玻色-费米超流混合体系中的相互作用调制隧穿动力学

研究了玻色-费米超流混合体系中的相互作用调制隧穿动力学特性,其中玻色子位于对称双势阱中,费米子位于对称双势阱中心的简谐势阱中.采用双模近似方法得到描述双势阱玻色-爱因斯坦凝聚的动力学特性方程组,并将其与简谐势阱中分子玻色-爱因斯坦凝聚的Gross-Pitaevskii方程进行耦合.通过对不同参数下玻色-费米混合体系中的隧穿现象进行数值研究,发现简谐势阱中费米子与双势阱中玻色子的相互作用使双势阱玻色-爱因斯坦凝聚的隧穿动力学特性更加丰富.不但驱使双势阱中玻色-爱因斯坦凝聚从类约瑟夫森振荡转变为宏观量子自囚禁,而且宏观量子自囚禁表现为三种不同的形式:相位与时间呈负相关并随时间单调减小的自囚禁、相位随时间演化有界的自囚禁以及相位与时间呈正相关并随时间单调增大的自囚禁.     

旋转受限相互作用玻色系统的相变温度及基态粒子占据率

利用局域密度近似(LDA)导出了简谐势阱中存在弱相互作用的旋转玻色气体发生玻色-爱因斯坦凝聚时的粒子数、相变温度和基态粒子占据率的解析表达式,探讨了粒子间相互作用对相变温度和基态粒子占据率的影响.计算表明,当粒子间的相互作用消失时,所有解析结果均能够与无相互作用的旋转理想玻色气体获得很好的一致.     

旋转对称W型势阱中玻色-爱因斯坦凝聚环

提出一旋转对称磁光阱,这一势阱束缚的玻色-爱因斯坦凝聚是环形的．分别对无相互作用和强排斥作用两种极端情况的基态波函数做了计算．分析了凝聚形成的转变温度. 关键词：     

三维非谐势阱中玻色-爱因斯坦凝聚的数值计算

本文从G-P平均势场理论出发,探讨了三维球对称非谐势阱中玻色-爱因斯坦凝聚(BEC)的G-P方程;用数值计算方法研究了三维球对称非谐势阱中原子间有相互作用的玻色-爱因斯坦凝聚气体的基态解;分析了非谐振势能项对玻色-爱因斯坦凝聚体的分布、能量和化学势的影响.     

简谐势阱中具有吸引相互作用原子体系的玻色-爱因斯坦凝聚

给出了简谐势阱中具有吸引相互作用原子体系的非线性定态薛定谔方程的基态解,得到了凝 聚原子数随能量本征值变化的双稳态曲线.并由此得到与实验报道相符的吸引型玻色-爱因斯 坦凝聚体所能包含的最大原子数. 关键词： 玻色-爱因斯坦凝聚 双稳态     

三维简谐势阱中玻色-爱因斯坦凝聚的边界效应

在定义特征长度的基础上,应用Euler–MacLaurin公式,研究了理想玻色气体在三维简谐势阱中玻色-爱因斯坦凝聚的边界效应.结果表明:粒子的凝聚分数由于有限尺度和有限粒子数效应而减小,修正的凝聚分数和凝聚温度由于边界效应存在一个极大值,选择优化的最佳势阱参数,可以有效提高凝聚分数和凝聚温度;热容量的跃变存在边界效应和粒子数效应,选择合理的势阱参数时,热容量的跃变存在一个极小值.导出了简谐势阱中有限理想玻色气体的状态方程,揭示了压强的各向异性(或各向同性)取决于简谐势频率的各向异性(或各向同性).     

三维非谐势阱中玻色－爱因斯坦凝聚的数值计算

本文从G－P平均势场理论出发,探讨了三维球对称非谐势阱中玻色－爱因斯坦凝聚(BEC)的G－P方程;用数值计算方法研究了三维球对称非谐势阱中原子间有相互作用的玻色－爱因斯坦凝聚气体的基态解;分析了非谐振势能项对玻色－爱因斯坦凝聚体的分布、能量和化学势的影响。     

Gross-Pitaevskii方程的数值解及其动力学研究

采用辛-打靶法求解了简谐势阱中碱金属玻色-爱因斯坦凝聚原子在T=0时的基态波函数,检验了波函数的稳定性,并研究了当简谐势阱突然改变时波函数随时间演化的动力学.     

非对称的玻色-爱因斯坦凝聚中的约瑟夫森结的动力学性质

利用半经典量子理论，研究了玻色-爱因斯坦凝聚体处于非对称的约瑟夫森结的动力学行为.结果表明双势阱中不同势阱的基态能量差与其相互作用能量的比率χ=0时，凝聚体表现为约瑟夫森效应；当χ≠0时，凝聚体中既存在量子宏观隧穿效应，又存在量子宏观局域效应. 关键词： 玻色爱因斯坦凝聚 约瑟夫森结 动力学性质     

非谐势阱中玻色-爱因斯坦凝聚的数值计算

从G-P平均势场理论出发,探讨了玻色-爱因斯坦凝聚(BEC)的G-P方程的一维形式,用数值计算方法研究了非谐势阱中非理想玻色凝聚气体的基态和第一激发态解.给出了能量随非线性系数的变化规律.     

Trading interactions for topology in scale-free networks

Scale-free networks with topology-dependent interactions are studied. It is shown that the universality classes of critical behavior, which conventionally depend only on topology, can also be explored by tuning the interactions. A mapping, gamma'=(gamma-mu)/(1-mu), describes how a shift of the standard exponent gamma of the degree distribution P(q) can absorb the effect of degree-dependent pair interactions J(ij)proportional to(q(i)q(j))(-mu). The replica technique, cavity method, and Monte Carlo simulation support the physical picture suggested by Landau theory for the critical exponents and by the Bethe-Peierls approximation for the critical temperature. The equivalence of topology and interaction holds for equilibrium and nonequilibrium systems, and is illustrated with interdisciplinary applications.     

Crossover Critical Behavior in the Three-Dimensional Ising Model

The character of critical behavior in physical systems depends on the range of interactions. In the limit of infinite range of the interactions, systems will exhibit mean-field critical behavior, i.e., critical behavior not affected by fluctuations of the order parameter. If the interaction range is finite, the critical behavior asymptotically close to the critical point is determined by fluctuations and the actual critical behavior depends on the particular universality class. A variety of systems, including fluids and anisotropic ferromagnets, belongs to the three-dimensional Ising universality class. Recent numerical studies of Ising models with different interaction ranges have revealed a spectacular crossover between the asymptotic fluctuation-induced critical behavior and mean-field-type critical behavior. In this work, we compare these numerical results with a crossover Landau model based on renormalization-group matching. For this purpose we consider an application of the crossover Landau model to the three-dimensional Ising model without fitting to any adjustable parameters. The crossover behavior of the critical susceptibility and of the order parameter is analyzed over a broad range (ten orders) of the scaled distance to the critical temperature. The dependence of the coupling constant on the interaction range, governing the crossover critical behavior, is discussed.     

Rotons in interacting ultracold bose gases

In three dimensions, noninteracting bosons undergo Bose-Einstein condensation at a critical temperature, T(c), which is slightly shifted by ΔT(c), if the particles interact. We calculate the excitation spectrum of interacting Bose systems, (4)He and (87)Rb, and show that a roton minimum emerges in the spectrum above a threshold value of the gas parameter. We provide a general theoretical argument for why the roton minimum and the maximal upward critical temperature shift are related. We also suggest two experimental avenues to observe rotons in condensates. These results, based upon a path-integral Monte Carlo approach, provide a microscopic explanation of the shift in the critical temperature and also show that a roton minimum does emerge in the excitation spectrum of particles with a structureless, short-range, two-body interaction.     

Influence of the range of interactions in thin magnetic structures

The properties of thin magnetic structures are influenced by many length scales that reflect both generic physics and chemical detail. A striking example is the experimentally determined shift of the critical temperature as a function of film thickness. While all systems experience a pronounced suppression in the transition temperature with decreasing film thickness, the magnitude of this shift cannot be reconciled with established theoretical results. By means of detailed Monte Carlo simulations, we address this discrepancy by investigating a model with long range interactions. The model also captures other features of real thin magnets, such as an almost linear temperature dependence for the surface magnetization. Our results demonstrate that the behavior of thin magnetic structures arises from a competition of length scales dictated by their slab-like geometry, the presence of surface boundaries, and crucially, the range of the interactions present.     

Competing interactions, the renormalization group, and the isotropic-nematic phase transition

We discuss 2D systems with Ising symmetry and competing interactions at different scales. In the framework of the renormalization group, we study the effect of relevant quartic interactions. In addition to the usual constant interaction term, we analyze the effect of quadrupole interactions in the self-consistent Hartree approximation. We show that in the case of a repulsive quadrupole interaction, there is a first-order phase transition to a stripe phase in agreement with the well-known Brazovskii result. However, in the case of attractive quadrupole interactions there is an isotropic-nematic second-order transition with higher critical temperature.     

Mutual influence between triel bond and cation–π interactions: an ab initio study

Using ab initio calculations, the cooperative and solvent effects on cation–π and B···N interactions are studied in some model ternary complexes, where these interactions coexist. The nature of the interactions and the mechanism of cooperativity are investigated by means of quantum theory of atoms in molecules (QTAIM), noncovalent interaction (NCI) index and natural bond orbital analysis. The results indicate that all cation–π and B···N binding distances in the ternary complexes are shorter than those of corresponding binary systems. The QTAIM analysis reveals that ternary complexes have higher electron density at their bond critical points relative to the corresponding binary complexes. In addition, according to the QTAIM analysis, the formation of cation–π interaction increases covalency of B···N bonds. The NCI analysis indicates that the cooperative effects in the ternary complexes make a shift in the location of the spike associated with each interaction, which can be regarded as an evidence for the reinforcement of both cation–π and B···N interactions in these systems. Solvent effects on the cooperativity of cation–π and B···N interactions are also investigated.     

Renormalization group solution of the one-dimensional classical Heisenberg model

It is shown that the method of Van Leeuwen^1,2) and Nauenberg and Nienhuis³) in the application of the renormalization theory to Ising-like spin systems, can easily be extended to include all one-dimensional classical spin systems with nearest neighbor interactions. The series for the free energy converges very rapidly towards the known exact value (for Heisenberg interaction), provided that the temperature is not too close to the critical temperature T = 0.     

Critical temperature of weakly interacting dipolar condensates

We calculate perturbatively the effect of a dipolar interaction upon the Bose-Einstein condensation temperature. This dipolar shift depends on the angle between the symmetry axes of the trap and the aligned atomic dipole moments, and is extremal for parallel or orthogonal orientations, respectively. The difference of both critical temperatures exhibits most clearly the dipole-dipole interaction and can be enhanced by increasing both the number of atoms and the anisotropy of the trap. Applying our results to chromium atoms, which have a large magnetic dipole moment, shows that this dipolar shift of the critical temperature could be measured in the ongoing Stuttgart experiment.     

Effect of elastic deformations on the critical behavior of disordered systems with long-range interactions

A field-theoretic approach is applied to describe behavior of three-dimensional, weakly disordered, elastically isotropic, compressible systems with long-range interactions at various values of a long-range interaction parameter. Renormalization-group equations are analyzed in the two-loop approximation by using the Padé-Borel summation technique. The fixed points corresponding to critical and tricritical behavior of the systems are determined. Elastic deformations are shown to changes in critical and tricritical behavior of disordered compressible systems with long-range interactions. The critical exponents characterizing a system in the critical and tricritical regions are determined.     

Magnetic particles with shifted dipoles

We present simulations and analytical calculations for a system of magnetic nano-particles that possess a dipole moment that is shifted out of the center of mass, towards the surface, leading to a further asymmetry of the dipolar interaction. In our contribution, we discuss the peculiarities of ground state small clusters, both, with and without an external magnetic field. Small clusters help us to get an insight into the inter-particle interactions and form a building block for studies of larger systems. Without external magnetic field, the ground state structure changes from chains and rings with parallel alignment of moments, usually observed in dipolar particles (Prokopieva et al., 2009 [1]), to pairs and triangles with close to anti-parallel orientation of moments, when the shift of the dipole is increased. We also present magnetization properties of larger systems at finite temperature and observe the influence of the shift in particular on the initial slope of the magnetization curve, namely, the initial susceptibility.