Conditions and features of the lasing in traps for the bose condensation of dipolar excitons

We study the conditions and features of the polariton mode lasing in traps for the Bose condensation of dipolar excitons. We discuss the spectral linewidth of lasing modes and the effects of spatial and spectral inhomogeneity of the exciton distribution. We study in detail the possibility of the polariton mode lasing in the vicinity of the Bose condensation threshold. We analyze the impact of the inhomogeneous broadening of the exciton line on the stability of stationary lasing. We also propose additional experiments aimed at obtaining more information on the polariton mode lasing in semiconductor structures for the Bose condensation of excitons.     

Kinetic properties of a system of spatially separated excitons and electrons in the presence of a Bose condensate of excitons

A system of interacting, spatially separated excitons and electrons is examined in the presence of a Bose condensate of excitons. The kinetic properties of the system that are governed by the interaction of excitations in the exciton subsystem with electrons are investigated. It is shown that a nonequilibrium distribution of excitations in the exciton subsystem gives rise to an induced electron current. Experimental observation of the kinetic phenomena described can provide new information on the exciton phase state. Zh. éksp. Teor. Fiz. 116, 1440–1449 (October 1999)     

Bose condensation of two-dimensional dipolar excitons: Simulation by the quantum Monte Carlo method

The Bose condensation of two-dimensional dipolar excitons in quantum wells is numerically studied by the diffusion Monte Carlo simulation method. The correlation, microscopic, thermodynamic, and spectral characteristics are calculated. It is shown that, in structures of coupled quantum wells, in which low-temperature features of exciton luminescence have presently been observed, dipolar excitons form a strongly correlated system.     

Phase diagram of the bose condensation of dipolar excitons in GaAs/AlGaAs quantum-well heterostructures

Experimental studies of the phase diagram of Bose condensation in a system of spatially indirect dipolar excitons in GaAs/AlGaAs quantum wells are reviewed. The properties of spatially periodic patterns arising in the luminescence of the exciton Bose condensate in a ring-shaped potential trap and the coherence of the condensate luminescence are discussed.     

Wannier-mott excitons in narrow-gap semiconductor heterostructures

Spectra of excitons in a bulk semiconductor and in a thin semiconducting layer are investigated in the two-band Dirac model. The dependences of the exciton binding energy on the energy gap and, in the two-dimensional case, on the layer width are obtained. The fine structures of exciton spectra are revealed.     

Anisotropic superfluidity in a dipolar Bose gas

We study the superfluid character of a dipolar Bose-Einstein condensate (DBEC) in a quasi-two dimensional geometry. We consider the dipole polarization to have some nonzero projection into the plane of the condensate so that the effective interaction is anisotropic in this plane, yielding an anisotropic dispersion relation. By performing direct numerical simulations of a probe moving through the DBEC, we observe the sudden onset of drag or creation of vortex-antivortex pairs at critical velocities that depend strongly on the direction of the probe's motion. This anisotropy emerges because of the anisotropic manifestation of a rotonlike mode in the system.     

Inelastic light scattering by dipolar excitons

The cross section for inelastic light scattering by a system of spatially indirect excitons is calculated. It is shown that there are two resonant scattering channels, one related to the transitions via the vacuum and the other via the two-exciton states of the crystal, and their contributions to the scattering amplitude cancel each other. As a result, there is no resonance enhancement of light scattering by excitons for photon energies in the vicinity of the band gap.     

Luminescence kinetics of dipolar excitons in circular traps

A detailed investigation of multiband superconductivity and Leggett’s mode in the Mg_1?x Al_xB₂ (0 ≤ x ≤ 0.45) system was carried out using tunneling and Andreev spectroscopy. Temperature dependences of superconducting gaps Δ_σ and Δ_π and their variation upon the degree of disorder and the Al concentration were studied. The dependence of the Leggett’s mode energy ε₀ upon the values of the gaps Δ_σ and Δ_π has been derived.     

Adiabatic Raman polariton in a Bose condensate

The Raman interaction of optical fields with a Bose condensate is studied in the adiabatic regime. A superposition of operators is found — one annihilating an atom in a metastable state and the other annihilating a photon in resonance with a transition from the ground state to an excited state — which is an adiabatic invariant of the problem (Raman polariton). Possible applications for Bose-condensate diagnostics and development of atomic lasers are proposed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 473–477 (10 October 1996)     

Trapped Bose condensate in a gravitational field

The Bose condensation of atoms in finite 1D and 2D parabolic traps placed in a gravitational field is considered. The distortion of the trap potential in this field is modeled by a combination of two rectangular 1D and 2D traps. The change in the critical temperature T _c is found with regard to the cutoff and renormalization of the spectrum of these model potentials. The shift of the critical temperature T _c in the gravitational field is calculated. The shift sign and magnitude depend on the way of introducing the gravitational field. For a certain choice, three critical temperatures can be sequentially observed. These temperatures can be attributed to three Bose condensations that occur in the cyclic motion of the trap along the Earth (I)-space (II)-Earth (III) route.     

High-density limit of quasi-two-dimensional dipolar Bose gas

We consider a simple model of the quasi-two-dimensional dipolar Bose gas confined in the one-dimensional square well potential. All dipoles are assumed to be oriented along the confining axis. By means of hydrodynamic approach it is shown that the general structure of the low-lying excitations can be analyzed exactly. We demonstrate that the problem significantly simplifies in the high-density limit for which the density profile in the confined direction as well as the leading-order contribution to the ground-state energy and spectrum of elementary excitations are calculated. The low-temperature result for the damping rate of the phonon mode is also presented.     

Strongly dipolar Bose-Einstein condensate of dysprosium

We report the Bose-Einstein condensation (BEC) of the most magnetic element, dysprosium. The Dy BEC is the first for an open f-shell lanthanide (rare-earth) element and is produced via forced evaporation in a crossed optical dipole trap loaded by an unusual, blue-detuned and spin-polarized narrowline magneto-optical trap. Nearly pure condensates of 1.5 × 10(4) (164)Dy atoms form below T = 30 nK. We observe that stable BEC formation depends on the relative angle of a small polarizing magnetic field to the axis of the oblate trap, a property of trapped condensates only expected in the strongly dipolar regime. This regime was heretofore only attainable in Cr BECs via a Feshbach resonance accessed at a high-magnetic field.     

Bose condensation of interwell excitons in double quantum wells

The luminescence of interwell excitons in double quantum wells GaAs/AlGaAs (n-i-n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells were studied as functions of density and temperature within the domains on the scale less than one micron. For this purpose, the surfaces of the samples were coated with a metallic mask containing specially prepared holes (windows) of a micron size an less for the photoexcitation and observation of luminescence. For weak pumping (less than 50 μW), the interwell excitons are strongly localized because of small-scale fluctuations of a random potential, and the corresponding photoluminescence line is inhomogeneously broadened (up to 2.5 meV). As the resonant excitation power increases, the line due to the delocalized excitons arises in a thresholdlike manner, after which its intensity linearly increases with increasing pump power, narrows (the smallest width is 350 μeV), and undergoes a shift (of about 0.5 μeV) to lower energies, in accordance with the filling of the lowest state in the domain. With a rise in temperature, this line disappears from the spectrum (T _c ≤ 3.4 K). The observed phenomenon is attributed to Bose-Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature range studied (1.5–3.4 K), the critical exciton density and temperature increase almost linearly with temperature.     

偶极玻色-爱因斯坦凝聚体中的各向异性耗散

针对偶极相互作用的玻色-爱因斯坦凝聚体,解析计算了点状杂质沿平行极化轴和垂直极化轴运动的能量耗散率,证明了在超流临界速度更大的方向上耗散率也更高.该结论为最近在162Dy原子气体中观测到的实验现象提供了理论支持.对于一般的运动方向,给出了耗散率在高速极限下以及临界速度附近的渐近形式.结合数值计算的结果,论证了耗散率随方向角的变化总是表现出与临界速度一致的各向异性.     

Calibrating dipolar interaction in an atomic condensate

We reexamine the topic of a dipolar condensate with the recently derived more rigorous pseudopotential for dipole-dipole interaction [Phys. Rev. A 67, 033607 (2003)]]. Based on the highly successful variational technique, we find that all dipolar effects estimated before (using the bare dipole-dipole interaction) become significantly larger, i.e., are amplified by the new velocity-dependent pseudopotential, especially in the limit of large or small trap aspect ratios. This result points to a promising prospect for detecting dipolar effects inside an atomic condensate.     

Scattering of neutrons on a Bose condensate

The spectrum of noncondensate excitations in neutron scattering on bosons is obtained in the framework of the Bogoliubov models both for liquid ⁴He and a dilute gas. The problem is solved using a path-integral representation of the partition function of the system. We describe the influence of scattering of neutrons on a Bose condensate in a stationary (time-independent) picture in the Gibbs equilibrium ensemble. This influence is a stationary boson response, and it depends on the initial neutron momentum k, transfer momentum p, and the neutron-boson interaction λ, which is related to the scattering length. The contribution of the neutrons to the initial Bogoliubov spectrum is found to be important for “quasi-elastic” scattering on the noncondensate, while the contribution of deep inelastic scattering is small; no contribution from elastic scattering on the Bose condensate is found. In the case of liquid Helium, the response is unlikely to be observable for all values p. On the other hand, for a gas one may expect a visible effect, in particular for a small momentum transfer p and a small density of the Bose condensate ϱ.     

Lasing threshold in traps for Bose-condensation of dipolar excitons

We consider exciton recombination lasing in heterostructure traps for Bose–Einstein condensation of dipolar excitons. We show that such structures suit well for class D lasers where cavity decay strongly exceeds polarization decay. We evaluate lasing threshold taking into account specific inhomogeneous broadening of the exciton spectral line owing to Bose–Einstein condensation phenomenon under quasi-equilibrium conditions.It is found that narrowing of the exciton momentum distribution just before the condensation onset considerably lowers lasing threshold. At the same time, it is pointed out that a subsequent formation of condensate itself does not help lasing much. We conclude that it is possible to achieve lasing on polariton modes in nowadays experiments aimed on Bose–Einstein condensation of excitons.     

Fermion-fermion interaction in a dilute gas-mixture Bose condensate

A mixture of a one-component Bose gas and two-component Fermi gas is considered at temperatures at which the Bose gas is completely condensed. Two fermions in such a mixture can interact with each other exchanging bosons from the condensate or supercondensate. The interaction potential, a change in the effective mass, the decay, and fermion spectrum are calculated in this quantum Fermi-Bose mixture.     

Anisotropic sound and shock waves in dipolar Bose-Einstein condensate

We study the propagation of anisotropic sound and shock waves in dipolar Bose-Einstein condensate in three dimensions (3D) as well as in quasi-two (2D, disk shape) and quasi-one (1D, cigar shape) dimensions using the mean-field approach. In 3D, the propagation of sound and shock waves are distinct in directions parallel and perpendicular to dipole axis with the appearance of instability above a critical value corresponding to attraction. Similar instability appears in 1D and not in 2D. The numerical anisotropic Mach angle agrees with theoretical prediction. The numerical sound velocity in all cases agrees with that calculated from Bogoliubov theory. A movie of the anisotropic wave propagation in a dipolar condensate is made available as supplementary material.