Resilient cluster Mott states in layered Nb$_3$Cl$_8$ against pressure-induced symmetry breaking

TL;DR

This study combines advanced computational methods to analyze how pressure affects the electronic and structural properties of Nb$_3$Cl$_8$, revealing the robustness of its cluster Mott insulating state despite symmetry breaking.

Contribution

It provides the first systematic theoretical analysis of pressure-induced symmetry breaking effects on the cluster Mott state in Nb$_3$Cl$_8$, combining DFT and DMFT methods.

Findings

Pressure reduces the charge gap but maintains the cluster Mott insulating state.

Local symmetry breaking occurs under pressure, changing from C_{3v} to C_s.

The gap reduction is due to increased bandwidth and decreased Coulomb interactions.

Abstract

In this work, by combining density-functional theory (DFT) with dynamical mean-field calculations (DMFT), we compare the crystal and electronic structures of the prototype cluster Mott insulator Nb$_{3}$Cl$_8$ at ambient and high-pressure. We explain the finite but significantly reduced charge gap experimentally observed at $P=9.7$ GPa. We reveal a local symmetry breaking of the Nb$_{3}$ trimer under pressure, reducing its symmetry from $C_{3v}$ to $C_{s}$. This leads to a strong bandwidth enhancement and a lift of band degeneracy. Crucially, despite the significant change of band details, the cluster Mott insulating state is robust against local symmetry breaking. We show that the experimental observed gap under pressure is still a cluster Mott gap and its reduced value stems from both increased bandwidth and reduced Coulomb interactions under pressure. Our study provides the first systematic theoretical elucidation of how pressure-induced symmetry breaking dictates the cluster Mott state, establishing a robust foundation for understanding the intricate relationship between symmetry, local/non-local correlations, and emergent quantum states in correlated cluster compounds.