球对称铷原子玻色-爱因斯坦凝聚中单极子模的朗道阻尼和频移

采用含时哈特里-福克-博戈留波夫近似研究球对称铷原子玻色-爱因斯坦凝聚中单极子模的朗道阻尼和频移,并用现有实验和数值模拟研究的粒子数和囚禁频率参量,解析计算了阻尼系数和频移大小及其它们的温度依赖。计算中,考虑元激发的实际弛豫及其各弛豫间的正交关系以获得阻尼和频移计算公式,把基态波函数取为高斯分布函数的一级近似以消除三模耦合矩阵元的发散。我们的计算结果与数值模拟结果和实验结果分别进行直接和间接地对比,讨论和说明了我们理论方法的合理性。     

雪茄形铷原子玻色-爱因斯坦凝聚中单极子模的朗道阻尼和频移

采用含时哈特里-福克-博戈留波夫近似研究雪茄形铷原子玻色-爱因斯坦凝聚中单极子模的朗道阻尼和频移. 通过考虑元激发的实际弛豫及其各弛豫间的正交关系改进原有方法, 并由此给出计算朗道阻尼和频移的新公式. 此外, 令凝聚体边界处动能密度为零代替令基态能量极小以改进原消除三模耦合矩阵元的方法. 通过这些改进, 同时计算阻尼和频移, 并讨论它们的温度依赖, 所得理论结果都与实验符合. 关键词： 玻色-爱因斯坦凝聚 朗道阻尼和频移 哈特里-福克-博戈留波夫近似 托马斯-费米近似     

强耦合光机械腔中的简正模式分裂和冷却

研究由辐射压力与驱动Fabry-Perot光学腔相耦合而产生的腔光机械动力学行为. 通过量子朗之万方程具体研究了机械振子的涨落光谱、机械阻尼与共振频移和基态冷却. 随着输入激光功率的增加,振子的涨落光谱呈现简正模式分裂的现象,并且数值模拟结果和实验结果相符合. 同时推导了有效机械阻尼和共振频移. 红移边带导致了机械模的冷却,蓝移边带引起了机械模的放大. 此外,引入一种近似机制来研究振子的基态冷却,并且考虑在解析边带机制下简正模式分裂对机械振子冷却的影响. 最后,数值讨论了初始浴温度、输入激光功率和机械品质因数这三个因素对机械振子冷却的影响. 关键词： 腔光机械 辐射压力 简正模式分裂 冷却     

Ultrasound Attenuation in s-wave Two-band Superconductors北大核心CSCD

s~±-波电子配对被认为很可能用于解释铁基超导体的配对机制.本文中,我们计算了s~±-波二带超导体的超声衰减系数,并利用T-矩阵近似研究了杂质的影响.我们的计算表明,当两带的能隙大小相当时,超声衰减-温度曲线中出现一个Hebel-Slichter峰.我们同时验证了核自旋弛豫率的研究中发现的低温下的共振束缚态效应.     

QL-Born迭代法电磁散射计算

积分方程法是多维电磁响应的数值模拟和反演计算的有效方法之一.在线性化散射电磁场积分方程的基础上,采用迭代Born近似法进行了电导率成像反演.针对迭代Born近似法反演依赖初始模型,而QL近似法无需给定初值的特点,提出了两步法(QL-Born迭代法),用QL近似法计算的结果作为Born迭代法的初始模型,避免人为设置初值,并进行了数值实验.     

硝基甲烷振动能量弛豫的集体模型

用分子动力学方法结合Dlott等人提出的"门槛模"理论研究集体相互作用下硝基甲烷振动能量弛豫过程.其中振动冷却过程与实验符合的很好.在振动激发过程的分子动力学模拟中观测到与实验一致的基频频移现象.用分子动力学方法从微观上详细地描述出分子"门槛模"振动激发过程.研究表明,在高温高压作用下,集体作用效应对多原子振动激发具有不可忽视的作用,能量传递过程中除了基频的作用外,强烈的非线性相互作用引起的振动模泛频也携带有大量的振动能,这些泛频也对分子振动能量传递产生重要影响.     

ICRF快波模转换及快波传播

本文通过分析离子回旋频段(ICRF)快波在模转换区传播的物理过程,导出了一个新的快波近似——完全快波近似二阶波方程。数值计算表明,它能给出与求解高阶方程非常一致的结果,尤其是在弱回旋阻尼(k_1较小)情况下,完全快波近似比快波近似有更满意的结果。最后讨论了二次谐波加热和少数成份离子加热的实例。     

飞秒光孤波传输系统中自频移的抑制

研究了飞秒光泳冲在光纤中传输时自频移的抑制问题；获得了在带宽限制放大器和非线性增益共同作用下脉宽与频移不随距离变化的近似稳态解。利用线性菌定性分析，获得了该菌态解的线性菌定条件。最后数值模拟了该稳态解在光纤系统中的传输，结果表明该菌态解在光纤系统中可稳定传输，喇曼自频移效应得到了较好的抑制。     

脉冲激光在有机分子材料中的共振传播

利用麦克斯韦-布洛赫耦合方程,研究了激光脉冲在4,4′-二甲氨基二苯乙烯分子材料中的共振传播,探讨了激光脉冲在传播中的时空演化情况.在数值计算中考虑了各种阻尼效应.     

一种分析楔形光纤的等效矩形近似-阶梯串联法

楔形光纤(WSF)是实现平面光波光路芯片入出端口与光纤高效连接的核心部件. 采用数值模拟方法分析其中光波传输演化过程,是优化设计光子器件耦合结构的重要基础. 提出基于等效矩形近似的三维阶梯串联法(ERA-SCM),将楔端沿传输方向细分,引入矩形波导近似,给出了细分后各段近似矩形波导的等效折射率;在此基础上,建立了阶梯串联法分析模型,分级给出WSF中光波传输过程与模场演变. 数值分析结果表明,ERA-SCM比有限差分束传播法(FD-BPM)能够更精确地分析非对称光纤和波导结构,描述其中光模场的演化. WSF出射光场实测结果表明,ERA-SCM数值模拟结果与实验结果的误差为1.9%,而FD-BPM的误差为4.5%. ERA-SCM是分析非对称光波导光波传输与模场演变的有效方法. 关键词： 平面光波光路 楔形光纤 等效矩形近似 阶梯串联法     

Landau Damping of Collective Mode in a Quasi-One-Dimensional Repulsive Bose-Einstein Condensate

We investigate the Landau damping of the collective mode in a quasi-one-dimensional repulsive Bose-Einstein condensate by using the self-consistent time-dependent Hartree-Fock-Bogoliubov approximation. We put forward a new method to calculate the Landau damping rate of the collective mode in the condensate and discuss the dependence of the Landau damping on temperature, on transverse trapping frequency, on atom number in the condensate, and on length of the system. Different from the usual calculation method for the three dimension system, our new calculation method is an interactive one by considering the practical relaxation of the elementary excitation. With little approximation, our theoretical calculation results agree with the experimental ones. Comparing with the usual calculation method, our theory is helpful to deduce the inter-particle interactions in damping phenomenon.     

Landau damping and frequency-shift of a quadrupole mode in a disc-shaped rubidium Bose–Einstein condensate

The damping and frequency-shift in Landau mechanism of a quadrupole mode in a disc-shaped rubidium Bose–Einstein condensate are investigated by using the Hartree–Fock–Bogoliubov approximation. The practical relaxations of the elementary excitations and the orthometric relation among them are taken into account to obtain advisable calculation formula for damping as well as frequency-shift. The first approximation of Gaussian distribution function is employed for the ground-state wavefunction to suitably eliminate the divergence of the analytic three-mode coupling matrix elements.According to these methods, both Landau damping rate and frequency-shift of the quadrupole mode are analytically calculated. In addition, all the theoretical results agree with the experimental ones.     

Landau damping of collective mode in a quasi-two-dimensional repulsive Boseben Einstein condensate

We investigate the Landau damping of the collective mode in a quasi-two-dimension repulsive Bose—Einstein condensate by using the self-consistent time-dependent Hatree—Fock—Bogoliubov approximation and a complete and orthogonal eigenfunction set for the elementary excitation of the system. We calculate the three-mode coupling matrix element between the collective mode and the thermal excited quasi-particles and the Landau damping rate of the collective mode. We discuss the dependence of the Landau damping on temperature, on atom number in the condensate, on transverse trapping frequency and on the length of the condensate. The energy width of the collective mode is taken into account in our calculation. With little approximation, our theoretic calculation results agree well with the experimental ones and are helpful for deducing the damping mechanics and the inter-particle interaction.     

Energy diffusion controlled reaction rate of reacting particle driven by broad-band noise

The energy diffusion controlled reaction rate of a reacting particle with linear weak damping and broad-band noise excitation is studied by using the stochastic averaging method. First, the stochastic averaging method for strongly nonlinear oscillators under broad-band noise excitation using generalized harmonic functions is briefly introduced. Then, the reaction rate of the classical Kramers' reacting model with linear weak damping and broad-band noise excitation is investigated by using the stochastic averaging method. The averaged It? stochastic differential equation describing the energy diffusion and the Pontryagin equation governing the mean first-passage time (MFPT) are established. The energy diffusion controlled reaction rate is obtained as the inverse of the MFPT by solving the Pontryagin equation. The results of two special cases of broad-band noises, i.e. the harmonic noise and the exponentially corrected noise, are discussed in details. It is demonstrated that the general expression of reaction rate derived by the authors can be reduced to the classical ones via linear approximation and high potential barrier approximation. The good agreement with the results of the Monte Carlo simulation verifies that the reaction rate can be well predicted using the stochastic averaging method.     

高斯光束照射下与矩孔相关的孔径衍射

依据基尔霍夫衍射理论,利用幂级数展开的方法,推导出傍轴条件下高斯光束的矩孔及单缝衍射的近似计算公式,并通过数值计算、数值模拟等方法对衍射场的基本特性进行了讨论。     

Spectral Statistics of the Rectangular Billiard with a Flux Line

The density of states of a rectangular billiard with an Aharonov–Bohm flux line in its center was calculated in the semiclassical approximation and was used for the calculation of the form factor in the diagonal approximation. The distribution of nearest level spacings and the form factor were calculated also numerically. For some values of the flux these were found to be close to the ones of the semi-Poisson statistics. The difference between the numerical results and the semiclassical ones were found to be much larger than for chaotic and for integrable systems within similar approximations. Possible reasons for these discrepancies are discussed.