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.

A study of the temperature dependence of luminescence from a two-dimensional electron system confined at an ultrahigh mobility single heterojunction reveals the importance of both many-body effects and of a thermally excited population in higher levels in determining the intensity of luminescence from states near the Fermi level.

Photoluminescence measurements of correlated states of two-dimensional electrons in GaAs/GaAlAs heterojunctions at low temperatures and in high magnetic fields are discussed in terms of many-body effects in the recombination process. The formation of incompressible states is studied by observing radiative transitions from densely and sparsely populated subbands. Optical evidence for the formation of an electron solid in the extreme quantum limit is presented.

Electrons confined at the interface of a GaAs/GaAlAs heterojunction form a 2D quantum electron liquid. Under a strong magnetic field a phase transition to an electron (Wigner) solid takes place in the low filling factor regime of the Fractional Quantum Hall Effect (FQHE). We describe experimental evidence for such electron solid formation obtained both by radiofrequency (RF) study of the low-frequency collective excitations and by conductivity measurements. A finite-threshold electric field for DC conduction reflecting the electron crystallite pinning in the sample random potential is found associated to a small gap in the solid phase low-lying collective excitation branch. The nu = 1/5 FQHE liquid reenters the solid domain at low temperature.

Abstract Electrons confined at the interface of a GaAs/GaAlAs heterojunction form a 2D quantum electron liquid. Under a strong magnetic field a phase transition to an electron (Wigner) solid takes place in the low filling factor regime of the Fractional Quantum Hall Effect (FQHE). We describe experimental evidence for such electron solid formation obtained both by radiofrequency (RF) study of the low-frequency collective excitations and by conductivity measurements. A finite-threshold electric field for DC conduction reflecting the electron crystalline pinning in the sample random potential is found associated to a small gap in the solid phase low-lying collective excitation branch. The v = 1 5 FQHE liquid reenters the solid domain at low temperature.

Photoluminescence measurements of two-dimensional electrons in GaAs/GaAlAs heterojunctions in the integer and fractional quantum Hall regimes are reviewed. Intensity and energy anomalies observed experimentally are discussed in terms of many-body effects in the recombination process. Optical evidence for the formation of an electron solid in the extreme quantum limit is presented.

Our recent optical detection of the integer and fractional quantum Hall effects in GaAs, by intrinsic band-gap photoluminescence at dilution refrigerator temperatures, is reviewed. This work has been extended to the extreme quantum limit where a photoluminescence peak develops close to Landau level filling factor v = 15 which correlates both with the onset of threshold behaviour in current-voltage characteristics of the two-dimensional electron system and a resonant radio-frequency absorption; the latter are quantitatively accounted for by a model of crystalline electronic structure broken up into domains. Preliminary mK transport experiments in intense, pulsed magnetic fields are also described, which establish a basis to access the electron solid phase transition in a hitherto unattainable region of the (B, T) plane.

We report a definitive optical detection, using band-gap photoluminescence, of the integer and fractional quantum Hall effects in GaAs by a comprehensive study of integer states from \ensuremath{\nu}=1 to 10 and the \ensuremath{\nu}=2/3 hierarchy out to the 5/9 daughter state, in an ultrahigh-mobility single heterojunction at 120 mK.

Spin configurations of fractions z<\ensuremath{\nu}<2 are examined by angular and n-dependent activation studies. Energy gaps \ensuremath{\Delta}(\ensuremath{\theta}) and intercepts ${\ensuremath{\sigma}}_{\mathrm{xx}}^{c}$(\ensuremath{\theta}) that probe the quasiparticle charge ${e}^{\mathrm{*}}$ quantify a dramatic difference between (4/3 and (5/3 states consistent with assignments (4/3\ensuremath{\uparrow}\ensuremath{\downarrow}, and (5/3\ensuremath{\uparrow}\ensuremath{\uparrow} (\ensuremath{\uparrow}\ensuremath{\downarrow},\ensuremath{\uparrow}\ensuremath{\uparrow}=zero, maximum polarization). A field-induced phase transition $_{3}^{4}\ensuremath{\rightarrow}_{3}^{4}$ (partial polarization) in which ${e}^{\mathrm{*}}$ changes from e/3 to e/5 is mapped out. The (7/5 state formed from e/3 quasiparticles is destroyed at the (4/3 transition. High-order assignments $_{5}^{7}$(\ensuremath{\uparrow}\ensuremath{\uparrow} or \ensuremath{\downarrow}), $_{5}^{8}$ \ensuremath{\uparrow}\ensuremath{\downarrow}, $_{7}^{10}$ \ensuremath{\uparrow}\ensuremath{\downarrow}, and $_{7}^{11}\mathrm{\ensuremath{\uparrow}}$ are consistent with experiment.