Special 290S/290K Quantum Materials Seminar speaker Dan Mao (Cornell), Monday, May 22 at 3:00 pm in 375 Physics North

Time/Venue Monday, May 22 at 3:00 pm in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman/Alessandra Lanzara
Title Superconducting and excitonic phase-stiffness for interacting isolated narrow bands
Abstract Inspired by the discovery of superconductivity in moir\'e materials with isolated narrow bandwidth electronic bands, I will analyze the question of what is the maximum attainable $T_c$ in interacting flat-band systems. I will focus specifically on the low-energy effective theory, where the density-density interactions are projected to the set of partially-filled flat bands. The resulting problem is inherently non-perturbative, given that the interaction energy scale can be comparable or larger than the band width of the low energy narrow bands, where the standard mean-field approximation is not applicable. Recently, we develop a Schrieffer-Wolff transformation based approach to compute the effective electromagnetic response and the superconducting phase-stiffness in terms of ``projected'' gauge-transformations and extend the formalism to compute the stiffness for excitonic superfluids. Importantly, our method requires neither any ``wannierization'' for the narrow bands of interest, regardless of their (non-)topological character, nor any knowledge of an underlying pairing symmetry, and can be set up directly in momentum-space. We use this formalism to derive upper bounds on the phase-stiffness for sign-problem-free models, where their values are known independently from numerically exact quantum Monte-Carlo computations. We also illustrate the analytical structure of these bounds for the superconducting and excitonic phase-stiffness for perfectly flat-bands with Landau-level-like wavefunctions.