Dimensionality-Dependent Exciton Dispersion in a Single-Band Mott Insulator

TL;DR

This study observes how exciton dispersion in Nb3Cl8 changes from quasi-two-dimensional to three-dimensional across a phase transition, highlighting the impact of dimensionality on exciton behavior.

Contribution

It provides the first direct experimental evidence of dimensionality-dependent exciton dispersion in a single-band Mott insulator, linking phase transition to exciton dynamics.

Findings

High-temperature phase shows quasi-2D linear exciton dispersion.

Low-temperature phase exhibits 3D parabolic exciton dispersion.

Dimensional mutation driven by increased interlayer coupling.

Abstract

Excitonic band structure is critical for investigating exciton dynamics. Theoretically, quantum effects from exchange scattering between electron-hole pairs significantly modulate exciton dispersion. Here, we report the direct observation of dimensionality-dependent exciton dispersion in a single-band Mott insulator Nb3Cl8 through high-resolution electron energy loss spectroscopy. In the high-temperature phase, the exciton in Nb3Cl8 hosts an exceptionally large binding energy, and exhibits clear quasi-two-dimensional massless linear dispersion. In contrast, in the low-temperature phase, the exciton splits into two bands, both displaying three-dimensional parabolic dispersion. These dramatic changes in the exciton dispersion stem from the dimensional mutation driven by a substantial enhancement of interlayer coupling across the phase transition. This Letter provides a clear and typical example of how exciton behavior evolves with dimensionality.