自旋与手征动力学的输运模拟研究

自旋相关的物理是多个研究领域的热门课题。核子和夸克都是自旋为1/2的费米子,在非对心重离子碰撞中受到自旋-轨道相互作用以及磁场的影响,会产生有趣的自旋动力学,特别是在垂直于反应平面方向上会出现自旋极化。在相对论重离子碰撞中,由于产生了极端的高温高密环境,夸克可以近似看作无质量粒子,此时自旋动力学过渡为手征动力学。在外界电磁场、涡旋场作用下以及在电荷与手征荷不对称的条件下,会产生一系列手征反常效应。本文介绍我们课题组基于输运模拟在自旋与手征动力学方面开展的一系列研究工作,包括中能重离子碰撞及相对论重离子碰撞中粒子的自旋极化、理想体系及相对论重离子碰撞中的手征磁波等。     

Rashba自旋轨道耦合下square-octagon晶格的拓扑相变

本文研究了各向同性square-octagon晶格在内禀自旋轨道耦合、Rashba自旋轨道耦合和交换场作用下的拓扑相变,同时引入陈数和自旋陈数对系统进行拓扑分类.系统在自旋轨道耦合和交换场的影响下会出现许多拓扑非平庸态,包括时间反演对称破缺的量子自旋霍尔态和量子反常霍尔态.特别的是,在时间反演对称破缺的量子自旋霍尔效应中,无能隙螺旋边缘态依然能够完好存在.调节交换场或者填充因子的大小会导致系统发生从时间反演对称破缺的量子自旋霍尔态到自旋过滤的量子反常霍尔态的拓扑相变.边缘态能谱和自旋谱的性质与陈数和自旋陈数的拓扑刻画完全一致.这些研究成果为自旋量子操控提供了一个有趣的途径.     

∧超核中∧粒子与核子的相互作用问题

本文从唯象的角度综述了自由Λ粒子与核子的相互作用,并讨论了Λ超核中Λ粒子与核子的有效相互作用。在本文中指出这种有效相互作用是与Λ超核的结构有关的。在Λ超核中Λ粒子的自旋-轨道耦合将远小于原子核中核子的自旋-轨道耦合。     

∧超核中∧粒子与核子的相互作用问题

本文从唯象的角度综述了自由Λ粒子与核子的相互作用,并讨论了Λ超核中Λ粒子与核子的有效相互作用。在本文中指出这种有效相互作用是与Λ超核的结构有关的。在Λ超核中Λ粒子的自旋-轨道耦合将远小于原子核中核子的自旋-轨道耦合。     

XAuBi (X=Eu, Ba)拓扑nodal chain半金属的第一性原理研究

由于在磁性材料体系中缺失时间反演对称性,导致nodal chain被破坏,所以nodal chain通常存在于非磁材料中。但是,磁性材料EuAuBi是与常规磁性材料不同。本工作以第一性原理计算为研究方法,预言了在不考虑自旋轨道相互作用时,磁性材料EuAuBi体系为新型拓扑nodal chain半金属;当考虑自旋轨道耦合时,EuAuBi会退化为外尔半金属。对于非磁材料BaAuBi来说,在不考虑自旋轨道相互作用时,它同样是一种拓扑nodal chain半金属;当考虑自旋轨道相互作用时,由于C3旋转对称性的存在,BaAuBi会退化为狄拉克半金属。在XAuBi (X=Eu, Ba)中发现nodal chain半金属,会促进对六角材料的拓扑性质研究以及开拓其新实际应用领域。     

强相互作用自旋-轨道耦合与夸克-胶子等离子体整体极化

在非对心相对论重离子碰撞中,参与反应的原子核物质系统具有巨大的初始轨道角动量,经过强相互作用的自旋-轨道耦合,这一巨大的轨道角动量可以转化为产生的夸克-胶子等离子体的整体极化.整体极化效应在理论上提出后,首先被美国布鲁克海文国家实验室的相对论重离子对撞机上的STAR实验所证实,激发了人们对相关问题的研究,成为重离子碰撞物理研究的一个新方向——重离子碰撞自旋物理.本文简单回顾了整体极化原始基本思想、理论计算体系与主要结果以及近几年的理论进展.     

金属表面Rashba自旋轨道耦合作用研究进展

自旋轨道耦合是电子自旋与轨道相互作用的桥梁, 它提供了利用外电场来调控电子的轨道运动、进而调控电子自旋状态的可能. 固体材料中有很多有趣的物理现象, 例如磁晶各向异性、自旋霍尔效应、拓扑绝缘体等, 都与自旋轨道耦合密切相关. 在表面/界面体系中, 由于结构反演不对称导致的自旋轨道耦合称为Rashba自旋轨道耦合, 它最早在半导体材料中获得研究, 并因其强度可由栅电压灵活调控而备受关注, 成为电控磁性的重要物理基础之一. 继半导体材料后, 金属表面成为具有Rashba自旋轨道耦合作用的又一主流体系. 本文以Au(111), Bi(111), Gd(0001)等为例综述了磁性与非磁性金属表面Rashba自旋轨道耦合的研究进展, 讨论了表面电势梯度、原子序数、表面态波函数的对称性, 以及表面态中轨道杂化等因素对金属表面Rashba自旋轨道耦合强度的影响. 在磁性金属表面, 同时存在Rashba自旋轨道耦合作用与磁交换作用, 通过Rashba自旋轨道耦合可能实现电场对磁性的调控. 最后, 阐述了外加电场和表面吸附等方法对金属表面Rashba自旋轨道耦合的调控. 基于密度泛函理论的第一性原理计算和角分辨光电子能谱测量是金属表面Rashba自旋轨道耦合的两大主要研究方法, 本文综述了这两方面的研究结果, 对金属表面Rashba自旋轨道耦合进行了深入全面的总结和分析.     

重离子碰撞中的对称势翻转

利用同位旋和动量相关的输运模型IBUU，以Sn,Sn同位素在低能与高能碰撞为例，在两种不同的对称势作用下，研究了重离子碰撞中的对称势翻转现象.我们发现从低能到高能在同位旋相分化、发射核子的中-质比、发射核子的双n/p比，中-质微分横向流观测量中均存在对称势翻转现象.对称势翻转效应的研究有利于确定对称能的密度依赖性. 关键词： 重离子碰撞 对称势翻转 对称能     

中能重离子碰撞中库仑相互作用对动量耗散的同位旋效应

利用同位旋相关的量子分子动力学模型，研究了中能重离子碰撞过程中的库仑相互作用对于 动量耗散的同位旋效应.计算结果表明: 由于库仑相互作用的存在，使得碰撞过程中的化学 不稳定区和力学不稳定区减小，明显地减弱了动量耗散；其减弱的程度与对称势的形式及强 度有关，对称势越强则动量耗散的减弱幅度越大.动量耗散灵敏地依赖于核子-核子碰撞截 面的同位旋相关性而较弱地依赖于对称势，这一特征不论有无库仑相互作用均存在. 关键词： 中能重离子碰撞 库仑相互作用 动量耗散 同位旋效应     

重离子碰撞中的矢量介子自旋排列

在非对心相对论重离子碰撞中,参与反应的系统具有巨大的轨道角动量,从而使产生的夸克胶子等离子体具有极强涡旋场,并通过自旋-轨道相互作用导致部分子的自旋极化,经过强子化导致重子的自旋极化以及矢量介子的自旋排列等可观测效应.矢量介子的自旋排列是指其自旋密度矩阵的00元素ρ00偏离1/3.在矢量介子衰变到两个赝标介子的过程中,衰变产物的极角分布只与ρ00有关,以此可以对自旋排列进行测量.理论研究表明,重离子碰撞过程中,重子的自旋极化反映了夸克自旋极化的时空平均效应,而矢量介子自旋排列则反映了夸克反夸克自旋极化的局域相空间关联.本文回顾了相对论重离子碰撞中矢量介子自旋排列的相关理论工作.重点以非相对论夸克融合模型为例,明确地计入夸克极化的相空间依赖性,展示了矢量介子自旋排列与夸克反夸克自旋极化特别是它们之间相空间关联的关系.本文还讨论了涡旋、电磁场、有效φ介子场以及它们的局域涨落对φ介子自旋排列的贡献,结果显示强作用场的时空关联效应是导致φ介子自旋排列的主要因素.矢量介子自旋排列为探索强相互作用物质和强相互作用场的性质提供了新途径.     

The heavy-ion spin-orbit interaction

An expression for the induced spin-orbit potential in a heavy-ion collision is derived by making and adiabatic approximation to the effective interaction in second-order perturbation theory. An explicit form for the induced spin-orbit interaction for the case of a projectile of arbitrary ground state spin S is given and is shown to be equivalent to the result obtained using the methods of geometrical magnetism.     

Spin Hall and spin Nernst effects in graphene with intrinsic and Rashba spin-orbit interactions

The spin Hall and spin Nernst effects in graphene are studied based on Green’s function formalism.We calculate intrinsic contributions to spin Hall and spin Nernst conductivities in the Kane-Mele model with various structures.When both intrinsic and Rashba spin-orbit interactions are present,their interplay leads to some characteristics of the dependence of spin Hall and spin Nernst conductivities on the Fermi level.When the Rashba spin-orbit interaction is smaller than intrinsic spin-orbit coupling,a weak kink in the conductance appears.The kink disappears and a divergence appears when the Rashba spin-orbit interaction enhances.When the Rashba spin-orbit interaction approaches and is stronger than intrinsic spin-orbit coupling,the divergence becomes more obvious.     

Slow cross-symmetry phase relaxation in complex collisions

We discuss the effect of slow phase relaxation and the spin off-diagonal S-matrix correlations on the cross-section energy oscillations and the time evolution of the highly excited intermediate systems formed in complex collisions. Such deformed intermediate complexes with strongly overlapping resonances can be formed in heavy-ion collisions, bimolecular chemical reactions, and atomic cluster collisions. The effects of quasiperiodic energy dependence of the cross sections, coherent rotation of the hyperdeformed ?(3 : 1) intermediate complex, Schrödinger cat states, and quantum-classical transition are studied for ²⁴Mg + ²⁸Si heavy-ion scattering.     

强磁场与涡旋场中的夸克胶子物质

相对论重离子碰撞可以产生高温的夸克胶子物质,同时也产生极强的电磁场和流体涡旋。在强电磁场和涡旋场中的夸克胶子物质呈现出新奇的宏观量子现象,比如手征磁效应、手征涡效应、手征分离效应、手征电分离效应、自旋极化现象等。它们一方面给我们提供了可以探测高温下量子色动力学的非平庸规范场拓扑结构、强相互作用的宇称破坏、夸克胶子物质中的自旋动力学等的实验手段,另一方面也与物理学其他分支,比如粒子物理、凝聚态物理、天体物理、冷原子物理等发生紧密联系,形成新的交叉研究领域。本文旨在对这些宏观量子现象的产生机制以及它们在相对论重离子碰撞中的探测等做一回顾和展望。特别地,我们揭示出重离子碰撞的磁场强度可以达到$10^{18}\sim 10^{20}$ G,流体涡旋可以达到$10^{22}$ s–1;这是我们已知当前宇宙中最强的磁场和流体涡旋。我们定量地对同量素碰撞实验做了分析,发现即便背景比例达到93%以上,当前的同量素碰撞实验仍然可在大约$3\sigma$的显著性水平上判断是否有手征磁效应的发生。我们系统地给出了满足因果律的自旋流体力学方程,并推导了其中的集体激发模式,这将有助于理解超子自旋极化中出现的符号问题。     

Dynamical effects of spin-dependent interactions in low- and intermediate-energy heavy-ion reactions

It is well known that noncentral nuclear forces, such as the spin–orbital coupling and the tensor force, play important roles in understanding many interesting features of nuclear structures. However, their dynamical effects in nuclear reactions are poorly known because only the spin-averaged observables are normally studied both experimentally and theoretically. Realizing that spin-sensitive observables in nuclear reactions may convey useful information about the in-medium properties of noncentral nuclear interactions, besides earlier studies using the time-dependent Hartree–Fock approach to understand the effects of spin–orbital coupling on the threshold energy and spin polarization in fusion reactions, some efforts have been made recently to explore the dynamical effects of noncentral nuclear forces in intermediate-energy heavy-ion collisions using transport models. The focus of these studies has been on investigating signatures of the density and isospin dependence of the form factor in the spin-dependent single-nucleon potential. Interestingly, some useful probes were identified in the model studies but so far there are still no data to compare with. In this brief review, we summarize the main physics motivations as well as the recent progress in understanding the spin dynamics and identifying spin-sensitive observables in heavy-ion reactions at intermediate energies. We hope the interesting, important, and new physics potentials identified in the spin dynamics of heavy-ion collisions will stimulate more experimental work in this direction.     

Polarization of Λ0 hyperons as a signature for the quark-gluon plasma

The momentum distribution of Λ⁰ hyperons produced from the quark-gluon plasma (QGP) in ultra-relativistic heavy-ion collisions is calculated in dependence on their polarization. The momentum distribution of Λ⁰ hyperons is defined by matrix elements of relativistic quark Wigner operators, which are calculated within the effective quark model with chiral U(3)×U(3) symmetry and the quark-gluon transport theory. We show that the polarization of the Λ⁰ hyperon depends on the spin of the strange quark that agrees well with the DeGrand-Miettinen model. We show that Λ⁰ hyperons, produced from the QGP, are fully unpolarized. This means that a detection of unpolarized Λ⁰ hyperons, produced in ultra-relativistic heavy-ion collisions, should serve as one of the signatures for the existence of the QGP in intermediate states of ultra-relativistic heavy-ion collisions.     

Spin physics at RHIC: Present and future

In 2001–2002 the relativistic heavy-ion collider (RHIC) at the Brookhaven National Laboratory (BNL) was first commissioned for polarized proton collisions. Polarized protons were injected into the RHIC, accelerated to 100 GeV, stored and the two beams were made to collide in four interaction regions. I will review the progress made by the RHIC spin program, followed by the physics goals for the next few years. After that I will present a brief overview of a proposal to build a high intensity polarized electron/positron beam facility at BNL which would enable deep inelastic scattering (DIS) experiments to be pursued at BNL by its collisions with the RHIC hadron beams.     

S-matrix spin and parity decoherence and damping of coherent nuclear rotation: quantum chaos in dissipative heavy-ion collisions?

We extend the statistical reaction with memory approach to study off-diagonal spin and parity S-matrix energy autocorrelation in dissipative heavy-ion collisions. It is suggested that S-matrix spin and parity decoherence results in (i) damping of the coherent nuclear rotation and (ii) is a manifestation of quantum chaos in dissipative heavy-ion collisions.     

Effects of symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei

Using an isospin-dependent transport model, we study the effects of nuclear symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei. We find that the density dependence of the nuclear symmetry energy affects significantly the nucleon emission times in these collisions, leading to larger values of two-nucleon correlation functions for a symmetry energy that has a stronger density dependence. Two-nucleon correlation functions are thus useful tools for extracting information about the nuclear symmetry energy from heavy-ion collisions.