MuSR and multi-probe studies of spin-charge interplays in emergent quantum phenomena

非技术摘要：超导是量子现象的一种突出的宏观表现，可应用于量子计算设备、无损功率传输、高场磁体和高速列车。1957年，用Bardeen，Cooper和Schrieffer(BCS)理论解释了锡、铝等简单金属的超导机制。然而，自从1986年铜氧化物系统被发现以来，许多“非常规”超导体(USC)被发现表现出BCS理论中意想不到的特征。它们中的许多都位于显示Mott金属-绝缘体转变(MIT)的材料附近。电(电荷)和磁(自旋)相互作用之间的强烈相互作用似乎是USC和MIT现象的关键。通过在选定的系统上进行介子自旋弛豫、中子散射和扫描隧道显微镜测量，并通过使用自1989年以来由国际物理学家协会发展的Uemura-Plot唯象来解释结果，本项目旨在阐明电荷-自旋相互作用并揭示USC和MIT的基本机制。该项目还旨在阐明维度和拓扑对这些系统和选定的新磁性系统相变的作用。这些项目涉及学生和年轻研究人员，有助于在主要的加速器设施的国际合作中开发人力资源。技术摘要：新兴量子材料，如非传统超导体、Mott跃迁、Skyrmion和拓扑系统，基于自旋、电荷、几何和拓扑的相互作用，呈现出新颖的量子现象。缪子自旋弛豫(MuSR)是研究这类系统的有力探针，它对超流密度和磁跃迁中的动态自旋涨落、静态有序参数、体积分数和局域自旋关联具有独特的敏感性。通过结合中子散射和扫描隧道显微镜(STM)的互补测量进行MuSR研究，PI旨在检验和澄清：(1)非超导Mott转变系统Ba(Co，Ni)S_2和Ni(S，Se)_2中存在类似于磁共振模式的高度纠缠的自旋-电荷间隙假说；(2)磁场诱导的Hg1201高T_c铜酸盐正常态的变化；(3)拓扑结构对MNSi和其他Skyrimon系统动态临界行为的影响；(4)具有Kagome晶格的Weyl半金属Co3(Sn，In)2S2的自旋涨落。PI还将这些实验工作与他关于非常规超导的现象学相结合，朝着促进对非常规超导的全面理解的最终目标迈进。此外，凝聚态物理前沿(FCMP)讲座和相关研讨会由PI组织，帮助来自世界各地的有动力的研究生/研究人员与主要贡献者一起学习凝聚态物理前沿的超导和磁性研究。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical abstract: Superconductivity is a prominent macroscopic manifestation of quantum phenomena, which can be applied to quantum computing devices, loss-free power-transmission, high-field magnets and high-speed trains. Superconducting mechanism for simple metals, such as Sn and Al, was explained by the theory of Bardeen, Cooper and Schrieffer (BCS) in 1957. However, since the discovery of cupper-oxide systems in 1986, many "unconventional" superconductors (USC) were found to exhibit signatures unexpected in BCS theory. Many of them are in the proximity to materials showing Mott metal-insulator transitions (MIT). Strong interplay between electric (charge) and magnetic (spin) interactions seems to be crucial for the USC and MIT phenomena. By performing muon spin relaxation, neutron scattering and scanning-tunneling microscopy measurements on selected systems, and by interpreting the results using Uemura-plot phenomenology developed by the PI since 1989, the present project aims to elucidate charge-spin interplays and unveil essential mechanisms of USC and MIT. The project also aims to clarify the role of dimensionality and topology on phase transitions of these systems and selected novel magnetic systems. Involving students and young researchers, these projects help develop human resources in international collaborations at major accelerator-based facilities. Technical abstract:Emergent quantum materials, such as unconventional superconductor (SC), Mott transition, Skyrmion and topological systems, exhibit novel quantum phenomena based on intertwining interplays of spin, charge, geometry and topology. Muon spin relaxation (MuSR) is a strong probe for studying these systems, with unique sensitivity to superfluid density in SC's and dynamic spin fluctuations, static order parameter, volume fraction and local spin correlations in magnetic transitions. By performing MuSR studies in combination with complementary measurements by neutron scattering and Scanning Tunneling Microscopy (STM), the PI aims to test and clarify: (1) a hypothesis of highly entangled spin-charge gap, analogous to the magnetic resonance mode, existing in non-superconducting Mott transition systems Ba(Co,Ni)S2 and Ni(S,Se)2; (2) magnetic-field-induced change in the normal state of Hg1201 high-Tc cuprates; (3) effect of topology in dynamic critical behavior of MnSi and other Skyrimon systems; and (4) spin fluctuations in topological Weyl semimetal Co3(Sn,In)2S2 with Kagome lattice. The PI also integrate these experimental efforts with his phenomenology on unconventional SC's towards the ultimate goal to advance comprehensive understandings of unconventional superconductivity. In addition, Frontiers of Condensed Matter Physics (FCMP) lectures and associated workshops organized by the PI facilitate motivated graduate students / researchers from over the world to study forefront CMP researches on superconductivity and magnetism with leading contributors.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.