Ab initio calculation of the shift photocurrent by Wannier interpolation

TL;DR

This paper introduces a first-principles Wannier-based algorithm for calculating shift-current responses in piezoelectric crystals, emphasizing gauge invariance, efficiency, and accurate optical matrix element treatment.

Contribution

It presents a novel, efficient, and gauge-invariant first-principles method for shift-current calculation using Wannier functions, improving accuracy over previous tight-binding approaches.

Findings

The algorithm is fully gauge invariant and computationally efficient.

Discarding off-diagonal position matrix elements causes significant errors.

Careful real-space embedding is crucial for accurate shift-current modeling.

Abstract

We describe and implement a first-principles algorithm based on maximally-localized Wannier functions for calculating the shift-current response of piezoelectric crystals in the independent-particle approximation. The proposed algorithm presents several advantages over existing ones, including full gauge invariance, low computational cost, and a correct treatment of the optical matrix elements with nonlocal pseudopotentials. Band-truncation errors are avoided by a careful formulation of $k\cdot p$ perturbation theory within the subspace of wannierized bands. The needed ingredients are the matrix elements of the Hamiltonian and of the position operator in the Wannier basis, which are readily available at the end of the wannierization step. If the off-diagonal matrix elements of the position operator are discarded, our expressions reduce to the ones that have been used in recent tight-binding calculations of the shift current. We find that this `diagonal' approximation can introduce sizeable errors, highlighting the importance of carefully embedding the tight-binding model in real space for an accurate description of the charge transfer that gives rise to the shift current.