Zero-point lattice expansion and band gap renormalization: Gr\"uneisen approach versus free energy minimization

TL;DR

This paper compares the Gr"uneisen approach and free energy minimization for calculating zero-point lattice expansion and its impact on band gap renormalization, demonstrating the efficiency and accuracy of the former across various materials.

Contribution

It introduces an efficient Gr"uneisen parameter-based formalism for ZPLE calculation, validated against free energy minimization, and highlights its importance in accurately estimating band gap ZPR.

Findings

Gr"uneisen approach agrees well with free energy minimization results.

ZPLE can contribute up to 80% of electron-phonon effects on band gaps.

Including ZPLE is crucial for accurate theoretical-experimental comparisons.

Abstract

The zero-point lattice expansion (ZPLE) is a small variation of the lattice parameters induced by the presence of phonons in a material compared to the static lattice picture. It contributes significantly to the zero-point renormalization (ZPR) of the band gap energy, but its consequences have not been investigated as thoroughly as those stemming from electron-phonon interactions. In the usual first-principles approach, one evaluates the ZPLE by minimizing the $T=0$ K Helmholtz free energy. In this work, we show that the formalism based on the Gr\"uneisen parameters, which commonly neglects zero-point effects, can be efficiently used to compute ZPLE for both isotropic and anisotropic materials at much lower computational cost. We systematically test this formalism on 22 cubic and wurtzite materials and obtain excellent agreement with free energy minimization results for both the ZPLE and the resulting band gap ZPR. We use our results to validate an empirical expression estimating the ZPLE-induced ZPR and unveil its sensitivity to the temperature range involved in estimating the ZPLE from experimental data. Our findings finally reveal that the ZPLE contribution to the band gap ZPR can reach 20% to more than 80% of the electron-phonon interaction contribution for heavier or more ionic materials, including materials containing light atoms. Considering both contributions on an equal footing is thus essential should one attempt to compare theoretical ZPR results with experimental data.