Density-dependent thermopower oscillations in mesoscopic two-dimensional electron gases

TL;DR

This study investigates how thermopower oscillations in mesoscopic 2DEGs depend on electron density, revealing a transition from Mott-like behavior to oscillatory regimes with sign changes, independent of resistance fluctuations.

Contribution

It demonstrates the density-dependent transition in thermopower behavior and suggests a universal nature of thermopower oscillations in mesoscopic 2DEGs, challenging existing mesoscopic fluctuation models.

Findings

Thermopower matches Mott prediction at high densities.

Oscillations and sign changes in thermopower at low densities.

No corresponding features in resistance measurements.

Abstract

We present thermopower $S$ and resistance $R$ measurements on GaAs-based mesoscopic two-dimensional electron gases (2DEGs) as functions of the electron density $n_s$. At high $n_s$ we observe good agreement between the measured $S$ and $S_{\rm{MOTT}}$, the Mott prediction for a non-interacting metal. As $n_s$ is lowered, we observe a crossover from Mott-like behaviour to that where $S$ shows strong oscillations and even sign changes. Remarkably, there are absolutely no features in $R$ corresponding to those in $S$. In fact, $R$ is devoid of even any universal conductance fluctuations. A statistical analysis of the thermopower oscillations from two devices of dissimilar dimensions suggest a universal nature of the oscillations. We critically examine whether they can be mesoscopic fluctuations of the kind described by Lesovik and Khmelnitskii in Sov. Phys. JETP. \textbf{67}, 957 (1988).