Quantum geometry and broken time-reversal symmetry

Talk by Mathias S. Scheurer 

Abstract

Analyzing the consequences of the quantum geometry induced by the momentum dependence of Bloch states has emerged as a very rich and active field in condensed matter physics. For instance, for the superfluid stiffness or the pairing mechanism, these geometric aspects can play an important role. In the first part of the talk, we will discuss that quantum geometry can also be essential for the disorder sensitivity of a superconductor, in particular when time-reversal symmetry is broken in the normal-state Bloch Hamiltonian. More specifically, we will identify "quantum geometric pair breaking", where any superconductor becomes susceptible to microscopically non-magnetic impurities, and formally relate it to the maximum possible localization of two-particle Wannier states. Further, in the presence of kinetic pair breaking, impurities can also enhance pairing, leading to an overall more complex, non-monotonic behavior of Tc with impurity concentration. We will illustrate the findings using rhombohedral graphene and superconducting altermagnets as examples. In the second part of the talk, we will discuss spatial inhomogeneities in altermagnets and derive an effective theory for the local electronic spectrum. We show that this leads to additional emergent electromagnetic fields that are absent in antiferromagnets and involve the real-space quantum metric of the underlying spin texture.  

Host: Morten Holm Christensen