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

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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.