[论文解读] Interacting line-node semimetal and spontaneous symmetry breaking

Bitan Roy|arXiv (Cornell University)|Jul 26, 2016

Cold Atom Physics and Bose-Einstein Condensates被引用 4

一句话总结

本文研究了在三维线节点半金属（LNSM）中，强关联的局域（U）和近邻（V）排斥作用如何诱导自发对称性破缺，从而引发连续量子相变进入有序相。表面鼓头态可依据U与V的相对大小发生电荷序或自旋序，甚至微弱的相互作用或长程库仑相互作用也能触发体相对称性破缺，对超导相及内部序的对称性具有重要意义。

ABSTRACT

strong onsite ( U) and nearest- neighbor (V ) repulsions, respectively. While onset of these two orderings from the semimetallic phase takes place through continuous quantum phase transitions, a rst order transition separates two ordered phases. By contrast, topologically protected drumhead shaped surface states can undergo charge or spin orderings, depending on relative strength of U and V , even when they are suciently weak. Such surface orderings as well as weak long range Coulomb interaction can be conducive to spontaneous symmetry breaking in the bulk for weaker interactions. We also discuss possible superconducting phases and internal symmetry among various orders in this system. Introduction: The mechanism of mass generation of elementary particles through spontaneous symmetry breaking nowadays leaving the territory of high energy physics curves its path through the fertile ground of con- densed matter physics, where a myriad of gapless phases is realized at low energies from complex band structures in solids. A celebrated example of such mass generation is the superconductivity, through which gapless excitations residing in the proximity to the Fermi surface acquire Majorana mass (1). Often the energy landscape available for electrons, of- fered by the periodic potential accommodated by im- mobile ions, displays band touching only at high sym- metry points in the Brillouin zone and gives rise to semimetallic phase when the chemical potential is pinned at band touching point (2). Although such band touch- ing is usually protected by underlying symmetries, a plethora of broken symmetry phases (BSPs), lacking discrete and/or continuous symmetries, can be realized when electronic interactions are taken into account. We here focus on a three dimensional system that supports linearly dispersing gapless excitations around an iso- lated two dimensional loop in the reciprocal space, the line-node semimetal (LNSM), and address the eects of short-range electronic interactions on such unconven- tional phase of matter. Recently developing interest in LNSM (3{10), along with its possible realization in var- ious materials (11{28), where the strength of electronic interactions varies over a wide range, besides fundamen- tal importance also endows timeliness to this quest. Due to linearly vanishing density of states (DOS) (29), any suciently

研究动机与目标

• 理解短程电子相互作用（特别是局域（U）和近邻（V）排斥）对三维线节点半金属（LNSM）稳定性与相变的影响。
• 研究在LNSM中拓扑保护的表面鼓头态如何根据U与V的相对强度发生电荷序或自旋序。
• 确定弱长程库仑相互作用或弱U与V是否仍能驱动LNSM体相的自发对称性破缺。
• 探索超导相的出现以及竞争序之间内部对称性的性质。

提出的方法

• 利用在动量空间中沿二维环形分布的线性色散激发的紧束缚模型分析LNSM相。
• 应用平均场理论研究强U与V相互作用对电子结构和对称性破缺的影响。
• 通过鼓头形表面模式的存在及其在不同U与V下对序的敏感性，研究表面态。
• 引入长程库仑相互作用，评估其在弱相互作用强度下稳定体相对称性破缺的作用。
• 利用对称性分析与序参量分类，探索可能的超导相及竞争序之间的内部对称性。
• 绘制相图以识别半金属相与有序相之间的连续量子相变，以及两有序相之间的的一阶相变。

实验结果

研究问题

• RQ1 局域（U）与近邻（V）排斥如何在线节点半金属中驱动自发对称性破缺？
• RQ2 是什么决定了表面鼓头态会形成电荷序还是自旋序，这种选择如何依赖于U与V的相对强度？
• RQ3 弱长程库仑相互作用或弱U与V是否仍能诱导LNSM体相的自发对称性破缺？
• RQ4 该体系中可能涌现出哪些类型的超导相，它们与底层序的对称性有何关联？
• RQ5 两有序相之间的相变本质是什么，它与从半金属相出发的相变有何不同？

主要发现

• 从半金属相开始的U与V驱动的序态通过连续量子相变出现。
• 一阶相变将两个不同的有序相分隔，表明相图具有非平凡结构，存在竞争序。
• 表面鼓头态可依据U与V的相对值发生电荷序或自旋序，即使在弱相互作用下亦然。
• 弱长程库仑相互作用可促进体相的自发对称性破缺，表明电子-电子相互作用无需很强即可引发此类效应。
• 该体系支持多种竞争序，包括潜在的超导相，其内部对称性取决于主要序的通道性质。
• 表面与体相物理的相互作用导致丰富的相图，即使在弱相互作用条件下，表面序态也能影响体相对称性破缺。