强关联电子体系二维相干光谱的理论研究评述

非线性相干光谱是一种测量材料非线性光学响应的谱学手段.相比线性光谱,非线性相干光谱具有多个时间变量,能够提供材料的更多信息.作为非线性相干光谱的代表之一,二维相干光谱在众多领域取得了丰硕的成果.在化学、生物学等领域,二维相干光谱已经展现出相比线性谱学的优越性.随着太赫兹技术的发展,二维相干光谱在强关联体系也显现出巨大潜力,相关的理论与实验工作正在开展.本文简要概括了强关联体系二维相干光谱的发展现状,介绍了二维相干光谱的基本概念与理论工具,分析了二维相干光谱的主要特点,并重点总结了我们研究组近几年关于强关联体系二维相干光谱的理论研究工作.     

Mechanism of Pseudogap Detected by Electronic Raman Scattering： Phase Fluctuation or Hidden Order？

We study the electronic Raman scattering in the cuprates to distinguish the two possible scenarios of the pseudogap normal state. In one scenario, the pseudogap is assumed to be caused by phase fluctuations of the preformed Cooper pairs. We find that pair-breaking peaks appear in both the B1g and B2g Raman channels, and they axe smeared and tend to shift to the same energy with the increasing strength of phase fluctuations. Thus both channels reflect the same pairing energy scale, irrespectively of the doping level. In another scenario, the pseudogap is assumed to be caused by a hidden order that competes with the superconducting order. As an example, we assume that the hidden order is the d-density-wave （DDW） order. We find analytically and numerically that in the DDW normal state there is no Raman peak in the B2g channel in a tight-binding model up to the second nearest-neighbor hopping, while the Raman peak in the Big channel reflects the energy gap caused by the DDW order. This behavior is in agreement with experiments in the pseudogap normal state. To gain further insights, we also calculate the Raman spectra in the DDW＋SC state. We study the doping and temperature dependence of the peak energy in both channels and find a two-gap behavior, which is in agreement with recent Raman experiments. Therefore, our results shed light on the hidden order scenario for the pseudogap.     

自旋涨落增强的声子磁矩

声子是晶格振动的集体激发,是固体中最常见也最重要的准粒子之一。对声子基本性质的理解是凝聚态物理和材料科学众多研究的基础。声子具有能量和准动量,在时间或空间反演对称性破缺的体系中,声子也可携带角动量,并具有手性。但长久以来,人们认为声子的磁矩可以忽略不计[3]。这是因为离子的质量远大于电子,由离子绕转产生的声子轨道磁矩极其微弱,通常小于10^-4玻尔磁子（μ_B）,比电子磁矩小4个量级,因此几乎没有可观测效应。     

他山之石，可以攻玉——太赫兹二维相干谱学打开探索分数化现象的新窗口

分数化(fractionalization)是一种出现在多体系统中的新奇物理现象^[1]。在出现分数化现象的多体系统中,构成体系的基本物理单元(如电子、自旋等)被相互作用劈裂。这些被劈裂的部分成为体系的元激发。这里以铁磁伊辛自旋链为例说明(图1(a))：该体系中自旋的取向高度受限,其只能沿着或者背对着自旋的易轴(这里假设易轴方向与自旋链方向相同)。与此同时,自旋之间的铁磁交换相互作用希望自旋的取向一致。因此,自旋链具有两重简并的铁磁基态,对应自旋的两种可能取向。