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The low temperature, high magnetic field phase of the two-dimensional electron system at the GaAs/GaAlAs heterojunction is distinguished by the appearance of both non-linear electrical conduction and a low frequency magneto-phonon branch with a small gap. The relationship between the gap and the threshold electric field for conduction is investigated and shown to be well accounted for by a simple model of a magnetically induced Wigner solid in a random field representing host disorder.

The 2D electron gas of a GaAs/GaAlAs heterojunction is confined to a small island by means of Schottky gates. Two gates provide adjustable tunnel barriers and a central gate controls the electron number of the island. Conductance oscillations, periodic with the electron number, are observed in the Coulomb blockade regime with negligible energy level spacing. At low temperature, the blockade of the conductance by the island is shown to be limited by the quantum process of co-tunneling.

Experimental studies of the magnetically induced Wigner solidification for electrons at high quality GaAs/Ga(Al)As heterojunctions in the extreme quantum limit and of the classical electron solidification for electrons on the helium surface are described. The use of a radiofrequency broadband spectroscopy technique to observe the low lying electronic collective excitations has led to the observation of the phase diagram of the quantum phase, the shear modulus and the specific heat of the classical phase.

We report the observation of low frequency collective excitations of the two-dimensional electron system in a GaAs/GaAlAs heterojunction in a magnetic field. The excitations are present for temperatures less than 2 K and fields corresponding to filling factors between ν = 1 2 and ν = 7 . Their dispersion relation is linear in the wave vector k, and shows no evidence of a gap. These modes vanish between the Hall plateaus and elsewhere their propagation velocity follows a H−1 1n(αH) field dependence. The features of these modes are consistent with a propagating perimeter wave in a quantum 2D Coulomb system.

Absolute specific heat measurements have been made on a 2D crystal of electrons condensed on the liquid helium surface. At melting no latent heat is detectable, suggesting a continuous transition.