Percolation from Quantum Metric in Flat-Band Delocalization

TL;DR

This paper investigates how disorder influences linear response conductivity in flat-band systems, revealing a critical delocalized regime linked to quantum metric percolation and connecting quantum geometry with classical percolation theory.

Contribution

It demonstrates that disorder-induced geometric conductivity in flat bands can be understood through a percolation model of quantum metric puddles, advancing quantum transport understanding.

Findings

Disorder modifies linear response conductivity via geometric contributions.

Identification of a critical delocalized regime characterized by finite geometric conductivity.

The critical delocalized regime coincides with a classical percolation universality class.

Abstract

The quantum metric is a fundamental ingredient of band quantum geometry and has recently at tracted intense interest, with most of its transport signatures appearing in the intrinsic second order nonlinear conductivity. In the clean limit, previous works argued that linear response conductivity is insensitive to the quantum metric, while the Berry curvature yields an intrinsic anomalous Hall con tribution. Here we combine analytic derivations with new numerics to show that disorder modifies the linear response conductivity dominated by geometric conductivity which is determined by the real space quantum metric. Focusing on a two dimensional multi-flatband stub-pyrochlore lattice, we identify a critical delocalized regime sandwiched between flat band localization and Anderson localization, characterized by finite geometric conductivity. Upon including spin orbit coupling, this regime evolves into a diffusive metallic phase, constituting a two dimensional inverse Anderson transition. Moreover, exploiting the connection between the real space quantum metric marker and the Wannier function spread, we construct a bond-percolation model on a square lattice. The resulting percolation region quantitatively coincides the critical delocalized regime, the exponent of which supports a classical percolation universality class. These findings suggest that flat band de localization can be understood as a classical percolation of quantum metric puddles. This advances our understanding of quantum geometric contributions to transport and establishes linear response measurements as a new avenue for accessing the quantum metric.