YQI Talk - Steven Gassner - University of Pennsylvania

 Surprising consequences of quantum geometry in non-flat bands: Light-induced switching of superconductivity and charge-flux-composite excitonic insulators
 

The momentum-space geometry of the Bloch states in electronic bands has gained attention as an important ingredient to correlated physics in systems with energetically flat bands. In this talk, I illustrate two distinct scenarios from my PhD work in which purely band-geometric features from non-flat bands have a surprising influence on the phenomenology of correlated states. In Part One, I propose a protocol whereby a metastable topological superconducting state may be accessed in a conventional superconductor by tailoring an ultrafast light pulse to drive a sub-gap collective mode.  In a minimal model, I show that the light-induced coupling to an odd-parity Bardasis-Schrieffer mode is dominantly controlled by the geometry of the states near the Fermi surface. In Part Two, I describe how an unconventional version of the “flux attachment” technique suggests the stability of an excitonic insulator with a fractional Hall conductivity of “1/m” near a band inversion between bands carrying relative angular momentum “m”. This suggests a design principle for the fractional quantum Hall effect beyond the mimicking of Landau levels. Beyond their primary goals, these two projects broaden the scope of geometric principles for designing Bloch bands with desired low-energy physics.