关联电子材料中的杂质与边界效应

Correlated electron materials host rich exotic states, e.g., unconventional superconductors, spin liquids and quantum critical phenomena, and are at the research frontier of condensed matter physics. The impurities and surfaces cannot be effectively screened due to the low carrier density. The bulk properties of materials can be strongly affected by specific impurities, which is closely related to the intrinsic properties of the materials, e.g., the superconductor pairing mechanism and the spin-orbit coupling. On the other hand, the deliberately introduced impurities, combined with local measurements, such as scanning tunneling spectroscopy and nuclear magnetic resonance, can be used as local probes to some of the key physical properties, e.g., the superconductor pairing symmetry and the fractionalized excitations in spin liquids. The impurities and surfaces can lead to exotic universal properties in the quantum critical regime, which are new and unconventional characteristics of quantum critical phenomena..We propose to study the impurity and surface effects in correlated electron materials with theoretical and numerical methods. The aims of this project are to explain the exotic impurity effect found in experiments on unconventional superconductors, e.g., Fe(Te,Se), to predict how to directly probe the fractionalized excitations in spin liquids using specific impurities, and to fully reveal the surface critical behavior and quantum entanglement in quantum phase transitions that go beyond the conventional phase transition theory.

关联电子材料呈现出丰富的新奇物态，如非常规超导、自旋液体、量子临界现象等，是凝聚态物理的研究前沿。因为载流子浓度较低，实际材料中的杂质与边界不能被有效屏蔽。特定的杂质会强烈影响材料的整体性质，这与超导配对机制、自旋-轨道耦合等材料的本征性质密切相关。另外，有意识地引入特定的杂质作为“探针”，结合扫描隧道谱、核磁共振等局域测量方法，可以原位测量材料对杂质的响应，探测材料的某些重要物理性质，如超导配对对称性、自旋液体的分数化激发等。杂质与边界会在量子临界区域导致新奇的普适性质，是对量子临界现象的新的、非传统的表征。.本项目拟结合理论与数值方法研究关联电子材料中的杂质与边界效应，以解释实验发现的Fe(Te,Se)等非常规超导体中新奇的杂质效应，为利用特定的杂质直接探测自旋液体的分数化激发提供理论指导与预言，全面揭示量子相变中超出传统相变理论描述的表面临界现象与量子纠缠特性。

关联电子系统呈现出丰富的新奇物态。系统中特定的杂质强烈影响材料的物性，杂质与边界会在量子临界区域导致新奇的普适性质，是对量子临界现象的新的、非传统的表征。本项目重点研究了量子自旋模型在量子临界点的表面临界现象、量子自旋液体候选材料H3LiIr2O6的物性、InAs/GaSb量子阱中的磁电导振荡等。此外，还研究了自对偶量子临界点的普适热力学特性等。.利用量子Monte Carlo模拟，系统研究了一大类具有O(3)对称性的二维量子自旋模型的量子相变及其在量子临界点的表面临界现象，发现多个新的表面临界普适类；研究了两类临界量子自旋链，发现系统的共形边条件是其对称富化（symmetry-enriched）临界态的更普遍的特征，并建立了量子临界态的一种新奇的体-边对应关系。.利用临界标度理论证明了自对偶量子临界点的一个普适热力学特性：自由能的普适标度函数的一阶导数在相变点为零，因此量子临界点处的热力学量Grüneisen系数在温度趋于零时不会发散。.结合第一性原理计算与模型分析，研究了自旋液体候选材料H3LiIr2O6中氢离子（质子）的量子动力学及其对材料磁性相互作用的影响，发现其基态为量子顺电态，并预言其低能激发可由红外光谱或介电响应探测。.与北京大学杜瑞瑞教授实验组合作，在二维拓扑绝缘体InAs/GaSb量子阱中观测到能隙中磁电导的反常振荡，并构造唯象理论模型定量解释了实验结果。这一唯象模型表明，杂质散射、电子相互作用等因素对于理解实验数据至关重要。.本项目在国内外重要学术期刊正式发表（接收）论文8篇，包括2篇发表于Physical Review Letters。