有限粒子数费米系统温度对顺磁性的影响

根据弱磁场中理想费米气体的粒子数N+的几率分布函数G(β,N+),运用理论解析与数值模拟相结合的方法,研究了有限粒子数理想费米系统的泡利顺磁性,给出了粒子数临界值和上界磁场的解析式,分析了温度和粒子数对顺磁性的影响.研究表明,温度趋于费米温度时,极限磁化率与平均磁化率的偏差减小;当温度高于费米温度时,极限磁化率与平均磁...     

关于平衡态下费米气体最小冷原子数目的讨论

应用统计物理等知识得到费米气体的定容热容量Cν的解析表达式，再利用热容量Cν与平衡态下冷原子的最小数目Nmin之间的关系分析讨论了弱相互作用费米气体在无外磁场和谐振势约束两种情况，给出了对这种系统的最小粒子数量级的估计。由此可知，在低温极限下，对于一个弱相互作用费米气体系统，无外磁场情况下至少要有量级为107的粒子才能构成一个热力学系统，而谐振势约束情况下则至少需要量级为105的粒子。     

任意子气体的微扰物态方程与外磁场中任意子气体的第二维里系数

用费曼路径积分方法直接计算外磁场中理想任意子气体和相互作用任意子气体的第二维里系数。用量子统计的格林函数方法求出了费米点附近理想任意子气体的一阶微扰物态方程。     

势阱中带电荷理想费米气体的热力学性质

从统计物理半经典近似方法出发,研究各向异性简谐势阱中理想带电荷费米气体系统的热力学性质,推导出了该体系的热力学势、比热、内能、磁场强度和磁化率随外加磁场的变化关系,进而分析了约束势阱对理想带电荷费米体系热力学性质的影响.     

弱磁场中弱相互作用费米气体的泡利顺磁性

根据赝势法和大数近似法导出弱磁场中弱相互作用费米气体的配分函数,并进行泡利顺磁性磁化率的解析计算,分析粒子间的相互作用对磁化率的影响.     

有限粒子数理想费米系统的磁化率

根据弱磁场中理想费米气体(忽略轨道运动)的正则配分函数Z(N),导出粒子数N 的几率分布函数G(β,N )。运用统计平均方法,解析出有限粒子数理想费米系统泡利顺磁性平均磁化率的表达式。然后通过计算热力学极限下不同温度范围的磁化率pχ,得到不同温度范围内的有限粒子数理想费米系统泡利顺磁性平均磁化率的解析式。研究结果表明,有限粒子数理想费米系统的平均磁化率小于热力学极限下的磁化率,温度愈高,平均磁化率愈小;在低温条件下,磁场愈强,平均磁化率愈小;在高温条件下,磁场愈强,平均磁化率愈大。系统的粒子数N也对平均磁化率有直接的影响,粒子数N愈大,系统的平均磁化率愈大。     

均匀重力场中弱相互作用费米气体的低温性质

根据半经典近似法和局域密度近似法,分别研究了均匀重力场中理想费米气体和弱相互作用费米气体的低温性质,得到了一些重要热力学量的一般解析表达式.对自由费米体系,均匀重力场中理想费米体系,均匀重力场中弱相互作用费米体系进行了比较,探讨了重力势场及粒子间的相互作用对体系性质的影响.     

有限重力场中理想Fermi气体的弱简并特性

研究有限重力场中弱简并理想费米气体的热力学性质.采用Thomas-Fermi半经典近似方法,从有限重力场中理想费米气体的状态密度出发,给出系统的化学势、内能和压强解析表达式,分析重力场对弱简并理想费米气体热力学性质的影响.结果表明,有限重力场的存在使弱简并理想费米气体热力学量出现一个附加的修正项.     

弱磁场中弱相互作用费米气体的相对论修正效应

基于量子统计方法，通过考虑单粒子能谱的相对论修正且引入无自旋自由费米子的非相对论自由能，导出弱磁场中弱相互作用费米气体的自由能，给出各种温度条件下系统热力学量的解析式，详细具体地展示相对论修正对热力学性质的影响，并分析其影响机理.研究表明，与非相对论比较，相对论修正总是降低化学势，在低温下也降低总能及热容量，而在高温下增加总能及热容量；除低温下的热容量外，总能及热容量的改变比例于磁场的平方.     

二维含铁磁耦合费米气体磁学性质的研究?

在平均场理论的框架下,推导出外磁场中具有铁磁耦和作用的二维费米气体的自洽方程,在此基础上,运用量子统计力学研究系统的磁学性质.结果表明总磁化强度密度随磁场强度的增大会出现极大值点,且随铁磁耦合强度增大,极大值点向磁场减弱的方向移动.总磁化强度密度也随Lande因子的增大,发生由抗磁性向顺磁性的转变.     

有限数目费米子体系的泡利顺磁性

研究弱相互作用费米体系的泡利顺磁性,导出磁化率的解析表达式．与理想体系比较,指出粒子间的相互作用加强了泡利顺磁性．通过分析系统的涨落,得到若要出现泡利顺磁现象,外磁场需达到一个临界值．     

Thermal stability conditions of a weakly interacting Fermi gas in a weak magnetic field

On the basis of the results derived from pseudopotential method and ensemble theory, thermal stability of a weakly interacting Fermi gas in a weak magnetic field are studied by using analytical method of thermodynamics. The exact analytical expressions of stability conditions at different temperatures are given, and the effects of interactions as well as magnetic field on stability of the system are discussed. It is shown that there is an upper-limit magnetic field for the stability of the system at low temperatures and there is an attractive dividing value at high temperatures. If attractive interaction is lower than the critical value, the stability of the system has no request for magnetic field, but if attractive interaction is higher than the dividing value, a lower-limit magnetic field exists for the stability of the system.     

均匀重力场中弱相互作用费米气体的热力学性质

根据局域密度近似法,研究了均匀重力场中弱相互作用费米气体高于费米温度条件下的性质,得到了一些重要热力学量的一般解析表达式,分析了系统的粒子数和能量密度的空间分布规律。对均匀重力场中理想费米体系,均匀重力场中弱相互作用费米体系进行了比较,探讨了重力势场及粒子间的相互作用对体系性质的影响。     

Creation of ultracold molecules from a Fermi gas of atoms

Following the realization of Bose-Einstein condensates in atomic gases, an experimental challenge is the production of molecular gases in the quantum regime. A promising approach is to create the molecular gas directly from an ultracold atomic gas; for example, bosonic atoms in a Bose-Einstein condensate have been coupled to electronic ground-state molecules through photoassociation or a magnetic field Feshbach resonance. The availability of atomic Fermi gases offers the prospect of coupling fermionic atoms to bosonic molecules, thus altering the quantum statistics of the system. Such a coupling would be closely related to the pairing mechanism in a fermionic superfluid, predicted to occur near a Feshbach resonance. Here we report the creation and quantitative characterization of ultracold 40K2 molecules. Starting with a quantum degenerate Fermi gas of atoms at a temperature of less than 150 nK, we scan the system over a Feshbach resonance to create adiabatically more than 250,000 trapped molecules; these can be converted back to atoms by reversing the scan. The small binding energy of the molecules is controlled by detuning the magnetic field away from the Feshbach resonance, and can be varied over a wide range. We directly detect these weakly bound molecules through their radio-frequency photodissociation spectra; these probe the molecular wavefunction, and yield binding energies that are consistent with theory.     

Vortices and superfluidity in a strongly interacting Fermi gas

Quantum degenerate Fermi gases provide a remarkable opportunity to study strongly interacting fermions. In contrast to other Fermi systems, such as superconductors, neutron stars or the quark-gluon plasma of the early Universe, these gases have low densities and their interactions can be precisely controlled over an enormous range. Previous experiments with Fermi gases have revealed condensation of fermion pairs. Although these and other studies were consistent with predictions assuming superfluidity, proof of superfluid behaviour has been elusive. Here we report observations of vortex lattices in a strongly interacting, rotating Fermi gas that provide definitive evidence for superfluidity. The interaction and therefore the pairing strength between two 6Li fermions near a Feshbach resonance can be controlled by an external magnetic field. This allows us to explore the crossover from a Bose-Einstein condensate of molecules to a Bardeen-Cooper-Schrieffer superfluid of loosely bound pairs. The crossover is associated with a new form of superfluidity that may provide insights into high-transition-temperature superconductors.