Hybrid cluster-expansion and density-functional-theory approach for optical absorption in TiO2

TL;DR

This paper introduces a hybrid computational method combining density-functional theory and cluster expansion to accurately predict optical absorption features in TiO2, emphasizing the role of excitonic states.

Contribution

It develops a novel hybrid approach integrating ab initio calculations with many-body theory for excitonic optical responses in semiconductors.

Findings

The method accurately reproduces TiO2's near-bandgap absorption features.

The observed features are attributed to a quadrupole-allowed 1s-exciton state.

Strong agreement with experimental data validates the approach.

Abstract

A combined approach of first-principles density-functional calculations and the systematic cluster-expansion scheme is presented. The dipole, quadrupole, and Coulomb matrix elements obtained from ab initio calculations are used as an input to the microscopic many-body theory of the excitonic optical response. To demonstrate the hybrid approach for a nontrivial semiconductor system, the near-bandgap excitonic optical absorption of rutile TiO2 is computed. Comparison with experiments yields strong evidence that the observed near-bandgap features are due to a dipole-forbidden but quadrupole-allowed 1s-exciton state.