# Time-dependent potential impurity in topological insulator

## TL;DR

This paper investigates how periodically driven potential impurities affect the surface states of two-dimensional topological insulators using continuum and lattice models, revealing changes in local density of states and impurity resonances.

## Contribution

It introduces a comparative analysis of impurity effects under periodic driving in continuum and lattice models of topological insulators, highlighting the emergence of impurity resonances and gap filling.

## Key findings

- Local density of states remains low-energy linear in the continuum model.
- Impurity resonances appear near the Fermi energy with increased driving amplitude.
- Edge and bulk impurity positions show different spectral behaviors.

## Abstract

We consider periodically driven potential impurities coupled to the surface states of a two-dimensional topological insulator. The problem is addressed by means of two models, out which the first model is an effective continuum Hamiltonian for the surface states, whereas the Kane-Mele lattice model is our second approach. While both models result in drastic changes in the local density of electron states with increasing amplitude and frequency of the driving field, the linearly low energy local density of electron states remains in the continuum model, however, with an increased Fermi velocity. The spectrum of the continuum model remains gapless under the emergence of new impurity resonances near the Fermi energy. The Kane-Mele lattice model represents a finite size system, with edge states appearing at the boundary of the system. We, thus, consider the impurity at two different positions, one at the boundary and one at the center of the lattice. In the former case, a reduction and broadening of the low energy local density of electron states result with increasing amplitude of the driving field. On the other hand, there are no new resonances emerging in the spectrum. In the latter case, the spectrum is gapped both in the absence of the impurity as well as for weak amplitudes of the driving field, while the gap tends to fill up with impurity states with increasing amplitude.