Off-diagonal long-range order in solid 4He

Measurements of the moment of inertia by Kim and Chan have found that solid (4)He acts like a supersolid at low temperatures. To understand the order in solid 4(He), we have used path integral Monte Carlo simulations to calculate the off-diagonal long-range order (ODLRO) [equivalent to Bose-Einstein condensation (BEC)]. We do not find ODLRO in a defect-free hcp crystal of (4)He at the melting density. We discuss these results in relation to proposed quantum solid trial functions. We conclude that the solid (4)He wave function has correlations which suppress both vacancies and BEC.     

生长及耗散波色-爱因斯坦凝聚中的怪波

利用达布变换法(Darboux transformation),解析的研究了生长及耗散波色-爱因斯坦凝聚(BEC)中的怪波.通过降维和无量纲化,将描述BEC的Gross-Pitaevskii (GP)方程转化成一维无量纲非线性薛定谔方程.利用达布变换,得到了一维非线性薛定谔方程的怪波解析解.根据解析结果,数值模拟了生长及耗散BEC中怪波的性质.结果表明,BEC中出现了一种典型的双洞怪波,并且BEC生长会延缓怪波的消失,而BEC的耗散会加速怪波的消失.     

玻色-爱因斯坦凝聚体中的怪波操控

利用Darboux变换法, 解析地研究了玻色-爱因斯坦凝聚体(BEC)中的怪波. 结果表明: 当谱参数等于非线性系数时, BEC中形成一种新型的单洞怪波; 而当谱参数小于非线性系数时, BEC中出现双洞怪波. 进一步地, 怪波的出现位置可通过调节周期性势阱的驱动频率和强度来控制. 此外, 随着原子间相互作用的减小, 怪波的最高幅度也随之降低. 相关结果可为预防怪波的危害提供帮助.     

玻色-爱因斯坦凝聚体中的怪波操控

利用Darboux变换法, 解析地研究了玻色-爱因斯坦凝聚体(BEC)中的怪波. 结果表明: 当谱参数等于非线性系数时, BEC中形成一种新型的单洞怪波; 而当谱参数小于非线性系数时, BEC中出现双洞怪波. 进一步地, 怪波的出现位置可通过调节周期性势阱的驱动频率和强度来控制. 此外, 随着原子间相互作用的减小, 怪波的最高幅度也随之降低. 相关结果可为预防怪波的危害提供帮助.     

Cover Picture: Fortschritte der Physik 8 / 2009

The cover page of 2009 shows high‐resolution interference “quantum carpet” patterns for the momentum wave function of an interacting Bose‐Einstein condensate (BEC). As time progresses (from back to front), the many‐body mean field interaction of the BEC first leads to a broadening of the wave function, but then also to a time‐varying interference structure by inducing site‐dependent nonlinear phase shifts when the BEC is confined in a spatially periodic potential. Imaging the wave function in momentum space for different times leads to a pattern that one reminds of a carefully woven carpet and is hence termed “quantum carpet”. Quantum carpets beautifully demonstrate the surprisingly high matter wave coherence of particle‐particle interactions in the zero‐temperature limit.     

玻色-爱因斯坦凝聚体中的怪波操控

利用Darboux变换法,解析地研究了玻色-爱因斯坦凝聚体(BEC)中的怪波.结果表明:当谱参数等于非线性系数时,BEC中形成一种新型的单洞怪波;而当谱参数小于非线性系数时,BEC中出现双洞怪波.进一步地,怪波的出现位置可通过调节周期性势阱的驱动频率和强度来控制.此外,随着原子间相互作用的减小,怪波的最高幅度也随之降低.相关结果可为预防怪波的危害提供帮助.     

The propagation of solitary waves in clouds of Bose--Einstein condensed atoms

The Gross--Pitaevskii equation which describes the motion of a Bose--Einstein condensed (BEC) atom in an elongated trap is solved analytically, and a solitary-wave solution is obtained in the low-density case without neglecting the effect of the interatomic interaction on the transverse function. It is shown that this effect leads to the velocity of the solitary wave slowing down and the profile of the solitary wave widening.     

Bose-Einstein condensation, phase coherence, and two-fluid behavior in 4He

It is shown that the assumption that the many particle wave functions describing superfluid 4He have some phase coherence properties of correlated basis functions provides a new explanation of the link between Bose-Einstein condensation and two fluid behavior. This assumption also implies that the condensate fraction is proportional to the superfluid fraction, in agreement with experiment, and provides a simple quantitative explanation of the anomalous reduction in spatial order and liquid density, observed as the temperature is lowered in superfluid 4He.     

Dynamics of quantum liquids in nanoporous media

We sketch our recent neutron scattering measurements of the phonon-roton (P-R) modes and Bose-Einstein condensation (BEC) of liquid ⁴He in porous media. The aim is to reveal the interdependence of BEC, well-defined P-R modes and superfluidity in helium confined to nanoscales and in disorder. In all porous media investigated to date, we observe well-defined P-R modes above T_c in the normal liquid phase, up to T_λ. Since well defined P-R modes are associated with BEC, this suggests that there is BEC above T_c in porous media. We interpret this as BEC localized to favorable regions separated by regions of normal fluid. At high pressures, p ≥25 bars, well defined P-R modes are no longer observed at lower wavevectors, Q ≤1.5 ?. At p ≈39 bars a roton is no longer observed. Work is in progress to explore whether loss of modes can be associated with a recently reported Quantum Phase Transition.     

On the asymptotic behavior of the pair correlation function for liquid4He

A new expression for the asymptotic pair correlation function of He II is proposed. Unlike the standard expressions, this contains explicitly the condensate fraction and leans heavily on the corresponding function for He I. The consequences for the long-wavelength liquid structure factor and the asymptotic effective interaction in the system are explored. In passing, the triplet and higher-order correlations are discussed briefly. Whenever possible, the connection with other treatments is pointed out.     

Structure and dynamics of water confined in silica hydrogels: X-ray scattering and dielectric spectroscopy studies

Liquid ⁴He immersed in porous media such as aerogel, Vycor, and Geltech silica are excellent examples of bosons in disorder and confinement. Of special interest is the impact of disorder on Bose-Einstein condensation (BEC), on the elementary excitations of the superfluid and on their connection to the superfluid properties. Indeed, the modifications induced by disorder can be used to reveal the interdependence of BEC, the excitations and superfluidity. To date, the superfluid properties in porous media are much more completely documented than BEC or the excitations. In this paper, we review measurements of the excitations by neutron scattering, focusing particularly on their temperature dependence and the existence of phonon-roton excitations at higher temperatures. The weight of single excitation response at higher temperatures suggests the existence of localized BEC above the superfluid-normal transition temperature in porous media. We sketch several recent predictions made for BEC, the excitations, and the superfluid properties in disorder. Connections with other Dirty Bose systems are made.Received: 1 January 2003, Published online: 21 October 2003PACS: 03.75.Kk Dynamic properties of condensates; collective and hydrodynamic excitations, superfluid flow - 61.12.Ex Neutron scattering (including small-angle scattering) - 67.40.Yv Boson degeneracy and superfluidity of ⁴He: Impurities and other defects     

High-Tc spin superfluidity in antiferromagnets

We report the observation of the unusual behavior of induction decay signals in antiferromagnetic monocrystals with Suhl-Nakamura interactions. The signals show the formation of the Bose-Einstein condensation (BEC) of magnons and the existence of spin supercurrent, in complete analogy with the spin superfluidity in the superfluid (3)He and the atomic BEC of quantum gases. In the experiments described here, the temperature of the magnon BEC is a thousand times larger than in the superfluid (3)He. It opens a possibility to apply the spin supercurrent for various magnetic spintronics applications.     

Structure and dynamics of the He 2 * (a 3Σ u + molecular complex in condensed phases of helium

An investigation is made of the absorption spectra of triplet metastable helium molecules in the a ³Σ _u ⁺ state in liquid ⁴He and ³He at various pressures and in dense ³He gas. An analysis of the spectrum corresponding to the a ³Σ _u ⁺ → c ³Σ _g ⁺ transition confirms the conclusion that there is a microscopic bubble surrounding the molecule in liquid helium. A simple approximation is proposed for the wave function of the valence electron of the molecule and the parameters of the bubble are determined for various experimental conditions. The coefficient of molecular recombination in liquid ³He and ⁴He was determined experimentally at various pressures and in dense cold ³He gas. The results show good agreement with the theory of mutual recombination limited by molecular diffusion under conditions of strong van der Waals interaction. It is shown that in the condensed phases of helium the polarization of the molecules under the action of the magnetic field does not lead to suppression of their mutual recombination, and this is confirmed experimentally.     

Deep inelastic scattering from quantum fluids

A simple and general argument is given for the deep inelastic behavior of the dynamic structure function of quantum fluids at zero temperature. The resulting high-energy tail is shown to give a reasonable fit to existing neutron data for liquid ⁴He.     

Spectrum of 3He atoms in superfluid 4He

It is suggested that at a wave vector near the inverse average interparticle separation, the spectrum of a ³He atom in superfluid ⁴He has a roton like dip. The results of light scattering and transport measurements in liquid ³He-⁴He mixtures can thereby be reconciled.     

Laser spectroscopic investigation of Ca atoms in liquid helium

Excitation and emission spectra of the 4s21S0 – 4s4p1P1 and 4s4p3PJ(J = 0, 1, 2) – 4s5s3S1 transitions of Ca atoms implanted not only in liquid 4He but also in liquid 3He have been measured. It has been found that the excitation spectra for liquid 3He show considerably smaller widths and peak shifts from transition wavelengths of a free Ca atom, in comparison with those for liquid 4He. These spectral characteristics have been well reproduced by our theoretical calculation based on a vibrating bubble model. This calculation has shown that, due to a significant difference in surface tension between the two liquids, the radius of a bubble formed around Ca in liquid 3He is larger than the one in 4He, and that this fact as well as the smaller number density of He atoms in liquid 3He cause weaker perturbation for Ca, resulting in the smaller peak shift and width for liquid 3He.     

Coherent spin precession in a separated solution of 3He in 4He

It is suggested that a coherently precessing spin structure in a normal Fermi liquid should be used to study the interface of two Fermi liquids. It is shown that the interface makes an additional contribution to the attenuation of the precessing structure. This contribution is determined by the kinetic coefficient which relates the magnetization flux across the interface to the magnetization jump at the interface. A relationship is established between this kinetic coefficient and the nature of the scattering of Fermi quasiparticles at the interface. Results of numerical calculations of the attenuation of the structure are presented for a specific object, a separated solution of ³He in ⁴He.     

不同能量下He原子与HCl分子碰撞激发分波截面的同位素效应研究

基于Hux1ey势函数的拟合势,通过精确度较高的密耦近似方法计算了入射能量为50 meV和150meV时,氦原子的四种同位素3He,4He,9He,10He与HCl分子碰撞体系的激发分波截面.通过分析不同能量下,各碰撞体系分波截面的差异,探讨了不同入射能量时He的同位素对He-HCl碰撞体系的分波截面的影响,总结出其分波截面随量子数和体系约化质量的变化规律.     

不同能量下He原子与HCl分子碰撞激发分波截面的同位素效应研究

基于Huxley势函数的拟合势,通过精确度较高的密耦近似方法计算了入射能量为50meV和150meV时,氦原子的四种同位素3He,4He,9He,10He与HCl分子碰撞体系的激发分波截面.通过分析不同能量下,各碰撞体系分波截面的差异,探讨了不同入射能量时He的同位素对He-HCl碰撞体系的分波截面的影响,总结出其分波截面随量子数和体系约化质量的变化规律.