Temperature-induced optical enhancement near a localization transition

TL;DR

This study investigates how temperature influences optical conductivity near a localization transition in quasiperiodic systems, revealing a discontinuous gap closure and resonant enhancement of low-frequency optical signals.

Contribution

It provides the first detailed analysis of finite-frequency optical conductivity near the metal-insulator transition in the Aubry-André model, highlighting a new activation mechanism for optical enhancement.

Findings

Optical gap closes discontinuously at the transition.

Strong low-frequency optical enhancement at certain resonant frequencies.

Thermal activation of Pauli-blocked transitions explains the enhancement.

Abstract

Quasiperiodic systems are an intermediate class of systems between periodic crystals and disordered systems, famously exhibiting metal-insulator transitions (MITs) even in one dimension. While their transport properties have been studied extensively, a systematic analysis of the finite-frequency optical conductivity near the critical point has been lacking. In this work, we carry out a detailed study of the optical conductivity in the paradigmatic Aubry-Andr\'e model. We find that the zero-temperature low-frequency optical signal is strongly restructured by the quasiperiodic potential, exhibiting an optical gap that closes discontinuously as the system approaches the MIT. Most strikingly, we uncover a mechanism for a strong enhancement of the low-frequency finite temperature optical conductivity at certain resonant frequencies. This enhancement stems from the thermal activation of Pauli-blocked transitions between strongly resonant van Hove singularities. This mechanism provides new insight into finite-frequency transport in quasiperiodic systems and a new pathway for manipulating optical properties near a localization transition. Furthermore, our findings establish the optical response as a powerful, experimentally accessible tool for probing non-trivial quasiperiodicity effects.