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A1: Low temperature materials

Giant Low-Temperature Thermopower in a Novel Correlated Electron Phase in Two-Dimensional Electron Systems

V. Narayan1, M. Pepper2, J. Griffiths1, G. A. C Jones1, H. E. Beere1 and D. A. Ritchie1
1Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK2Department of Electrical and Electronic Engineering, University College London, Torrington Place, London WC1E 7JE

We present thermopower (S) and electrical resistance (R) measurements in a GaAs-based two-dimensional electron system (2DES) of mesoscopic dimensions in the temperature (T) range 0.3 K to 2 K. We find that as the density ns is varied, S obeys the Mott formula closely at high ns. At a critical value of ns, however, S shows an abrupt departure from Mott-like behaviour growing steeply in magnitude and exceeding the predicted Mott value by nearly three orders of magnitude. Accompanied by this rapid growth are strong, reproducible oscillations and even sign changes in S which, remarkably, are completely absent in R. The ns value at which the oscillations set in is constant over several devices, suggesting this to be an electron-interaction-driven phenomenon rather than a disorder-driven one.

We investigate the T-dependence of S in the low-ns state and find, surprisingly, the 2DEG to be manifestly metallic as evidenced by the linear T-dependence of S below 0.7 K. Even more surprisingly, this metallic character persists to the lowest achievable ns where R >> h/e2 the quantum of resistance. Second, in this ns-regime we observe phonon drag to become significant at relatively low T, signifying a sharp suppression in the effective number of carriers. We argue that these observations indicate a novel correlated electron phase in which collective electron excitations mediate the electrical and thermal transport.

Importantly, the measured S values in this phase are exceedingly large, reaching as much as 100 mV/K at 1.5 K. We explore the applicability of this phase in cryogenic Peltier cooling and also present initial experimental results on the measurement of the thermal conductivity of the 2DES at T = 100 mK, and thus towards estimating its thermoelectric figure-of-merit.