# Highly tunable band structure in ferroelectric R-stacked bilayer WSe2

## TL;DR

Researchers studied the tunable electronic properties of a special two-layer material, revealing how its structure can be controlled for future quantum devices.

## Contribution

The study experimentally quantifies the tunable band structure and ferroelectric domain switching in rhombohedral-stacked WSe2 bilayers.

## Key findings

- Exciton spectroscopy confirms type-II band alignment with conduction and valence band edges at Λ and K valleys.
- Ferroelectric domains AB and BA coexist and respond to displacement fields through excitonic hybridization.
- Electric-field-driven domain switching alters the valence band maximum, demonstrating controllable polarization.

## Abstract

Transition metal dichalcogenide homobilayers unite two frontiers of quantum materials research: sliding ferroelectricity, arising from rhombohedral stacking, and moiré quantum matter, emerging from small-angle twisting. The spontaneous polarization of ferroelectric rhombohedral stacked homobilayers produces a highly tunable band structure, which, together with strain-induced piezoelectricity, governs the topology and correlated electronic phases of twisted bilayers. Here we present a systematic low-temperature optical spectroscopy study of rhombohedral stacked bilayer WSe2 to quantitatively establish its fundamental electronic and ferroelectric properties. Exciton and exciton-polaron spectroscopy under doping reveals a pronounced electron-hole asymmetry that confirms type-II band alignment, with the conduction and valence band edges located at the Λ and K valleys, respectively. Through distinct excitonic responses and tunable interlayer-intralayer exciton hybridization under displacement fields, we uncover the coexistence of AB and BA ferroelectric domains. Using exciton-polarons as a probe, we directly measure the intrinsic polarization field and extract the interlayer potential. Finally, we demonstrate electric-field-driven symmetric switching of the valence band maximum, attributed to ferroelectric domain switching. These results provide a complete experimental picture of the band alignment, spontaneous polarization field, and domain dynamics of rhombohedral stacked WSe2, establishing key parameters to understand twisted bilayers and enabling new ferroelectric and excitonic device opportunities.

Authors uncover the tunable band structure in ferroelectric rhombohedral-stacked bilayer WSe2 by optical spectroscopy, quantifying spontaneous polarization and demonstrating electric-field-driven domain switching for quantum device applications.