We observe the total filling factor nuT=1 quantum Hall state in a bilayer two-dimensional electron system with virtually no tunneling. We find thermally activated transport in the balanced system with a monotonic increase of the activation energy with decreasing d/lB below 1.65. In the imbalanced system we find activated transport in each of the layers separately, yet the activation energies show a striking asymmetry around the balance point, implying a different excitation spectrum for the separate layers forming the condensed state.

We observe the transition from a spin-unpolarized to a polarized nu=2/3 fractional quantum Hall state at low currents (<5 nA), recently described in terms of quantum Hall ferromagnetism, versus density and parallel magnetic field. At larger currents the time and current dependent huge longitudinal resistance (HLR) is always initiated at the transition. Transport in the HLR regime is linear and the amount of current-induced nuclear polarization in the HLR is comparable to the thermal nuclear polarization at approximately 20 mK and 10 T. A current-induced disorder in the nuclear polarization is speculated to cause the enhanced resistance in the HLR regime.

Magneto-transport and drag measurements on a quasi-Corbino 2D electron bilayer at the systems total filling factor 1 (νT =1) reveal a drag voltage that is equal in magnitude to the drive voltage as soon as the two layers begin to form the expected νT =1 exciton condensate. The identity of both voltages remains present even at elevated temperatures of 0.25 K. The conductance of the drive layer vanishes only in the limit of strong coupling between the two layers and at T→0 K which suggests the presence of an excitonic circular current. When two closely spaced two-dimensional electron systems (electron bilayer) are exposed to a perpendicular magnetic field B so that each layer has a filling factor close to 1/2 and the relative distance between interlayer electrons, parameterized by the ratio d/l B (d: layer separation, l B = /eB = 1/ √ 2πn T : magnetic length with n T as the total density), is sufficiently small, a new quantum Hall (QH) state characterized by the total filling factor 1 (ν T =1) can be observed. Both theoretically and experimentally it is found that this novel state in bilayers occurs at a d/l B ratio of less than ≈ 2 [1, 2]. In the limit of comparably small tunneling, its origin is dominated by Coulomb interactions, where the electrons in the two layers form a strongly correlated many-body state to minimize their exchange energy. In this state and in the low temperature limit, spontaneous interlayer phase coherence develops, driving all electrons in a quantum mechanical superposition of the layer eigenstates sharing the same macroscopic phase φ [3,4]. However, the predicted Kosterlitz-Thouless type of phase-transition [12] has not yet been unequivocally demonstrated in experiment. After a particle-hole transformation that changes the sign of the interactions from repulsive to attractive, this state can be regarded as an excitonic condensate, where each electron is bound to a "vacant state" in the opposite layer. Interlayer drag experiments, where a constant current is passed through one of the layers ("drive layer") and the induced longitudinal and transverse voltage drop in the other layer ("drag layer") is measured [2], have revealed a Hall drive and drag which approaches a quantized value of h/e 2 at ν T = 1 in a temperatureactivated fashion. The quantization of the Hall drag is an indirect indication of a superfluid mode of excitons [5], which can be viewed as either an uniform flow of interlayer excitons or as a counter flow of electrons in the opposite layers.In standard Hall bars the occurrence of the ordinary integer QH effect with the vanishing of the longitudinal resistance and the quantization of the Hall resistance can be explained in terms of one-dimensional (dissipationless) edge channels [6,7]. However, in the case of the ν T = 1 and its associated superfluid transport mode, it cannot be ruled out from the Hall bar data that a dissipationless quasi-particle current at the sample edges is responsible for the observed effects [8].In this paper, we report on interlayer drag measurements...

Direct electron spin resonance (ESR) on a high mobility two dimensional electron gas in a single AlAs quantum well reveals an electronic g-factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 Gauss. The ESR amplitude and its temperature dependence suggest that the signal originates from the effective magnetic field caused by the spin orbit-interaction and a modulation of the electron wavevector caused by the microwave electric field. This contrasts markedly to conventional ESR that detects through the microwave magnetic field. [5,6], without the need for Ohmic contacts to the samples. Moreover, from the dependence of the g-factor anisotropy on Fermi wavevector and from the dependence of the g-factor on angle between microwave field and static magnetic field, recently the (tiny) Bychkov-Rashba spin-orbit interaction of 2D electrons in Si/SiGe samples could be determined [7,8]. In this paper we show that in high mobility 2D samples, this spin-orbit interaction allows to resonantly manipulate the electron spin by means of GHz electric fields.Direct ESR on a two dimensional electron gas (2DEG) has proved difficult because of the typically small number of spins in the 2DEG. So far it has been restricted to Si (either in Si/SiC or in Si/SiGe samples) because of its favourable physical properties. As the sensitivity of ESR is proportional to the inverse of the linewidth squared, narrow linewidths are a prerequisite. In Si linewidths down to 3 µT are observed [8], as little T 1 -broadening occurs. This is because Si has a rather small spin-orbit (SO) interaction. Also it has only one isotope with nuclear spin ( 29 Si) which additionally has only a small natural abundance (4.7 %). This contrasts markedly to the III-V semiconductors where there are many isotopes with nuclear spin ( 69 Ga, 71 Ga, 27 Al, 75 As, 115 In, 31 P etc.) with large natural abundance, many of which have a strong SO coupling. This leads to considerable line broadening and at low temperatures, where ESR usually has the best sensitivity, to large hyperfine fields that vary slowly with time. Consequently direct ESR has never been demonstrated on 2D electrons in III-V semiconductors.Here, we present the first direct ESR on a 2DEG in a III-V semiconductor. We study ESR of high mobility 2D electrons in a single AlAs quantum well. At 9.35 GHz and at 34 GHz g-factors of 1.991 and 1.989 were determined respectively. By rotating the sample in the cavity we demonstrate that our ESR originates from the microwave electric field (E 1 -field) and not from the microwave magnetic field (B 1 -field). For small power (P ) of the E 1 -field, the ESR follows a P 0.5 -law, but for larger powers, the exponent increases to ∼1. The temperature dependence of the ESR is much stronger than the 2D magnetisation expected for such a system [2]. Our observations can be explained by assuming that the spin transitions occur through the effective magnetic field caused by SO interaction and the modulation of the electron wavevector around k F induced by the mi...

We report on the selectivity to spin in a drag measurement. This selectivity to spin causes deep minima in the magneto-drag at odd fillingfactors for matched electron densities at magnetic fields and temperatures at which the bare spin energy is only one tenth of the temperature. For mismatched densities the selectivity causes a novel 1/B-periodic oscillation, such that negative minima in the drag are observed whenever the majority spins at the Fermi energies of the two-dimensional electron gasses (2DEGs) are anti-parallel, and positive maxima whenever the majority spins at the Fermi energies are parallel.