When Cubic Is Not Isotropic: Phonon-Exciton Decoupling in CuInSnS$_4$ Single Crystals

TL;DR

This study reveals that intrinsic atomic disorder in cubic CuInSnS$_4$ crystals causes a decoupling of phononic and excitonic properties, enabling polarization-sensitive optical functionalities despite overall cubic symmetry.

Contribution

It demonstrates phonon-exciton decoupling due to local symmetry breaking from cation disorder, combining experimental and first-principles methods.

Findings

Phonons remain symmetry-averaged and homogeneous on the nanoscale.

Photoluminescence shows polarization anisotropy linked to local symmetry breaking.

Disorder localizes excitons while vibrational coherence is preserved.

Abstract

Atomic-scale disorder can create hidden optical anisotropy even in crystals that are structurally cubic on average. Here, we show that CuInSnS$_4$ single crystals host locally symmetry-broken environments arising from intrinsic In/Sn cation disorder, which affect vibrational and excitonic properties in markedly different ways. Combining polarization- and temperature-dependent Raman spectroscopy, infrared near-field microscopy, steady-state and time-resolved photoluminescence, and first-principles calculations, we find that phonons remain largely symmetry-averaged and locally homogeneous on the nanoscale. In contrast, photoluminescence reveals a lower-energy band-tail emission with pronounced polarization anisotropy following a well-defined angular symmetry, highlighting the strong sensitivity of excitonic states to local symmetry breaking. This phonon-exciton decoupling reveals that intrinsic disorder can localize excitons while preserving vibrational coherence and dielectric homogeneity, thereby opening new opportunities for polarization-sensitive light sources, anisotropic photodetectors, and exciton-based optical functionalities even in nominally cubic multinary semiconductors.