扫描磁场中玻色-爱因斯坦凝聚体系的奇异自旋隧穿

用平均场的方法，研究了线性扫描磁场中自旋-1玻色-爱因斯坦凝聚体系的自旋隧穿.集中考虑⁸⁷Rb这种典型的碱金属原子凝聚体，根据外磁场扫描率的不同，研究了它的隧穿动力学.在慢扫描(即绝热条件)和快扫描条件下，体系无隧穿现象.对中等大小的扫描率，发现隧穿现象，且这个隧穿动力学对磁场扫描率非常的敏感，表现为看似混沌的隧穿区的存在.然而，把这个看似混沌的区域放大，发现在扫描率精度为10^-8T/s的量级上，隧穿率对磁场扫描率的关系实际是有规律的类周期结构.此外，还发现，实     

玻色-爱因斯坦凝聚体系中的混沌隧穿行为

研究了双势阱玻色-爱因斯坦凝聚(BEC)系统在外加周期调制下的混沌相变过程,着重讨论了混沌现象对BEC系统隧穿的影响.当外加调制频率与系统固有频率达到共振时,相平面会出现不稳定现象,即混沌现象.在量子情况下,研究了系统的Husimi函数随时间的演化.研究发现：当混沌现象出现时,系统中粒子间相互作用增大,使得混沌区域扩大,进而引起混沌辅助隧穿程度的变化. 关键词： 玻色-爱因斯坦凝聚 混沌 隧穿     

偶极旋量玻色-爱因斯坦凝聚体在外场中的自旋混合动力学

探讨了外场中偶极旋量玻色-爱因斯坦凝聚体的静态和动力学行为.研究结果表明,可以调节外场、自旋交换相互作用和偶极-偶极相互作用来调控三组分之间的隧穿,控制布居数动力学演化范围和相对相位动力学行为. 关键词： 玻色-爱因斯坦凝聚 偶极-偶极相互作用 自旋混合动力学     

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.     

三势阱玻色爱因斯坦凝聚体三体复合效应的数值研究

文章中着重研究在三势阱中的凝聚体所受到的非弹性碰撞相互作用,考虑有三体复合耗散和原子填充时三势阱中玻色-爱因斯坦凝聚体动力学性质,研究了三势阱中玻色-爱因斯坦凝聚体系的自囚禁与宏观量子遂穿问题,用数值分析和解析推导的方法,发现自俘获和宏观量子隧穿现,另外,在自俘获和宏观量子隧穿进程中发现了量子拍现象,并对这些现象给出合理的解释。     

两分量玻色-爱因斯坦凝聚系统的自旋压缩

在周期性脉冲撞击的两分量玻色－爱因斯坦凝聚系统中研究了自旋压缩动力学性质,结果表明：原子自旋压缩动力学能够揭示相空间的混沌和规则结构,即当初始相干态处在混沌区域时,自旋压缩在很短时间后消失,而当初始相干态处在规则区域时,自旋压缩则存在很长时间.特别是随着时间的演化,系统的平均自旋方向的分布和摆动也与初态所处的空间结构有着密切的联系.最后,研究了相空间的整体自旋压缩动力学,得到了一种较好的量子-经典对应. 关键词： 玻色-爱因斯坦凝聚 混沌 自旋压缩 平均自旋方向     

双势阱中玻色-爱因斯坦凝聚的绝热隧穿

研究了玻色-爱因斯坦凝聚体在双势阱中随着能级差绝热循环变化而发生的绝热隧穿.发现当相互作用较强且初态选择为凝聚体全部置于较浅势阱时,演化过程破坏绝热定理,而演化结果有可能回到初态,也有可能不回到初态,取决于演化周期的选择;另外还发现演化过程表现出对初态选择的依赖,具有不对称的特征.利用能级图和相图,对上述现象给出了解释. 关键词： 玻色-爱因斯坦凝聚 Landau-Zener模型 绝热隧穿     

旋量玻色-爱因斯坦凝聚体拓扑性质的研究进展

实现玻色-爱因斯坦凝聚的原子大多具备内部自旋自由度,在光势阱下原子内部自旋被解冻,从而使原子可以凝聚到各个超精细量子态上,形成旋量玻色-爱因斯坦凝聚体.灵活的自旋自由度成为体系相关的动力学变量,可以使体系出现新奇的拓扑量子态,如自旋畴壁、涡旋、磁单极子、斯格明子等.本文综述了旋量玻色-爱因斯坦凝聚的实验和理论研究,旋量玻色-爱因斯坦凝聚体中拓扑缺陷的种类,以及两分量、三分量玻色-爱因斯坦凝聚体中拓扑缺陷的研究进展.     

对称双势阱玻色-爱因斯坦凝聚系统在周期驱动下的动力学相变及其量子纠缠熵表示

研究了在对称双势阱玻色-爱因斯坦凝聚体系粒子间相互作用项上外加周期调制而引起的系统动力学相变,特别地研究了该系统通向混沌的相变过程.发现在一定驱动参数下,当外加调制频率与系统固有频率达到共振时,相平面会出现不稳定性现象,即混沌.在混沌区域,粒子在各量子态随机分布,平均布居数差在零附近波动.特别地,研究表明,混沌现象的出现可以用量子纠缠熵来表征,混沌现象出现时,两种平均纠缠熵都趋于它们的最大值. 关键词： 玻色-爱因斯坦凝聚 双势阱 混沌 纠缠熵     

自旋-轨道耦合玻色-爱因斯坦凝聚势垒散射特性的研究

对自旋-轨道耦合玻色-爱因斯坦凝聚中的双势垒散射问题进行了研究, 得到了系统透射系数的解析表达式, 并对如何克服Klein隧穿以及如何束缚Dirac粒子进行了讨论并给出囚禁Dirac粒子的实验方案. 此外, 运用时间劈裂谱方法对Dirac粒子势垒散射问题进行了数值模拟. 分析了Dirac粒子分别在势垒Klein阻塞区域中心以及边缘的透射情况. 最后从排斥和吸引相互作用两方面研究了非线性相互作用对于Dirac粒子演化的影响, 结果表明弱非线性相互作用对散射特性的影响非常小, 而强非线性相互作用会彻底破坏波包的动量分布, 从而改变Dirac粒子的势垒散射效果． 关键词： 自旋-轨道耦合 Klein隧穿 势垒散射 玻色-爱因斯坦凝聚     

Spin tunneling properties in mesoscopic magnets: effects of a magnetic field

The tunneling of a giant spin at excited levels is studied theoretically in mesoscopic magnets with a magnetic field at an arbitrary angle in the easy plane. Different structures of the tunneling barriers can be generated by the magnetocrystalline anisotropy, the magnitude and the orientation of the field. By calculating the nonvacuum instanton solution explicitly, we obtain the tunnel splittings and the tunneling rates for different angle ranges of the external magnetic field ( θ _H = π/2 and π/2 < θ _H < π). The temperature dependences of the decay rates are clearly shown for each case. It is found that the tunneling rate and the crossover temperature depend on the orientation of the external magnetic field. This feature can be tested with the use of existing experimental techniques. Received 12 March 2001 and Received in final form 18 October 2001     

Control of the nuclear spin of the <Superscript>129</Superscript>Xe and <Superscript>131</Superscript>Xe isotopes in the spin-exchange interaction with <Superscript>87</Superscript>Rb atoms

Nuclear spin dynamics of the ¹²⁹Xe and ¹³¹Xe isotopes in an external magnetic field B ₀ is considered. Nuclear spin is pumped by the laser through ⁸⁷Rb, which transfers the electron spin to the ¹²⁹Xe and ¹³¹Xe nuclei in the spin-exchange interaction. The nuclear spin dynamics is controlled with a transverse magnetic field that causes nuclear magnetic resonance in both ¹²⁹Xe and ¹³¹Xe isotopes. Numerical calculations are performed to find conditions at which the transverse component of the nuclear spin in the established motion is of maximum and the slope angle relative to the vector of the constant magnetic field B ₀ is 45°. This regime is taken to be optimal for simulation of practical applications. It is also found that the pump of the nuclear spin of xenon is strongly attenuated when the rubidium polarization vector is turned to the plane perpendicular to the external magnetic field vector B ₀.     

Quantum antidot as an electrometer:Observation of fractional charge

In experiments on resonant tunneling through a quantum antidot in the quantum Hall (QH) regime, we observe periodic conductance peaks both versus magnetic field and a global gate voltage, i.e., electric field. Each conductance peak can be attributed to tunneling through a quantized antidot-bound state. The fact that the variation of the uniform electric field produces conductance peaks implies that the deficiency of the electrical charge on the antidot is quantized in units of charge of quasiparticles of surrounding QH condensate. The period in magnetic field gives the effective area of the antidot state through which tunneling occurs, the period in electric field (obtained from the global gate voltage) then constitutes a direct measurement of the charge of the tunneling particles. We obtain electron charge C in the integer QH regime, and quasiparticle charge C for the QH state.     

Landau-Zener tunneling of fermi superfluid gases in deep BEC regime

The dynamical phenomena of the Landau–Zener tunneling of the superfluid fermi gases in a double-well potential in deep BEC regime is investigated in this paper. The contribution of the higher order term representing the lowest approximation of beyond mean field corrections is considered. By using the corresponding classical Hamiltonian of the system, the fixed points and adiabatic energy levels are studied. The critical parameter for the number of fixed points evolution from two to four is obtained. For the nonlinear Landau–Zener tunneling, the tunneling probability with sweeping rate α$\alpha$ is numerically given. At the adiabatic limit, the tunneling probability is not zero in certain regime, while it goes to zero in the other regime. Exponential dependence of the transition probability on the sweeping rate α$\alpha$ in certain case is obtained.     

μSR on the novel heavy‐fermion system Nd2-yCeyCuO4

The magnetic correlations in Nd_2-yCe_yCuO₄ have been studied by zero and longitudinal field μSR. Nd_1.8Ce_0.2CuO₄ reveals a saturation of the muon relaxation rate below 0.5 K. Even down to 70 mK LF spectra evidence spin fluctuations in the dynamic regime with a rate of \sim 10⁹ s^-1 excluding long range magnetic order or spin glass freezing. The average Nd moment is estimated to be \approx 0.2\mu_B, i.e., strongly reduced from the value determined for the ground state Kramers doublet of Nd₂CuO₄. Extending former μSR measurements on Nd₂CuO₄, a gradual enhancement of the internal field has been detected below 10 K which is accompanied by an increase of the relaxation rate. These results are attributed to the development of ordered Nd moments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Magnetic relaxation in superfluid<Superscript>3</Superscript>He-B at very low temperatures

Main properties of the spin supercurrents and coherent precession of magnetization in the superfluid³He-B in hydrodynamic regime seem to be very well understood now. But recently surprisingly new unpredicted phenomena such as, for example, “catastrophic” relaxation, persistent spin precession, very strong magnetic relaxation, etc., have been observed in³He-B at ultralow temperatures in so-called non-hydrodynamic regime using both pulse and cw-NMR techniques. This paper deals with some of these new phenomena (a “linear term” in magnetic relaxation and a reduction of magnetization of coherent precession with magnetic field gradient) observed by cw-NMR technique, compares these results with new effects found by pulse NMR and speculates about the nature of these new phenomena.     

Nondemolition measurements of a single quantum spin using Josephson oscillations

We consider a single localized spin-1/2 between the singlet superconducting leads of a Josephson junction (e.g., a superconducting STM). For the spin subject to a dc magnetic field B parallel z, we study the spin dynamics and the possibility to measure the spin state via transport through the junction embedded in a dissipative circuit. Turning on the tunneling or a voltage bias induces oscillations of the Josephson current, with an amplitude sensitive to the initial value of the z component of the spin, S(z)=+/-1/2. At low temperatures, when effects of quasiparticles are negligible, this procedure realizes a quantum nondemolition measurement of S(z).     

Influence of in-plane magnetic field on spin polarization in the presence of the oft-neglected k-Dresselhaus spin-orbit coupling

The influence of in-plane magnetic field on spin polarization in the presence of the oft-neglected k³-Dresselhaus spin-orbit coupling was investigated. The k³-Dresselhaus term can produce a limited spin polarization. The in-plane magnetic field plays a great role in the tunneling process. It can generate the perfect spin polarization of the electrons and the ideal transmission coefficient for spin up and down simultaneously. In energy scale, complete separation between spin up and down resonance was obtained by a relatively higher in-plane magnetic field while a comparatively lower in-plane magnetic field vanishes the spin separation. On the other hand, the spin relaxation can be suppressed by compensating the oft-neglected k³-Dresselhaus spin orbit coupling using a relatively lower in-plane magnetic field.     

Spin configurations in nanostructured magnetic rings: From DC transport to GHz spectroscopy

We report three different experiments performed on permalloy nanorings. They address either (a) the stray-field and (b) the magnetoresistance characteristics of an individual nanoring, or (c) the magnetization dynamics of a periodic array of identical rings. Interestingly, in all three properties covering the frequency range from DC to 20 GHz we observe irreversible switching and hysteretic behavior. Localized spin waves, which are excited by ferromagnetic resonance in different ring segments, are found to obey an intriguing magnetic field dispersion. We attribute this to the characteristic spin configurations in the nanorings, i.e., the onion and the vortex states, which are controlled by the external magnetic field.