# Tuning topology in thin films of topological insulators by strain gradients

## TL;DR

This paper theoretically demonstrates how strain gradients in thin films of topological insulators can induce phase transitions between different topological phases, offering a new way to control topological states.

## Contribution

It introduces a theoretical framework showing strain-gradient effects can switch topological phases in 3D topological insulator thin films, including Bi2Se3 and alloys.

## Key findings

- Strain gradients induce phase transitions from quantum spin-Hall to trivial insulators.
- In alloys with reduced band gaps, strain gradients promote non-trivial topological phases.
- Strain-gradient effects could enable flexomagnetic coupling in magnetic topological insulators.

## Abstract

We theoretically show that the coupling of inhomogeneous strains to the Dirac fermions of three-dimensional topological insulators (3DTI) in thin film geometries results in the occurrence of phase transitions between topologically distinct insulating phases. By means of minimal k dot p models for strong 3DTI in the Bi 2 Se 3 materials class, we find that in thin films of stoichiometric materials a strain-gradient induced structure inversion asymmetry drives a phase transition from a quantum spin-Hall phase to a topologically trivial insulating phase. Interestingly, in alloys with strongly reduced bulk band gaps strain gradients have an opposite effect and promote a topologically non-trivial phase from a parent normal band insulator. These strain-gradient assisted switchings between topologically distinct phases are expected to yield a flexomagnetic coupling in magnetic topological insulator thin films.