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

量子力学最大的成就之一是发现了宏观量子态——数十亿电子自发地落入同一个量子态，没有任何中心协调。最突出的例子之一是超导性，其中许多电子对在材料中无任何阻力地相干移动，因此不会产生热损失。发生这种情况有两种方式：两个电子首先紧密结合到一个分子中，然后与其他分子形成相干性；或者许多电子同时发生配对和结合。这两种体制是宏观相干态形成的两种极限情况，它们在激光、超导量子比特和超级磁铁等基础现代技术中被广泛观察和使用。该项目旨在实现介于这两个极限之间的可调谐材料，在这种交叉中探索物质的新状态及其电子结构。来自当地初高中的学生将通过超导和激光的互动讲座和演示参与到这个项目中来。本地大学本科生和研究生将与研究团队共同开发课程，并共同领导会议，以扩大参与范围。技术摘要bardeen - coopero - schrieffer （BCS）机制和玻色-爱因斯坦凝聚（BEC）是实现相互作用费米子宏观量子相干态的一条连续路径的两种极限场景，其中系统中的所有粒子同时占据相同的多体基态。在固态系统中，这些限制场景可以驱动金属到超导体和金属到绝缘体的相变。尽管有巨大的理论兴趣，但在固态系统中这种交叉的实验实现是稀缺和有争议的。在这项研究中，该团队的目标是实现、理解和调整新型体和薄膜固态模型系统中的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。该项目不仅揭示了波函数“相”在宏观量子态中的关键作用，而且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。

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.