NSF-BSF: Spectroscopy of excitonic insulators with direct gap

摘要：组成材料的粒子之间的相互作用可能导致令人惊讶的结果，其中材料作为一个整体的性质与其组成部分的性质不同。所讨论的粒子可以是电子，也可以是电子空穴，即材料原子晶格中缺少的电子。材料中电子和空穴之间的相互作用可以导致这种新性质的出现——这是凝聚态物理学积极探索的前沿。这种有趣材料的一个例子是激子绝缘体，激子是材料中由于电子和空穴之间的吸引相互作用而出现的新的类粒子实体。这样的激子有望在足够低的温度下达到一种新的量子态，称为“激子凝聚”。关于它存在的理论预测从未得到确凿的实验证据的充分证实。这项工作旨在为激子凝聚提供直接和令人信服的证据。本项目还将为低温高分辨率光谱领域的博士后和本科生提供全面的培训，他们将受益于国际合作带来的经验。技术摘要：Rutgers-Technion NSF-BSF合作研究了导致固体中激子绝缘体难以捉摸状态的条件，通过使用输运，角度分辨光发射（ARPES）和偏振分辨光散射（拉曼）光谱的组合来同时研究该状态的性质和动力学。目的是推导出激子绝缘体的完整相图，其中包括与半金属到绝缘体过渡交织在一起的结构不稳定性，并建立激子凝聚的合理独特制度，确定潜在的微观机制，并测试相关理论。从本研究中获得的经验教训将进一步促进对其他强相互作用量子系统中自组织的更广泛理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical Abstract:Interactions between particles making up a material can lead to surprising consequences, where the properties of the material as a whole are distinct from the properties of its constituents. The particles in question can be electrons, or electron holes, which are missing electrons in the material's atomic lattice. Interactions between electrons and holes in materials can lead to the emergence of such new properties – a frontier actively explored by condensed matter physics. An example of such interesting material is the excitonic insulator, with excitons being new particle-like entities in the material that appear due to the attractive interaction between electrons and holes. Such excitons are expected to achieve a novel quantum state, called "excitonic condensate" at sufficiently low temperatures. Theoretical predictions of its existence have never been fully confirmed by conclusive experimental evidence. This work aims at providing direct and convincing evidence for the excitonic condensate. This project also provides comprehensive training to postdocs and undergraduate students in low-temperature high-resolution spectroscopies, who will benefit from experiences brought by international collaborative efforts.Technical Abstract:The Rutgers-Technion NSF-BSF collaboration explores the conditions leading to the elusive state of excitonic insulator in solids by employing a combination of transport, angle-resolved photoemission (ARPES) and polarization-resolved light scattering (Raman) spectroscopies to concurrently study the properties and the dynamics of this state. The objective is to to derive the complete phase diagram for excitonic insulators that includes a structural instability intertwined with a semimetal-to-insulator transition and to establish plausible distinct regimes of the exciton condensation, identify the underlying microscopic mechanisms, and test the relevant theories. The lessons learned from this study will further promote broader understanding of self-organization in other strongly interacting quantum systems.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.