Quantum Localization Effects in Non-Degenerate 2D Electron Gases

Abstract

Electrons sprayed onto the surface of solid hydrogen form a highly two-dimensional electron gas (2DEG) whose density can be varied over several orders of magnitude to cover a wide range of e - e interaction energy. For carrier densities below 10⁸ cm ^-2 and temperatures of a few Kelvin the system is essentially non-interacting. In the other extreme, densities of several times 10¹⁰ cm ^-2 can be realized on hydrogen, which at temperatures below IK puts the system well into the strongly interacting Wigner crystal regime [1]. The focus of this chapter will be primarily on the effects of disorder on the transport properties of a weakly interacting two-dimensional electron gas (2DEG) formed on solid hydrogen. This system has several unique qualities that separate it from the more traditional disordered metals and semiconductors. The first, as mentioned above, is that the non-interacting regime is a natural region of parameter space for electrons on cryogenic surfaces. In fact, all of the experiments described below were carried out with carrier densities n ₀ ≤10⁸cm^-2 corresponding to an e-e interaction energy of only a few Kelvin. Wigner crystallization occurs at an interaction energy of ~130 k _B T [1]. A second important distinguishing characteristic of this system is that at temperatures of interest, the 2DEG is completely non-degenerate thus allowing us to separate out quantum statistics effects from single-particle quantum effects. Finally, and perhaps most importantly, the 2DEG on hydrogen system can be disordered by the introduction of ambient helium gas into the experimental cell.