Loss of superfluidity in a Bose-Einstein condensate via forced resonant oscillations

We predict the loss of superfluidity in a Bose-Einstein condensate in an axially symmetric harmonic trap alone during resonant collective oscillations via a classical dynamical transition. The forced resonant oscillation can be initiated by (a) periodic modulation of the atomic scattering length with a frequency that equals twice the radial trapping frequency or multiples thereof, or by (b) periodic modulation of the radial trapping potential with a frequency that equals the radial trapping frequency or multiples thereof. Suggestion for future experiment is made. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Loss of superfluidity in a Bose-Einstein condensate on an optical lattice via a novel classical phase transition

We predict the loss of superfluidity in a Bose-Einstein condensate (BEC) trapped in a combined optical and axially-symmetric harmonic potentials during a resonant collective excitation initiated by a periodic modulation of the atomic scattering length a, when the modulation frequency equals twice the radial trapping frequency or multiples thereof. This classical dynamical transition is marked by a loss of superfluidity in the BEC and a subsequent destruction of the interference pattern upon free expansion. Suggestion for future experiment is made. (C) 2003 Elsevier B.V. B.V. All rights reserved.

HIGH-TEMPERATURE SUPERFLUIDITY WITH ABNORMAL FERMIONIC OCCUPANCY

We examine a Lipkin based two-level pairing model at finite temperature and in the thermodynamic limit. Whereas at T = 0 the model exhibits a superconducting ground state for sufficiently high values of the coupling constant, a partially superconducting phase in which some of the particles are paired, is found to survive at high temperatures in a special treatment. This phase is a mixture of abnormally-occupied eigenstates, which lie at higher energy, of the interactionless model Hamiltonian.

Exotic superfluid states of ultra-cold Fermi gases

This is a study of ultra-cold Fermi gases in different systems. This thesis is focused on exotic superfluid states, for an example on the three component Fermi gas and the FFLO phase in optical lattices. In the two-components case, superfluidity is studied mainly in the case of the spin population imbalanced Fermi gases and the phase diagrams are calculated from the mean-field theory. Different methods to detect different phases in optical lattices are suggested. In the three-component case, we studied also the uniform gas and harmonically trapped system. In this case, the BCS theory is generalized to three-component gases. It is also discussed how to achieve the conditions to get an SU(3)-symmetric Hamiltonian in optical lattices. The thesis is divided in chapters as follows: Chapter 1 is an introduction to the field of cold quantum gases. In chapter 2 optical lattices and their experimental characteristics are discussed. Chapter 3 deals with two-components Fermi gases in optical lattices and the paired states in lattices. In chapter 4 three-component Fermi gases with and without a harmonic trap are explored, and the pairing mechanisms are studied. In this chapter, we also discuss three-component Fermi gases in optical lattices. Chapter 5 devoted to the higher order correlations, and what they can tell about the paired states. Chapter 6 concludes the thesis.

Nonclassical rotational inertia in a two-dimensional bosonic solid containing grain boundaries

We study the occurrence of nonclassical rotational inertia (NCRI) arising from superfluidity along grain boundaries in a two-dimensionalbosonic system. We make use of a standard mapping between the zero-temperature properties of this system and the statistical mechanics of interacting vortex lines in the mixed phase of a type-II superconductor. In the mapping, the liquid phase of the vortex system corresponds to the superfluid bosonic phase. We consider numerically obtained polycrystalline configurations of the vortex lines in which the microcrystals are separated by liquidlike grain-boundary regions which widen as the vortex system temperature increases. The NCRI of the corresponding zero-temperature bosonic systems can then be numerically evaluated by solving the equations of superfluid hydrodynamics in the channels near the grain boundaries. We find that the NCRI increases very abruptly as the liquid regions in the vortex system (equivalently, superfluid regions in the bosonic system) form a connected, system-spanning structure with one or more closed loops. The implications of these results for experimentally observed supersolid phenomena are discussed.

Holographic Superfluids and Solitons

Superfluidity is perhaps one of the most remarkable observed macroscopic quantum effect. Superfluidity appears when a macroscopic number of particles occupies a single quantum state. Using modern experimental techniques one dark solitons) and vortices. There is a large literature on theoretical work studying the properties of such solitons using semiclassical methods. This thesis describes an alternative method for the study of superfluid solitons. The method used here is a holographic duality between a class of quantum field theories and gravitational theories. The classical limit of the gravitational system maps into a strong coupling limit of the quantum field theory. We use a holographic model of superfluidity to study solitons in these systems. One particularly appealing feature of this technique is that it allows us to take into account finite temperature effects in a large range of temperatures.

Out of equilibrium plasticity dynamics and the annealing of supersolidity in solid He-4

We present a numerical study of a continuum plasticity field coupled to a Ginzburg-Landau model for superfluidity. The results suggest that a supersolid fraction may appear as a long-lived transient during the time evolution of the plasticity field at higher temperatures where both dislocation climb and glide are allowed. Supersolidity, however, vanishes with annealing. As the temperature is decreased, dislocation climb is arrested and any residual supersolidity due to incomplete annealing remains frozen. Our results may provide a resolution of many perplexing issues concerning a variety of experiments on bulk solid He-4.

Frustration in Optical Lattices of Interacting Bosons

The realization of optical lattices of cold atoms has opened up the possibility of engineering interacting lattice systems of bosons and fermions, stimulating a frenzy of research over the last decade. More recently, experimental techniques have been developed to apply synthetic gauge fields to these optical lattices. As a result, it has become possible to study quantum Hall physics and the effects of frustration in lattices of cold atoms. In this article we describe the combined effect of frustration and interactions on the superfluidity of bosons. By focussing on a frustrated ladder of interacting bosons, we show that the effect of frustration is for ``chiral'' order to develop, which manifests itself as an alternating pattern of circulating supercurrents. Remarkably, this order persists even when superfluidity is lost and the system enters a Mott phase giving rise to a novel chiral Mott insulator. We describe the combined physics of frustration and interactions by studying a fully frustrated one dimensional model of interacting bosons. The model is studied using mean-field theory, a direct quantum simulation and a higher dimensional classical theory in order to offer a full description of the different quantum phases contained in it and transitions between the different phases. In addition, we provide physical descriptions of the chiral Mott insulator as a vortex-anitvortex super solid and indirect excitonic condensate in addition to obtaining a variational wavefunction for it. We also briefly describe the chiral Mott states arising in other microscopic models.

Coulomb drag and tunneling studies in quantum Hall bilayers

The bilayer quantum Hall state at total filling factor ν_T=1, where the total electron density matches the degeneracy of the lowest Landau level, is a prominent example of Bose-Einstein condensation of excitons. A macroscopically ordered state is realized where an electron in one layer is tightly bound to a "hole" in the other layer. If exciton transport were the only bulk transportmechanism, a current driven in one layer would spontaneously generate a current of equal magnitude and opposite sign in the other layer. The Corbino Coulomb drag measurements presented in this thesis demonstrate precisely this phenomenon.

Excitonic superfluidity has been long sought in the ν_T=1 state. The tunneling between the two electron gas layers exihibit a dc Josephson-like effect. A simple model of an overdamped voltage biased Josephson junction is in reasonable agreement with the observed tunneling I-V. At small tunneling biases, it exhibits a tunneling "supercurrent". The dissipation is carefully studied in this tunneling "supercurrent" and found to remain small but finite.

Transport parameters in neutron stars from in-medium N N cross sections

We present a numerical study of shear viscosity and thermal conductivity of symmetric nuclear matter, pure neutron matter, and beta-stable nuclear matter, in the framework of the Brueckner theory. The calculation of in-medium cross sections and nucleon effective masses is performed with a consistent two- and three-body interaction. The investigation covers a wide baryon density range as needed in the applications to neutron stars. The results for the transport coefficients in beta-stable nuclear matter are used to make preliminary predictions on the damping time scales of nonradial modes in neutron stars.

Isospin-dependent pairing interaction from nuclear matter calculations

The isospin dependence of the effective pairing interaction is discussed on the basis of the Bardeen, Cooper, and Schrieffer theory of superfluid asymmetric nuclear matter. It is shown that the energy gap, calculated within the mean field approximation in the range from symmetric nuclear matter to pure neutron matter, is not linearly dependent on the symmetry parameter owing to the nonlinear structure of the gap equation. Moreover, the construction of a zero-range effective pairing interaction compatible with the neutron and proton gaps in homogeneous matter is investigated, along with some recent proposals of isospin dependence tested on the nuclear data table.

用RIMS研究低能He2+与Ar碰撞的多电子过程

用反冲离子动量谱仪研究了低能He2+与Ar碰撞的多电子交换过程。该技术的独特优势是对反冲离子末态动量的完全测量。实验上鉴别了单电子俘获SC，双电子俘获DC和转移电离TI各子过程，并得到了末态量子态布居信息，首次测量了该体系各反应道的角微分截面及直接与碰撞参数相关的信息。实验发现电子主要俘获到入射离子基态或单激发态，反冲离子处于单激发态或多激发态，并有较大几率形成空心离子。研究表明电荷交换过程中存在较强的电子-电子关联作用。角分布随反冲离子电荷态增加而变宽，并向大角度方向移动，表明碰撞参数减小，相互作用加剧。SC和DC与MCBM理论角微分截面符合很好，说明在大碰撞参数时经典近似有一定合理性。不同碰撞参数范围内反冲纵向动量分布表明，SC在碰撞参数7.2a.u.≥b≥3.6a.u.时发生机率最大，随碰撞参数减小，靶离子有不断向更高激发态过渡的趋势

三体核力对核物质中基态关联效应和中子星物质中1S0态质子超流性的影响

本论文介绍了极端条件下核物质性质研究的现状以及目前常用的几种微观核多体方法，系统描述了核物质中的基态关联效应、温度效应对单核子势的影响和同位旋非对称核物质 态中子、质子超流性，重点考虑了利用介子交换流方法建立的微观三体核力所产生的影响。利用BHF和BCS的理论方法,计算了同位旋非对称核物质中 态中子和质子的对关联能隙，着重讨论了三体核力的影响。结果表明，三体核力对同位旋非对称核物质中 态的中子超流性影响相对较小，但是对 态的质子超流性具有重要影响，其效应随总核子数密度的增大而迅速增强。随着同位旋非对称度 的增加，中子能隙向低密移动，能隙峰值逐渐增大，并且这种效果随着 的增加而逐渐减弱，而质子的情况刚好相反。另外还参与了 稳定中子星物质中超流性的研究。利用质量算子空穴线展开，通过计算不同温度和密度下的核物质中单核子势和核子有效质量，重点研究和讨论了基态关联效应和三体核力贡献对热核物质中单核子势的影响，研究表明：基态关联效应提供了一个排斥效应，温度和三体核力都削弱了基态关联效应

中子及中子星物质中的3PF2态超流性和微观三体核力效力效应

本论文介绍了当前中子及中子星物质中超流性研究的背景及现状、核多体理论Brueckner-Hartree-Fock及同位旋相关的Brueckner-Hartree-Fock方法，以及利用BHF和BCS理论计算中子及中子星物质中的对关联能隙方法。系统计算并描述了中子及中子星物质中子的3PF2态超流性，并重点考虑了利用介子交换流方法建立的微观三体核力所产生的影响。 我们的研究结果表明：三体核力对中子物质中3PF2态中子超流性有强烈的增强效应。 当在BCS能隙方程中采用自由粒子能谱近似时，三体核力使相应的对关联能隙峰值增加了77%(由0.64MeV增大到1.13MeV)；当采用自洽BHF单粒子能谱时，三体核力导致相应的对关联能隙峰值由0.22MeV增大到0.50MeV，增加了约127%。 三体核力使中子星物质中3PF2态中子超流能隙随着密度的增大而单调递增。当采用自洽BHF单粒子能谱时不考虑三体核力时，对关联能隙峰值在密度约1.9fm-3时有峰值0.19MeV。而在这个两体力导致的能隙的峰值密度，三体核力导致相应的对关联能隙由0.19MeV增大到0.36MeV

三体核力影响下的核物质状态方程及中子星物质中的中子质子超流性

本论文介绍了极端条件下核物质研究的现状以及目前常用的几种核多体方法，系统描述了冷、热非对称核物质的状态方程和刀稳定中子星物质中的15。态中子和质子超流性，特别是微观三体核力对此所产生的影响。通过引入微观三体核力，扩展了有限温度的BI．tleclcller-Hal'tree-Fock（FTBHF）理论。利用这一扩展的理沦，详细研究了同位旋非对称热核物质的状态方程、液气相变临界现象以及三体核力对此所产生的影响。在不同的中质比条件下，重点讨论了热核物质液气相变的临界温度和动力学不稳定区域的温度、同位旋相关性。由此表明，三体核力的引入在一定程度上降低了液气相变的临界温度值，在固定的温度和密度下，非对称核物质的压弧随同位旋非对称度的增加而单调的增力日，而且随着核物质温度的升高和非对称度的增加，动力学不稳定区域逐渐缩小。通过与其它理论模型（特别是Di1'ac-BHF方法）所预言的结果相比较，就目前扩展的包含三体核力修正的FTBHF理论与Dilac-BHF方法所计算的临界温度的差异问题，文中给出一种可能的解释。通过计算热核物质的单粒子结合能，给出了有限温度条件下对称能的计算方式，并且细致研究了不同温度、密度下的对称能以及三体核力在高密度区域对对称能的影响。结果表明，微观三体核力强烈影响着高密度区域的对称能，使其对温度的变化更加明显。此外，其它重要物理量（例如中子和质子的单粒子势能、有效质量等）的同位旋依赖性和温度、密度相关性在文中也被详细的讨论。利用质量算子的空穴线展井，表明了在基态关联所导致的对单核子势的重排修正项影响下的HLlgenholtz-VanHove(HVH)定理的恢复程度，并且进一步计算了中子和质子化学势。并且以包含兰体核力的FTBI-方法为基础，研究了热核物质中重排项的密度和温度依赖性并讨论了三体核力对重排项的影响。通过计算不同温度和密度下的核物质中单核子势和核子有效质量，特别是研究和讨论了基态关联效应和三体核力贡献对热核物质中单核子势的影响，表明了基态关联和三体核力对单核子势修正的重要性。利用BHF和BCS的理论方法，计算了β稳定中子星物质中处于150态的中子和质子的对关联能隙，着重研究和讨论了三体核力的影响。结果表明，三体核力对刀稳定物质中｛s0态中子超流性的影响相对较小，但是对，S0态质子超流性具有重要影响，其效应随核子数密度的增大而迅速增强。三体核力的主要作用是强烈地抑制了高密度区刀稳定中子星物质中的150态质子超流性，而且三体核力对中子星物质中，So态超流相的抑制效应主要是通过质子或中子的有效对相互作用而起作用的。