Linear temperature dependence of conductivity in the apparent insulating regime of dilute two-dimensional holes in GaAs

Abstract

The conductivity of extremely high mobility dilute two-dimensional holes in GaAs changes linearly with temperature in the insulating side of the metal-insulator transition. Hopping conduction, characterized by an exponentially decreasing conductivity with decreasing temperature, is not observed when the conductivity is smaller than e 2 /h. We suggest that strong interactions in a regime close to the Wigner crystallization must be playing a role in the unusual transport.In recent years, there has been great interest in the transport of low-density two-dimensional ͑2D͒ charge carrier systems. In contradiction to the scaling theory of localization 1 which predicted that all states in 2D in the absence of electron-electron interactions are localized, experimental observations of ''metallic'' behavior showing increasing conductivity () with decreasing temperature ͑T͒ (d/dTϽ0) and an apparent metal-insulator transition ͑MIT͒ as the carrier density is lowered have been reported on many lowdensity, low disorder 2D systems. 2 Although there have been many experimental and theoretical studies trying to understand this metallic behavior, there is still no consensus on its origin to date.On the other hand, the ''insulating'' behavior (d/dT Ͼ0) for lower densities in these low disorder systems has not been studied as extensively as the metallic behavior. In highly disordered systems, the transport in the insulating regime is described by variable range hopping 3,4 ͑VRH͒ among the localized states, and the T dependence of is expressed as (T)ϭ 0 exp͓Ϫ(T 0 /T) x ͔, where 0 is a prefactor and T 0 is a characteristic temperature. The exponent x depends on the density of states ͑DOS͒ at the Fermi energy. For a constant DOS in the absence of interactions, xϭ1/3 corresponding to the Mott VRH, 3 and for a DOS which has a Coulomb gap at the Fermi energy due to interactions, xϭ1/2 corresponding to the Efros-Shklovskii VRH. 4 In clean systems with low disorder, however, the origin of the insulating behavior (d/dTϾ0) in low-density regime, where the interactions between carriers are strong, could be significantly different from that described by simple localization. In the ideal case, it was predicted that the ground state of the system in the low-density limit is a Wigner crystal, 5 which can be pinned by even a tiny amount of impurities in the system.We have recently studied the MIT of extremely high mobility dilute 2D holes in GaAs and presented a detailed analysis for the T dependence of on the metallic side of the transition. 6 The metallic behavior persisted up to r s ϭ57, much higher than 37 predicted for Wigner crystallization ͓r s is the interaction energy to Fermi energy ratio, given by r s ϭ(p) Ϫ1/2 m*e 2 /4ប 2 ⑀, where p is the hole density, m* effective mass, and ⑀ the dielectric constant͔. With the persistence of metallic behavior up to such a large r s value, the question arises regarding the nature of the insulating behavior observed in this 2D system for r s reaching 80. To address this question, we present the T dependen...

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

“…The portion above T c is approximately linear, consistent with previous findings. 11,13,14 This seems to agree with the viscous flow model first developed for helium atoms 15 and later extended to GaAs 2D hole systems 19 and to strongly correlated electron liquids in a semiquantum limit (assuming T > T M ,T F ).…”

“…11,13,14 For fixed p below 7 × 10 9 cm −2 , we first observed a conductivity dip as T is lowered across some characteristic value T c . Remarkably, when p is lowered below 4 × 10 9 cm −2 , the dip evolves into a kink.…”

“…holes in HIGFETs to a lowest density of 6 × 10 8 cm −2 , 11,13 and in SiGe systems down to 3 × 10 10 cm −2 . 14 Compared to the disorder domination and the comparable disorderinteraction interplay in MIT, these findings signify a prominent interaction that possibly drives the system from a FL into a WC regime.…”

“…At first glance, for p 2.5 × 10 9 cm −2 , σ (T ) exhibits a linearlike relationship above 100 mK, in seeming agreement with the previous findings. 11,13,14 For lower p, σ (T ) becomes power-law like. 12 However, a close examination within the linear region reveals a fine feature that becomes obvious in a zoom-in view shown in Fig.…”

Spin-orbit coupling and transport in strongly correlated two-dimensional systems

“…The devices adopted are 2D holes in undoped p-channel 100 HIGFET [11][12][13] patterned into 1 mm × 6 mm Hallbar shapes. As illustrated in Fig.…”