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We have performed microwave photoresistance measurements in high mobility GaAs/AlGaAs quantum wells and investigated the value of the effective mass. Surprisingly, the effective mass, obtained from the period of microwave-induced resistance oscillations, is found to be about 12% lower than the band mass in GaAs, m(b)*. This finding provides strong evidence for electron-electron interactions which can be probed by microwave photoresistance in very high Landau levels. In contrast, the measured magnetoplasmon dispersion revealed an effective mass which is close to m(b)*, in accord with previous studies. DOI: 10.1103/PhysRevB.87.161307

We fabricated a He-3 immersion cell for transport measurements of semiconductor nanostructures at ultra low temperatures and in strong magnetic fields. We have a new scheme of field-independent thermometry based on quartz tuning fork Helium-3 viscometry which monitors the local temperature of the sample's environment in real time. The operation and measurement circuitry of the quartz viscometer is described in detail. We provide evidence that the temperature of two-dimensional electron gas confined to a GaAs quantum well follows the temperature of the quartz viscometer down to 4 mK. (C) 2011 American Institute of Physics. [doi:10.1063/1.3586766]

We report the unambiguous observation of a fractional quantum Hall state in the Landau level of a two-dimensional-hole sample at the filling factor v = 8/3. We identified this state by a quantized Hall resistance and an activated temperature dependence of the longitudinal resistance and found an energy gap of 40 mK. Notably, the particle-hole conjugate state at filling factor v = 7/3 in our sample did not develop down to 6.9 mK. This observation is contrary to that in electron samples, in which the 7/3 state is typically more stable than the 8/3 state. We present evidence that the asymmetry between the 7/3 and 8/3 states in our hole sample is due to Landau-level mixing.

We report the observation of a new fractional quantum Hall state in the second Landau level of a two-dimensional electron gas at the Landau level filling factor nu = 2 + 6/13. We find that the model of noninteracting composite fermions can explain the magnitude of gaps of the prominent 2 + 1/3 and 2 + 2/3 states. The same model fails, however, to account for the gaps of the 2 + 2/5 and the newly observed 2 + 6/13 states suggesting that these two states are of exotic origin.

We report the observation of a resonance in the microwave spectra of the real diagonal conductivities of a two-dimensional electron system within a range of similar to +/- 0.015 from filling factor v = 1/3. The resonance is remarkably similar to resonances previously observed near integer v, and is interpreted as the collective pinning mode of a disorder-pinned Wigner solid phase of e/3-charged carriers.

We investigate effects of lateral confinement on spin splitting of energy levels in 2D hole gases grown on [311] GaAs. We found that lateral confinement enhances anisotropy of spin splitting relative to the 2D gas for both confining directions. Unexpectedly, the effective g factor does not depend on the 1D energy level number N for B parallel to[0 (1) over bar1] while it has strong N dependence for B parallel to[(2) over bar 33]. Apart from quantitative difference in the spin splitting of energy levels for the two orthogonal confinement directions, we also report qualitative differences in the appearance of spin-split plateaus, with nonquantized plateaus observed only for the confinement in the [0 (1) over bar1] direction. In our samples we can clearly associate the difference with anisotropy of spin-orbit interactions.

We use spatial spin separation by a magnetic focusing technique to probe the polarization of quantum point contacts. The point contacts are fabricated from p-type GaAs/AlGaAs heterostructures. A finite polarization is measured in the low density regime, when the conductance of a point contact is tuned to < 2e(2)/h. Polarization is stronger in samples with a well defined '0.7 structure'.

Recent experiments have shown that two-dimensional electron systems with an externally applied magnetic field are an extremely rich ground for many-body physics. In particular, when only two of the Landau levels (LL) are filled an intricate magnetoresistance is found. This result stems from an interesting competition of electronic phases such as fractional quantum Hall liquids, reentrant integer Hall states, and unique quantized states at even denominator LL filling factors. We present a brief review of the transport properties of these electronic phases and discuss in detail the effects of an added in-plane magnetic field. (c) 2006 Elsevier B.V. All rights reserved.

We use spatial spin separation by a magnetic focusing technique to probe the polarization of quantum point contacts. The point contacts are fabricated from p-type GaAs/AlGaAs heterostructures. A finite polarization is measured in the low-density regime, when the conductance of a point contact is tuned to < 2e(2)/h. Polarization is stronger in samples with a well-defined "0.7 structure."

We review three novel techniques whereby the highly uniform two-dimensional films produced by molecular beam epitaxy (MBE) can be further patterned into nanowires or nanostructures having quantum confinement in all three dimensions. These techniques have the potential of greatly extending the power and utility of the MBE method. (c) 2005 Lucent Technologies Inc.