J.W. Campbell

Termination of the quantized Hall effect by the electron solid

Abstract The diagonal and Hall resistance were studied at quantized-Hall-effect-to-insulator transitions in a dilute 2D electron system in Si(100), in the milli-kelvin temperature range. The Hall resistance remains close to its classical value, H/nec, even when the temperature-activated diagonal resistivity ρ xx rises to 4 × 10 6 ohm/square. Experimental data show that the insulating state may develop directly from at least 3 metallic states with quantized Hall resistance at filling factors ν = 1, 2 and 6. Persistence of well resolved Landau levels up to the final insulating stage makes it possible to trace the delocalized states at the QHE to insulator transition. Within experimental uncertainty no evidence for their exit through the Fermi energy has been found. The results are discussed in terms of recent theoretical models for the transition to the insulating state.

Transport properties of a 2D electron solid in Si-MOSFETs

Abstract Recently we reported the observation of magnetic field induced transitions between insulator and integer quantum Hall effect states in very high mobility Si-MOSFETs at temperatures below 0.5 K and dilute electron concentrations n s below 10 11 cm −2 . We present results of activation energy, current-voltage characteristics and threshold field measurements which are attributed to sliding transport in a pinned electron solid (ES) forming near half filled Landau levels at low magnetic fields in Si-MOSFETs.

Collective insulating state at zero magnetic field in a dilute 2D electron system

Abstract We present experimental evidence for the low temperature collective insulator state which sets in upon lowering the electron density at zero magnetic field in high mobility Si-inversion layers. Data on temperature activated and threshold electric field dependent dc conduction are fitted well by a model of transport in a Wigner solid provided by the motion of dislocation pairs. We report weak narrowband noise with a current dependent frequency spectrum on a background of broadband noise.

Electron solid at zero magnetic field in a dilute 2D electron system in Si

Abstract We report on the study of a collective insulating state which develops in a dilute 2D electron system in high-mobility Si-MOSFETs at zero magnetic field upon decreasing electron density. The electron solid exhibits a crossover to a single-particle insulator with increasing disorder. Our data are well explained by a model for a pinned Wigner solid.

Amplitude-dependent effects of the acoustoelectric voltage and acoustic charge transport in GaAsAlGaAs heterostructure in the quantum Hall regime

Abstract The acoustoelectric effect induced by the coupling between a surface acoustic wave and a two-dimensional electron system is investigated on GaAs AlGaAs heterojunctions at 4.2 K in the quantum Hall effect regime. This work was performed to study two-dimensional acoustoelectric non-linearities. We report on amplitude-dependent effects observed in the acoustoelectric voltage in the open geometry. These effects are qualitatively similar to those found on a CdS thick film. We present data suggesting that an acoustic charge-transport behaviour could be present in our experiment. This charge-transport signature provides further evidence for the presence of non-linear effects since the estimated electric field threshold value for non-linear effects reproduced similar non-linear behaviour observed recently in the three-dimensional (3D) piezoelectric semiconductor InSb.

Electric field induced non-ohmic conduction in the 2D insulating phase

Abstract Threshold conduction was studied in Si MOSFET samples with mobility ranging from 5000 to 55 000 cm 2 /V · s, and in the wide temperature range 20 mK-4 K. We found that non-ohmic conduction is qualitatively different in low and high mobility samples near and far from the critical density. In the high mobility samples we observed a novel feature, in that the differential resistance changes sign slightly above the threshold.

Termination of the integer QHE by an electron solid in a dilute 2D electron system at mK-temperatures

Upon decreasing the density, the 2D electron system exhibits a transition from the metallic state, characterized by the Quantum Hall effect, to the collective insulating state (electron solid), interrupted by the Quantum Hall effect at integer filling factors ν=1, 2, and 6. The electron solid shows many features attributed to the pinned Wigner crystal. On the basis of the longitudinal and Hall resistance measurements through these reentrant transitions, we discuss the pathway for the disappearance of delocalized states.

Nonlinear transport and generated narrowband noise in the 2D electron solid in Si

Abstract We report on the observation of the narrowband noise generated in the collective insulating phase in a dilute 2D electron system in Si, which is directly indicative of the short-range ordered state. Our data on the nonlinear threshold conduction and its temperature activated behaviour are fitted well by the picture of depinning of an electron crystal via generation and motion of dislocation pairs.

Activated transport in the 2D electron solid in Si

Abstract We describe the characteristics of the temperature activated dc-transport observed in the insulating phase which occurs at dilute concentrations and low temperatures in high mobility 2D electron systems in Si. Our data show the existence of two different temperature regimes, one of which can be fitted well using the phase slippage model commonly applied to charge density waves.