Quantum capacitance anomalies of two-dimensional non-equilibrium states under microwave irradiation

TL;DR

This study investigates quantum capacitance oscillations in high-mobility 2D electron systems under microwave irradiation, revealing anomalous behavior and multi-photon processes that challenge existing equilibrium theories.

Contribution

It provides the first direct measurement of quantum capacitance in non-equilibrium 2D electron states under microwave irradiation, highlighting anomalous oscillations and multi-photon effects.

Findings

Quantum capacitance oscillates with MIRO but oppositely to equilibrium expectations.

Anomalous phenomena suggest non-equilibrium effects, domains, and inhomogeneity induced by microwaves.

Detection of multi-photon processes around j=1/2 under intense microwave irradiation below 30 GHz.

Abstract

We report our direct study of the compressibility on ultrahigh mobility two-dimensional electron system ($\mu_{e} \sim 1 \times 10^{7}$ cm$^{2}$/Vs) in GaAs/AlGaAs quantum wells under microwave (MW) irradiation. The field penetration current results show that the quantum capacitance oscillates with microwave induced resistance oscillations (MIRO), however, the trend is opposite with respect to the compressibility for usual equilibrium states in previous theoretical explanations. The anomalous phenomena provide a platform for study on the non-equilibrium system under microwave, and point to the current domains and inhomogeneity induced by radiation. Moreover, the quantum capacitance indication for multi-photon process around $j = 1/2$ is detected under intensive microwave below 30 GHz.