的玻色系统液氦没有自旋,不表现磁性,玻色系统的磁性很少被关注.碱金属原子气体玻色-爱因斯坦凝聚的实现,在开辟了冷原子物理研究领域的同时,也打开了研究玻色系统磁性的大门.这是因为碱金属原子通常具有超精细结构,是旋量玻色气体,能够展示磁性.文章通过对比费米气体的相关结果,介绍了旋量玻色气体磁性的研究概况和最新进展,特别是铁磁性玻色气体的磁性相变以及在低温下铁磁性凝聚体的动力学特征.     

Fermion-mediated long-range interactions between bosons stored in an optical lattice

We describe a new method for creating spin-dependent long-range interactions between atomic ultra-cold neutral bosons—specifically ⁸⁷Rb—in an optical lattice. In this proposal, the bosonic system is immersed in a spin-polarized degenerate Fermi gas (almost perfectly non-interacting), here ⁶Li. We first show that the bosons acquire a long-range interaction analogous to Ruderman–Kittel–Kasuya–Yosida interaction in solids. The resulting fermion-mediated Bose–Bose interaction, which can depend on the bosons’ spin state, is tunable using inter-species Feshbach resonance. When the bosons are subject to a suitable optical lattice, 3-body loss processes are greatly suppressed. We conclude by showing that these interactions can lead to a supersolid phase for single-spin Bose system, and also to a fully tunable transverse field Ising model for a two-component Bose system.     

On the critical temperature in a Boson-Fermion mixture with attraction between the components

The dependence of the critical temperature shift in the Bose component of a degenerate Bose-Fermi gas mixture in a harmonic trap on the number of bosons and fermions has been obtained on the basis of the effective Hamiltonian of the Bose subsystem. The presence of the Fermi component leads to a qualitatively new behavior of the shift, as compared to the case of a single interacting Bose gas in the trap. Namely, the T _c(N) dependence has a different curvature in the presence of the Fermi component.     

Thermodynamics of a two-dimensional interacting Bose gas trapped in a quartic potential

We have studied the Bose-Einstein condensation (BEC) of an interacting Bose gas confined in a two-dimensional (2D) quartic potential by using a mean-field, semiclassical two-fluid model. A thermodynamic analysis including the chemical potential, condensate fraction, total energy, and specific heat has been carried out by considering different values of the interaction strength. Finally, we have found that the behaviour of the condensate fraction and specific heat of quartically trapped bosons differs from those of bosons trapped in a harmonic potential.     

Do Bosons Condense in a Homogeneous Magnetic Field?

It has been known since the paper⁽²⁶⁾ and then due to a rigorous result⁽³⁾ that the answer to the question in the title is negative for a three-dimensional “ideal gas of charged bosons”. The present paper adds a new rigorous result in this direction. We show that the answer to the question becomes positive, if this “ideal gas of charged bosons” is simultaneously embedded in an appropriate periodic external potential. We prove that it is true for the Perfect Bose Gas (PBG), as well as for the Imperfect Bose Gas with a Mean-Field repulsive particle interaction.     

Quantum Breathers in Anisotropy Ferromagnetic Chains with Second-Order Coupling

Under considering the next-nearest-neighbor interaction, quantum breathers in one-dimensional anisotropy ferromagnetic chains are theortically studied. By introducing the Dyson-Maleev transformation for spin operators, a map to a Heisenberg ferromagnetic spin lattice into an extended Bose–Hubbard model can be established. In the case of a small number of bosons, by means of the numerical diagonalization technique, the energy spectrum of the corresponding extended Bose–Hubbard model containing two bosons is calculated. When the strength of the single-ion anisotropy is enough large, a isolated single band appears. This isolated single band corresponds to two-boson bound state, which is the simplest quantum breather state. It is shown that the introduction of the next-nearest-neighbor interaction will lead to interesting band structures. In the case of a large number of bosons, by applying the time-dependent Hartree approximation, quantum breather states for the system is constructed. In this case, the effect of the next-nearest-neighbor interaction on quantum breathers is also analyzed.     

Ultracold Atomic Fermi–Bose Mixtures in Bichromatic Optical Dipole Traps: A Novel Route to Study Fermion Superfluidity

The study of low density, ultracold atomic Fermi gases is a promising avenue to understand fermion superfluidity from first principles. One technique currently used to bring Fermi gases in the degenerate regime is sympathetic cooling through a reservoir made of an ultracold Bose gas. We discuss a proposal for trapping and cooling of two-species Fermi–Bose mixtures into optical dipole traps made from combinations of laser beams having two different wavelengths. In these bichromatic traps it is possible, by a proper choice of the relative laser powers, to selectively trap the two species in such a way that fermions experience a stronger confinement than bosons. As a consequence, a deep Fermi degeneracy can be reached having at the same time a softer degenerate regime for the Bose gas. This leads to an increase in the sympathetic cooling efficiency and allows for higher precision thermometry of the Fermi–Bose mixture.     

Low-temperature magnetization in nanofilms

The low-temperature magnetization of a film was analyzed by the use of exact Bose representation of spin operators that does not suffer from the presence of unphysical states. The magnetization of thin films has exponentially small temperature correctness, so that Dyson’s proof about exponentially small correction coming from two bosons at ideal lattice point cannot be used in film analyses. The main conclusions of this work are that magnetic lattice of a thin film is more rigid than the macroscopic lattice and that the autoreduction process (the three layer film divides into two layer subfilms) takes place in the film.     

Stability of the Bose system in Bose-Fermi mixture with attraction between bosons and fermions

An effective Hamiltonian for the Bose system in the mixture of ultracold atomic clouds of bosons and fermions is obtained by integrating out the Fermi degrees of freedom. An instability of the Bose system is found in the case of attractive interaction between components that is in good agreement with an experiment on the bosonic ⁸⁷Rb and fermionic ⁴⁰K mixture.     

Phase separation in the two-dimensional bosonic Hubbard model with ring exchange

We show that soft-core bosons in two dimensions with a ring exchange term exhibit a tendency for phase separation. This observation suggests that the thermodynamic stability of normal Bose liquid phases driven by ring exchange should be carefully examined.     

Universality of the one-dimensional Bose gas with delta interaction

We consider several models of interacting bosons in a one-dimensional lattice. Some of them are not integrable like the Bose-Hubbard others are integrable. At low density all of these models can be described by the Bose gas with delta interaction. The lattice corrections corresponding to the different models are contrasted.     

Theory of a weakly nonideal Bose gas in a magnetic field

This paper investigates Bose-Einstein condensation of an ideal gas of finite-spin bosons in an external magnetic field. We generalize Bogolyubov’s theory of a weakly nonideal Bose gas to the case where the gas of finite-spin bosons is located in an external magnetic field. We find the corresponding quasiparticle spectrum and formulate the superfluidity criterion for the boson gas. The magnetization of the weakly nonideal Bose gas is also determined. Finally, we specify a method of studying kinetic processes that take place in a weakly nonideal Bose gas. Zh. éksp. Teor. Fiz. 113, 918–929 (March 1998)     

多粒子玻色关联对夸克胶子等离子体颗粒源的检测

对由空间离散分布的夸克－胶子等离子体热滴发射的玻色子，本文进行了多粒子玻色关联的蒙特卡罗模拟研究．与两粒子玻色关联相比，多粒子玻色关联能够更敏感地检测玻色子发射源的颗粒性．通过对多粒子可观测量的分析，能够获得夸克－胶子等离子体和强子气体混合相存在的检测信号．     

Multiparticle Bose Correlations: Probes for Granular Source of Quark-Glunon-Plasma Droplets

The multiparticle Bose correlations of bosons emitted from dispersed thermal droplets of quark-gluon plasma are simulated by Monte Carlo.Multiparticle Bose correlations can offer more sensitivity to probe the granularity of the Bosonemitting source than two-particle Bose correlations.A promising signal of the existence of a mixed phase of quark-gluon plasma and hadronic gas can be obtained from multiparticle observables.     

Monte Carlo simulation of systems of identical bosons

Methodical questions concerning the Monte Carlo simulation of quantum mechanical systems are discussed. The optimum value of the maximum shift on one step of the Metropolis algorithm is found theoretically. In the one-dimensional case it equals 2.62√a/m (here a is the time step and m is the mass); in the three-dimensional case it equals 1.7√a/m. The generalization of the standard Metropolis algorithm to the case of systems of identical particles is suggested. This method has a rather restricted applicability region for fermions (no large number of particles and no strong degeneracy). In the case of bosons, however, it enables one to investigate many-particle interacting systems at any temperature, starting from first principles. The feasibility of the method is tested on some simple examples: two identical particles in the external field, and a one- and three-dimensional Bose gas. In the last case the Bose condensation has been seen.     

Quantum Two-breathers Formed by Ultracold Bosonic Atoms in Optical Lattices

Two-discrete breathers are the bound states of two localized modes that can appear in classical nonlinear lattices. I investigate the quantum signature of two-discrete breathers in the system of ultracold bosonic atoms in optical lattices, which is modeled as Bose–Hubbard model containing n bosons. When the number of bosons is small, I find numerically quantum two-breathers by making use of numerical diagonalization and perturbation theory. For the cases of a large number of bosons, I can successfully construct quantum two-breather states in the Hartree approximation.     

Non-autonomous matter-wave solitons in hybrid atomic–molecular Bose–Einstein condensates with tunable interactions and harmonic potential

In this paper, we investigate matter-wave solitons in hybrid atomic–molecular Bose–Einstein condensates with tunable interactions and external potentials. Three types of time-modulated harmonic potentials are considered and, for each of them, two groups of exact non-autonomous matter-wave soliton solutions of the coupled Gross–Pitaevskii equation are presented. Novel nonlinear structures of these non-autonomous matter-wave solitons are analyzed by displaying their density distributions. It is shown that the time-modulated nonlinearities and external potentials can support exact non-autonomous atomic–molecular matter-wave solitons.     

A possibility of exciton luminescence with Boson correlation in a coupled QW

We have observed an anomaly in the low-temperature photoluminescence of dipole-oriented long-lived excitons in a coupled quantum well under an electrical bias. The most plausible explanations of these effects so far offered is a normal-to-superfluid transition of a two-dimensional Bose system in a random potential with repulsive interaction between bosons.