弯曲应变下六角晶格量子反铁磁体的赝朗道能级

利用线性自旋波理论和量子蒙特卡罗方法研究了弯曲应变下六角晶格量子反铁磁体的赝朗道能级.通过线性自旋波理论,发现磁赝朗道能级出现在磁子能谱的高能端,其能级间距与能级指数的平方根成正比.线性自旋波理论和量子蒙特卡罗方法都显示,尺寸相同时随着应变强度的逐渐增加,局域磁化强度逐渐减弱,应变强度相同的条件下反铁磁序在y方向上连续减弱,因为上边界处的海森伯链解耦为孤立的垂直链,导致上边界附近的磁序被破坏.量子蒙特卡罗方法提供了更精确的反铁磁序演化:在特定应变强度下上边界处垂直关联不变,水平关联增加,从而影响磁化强度,使局域磁化在上边界处呈上翘趋势.研究结果有助于理解弯曲应变对自旋激发的影响,并可能在二维量子磁性材料实验中得以实现.     

磁性硅烯超晶格中电场调制的谷极化和自旋极化

研究了基于硅烯的静电势超晶格、铁磁超晶格、反铁磁超晶格中谷极化、自旋极化以及赝自旋极化的输运性质,分析了铁磁交换场、反铁磁交换场以及化学势对输运性质的影响,讨论了电场对谷极化、自旋极化以及赝自旋极化的调控作用.结果表明:当3种超晶格的晶格数达到10以上时,在硅烯超晶格中很容易实现100%的谷极化、自旋极化和赝自旋极化,而且通过调节超晶格上的外加电场可以使极化方向发生翻转,从而在硅烯超晶格中实现外电场对谷自由度、自旋自由度以及赝自旋自由度的操控.     

超强磁场下电子朗道能级稳定性及对电子费米能的影响

磁星是一类由磁场供能、强磁化的中子星,其内部磁场远高于电子的量子临界磁场.本文通过引入电子朗道能级的稳定性系数g_n,讨论了在磁星环境下电子的朗道能级的稳定性及其对电子费米能E_F(e)的影响;研究发现,磁场越强,电子的朗道能级越不稳定,最大的朗道能级级数n_(max)越小;朗道能级数n越大,能级稳定性系数g_n越小.根据朗道能级的稳定性系数g_n随磁场的增加而减小的要求,电子费米能表达式的磁场指数β必须是正数.通过引入Dirac-δ函数,推导出超强磁场下的简并的、相对论电子费米能的一般表达式,修正了E_F(e)的特解.新的特解给出磁场指数β=1/6;特解的适用范围ρ≥10~7g·cm~(-3),Bcr≤B≤10~(17)G.本文结果将有助于中子星内部弱相互作用过程(包括修正的URCA反应和电子俘获)和磁星磁热演化机理的研究.     

耦合电磁场对石墨烯量子磁振荡的影响

我们研究了包含自旋轨道耦合与杂质散射在内的石墨烯量子磁振荡对外加电磁场的响应.我们发现,石墨烯中自旋轨道耦合、电磁场以及边界共同修正了朗道能谱,且当电场与磁场比值超过某一临界值时,量子磁振荡会突然消失,这与非相对论二维电子气的情况显著不同.这种现象可以通过朗道量子化轨道由封闭转化为开放的半经典理论来解释.此外,我们还发现杂质散射和温度的共同作用会使得磁振荡振幅衰减.我们的结果可用于分析石墨烯及其类似结构(硅烯、锗烯、锡烯等)的费米能级与朗道能谱的相互作用,进而探测自旋轨道耦合引起的能隙.     

二维环状无限深势阱中的朗道能级

对于带电粒子在磁场中的运动,在各种教科书上都有详细的阐述,但是对于限制在二维无限深势阱的带电粒子,现在流行的各种量子力学教科书上都没有阐述.本文主要讨论二维无限深势阱中的能级和朗道能级.     

非对易量子力学中中性原子在外电磁场中的朗道能级量子化

在非对易量子力学的框架中研究了中性原子在外加电磁场中运动时的朗道能级量子化问题.首先给出了中性原子体系在非对易量子力学中的哈密顿量,然后分别求解了非对易空间和非对易相空间中的薛定谔方程,并得到相应的朗道能级和本征波函数,同时给出了由于空间非对易性引起的能量修正项.     

不均匀磁场中单层二硫化钼的朗道能谱

单层二硫化钼是一种新型的类石墨烯材料,其内部较强自旋轨道耦合作用使它表现出许多不同于单层石墨烯的物理性质.本文从量子力学的基本概念出发,借助特殊函数方程理论,研究了在不均匀垂直磁场作用下单层二硫化钼中奇异的朗道能级结构.     

自旋-轨道耦合作用下玻色-爱因斯坦凝聚在量子相变附近的朗道临界速度

各向异性超流体中的朗道临界速度并非简单地由运动方向的元激发能谱决定.在自旋-轨道耦合作用下的双分量玻色-爱因斯坦凝聚中,当系统跨过平面波相与零动量相之间的量子相变时,尽管超流声速连续变化,但垂直于自旋-轨道耦合方向的朗道临界速度会出现跳变,跳变幅度随自旋相互作用强度单调增加.根据线性响应理论,计算了凝聚体中运动杂质在不同速度下的能量耗散率,提出可以通过能量耗散观测临界速度在量子相变处的不连续性.     

不均匀磁场和垂直电场作用下单层硅烯中的朗道能级

从量子力学的基本概念出发,借助特殊函数方程理论,研究了在不均匀磁场和垂直电场作用下单层硅烯中的朗道能谱.结果表明外电场调制的自旋轨道耦合作用使得该体系中的朗道能级结构表现出一些新奇的特征.     

New physics in high Landau levels

Recent magneto-transport experiments on ultra-high mobility 2D electron systems in GaAs/AlGaAs heterostructures have revealed the existence of whole new classes of correlated many-electron states in highly excited Landau levels. These new states, which appear only at extremely low temperatures, are distinctly different from the familiar fractional quantum Hall liquids of the lowest Landau level. Prominent among the recent findings are the discoveries of giant anisotropies in the resistivity near half-filling of the third and higher Landau levels and the observation of re-entrant integer quantum Hall states in the flanks of these same levels. This contribution will survey the present status of this emerging field.     

Strongly anisotropic electronic transport in higher Landau levels

Low-temperature, electronic transport in higher Landau levels (N>1) in a two-dimensional electron system is strongly anisotropic. At half-filling of either spin level of such Landau levels ( etc.) the magnetoresistance either collapses to form a deep minimum or is peaked in a sharp maximum, depending on the in-plane current direction. The anisotropic axis can be reoriented by applying an in-plane magnetic field of 1–2 T strength. The magnetoresistance at and (N=1) is initially isotropic but an in-plane field induces a strong anisotropy. Our observations are strong evidence for a new many-electron phase in higher Landau levels, which forms spontaneously or can be induced by an in-plane field.     

Landau levels as mass excitations in relativistic quantum theory

We show that, in a manifestly covariant quantum theory, the mass spectrum of a single charged particle moving in a constant magnetic field contains a discrete part, corresponding to mass excitations associated with the field, and a continuous part associated with the free motion parallel to the field.     

Landau levels in uniaxially strained graphene: A geometrical approach

The effect of strain on the Landau levels (LLs) spectra in graphene is studied, using an effective Dirac-like Hamiltonian which includes the distortion in the Dirac cones, anisotropy and spatial-dependence of the Fermi velocity induced by the lattice change through a renormalized linear momentum. We propose a geometrical approach to obtain the electron’s wave-function and the LLs in graphene from the Sturm–Liouville theory, using the minimal substitution method. The coefficients of the renormalized linear momentum are fitted to the energy bands, which are obtained from a Density Functional Theory (DFT) calculation. In particular, we evaluate the case of Dirac cones with an ellipsoidal transversal section resulting from uniaxially strained graphene along the Arm-Chair (AC) and Zig-Zag (ZZ) directions. We found that uniaxial strain in graphene induces a contraction of the LLs spectra for both strain directions. Also, is evaluated the contribution of the tilting of Dirac cone axis resulting from the uniaxial deformations to the contraction of the LLs spectra.     

Landau levels of scalar QED in time-dependent magnetic fields

The Landau levels of scalar QED undergo continuous transitions under a homogeneous, time-dependent magnetic field. We analytically formulate the Klein–Gordon equation for a charged spinless scalar as a Cauchy initial value problem in the two-component first order formalism and then put forth a measure that classifies the quantum motions into the adiabatic change, the nonadiabatic change, and the sudden change. We find the exact quantum motion and calculate the pair-production rate when the magnetic field suddenly changes as a step function.     

Influences of strains on the formation of the quasi-Dirac cone and the Landau levels in black phosphorus

A tunable tight-binding (TB) Hamiltonian, which can be tuned continuously by applying the strains, is proposed based on the two tight-binding Hamiltonian models used to describle the pristine and compressed black phosphorus (BP). This tunable TB Hamiltonian should be of fundamental significance in two aspects: i) A general tunable TB Hamiltonian making it possible to study the BP more conveniently and deeply; ii) An extension of the study of the strain effect from the monolayer phosphorene to the multilayer phosphorene and even the bulk BP in the presence of both the in-plane and out-of-plane strains. Furthermore, based on the tunable TB Hamiltonian we can find the strain-induced quasi-Dirac cone, and clearly disclose how the strains affect the formation of the quasi-Dirac cone. Then we investigate the Landau levels (LLs) of BP under different magnetic fields, uncovering the linear, square-root, or complex dependence of the LLs on the LL number or the magnetic field. Moreover, the double-step and triple-step transitions of the LLs are found to be induced by the strains for the BP in the in-plane and out-of-plane magnetic fields. This research successfully incorporates the out-of-plane strain effect into the tunable TB Hamiltonian, enabling the comprehensive investigation of the strain-correlated effect in BP.     

门电压控制的硅烯量子线中电子输运性质

硅烯是由单层硅原子形成的二维蜂窝状晶格结构,具有石墨烯类似的电学性质,由于硅烯中存在比较强的自旋轨道耦合而备受关注.本文利用非平衡格林函数方法研究了门电压控制的硅烯量子线中电子输运性质和能带结构.研究发现,只有在较强的门电压下,而且硅烯量子线具有较好的锯齿形或扶手椅形边界而不存在额外硅原子时,硅烯量子线中才存在无能隙的自旋极化边缘态.另外,计算结果表明这种门电压控制的硅烯量子线中边缘态在每个能谷处自旋是极化的.这些计算结果将为实验上利用电场制作硅烯纳米结构提供理论支持.     

Light-induced unconventional Landau levels of ultracold fermions in a trilayer honeycomb lattice

The spectrum of cold fermionic atoms is studied in a trilayer honeycomb optical lattice subjected to a perpendicular effective magnetic field,which is created with optical means. In the low energy approximation,the spectrum shows unconventional Landau levels,which are proportional to the 3/2 power of integer numbers. The zoro modes exist and the quasiparticles are chiral. It is also proposed to identify the unconventional Landau levels via probing the dynamic structure factor of the system with Bragg spectr...     

Discrete coherent states for higher Landau levels

We consider the quantum dynamics of a charged particle evolving under the action of a constant homogeneous magnetic field, with emphasis on the discrete subgroups of the Heisenberg group (in the Euclidean case) and of the SL(2,R)$SL(2,\mathbb{R})$ group (in the Hyperbolic case). We investigate completeness properties of discrete coherent states associated with higher order Euclidean and hyperbolic Landau levels, partially extending classic results of Perelomov and of Bargmann, Butera, Girardello and Klauder. In the Euclidean case, our results follow from identifying the completeness problem with known results from the theory of Gabor frames. The results for the hyperbolic setting follow by using a combination of methods from coherent states, time-scale analysis and the theory of Fuchsian groups and their associated automorphic forms.