Optical readout of the chemical potential of two-dimensional electrons

TL;DR

This paper introduces an optical method to precisely measure the chemical potential of 2D materials, enabling detailed exploration of their quantum states and inhomogeneities with high spatial and temporal resolution.

Contribution

The authors develop a novel optical readout technique for the chemical potential of 2D materials, demonstrating high sensitivity and spatial imaging capabilities.

Findings

Detected a correlated insulating state at one hole per moire unit cell

Observed evolution from Mott to charge-transfer insulator with electric field

Quantified spatial inhomogeneity of the sample

Abstract

The chemical potential u of an electron system is a fundamental property of a solid. A precise measurement of u plays a crucial role in understanding the electron interaction and quantum states of matter. However, thermodynamics measurements in micro and nanoscale samples are challenging because of the small sample volume and large background signals. Here, we report an optical readout technique for u of an arbitrary two-dimensional (2D) material. A monolayer semiconductor sensor is capacitively coupled to the sample. The sensor optical response determines a bias that fixes its chemical potential to the band edge and directly reads u of the sample. We demonstrate the technique in AB-stacked MoTe2/WSe2 moire bilayers. We obtain u with DC sensitivity about 20 ueV/sqrt(Hz), and the compressibility and interlayer electric polarization using AC readout. The results reveal a correlated insulating state at the doping density of one hole per moire unit cell, which evolves from a Mott to a charge-transfer insulator with increasing out-of-plane electric field. Furthermore, we image u and quantify the spatial inhomogeneity of the sample. Our work opens the door for high spatial and temporal resolution measurements of the thermodynamic properties of 2D quantum materials.