Conductivity kinks in the transport of ultradilute two-dimensional GaAs hole systems in zero field

Abstract

High-purity two-dimensional hole systems of dilute charge concentrations are realized in GaAs field-effect transistors. For fixed charge densities below 4 × 10 9 cm −2 , transport measurement reveals a conductivity (σ ) kink while sweeping temperature across some characteristic value T c , where the derivative dσ/dT exhibits a discontinuous step. Moreover, T c has a piecewise density dependence separated approximately by the critical density of the apparent metal-to-insulator transition.Low-temperature charge transport studies of high-purity electron systems encompass fundamental subjects of disorder 1 and the electron-electron interaction. A fascinating subject on interaction-driven critical phenomenon is the Wigner crystallization (WC) 2,3 of electrons in a low disordered environment. Although the WC was first observed for electrons on helium in 1979, 4 a Fermi liquid (FL)-to-WC transition has never been demonstrated. 5 Important questions on the nature of the transition remain unanswered. From the experimental standpoint, semiconductor bulk materials offer a broad tunability of charge density. However, such a transition has never been observed through varying carrier concentrations even in the high-quality materials. Apart from the complexity associated with the disorder, which competes with or even dominates over interaction, an important question is whether there exist intermediate phases that hinder a direct first-order transition. 6,7 Early measurements of dilute two-dimensional (2D) systems demonstrate activated conduction that is understood through the well-known Anderson localization caused by a dominating random disorder. 1,8 As cleaner systems later became available, systems with lower charge densities are more accessible. Interaction becomes important because the interaction parameter r s , the ratio of the Coulomb potential E ee and the Fermi energy E F , increase with decreasing density. As a result, the measured T dependence of the resistivity ρ(T ) exhibits a metal-to-insulator transition (MIT) as the charge density is lowered across a critical value n c . 9 It is generally believed that MIT is caused by a complex disorder-interaction interplay that is still not well understood. 5,10 However, ρ(T ) for the lowest charge densities on the insulating side remains activated, signifying the same disorder domination.Recently, with improved semiconductor growth techniques, higher-purity 2D systems are realized for accessing even more dilute charges: A 2D hole concentration of 6 × 10 8 cm −2 has been realized in undoped GaAs/AlGaAs heterojunctioninsulated-gate field-effect transistors (HIGFETs). 11 The maximum r s is beyond 90 if an effective mass m * of ∼0.3m 0 is assumed. Interestingly, an unexpected resistivity (ρ)-T relationship has been observed on both sides of the apparent MIT. On the metallic side, an insulatorlike behavior indicated by a rising ρ with cooling appears for T below 30 mK where ρ min is found. 12 On the insulating side, nonactivated collective behaviors, specifically the linear and ...