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

We present thermopower S and resistance R measurements on GaAs-based mesoscopic two-dimensional electron gases as functions of the electron density n_s. At high n_s we observe good agreement between the measured S and S_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 (1988 Sov. Phys. JETP 67 957).