CAREER: Towards in situ control of BCS-BEC crossover in solid state systems

非技术摘要量子力学最大的成功之一是发现了宏观量子态--数十亿电子在没有任何中心配位的情况下自发地落入同一量子态。最突出的例子之一是超导电性，其中许多电子对在材料中相干地移动而没有任何阻力，因此不会产生热损失。这有两种方式发生：两个电子首先紧密地结合到一个分子中，然后与其他分子形成连贯性；或者多个电子同时形成配对和结合。这两种机制是宏观相干态形成的两种极限情况，它们被广泛地看到并用于激光、超导量子比特和超磁体等基石现代技术中。该项目旨在实现跨越这两个极限的可调材料，在这种交叉过程中探索物质的新状态及其电子结构。来自当地初中和高中的学生将通过超导和激光的互动讲座和演示来参与这个项目。当地大学本科生和研究生将共同开发课程，并与研究团队共同领导课程，以获得更广泛的参与。技术摘要Bardeen-Cooper-Schrieffer(BCS)机制和玻色-爱因斯坦凝聚体(BEC)是实现相互作用费米子宏观量子相干态的一条连续路线的两种限制场景，即系统中的所有粒子同时占据相同的多体基态。这些限制方案体现在固态系统中，可以驱动从金属到超导体和从金属到绝缘体的相变。尽管在理论上引起了极大的兴趣，但在固态系统中实现这种交叉的实验还很少，而且存在争议。在这项研究中，该团队的目标是在新型块体和薄膜固态模型系统中实现、理解和调整BCS-BEC交叉，特别强调热力学和光谱研究，因为系统通过交叉在现场进行调谐。该项目不仅阐明了波函数“相”在宏观量子态中的关键作用，还开发了新的材料平台和新的热力学技术来研究三维和准二维的现象。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical abstractAmong the biggest triumphs of quantum mechanics was the discovery of macroscopic quantum states - billions of electrons spontaneously fall into the same quantum state without any central coordination. One of the most prominent examples is superconductivity, where many electron pairs coherently move through the material without any resistance hence generating no heat loss. There are two ways for this to occur: two electrons first tightly bind into a molecule, then develop coherence with other molecules; or many electrons develop pairing and binding all at once. These two regimes are two limiting cases of how macroscopic coherent states can form, which are widely seen and used in cornerstone modern technologies such as lasers, superconducting qubits, and supermagnets. This project aims to realize tunable materials straddled between these two limits, exploring novel states of matter and their electronic structure during this crossover. Students from local middle and high-schools will be engaged in this project through interactive lectures and demonstrations of superconductivity and lasing. Local college undergraduate and graduate students will co-develop the curriculum and co-lead the sessions with the research team for broader participation.Technical abstractBardeen-Cooper-Schrieffer (BCS) mechanism and Bose-Einstein Condensate (BEC) are two limiting scenarios of one continuous route to realize macroscopic quantum coherent state of interacting fermions, where all particles in a system simultaneously occupy the same many-body ground state. Incarnated in solid state systems, these limiting scenarios can drive metal-to-superconductor and metal-to-insulator phase transitions. Despite immense theoretical interest, experimental realization of this crossover in solid state systems is scarce and controversial. In this research, the team aims to realize, understand, and tune BCS-BEC crossover in novel bulk and thin film solid state model systems, with a special emphasis on thermodynamic and spectroscopic investigations as the systems are tuned in situ through the crossover. This project not only sheds light on the crucial role of the wave function "phase" in macroscopic quantum states, but also develops new material platforms and novel thermodynamic techniques to investigate the phenomenon in both three and quasi-two dimensions.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.