Bose-Einstein condensation of S = 1 nickel spin degrees of freedom in NiCl2-4SC(NH2)2

It has recently been suggested that the organic compound NiCl2-4SC(NH2)2 (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni S = 1 spins exhibit three-dimensional XY antiferromagnetism above a critical field H(c1) approximately 2 T. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law H - H(c1) proportional variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.     

Collective Excitations of Low Dimensional Trapped Bose-Condensed Gas

In this paper, we investigate excited characteristic of the weakly interacting quasi-one-dimensional (1D) and quasi-two-dimensional (2D) Bose-Einstein condensation (BEC) in harmonic potential trap. The energy spectrum and the analytical expression of the sound velocity are obtained and analyzed. Compared with 3-Dimensional homogeneous Bose-condensed gas occasion, the sound velocity of 2D Bose-Einstein condensation in harmonic potential trap is smaller.     

Evolution from BCS to BEC superfluidity in p-wave Fermi gases

We consider the evolution of superfluid properties of a three-dimensional p-wave Fermi gas from a weak coupling Bardeen-Cooper-Schrieffer (BCS) to strong coupling Bose-Einstein condensation (BEC) limit as a function of scattering volume. At zero temperature, we show that a quantum phase transition occurs for p-wave systems, unlike the s-wave case where the BCS to BEC evolution is just a crossover. Near the critical temperature, we derive a time-dependent Ginzburg-Landau (GL) theory and show that the GL coherence length is generally anisotropic due to the p-wave nature of the order parameter, and becomes isotropic only in the BEC limit.     

Lasing behaviour from the condensation of polaronic excitons in a ZnO nanowire

Atoms under optical and magnetic trapping in a limited space at a very low temperature can lead to Bose-Einstein condensation (BEC),even in a one-dimensional (1D) optical lattice. However,can the confinment of dense excitons in a 1D semiconductor microstructure easily reach the excitonic BEC A lightly Mn(Ⅱ)-doped ZnO nanowire under a femtosecond laser pulse pump at room temperature produces single-mode lasing from coherent bipolaronic excitons,which is much like a macroscopic quantum state due to the condensation of the bipoaronic excitons if not real BEC. In this process,longitudinal biphonon binding with the exciton plays an important role. We revisit this system and propose possibility of bipolaronic exciton condensation. More studies are needed for this condensation phenomenon in 1D microcavity systems.     

Direct measurement of the Bose-Einstein condensation universality class in NiCl2-4SC(NH2)2 at ultralow temperatures

In this work, we demonstrate field-induced Bose-Einstein condensation (BEC) in the organic compound NiCl2-4SC(NH2)_{2} using ac susceptibility measurements down to 1 mK. The Ni S=1 spins exhibit 3D XY antiferromagnetism between a lower critical field H_{c1} approximately 2 T and a upper critical field H_{c2} approximately 12 T. The results show a power-law temperature dependence of the phase transition line H_{c1}(T)-H_{c1}(0)=aT;{alpha} with alpha=1.47+/-0.10 and H_{c1}(0)=2.053 T, consistent with the 3D BEC universality class. Near H_{c2}, a kink was found in the phase boundary at approximately 150 mK.     

Bose-Einstein condensation in the relativistic ideal Bose gas

The Bose-Einstein condensation (BEC) critical temperature in a relativistic ideal Bose gas of identical bosons, with and without the antibosons expected to be pair-produced abundantly at sufficiently hot temperatures, is exactly calculated for all boson number densities, all boson point rest masses, and all temperatures. The Helmholtz free energy at the critical BEC temperature is lower with antibosons, thus implying that omitting antibosons always leads to the computation of a metastable state.     

Signature of Critical Point in Momentum Profile of Trapped Ultracold Bose Gases

We present a new method to identify the critical point for the Bose-Einstein condensation(BEC) of a trapped Bose gas.We calculate the momentum distribution of an interacting Bose gas near the critical temperature,and find that it deviates significantly from the Gaussian profile as the temperature approaches the critical point.More importantly,the standard deviation between the calculated momentum spectrum and the Gaussian profile at the same temperature shows a turning point at the critical point,which can be used to determine the critical temperature.These predictions are aiso confirmed by our BEC experiment for magnetically trapped ~(87)Rb gases.     

二维简谐势阱中旋转理想玻色气体热力学性质的修正

利用截断求和方法修正了二维简谐势阱中旋转理想玻色气体的热力学性质.对玻色-爱因斯坦凝聚(BEC)临界温度的修正表明:旋转框架下的BEC临界温度随旋转频率增大而快速趋近于零,到达势阱特征频率时,基态将会发生从BEC态到强关联非凝聚态的转变;由合成磁场引起的旋转对BEC临界温度的影响则要弱得多.对旋转导致的抗磁性的修正表明:磁化强度随旋转频率和合成磁场的增大而增强.利用截断求和方法计算的结果与考虑有限尺度效应的修正结果获得了很好的一致.     

Critical point of an interacting two-dimensional atomic Bose gas

We have measured the critical atom number in an array of harmonically trapped two-dimensional (2D) Bose gases of rubidium atoms at different temperatures. We found this number to be about 5 times higher than predicted by the semiclassical theory of Bose-Einstein condensation (BEC) in the ideal gas. This demonstrates that the conventional BEC picture is inapplicable in an interacting 2D atomic gas, in sharp contrast to the three-dimensional case. A simple heuristic model based on the Berezinskii-Kosterlitz-Thouless theory of 2D superfluidity and the local density approximation accounts well for our experimental results.     

Two-species fermion mixtures with population imbalance

We analyze the phase diagram of uniform superfluidity for two-species fermion mixtures from the Bardeen-Cooper-Schrieffer to Bose-Einstein condensation (BEC) limit as a function of the scattering parameter and population imbalance. We find at zero temperature that the phase diagram of population imbalance versus scattering parameter is asymmetric for unequal masses, having a larger stability region for uniform superfluidity when the lighter fermions are in excess. In addition, we find topological quantum phase transitions associated with the disappearance or appearance of momentum space regions of zero quasiparticle energies. Lastly, near the critical temperature, we derive the Ginzburg-Landau equation and show that it describes a dilute mixture of composite bosons and unpaired fermions in the BEC limit.     

势场中玻色-爱因斯坦凝聚的临界温度

给出了不同于文献的势场中玻色-爱因斯坦凝聚临界温度表达式.结果揭示了势场中理想玻色子气体凝聚的临界温度与势场之间的关系，表明势场中临界温度正比于无势场情况下临界温度T０ｃ，还给出了势场的有效性判据.势场的有效性是势场与玻尔兹曼常数k和无势场情况下临界温度Ｔ０ｃ乘积ｋＴ０ｃ的比较.当势场接近或大于ｋＴ０c时，临界温度会有效增加;当势场远小于ｋＴ０c时, 势场是无效的. 关键词： 玻色-爱因斯坦凝聚 临界温度 势阱     

Bose-Einstein condensation of bouncing balls

Microscopic bouncing balls, i.e., particles confined within a positive one-half-dimensional gravitational potential, display Bose-Einstein condensation (BEC) not only in the thermodynamic limit but also in the case of a finite number of particles, and the critical temperature with a finite number of particles is higher than that in the thermodynamic limit. This system is different from the one-dimensional harmonic potential one, for which the standard result indicates that the BEC is not possible unless the number of particles is finite.     

谐振势阱中旋转广义玻色系统的热力学性质

以非广延Tsallis统计理论为基础,导出了广义玻色-爱因斯坦统计分布表达式,并用其分别讨论了三维和二维谐振势阱约束的旋转广义玻色气体的热力学性质.结合系统粒子数、玻色-爱因斯坦凝聚(BEC)临界温度、基态粒子占据率和比热等物理量的解析表达式,分析了非广延参数和势阱旋转频率等因素对系统热力学性质的影响.     

Bose-einstein condensation in quasi-2D trapped gases

We discuss Bose-Einstein condensation (BEC) in quasi-2D trapped gases and find that well below the transition temperature T(c) the equilibrium state is a true condensate, whereas at intermediate temperatures T相似文献   

Translationally noninvariant path integral for Bose-Einstein condensate

The Bogolyubov [Hartree-Fock-Bogolyubov (HFB)] method performs the one-particle (mean-field) approximation in the theory of Bose-Einstein condensation (BEC). Various generalizations of this method are possible. Apart from a nonlinear theory, taking the correlation effects into consideration, the HFB approximation for translationally noninvariant systems describes an instructive phenomenon. This paper is devoted to the treatment of two cases: superfluid ⁴He in porous media and atomic BEC in traps subjected to the gravitational field. Both these systems show the dependence of a critical BEC temperature T _c on their nonuniform properties in space.     

High temperature BEC with photon-like atomic polaritons

We consider the problem of high temperature (room and beyond) true Bose-Einstein condensation (BEC) of trapped ideal 1D gas of low branch (LB) atomic polaritons for the system of two-level atomic ensemble interacting with a quantized single-mode electromagnetic field in the presence of optical collisions (OC) with buffer gas atoms. We discuss the application of biconical waveguide cavity (BWC) for observing predicted effects.     

Towards a theory of superconductivity based on collective Cooper pairs

Superconductivity could be seen as a Bose-Einstein condensation (BEC) of Cooper pairs. However, the creation and annihilation operators of Cooper pairs do not satisfy the bosonic commutation relations and then, the mentioned viewpoint has a weakness in its foundation. In this work, we introduce the concept of collective Cooper pairs (CCP) as linear combinations of Cooper pairs and prove their bosonic nature at the dilute limit. This bosonic nature is given rise from their diffuse character on the Cooper pairs, which permits the accumulation of many collective pairs at a single quantum state. Moreover, the superconducting ground state proposed by Bardeen, Cooper and Schrieffer (BCS) can be written in terms of these collective Cooper pairs, which means that the BCS theory is consistent with a possible BEC theory of superconductivity based on collective Cooper pairs. Finally, we calculate the energy spectra and the BEC critical temperature of CCP.     

Pre-formed Cooper pairs and Bose-Einstein condensation in cuprate superconductors

A two-dimensional (2D) assembly of noninteracting, temperature-dependent, pre-formed Cooper pairs in chemical/thermal equilibrium with unpaired fermions is examined in a binary boson-fermion statistical model as the Bose-Einstein condensation (BEC) singularity temperature T_c is approached from above. Compared with BCS theory (which is not a BEC theory) substantially higher T_cs are obtained without any adjustable parameters, that fall roughly within the range of empirical T_cs for quasi-2D cuprate superconductors.     

Bose-Einstein condensation in solid 4He

We present neutron scattering measurements of the atomic momentum distribution n(k) in solid helium under a pressure p=41 bar (molar volume Vm=20.01+/-0.02 cm3/mol) and at temperatures between 80 and 500 mK. The aim is to determine whether there is Bose-Einstein condensation (BEC) below the critical temperature, Tc=200 mK, where a superfluid density has been observed. Assuming BEC appears as a macroscopic occupation of the k=0 state below Tc, we find a condensate fraction of n0=(-0.10+/-1.20)% at T=80 mK and n0=(0.08+/-0.78)% at T=120 mK, consistent with zero. The shape of n(k) also does not change on crossing Tc within measurement precision.     

匀强磁场与简谐势阱中的低维荷电自旋-1玻色气体

采用截断求和法和半经典近似,以二维理想玻色气体为例,研究了磁场和简谐势阱中低维荷电自旋-1玻色子的相变及磁性质．结果表明,电荷-磁场和自旋-磁场作用的竞争导致玻色-爱因斯坦凝聚临界温度随磁场的增大先略微上升后缓慢下降．截断求和法能够有效的改进半经典近似的不足．最后,讨论了磁化强度由抗磁性到顺磁性的转变及自旋因子临界值随磁场和温度的变化．