We report the observation of a metal-insulator transition at B 0 in a high mobility two dimensional hole gas in a GaAs-AlGaAs heterostructure. A clear critical point separates the insulating phase from the metallic phase, demonstrating the existence of a well defined minimum metallic conductivity s min 2e 2 ͞h. The s͑T͒ data either side of the transition can be "scaled" onto one curve with a single parameter T 0 . The application of a parallel magnetic field increases s min and broadens the transition. We argue that strong electron-electron interactions (r s Ӎ 10) suppress quantum interference corrections to the conductivity. [S0031-9007(98)05325-3] PACS numbers: 73.20.Dx, 71.30. + h, 73.20.Fz In the mid-1970s experiments on silicon inversion layers produced considerable evidence for the existence of a metal-insulator transition in 2D and a minimum metallic conductance, s min [1-3]. The decay constants of localized state wave functions were investigated, and it was shown that when the number of localized electrons exceeded 2 3 10 11 cm 22 the location of the mobility edge was determined by electron-electron interactions and increased with increasing carrier concentration. Subsequent theoretical work in 1979 suggested that all states in 2D were localized [4] and that phase incoherent scattering imposed a cutoff to a localized wave function giving a logarithmic correction to metallic conduction (weak localization) which was widely observed and used to obtain very detailed information on the various types of electronelectron scattering in all three dimensions [5,6]. However, in order to investigate the logarithmic correction at low, but accessible, temperatures it was necessary to use samples with low mobility so that the elastic scattering length l was small [7]. In view of the success of the theory it was then assumed that the earlier high mobility samples did not show a logarithmic correction because the phase coherence length l f was not greater than the elastic scattering length, but that if experiments could be performed at much lower temperatures (beyond the capability of cryogenics) then the logarithmic correction would be found.Recent experimental results have raised this issue again and indicate that states in 2D are not always localized with strong evidence for a metal-insulator transition in high mobility Si metal-oxide-semiconductor field-effect transistors (MOSFETs) [8]. It was found that the resistivity on both the metallic and insulating sides of the transition varied exponentially with decreasing temperature, and that a single scaling parameter could be used to collapse the data on both sides of the transition onto a single curve. While the exact nature of the transition is presently not understood, there have been several reports of similar scaling and duality between the resistivity (and conductivity) on opposite sides of the transition, both for electrons in Si MOSFETs [9,10] and for holes in SiGe quantum wells [11]. In all of these reports electron-electron interactions are known to be import...

A detailed investigation of the metallic behaviour in high quality GaAs-AlGaAs two dimensional hole systems reveals the presence of quantum corrections to the resistivity at low temperatures. Despite the low density (rs > 10) and high quality of these systems, both weak localisation (observed via negative magnetoresistance) and weak hole-hole interactions (giving a correction to the Hall constant) are present in the so-called metallic phase where the resistivity decreases with decreasing temperature. The results suggest that even at high rs there is no metallic phase at T=0 in two dimensions.PACS numbers: 73.40. Qv, 71.30.+h, 73.20.Fz Since the claimed observation of metallic behaviour in strongly interacting two-dimensional (2D) systems over 5 years ago 1 experimentalists have tried to provide data from which an understanding of the conduction processes in high quality 2D systems can be obtained. Initial studies of these new strongly interacting systems revealed that the resistivity data can be "scaled" over a wide range of temperatures indicating the presence of a true phase transition between insulating and metallic states 1-3 . Following this an empirical formula for ρ(T) has been put forward which fits all the available experimental data of the metallic state 4,5 . This formula describes a saturation of the resistivity as the temperature is reduced, giving a finite resistance at T=0, further testifying the existence of a 2D metallic state. Despite these results the nature of the metallic state and whether it really persists to the zero of temperature remains unclear. Early theoretical 6,7 and experimental 8 studies of weakly interacting systems (low r s ) revealed that the presence of any disorder would give rise to logarithmic corrections to the conductivity. Since these corrections become increasingly important as T is reduced the question of what happens to the metallic behaviour as T→ 0 in 2D systems remains. This paper reports the observation of both weak localisation and weak hole-hole interactions in the "metallic" phase of a high quality 2D GaAs hole system. First we demonstrate that the system studied here exhibits all of the characteristics previously associated with the 2D "metal"-insulator transition. Magnetoresistance measurements are then used to extract the logarithmic corrections to the Drude conductivity at low temperatures. The data show that: (1) the anomalous exponential decrease of resistivity with decreasing temperature in the metallic phase is not due to quantum interference or strong interaction effects, (2) phase coherence is preserved in the metallic regime with evidence for normal Fermi liquid behaviour, and (3) hole-hole interactions provide a localising correction to the conductivity.The sample used here is a gated, modulation doped GaAs quantum well grown on a (311)A substrate 3 . Four terminal magnetoresistance measurements were performed at temperatures down to 100mK using low frequency (4 Hz) ac lockin techniques and currents of 0.1-5 nA to avoid electron heating. The hol...

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There have been predictions that for closely spaced two-dimensional electron gases ͑2DEG's͒, charge transfer between the 2DEG's can arise from electron interactions due to exchange and correlation ͓Ruden and Wu, Appl. Phys. Lett. 59, 2165 ͑1991͔͒. In this study, we are able to determine accurately the charge in each of the 2DEG's of a triple-quantum-well structure. The results demonstrate that as the charge in one 2DEG is reduced, by applying a negative gate voltage the adjacent layer can gain charge; thus charge is effectively transferred between the 2DEG's. This type of charge transfer cannot be explained by a simple noninteracting model of the structure, but can be modeled by including intralayer electron interactions in the form of exchange and correlation correction terms within the local-density approximation. The inclusion of these many-body interaction terms into our model leads to a good qualitative agreement with the experimental data, confirming that the origin of the charge transfer is electron interactions.

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The tunneling current between an electron gas with a periodic potential in two dimensions and a plain two-dimensional electron system (2DES) has been studied. The strength of the periodic potential, the subband energy of the plain 2DES, and an applied in-plane magnetic field were varied, mapping the Fourier transform of the periodic wave function. Periodic peaks were observed and explained by translations in the reciprocal lattice. When the potential was strongly modulated to form an array of antidots, commensurability peaks were seen in lateral transport, but, as expected, not in tunneling.

Independent contacts have been produced to all three two-dimensional electron gases (2DEGs) in a triple-quantum-well structure. This has been achieved by using in situ focused ion beam implantation followed by molecular beam epitaxial regrowth. Lateral transport studies of the individual layers have demonstrated resonant coupling between the high-mobility 2DEGs. Large resistance changes are observed due to coupling between the layers and these can be enhanced by having all three 2DEGs on resonance. A versatile device can thus be fabricated for studying electron interactions.