By analyzing the temperature ͑T͒ and density ͑n͒ dependence of the measured conductivity ͑͒ of twodimensional ͑2D͒ electrons in the low-density ͑ϳ10 11 cm −2 ͒ and temperature ͑0.02-10 K͒ regimes of highmobility ͑1.0 and 1.5ϫ 10 4 cm 2 / Vs͒ Si metal-oxide-semiconductor field-effect transistors, we establish that the putative 2D metal-insulator transition is a density-inhomogeneity-driven percolation transition where the density-dependent conductivity vanishes as ͑n͒ ϰ ͑n − n p ͒ p , with the exponent p ϳ 1.2 being consistent with a percolation transition. The "metallic" behavior of ͑T͒ for n Ͼ n p is shown to be well described by a semiclassical Boltzmann theory, and we observe the standard weak localization-induced negative magnetoresistance behavior, as expected in a normal Fermi liquid, in the metallic phase.The so-called two-dimensional ͑2D͒ metal-insulator transition ͑MIT͒ has been a subject 1,2 of intense activity and considerable controversy ever since the pioneering experimental discovery 3 of the 2D MIT phenomenon in Si metaloxide-semiconductor field-effect transistors ͑MOSFETs͒ by Kravchenko and Pudalov some 15 years ago. The apparent MIT has now been observed in almost all existing 2D semiconductor structures, including Si MOSFETs, 3,4 electrons, 5-7 and holes [8][9][10][11] in GaAs/AlGaAs, and electrons in Si/SiGe ͑Refs. 12 and 13͒ systems. The basic phenomenon refers to the observation of a carrier density-induced qualitative change in the temperature dependence of the resistivity ͑n , T͒, where n c is a critical density separating an effective "metallic" phase ͑n Ͼ n c ͒ from an "insulating" phase ͑n Ͻ n c ͒, exhibiting d / dT Ͼ 0͑Ͻ0͒ behavior typical of a metal ͑insulator͒.The high-density metallic behavior ͑n Ͼ n c ͒ often manifests in a large ͑by 25% for electrons in GaAs/AlGaAs heterostructures to factors of 2-3 in Si MOSFETs͒ increase in resistivity with increasing temperature in the lowtemperature ͑0.05-5 K͒ regime where phonons should not play much of a role in resistive scattering. The insulating regime, at least for very low ͑n Ӷ n c ͒ densities and temperatures, seems to be the conventional activated transport regime of a strongly localized system. The 2D MIT phenomenon occurs in relatively high-mobility systems, although the mobility values range from 10 4 cm 2 / Vs ͑Si MOSFET͒ to 10 7 cm 2 / Vs͑GaAs/ AlGaAs͒ depending on the 2D system under consideration. The 2D MIT phenomenon is also considered to be a low-density phenomenon although, depending on the 2D system under consideration, the critical density n c differs by 2 orders of magnitude ͑n c ϳ 10 11 cm −2 in 2D Si and ϳ10 9 cm −2 in high-mobility GaAs/AlGaAs heterostructures͒. The universal features of the 2D MIT phenomenon are ͑1͒ the existence of a critical density n c distinguishing an effective high-density metallic ͑d / dT Ͼ 0 for n Ͼ n c ͒ phase from an effective low-density insulating ͑d / dT Ͻ 0 for n Ͻ n c ͒ phase, and ͑2͒ while the insulating phase for n Ͻ n c seems mostly to manifest the conventional activated transport be...

We perform Landau-Zener-Stückelberg-Majorana (LZSM) spectroscopy on a system with strong spin-orbit interaction (SOI), realized as a single hole confined in a gated double quantum dot. Analogous to electron systems, at a magnetic field B=0 and high modulation frequencies, we observe photon-assisted tunneling between dots, which smoothly evolves into the typical LZSM funnel-shaped interference pattern as the frequency is decreased. In contrast to electrons, the SOI enables an additional, efficient spin-flip interdot tunneling channel, introducing a distinct interference pattern at finite B. Magnetotransport spectra at low-frequency LZSM driving show the two channels to be equally coherent. High-frequency LZSM driving reveals complex photon-assisted tunneling pathways, both spin conserving and spin flip, which form closed loops at critical magnetic fields. In one such loop, an arbitrary hole spin state is inverted, opening the way toward its all-electrical manipulation.

Using a combination of heat pulse and nuclear magnetic resonance techniques we demonstrate that the phase boundary separating the interlayer phase coherent quantum Hall effect at νT = 1 in bilayer electron gases from the weakly coupled compressible phase depends upon the spin polarization of the nuclei in the host semiconductor crystal. Our results strongly suggest that, contrary to the usual assumption, the transition is attended by a change in the electronic spin polarization.A remarkable quantum fluid emerges from bilayer twodimensional electron systems (2DES) in perpendicular magnetic fields B when the layer separation is small and the total density of electrons N T in the bilayer equals the degeneracy eB/h of a single spin-resolved Landau level produced by the field. Inter-and intralayer Coulomb interactions are of comparable strength in this fluid and result in spontaneous interlayer quantum coherence among the electrons in the system. The system may be viewed in several equivalent ways, including as a pseudospin ferromagnet[1] or a superfluid of interlayer excitons [2]. In addition to exhibiting the integer quantized Hall effect (QHE) when parallel currents flow in the two layers, this collective state displays a number of other very unusual transport properties, including Josephson-like interlayer tunneling[3] and a diverging conductivity for counterflowing currents in the two layers as the temperature is reduced toward zero [4,5].As the layer separation is increased, the excitonic phase first weakens and then gives way to a non-QHE, weaklycoupled phase lacking interlayer coherence [6]. When the layer separation is very large this phase consists of two independent 2D electron systems. For equal layer densities, each 2DES is at Landau level filling fraction ν = 1/2 and is well-described as a metallic state of composite fermions [7]. Closer to the critical layer separation the situation is much less clear. Recent experiments have revealed a strong enhancement of interlayer drag [8] in the vicinity of the transition and that the critical layer separation increases when small anti-symmetric layer density imbalances are imposed [9,10]. Although these findings are consistent with recent theoretical work [11,12,13], the precise nature of the transition is unknown. Fundamental questions, such as the order of the transition, how many phases actually exist, and what their electronic structures are near the critical point(s), remain unanswered.A common simplifying assumption has been that the electron spins in the bilayer 2DES at ν T = 1 are frozen out by the Zeeman energy. While this is perhaps reasonable in the gapped excitonic phase at small layer separation, at large separation it conflicts with the several reports of incomplete polarization at ν = 1/2 in single layer 2D systems at low density [14,15,16]. Given the poor current understanding of the phase transition between the excitonic superfluid and the non-QHE phases at ν T = 1, this question of spin configuration looms large. Here we report compelling evide...

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Hole spins have recently emerged as attractive candidates for solid-state qubits for quantum computing. Their state can be manipulated electrically by taking advantage of the strong spinorbit interaction (SOI). Crucially, these systems promise longer spin coherence lifetimes owing to their weak interactions with nuclear spins as compared to electron spin qubits. Here we measure the spin relaxation time T 1 of a single hole in a GaAs gated lateral double quantum dot device. We propose a protocol converting the spin state into long-lived charge configurations by the SOI-assisted spin-flip tunneling between dots. By interrogating the system with a charge detector we extract the magnetic-field dependence of T 1 ∝ B −5 for fields larger than B = 0.5 T, suggesting the phonon-assisted Dresselhaus SOI as the relaxation channel. This coupling limits the measured values of T 1 from~400 ns at B = 1.5 T up tõ 60 μs at B = 0.5 T.