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Exotic Quantum Criticalities in Low Dimensions and Systems with Unusual Quantum Many-Body Thermalization

低维中的奇异量子临界点和具有不寻常量子多体热化的系统

基本信息

批准号：
2001186
负责人：
Olexei Motrunich
金额：
$ 36.85万
依托单位：
California Institute of Technology
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2020
资助国家：
美国
起止时间：
2020-09-01 至 2024-08-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=2001186&HistoricalAwards=false
关键词：
Exotic Quantum Criticalities Low Dimensions

项目摘要

NON-TECHNICAL SUMMARYThis award supports theoretical research studying exotic collective behaviors involving many quantum particles. At absolute zero temperature, such systems can organize into myriad quantum states of matter---quantum phases---depending on details of interactions between particles. While the most common of such phases and their transformations are well understood, some quantum phase transitions do not fit into conventional paradigms, most notably continuous transitions between phases with mismatched orders as can occur in quantum magnets. Yet another class of phases---so-called gapless spin liquids with emergent itinerant quasiparticles---are proposed to occur in several antiferromagnetic solids that defy ordering even at zero temperature. (Here, "liquid" refers to randomness in the directions of the electrons' magnetism inside a solid.) The PI's group will pursue tractable examples of such unusual quantum phase transitions in one and two dimensions and also questions motivated by recent experiments in candidate spin-liquid materials.A different aspect of collective quantum behavior is evolution in time when a system of many particles is started in a non-equilibrium state with finite energy density above zero temperature. This has come to the fore as experimentalists are engineering systems with many degrees of freedom that behave quantum-mechanically, in part in an effort to develop quantum technologies. A fundamental idea to describe such an evolution is that of dynamical thermalization and the closely related eigenstate thermalization hypothesis, which offers a way of understanding why time flows only in one direction. A notable exception to thermalization---so-called many-body localization---occurs in the presence of a large number of impurities. A challenging question, also of technological interest, is whether thermalization can be completely or at least very strongly suppressed in the absence of impurities. The PI's group will continue work on understanding pre-thermalization phenomena and also on so-called quantum many-body scar states that violate the eigenstate thermalization hypothesis.Projects under this award will initiate graduate students to research at the frontier of quantum many-particle physics and will teach them a variety of analytical and numerical approaches that will prepare them for future careers in quantum physics and quantum engineering areas.TECHNICAL SUMMARYThis award supports theoretical research studying different facets of quantum many-body physics: 1) unconventional quantum phase transitions and critical phases at zero temperature, and 2) unusual thermalization dynamics at non-zero energy density above zero temperature.1) While by now we have good ``fixed-point'' pictures of myriad gapped phases, both conventional and exotic, we do not have as good an understanding of quantum phase transitions in many cases. Among these, so-called ``deconfined quantum critical points'' that lie beyond the Landau paradigm have fascinated researchers for more than fifteen years; however, finding truly controlled instances of such critical points has been controversial. Another challenging open problem is the description of gapless fractionalized phases, which are believed to be critical phases that do not require fine-tuning. The PI's group will search for tractable examples of unconventional quantum critical points in low dimensions and will also pursue projects motivated by recent experiments in candidate gapless spin-liquid materials.2) This award will also pursue research in quantum many-body dynamics and thermalization in closed quantum systems. For many years, the interest in such questions had been primarily theoretical, as they relate to foundations of statistical mechanics and notions of quantum chaos. However, modern experiments in engineered quantum systems such as cold atom arrays, trapped ions, dipolar spin impurities, etc., have already achieved well-isolated systems with many quantum degrees of freedom, and are already finding unexpected phenomena opening new exciting directions in the field. In this context, the PI's group will specifically pursue examples of unusual thermalization in systems without disorder and will explore questions of pre-thermalization and quantum many-body scar states. The PI's group studies concrete systems to address challenging open questions and combines and develops both analytical and numerical tools to achieve this.Projects under this award will initiate graduate students to research at the frontier of condensed-matter physics. The students will learn a variety of analytical and numerical approaches to quantum many-body systems that will prepare them for future careers in quantum physics and quantum engineering areas.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.

该奖项支持涉及许多量子粒子的奇异集体行为的理论研究。在绝对零度的温度下，这样的系统可以根据粒子之间相互作用的细节组织成无数种物质的量子态——量子相。虽然最常见的这些相和它们的转换被很好地理解，但一些量子相变并不适合传统的范式，最明显的是在量子磁体中可能发生的顺序不匹配的相之间的连续转变。然而，另一类相——具有涌现的流动准粒子的所谓无间隙自旋液体——被提出出现在几种反铁磁固体中，即使在零温度下也不有序。（这里的“液体”指的是固体内部电子磁性方向的随机性。）PI的小组将在一维和二维中寻找这种不寻常的量子相变的易于处理的例子，并在最近的候选自旋液体材料实验中提出问题。集体量子行为的另一个方面是由许多粒子组成的系统在零温度以上以有限能量密度的非平衡状态开始时的时间演化。当实验学家正在设计具有量子力学行为的多个自由度的系统时，这一点已经浮出水面，部分原因是为了开发量子技术。描述这种演化的一个基本思想是动态热化和与之密切相关的本征态热化假说，它提供了一种理解为什么时间只向一个方向流动的方法。热化的一个明显例外——所谓的多体局部化——发生在大量杂质存在的情况下。一个具有挑战性的问题，也是技术上的兴趣所在，是在没有杂质的情况下，热化是否可以完全或至少非常强烈地抑制。PI的团队将继续致力于理解预热化现象，以及违反本征态热化假设的所谓量子多体疤痕态。该奖项下的项目将启动研究生在量子多粒子物理前沿的研究，并将教授他们各种分析和数值方法，为他们未来在量子物理和量子工程领域的职业生涯做好准备。该奖项支持研究量子多体物理不同方面的理论研究：1)非常规的量子相变和零温度下的临界相，以及2)零温度以上非零能量密度下的非常规热化动力学。1)虽然到目前为止，我们已经有了无数间隙相的“定点”图像，既有传统的，也有奇异的，但在许多情况下，我们对量子相变还没有很好的理解。其中，超越朗道范式的所谓“定义量子临界点”已经吸引了研究人员超过15年；然而，找到这些临界点的真正受控实例一直存在争议。另一个具有挑战性的开放问题是描述无间隙分馏相，这被认为是不需要微调的关键相。PI的小组将在低维中寻找可处理的非常规量子临界点的例子，并将在候选无间隙自旋液体材料的最新实验中开展项目。2)该奖项还将对量子多体动力学和封闭量子系统的热化进行研究。多年来，对这些问题的兴趣主要是理论性的，因为它们涉及到统计力学的基础和量子混沌的概念。然而，在工程量子系统中的现代实验，如冷原子阵列、捕获离子、偶极自旋杂质等，已经实现了具有许多量子自由度的良好隔离系统，并且已经发现了意想不到的现象，为该领域开辟了新的令人兴奋的方向。在这种情况下，PI的小组将专门研究无无序系统中异常热化的例子，并将探索预热化和量子多体疤痕态的问题。PI的小组研究具体的系统来解决具有挑战性的开放问题，并结合和开发分析和数值工具来实现这一目标。该奖项下的项目将鼓励研究生在凝聚态物理的前沿进行研究。学生将学习量子多体系统的各种分析和数值方法，为他们未来在量子物理和量子工程领域的职业生涯做好准备。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。