High Resolution Spectroscopy of Two-Dimensional Electron Systems

TL;DR

This paper introduces an advanced spectroscopic technique to measure the single-particle density of states in two-dimensional electron systems with high precision, revealing new insights into their correlated quantum states.

Contribution

An improved time domain capacitance spectroscopy method enabling high-resolution measurements of the SPDOS in 2DES, including novel observations of spin gaps and Landau level splitting.

Findings

First direct measurement of the spin gap in 2DES.

Observation of exchange splitting of Landau levels away from the Fermi surface.

High-fidelity spectral data of correlated electron states.

Abstract

Spectroscopic methods involving the sudden injection or ejection of electrons in materials are a powerful probe of electronic structure and interactions. These techniques, such as photoemission and tunneling, yield measurements of the "single particle" density of states (SPDOS) spectrum of a system. The SPDOS is proportional to the probability of successfully injecting or ejecting an electron in these experiments. It is equal to the number of electronic states in the system able to accept an injected electron as a function of its energy and is among the most fundamental and directly calculable quantities in theories of highly interacting systems. However, the two-dimensional electron system (2DES), host to remarkable correlated electron states such as the fractional quantum Hall effect, has proven difficult to probe spectroscopically. Here we present an improved version of time domain capacitance spectroscopy (TDCS) that now allows us to measure the SPDOS of a 2DES with unprecedented fidelity and resolution. Using TDCS, we perform measurements of a cold 2DES, providing the first direct measurements of the single-particle exchange-enhanced spin gap and single particle lifetimes in the quantum Hall system, as well as the first observations of exchange splitting of Landau levels not at the Fermi surface. The measurements reveal the difficult to reach and beautiful structure present in this highly correlated system far from the Fermi surface.