Time-resolved Hall conductivity of pulse-driven topological quantum systems

TL;DR

This paper investigates the time-dependent Hall response in a 2D honeycomb lattice under pulsed perturbations, revealing dynamical features of topological transitions and Floquet effects on ultrafast timescales.

Contribution

It introduces a detailed analysis of the transient Hall response in topological systems driven by pulses, highlighting dynamical Floquet gaps and non-adiabatic effects.

Findings

Oscillating dynamical Hall response depends on doping and occupation.

Photon-dressed topological bands show features within femtoseconds.

Remnant Hall response persists even with few-cycle pulses.

Abstract

We address the question of how the time-resolved bulk Hall response of a two dimensional honeycomb lattice develops when driving the system with a pulsed perturbation. A simple toy model that switches a valley Hall signal by breaking inversion symmetry is studied in detail for slow quasi-adiabatic ramps and sudden quenches, obtaining an oscillating dynamical response that depends strongly on doping and time-averaged values that are determined both by the out of equilibrium occupations and the Berry curvature of the final states. On the other hand, the effect of irradiating the sample with a circularly-polarized infrared pump pulse that breaks time reversal symmetry and thus ramps the system into a non-trivial topological regime is probed. Even though there is a non quantized average signal due to the break down of the Floquet adiabatical picture, some features of the photon-dressed topological bands are revealed to be present even in a few femtosecond timescale. Small frequency oscillations during the transient response evidence the emergence of dynamical Floquet gaps which are consistent with the instantaneous amplitude of the pump envelope. On the other hand, a characteristic heterodyining effect is manifested in the model. The presence of a remnant Hall response for ultra-short pulses that contain only a few cycles of the radiation field is briefly discussed.