Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Q44: BCS-BEC Crossover in Quantum MaterialsInvited Live Streamed
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Sponsoring Units: DCMP DMP Chair: Andrey Chubukov, University of Minnesota Room: McCormick Place W-375C |
Wednesday, March 16, 2022 3:00PM - 3:36PM |
Q44.00001: BCS-BEC crossover in FeSe-based superconductors Invited Speaker: Yuji Matsuda The physics of the crossover between weak-coupling BCS and strong-coupling BEC limits gives a unified framework of quantum-bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems but has been challenging to find a solid-state realization. Here we show that superconducting Fe(Se1-xSx) offers the possibility to enter the previously unexplored realm where the two energies, Fermi energy εF, and superconducting gap Δ, become comparable, indicating that this system is deep inside the BCS–BEC crossover regime [1]. Through scanning tunneling microscopy and laser-excited angle-resolved photoemission spectroscopy, we demonstrate that εF of Fe(Se1-xSx) is extremely small, with the ratio Δ/εF∼0.3-1 for all bands [2][3]. We discuss several unusual superconducting properties associated with the crossover, including non-Gaussian superconducting fluctuations [4], pseudogap, quantum vortex core [5], FFLO phase [6], and unusual Bogoliubov quasiparticle band dispersions [3]. Some of these properties are not expected for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in the multiband system. |
Wednesday, March 16, 2022 3:36PM - 4:12PM |
Q44.00002: Gate-controlled BCS-BEC crossover in a two-dimensional superconductor Invited Speaker: Yoshihiro Iwasa The Bardeen-Cooper-Schrieffer (BCS) condensation and Bose-Einstein condensation (BEC) are the two limiting ground states of paired Fermion systems, and the crossover between these two limits has been a source of excitement for both fields of high temperature superconductivity and cold atom superfluidity. Here we report the two-dimensional (2D) BCS-BEC crossover realized in a gate-controlled superconductor, electron doped layered material ZrNCl, and the associated transport properties. To observe this phenomenon, we utilized an ionic gating method, which is well known as a powerful tool to control the carrier density in a large scale and induced 2D superconductivity. |
Wednesday, March 16, 2022 4:12PM - 4:48PM |
Q44.00003: Are there bounds on the superconducting transition temperature? Invited Speaker: Mohit Randeria Understanding limits on the superconducting transition temperature Tc is a question of fundamental and practical importance. I will begin by describing developments in quantum materials and in the BCS-BEC crossover in ultracold atoms that challenge conventional wisdom on what controls Tc. I will then describe exact upper bounds [1] on the BKT Tc for two dimensional (2D) systems expressed in terms of the optical spectral weight that are valid for any pairing mechanism or strength. The 2D bound takes a particularly simple form for parabolic dispersion -- Tc cannot exceed one-eighth the Fermi temperature – which has been realized in recent experiments on Li:ZrNCl. I will next describe results for multi-band systems with complex band structures and discuss applications to 2D superconducting materials including monolayer FeSe/STO and magic angle twisted bilayer graphene. Finally, I will describe the generalization [2] of the optical spectral weight bounds for trivial and topological flat bands in 2D which involve the quantum geometry of electronic wave functions. I will conclude by describing the challenges in deriving bounds on Tc in 3D, which remains an open problem. |
Wednesday, March 16, 2022 4:48PM - 5:24PM |
Q44.00004: BCS-BEC crossover and the topological band structure of Fe(Se,Te) Invited Speaker: Amit Kanigel Fe1+ySexTe1-xis a nearly compensated semimetal, where both electron and hole pockets are very shallow, with Fermi energies, εF, of only a few meV. We realize the BCS-BEC crossover by tuning the Fermi energy via chemical doping, which permits us to systematically change Δ/εFfrom 0.16 to 0.50, where Δ is the superconducting gap. We use angle-resolved photoemission spectroscopy to measure the Fermi energy, the SC gap and characteristic changes in the SC state electronic dispersion as the system evolves from a BCS to a BEC regime. |
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