The existence of the magnetic-field-induced liquid-to-solid phase transition in an extreme quantumlimit 2D electron plasma is established for electrons at a high-quality GaAs/GaAlAs heterojunction by detection of a gapless magnetophonon excitation branch with radio-frequency spectroscopy. The phase diagram, determined for Wigner-Seitz to Bohr radius ratio 1.6 <») limit. PACS numbers: 73.40.Kp, 64.70.-p, 73.20.MfA zero-temperature electron plasma confined to a plane shows unusually varied physical properties as a function of density ris and perpendicular magnetic field H, In zero field and high density, the zero-point energy arising from the Pauli exclusion principle dominates, resulting in a liquid state with Fermi energy Ef^^h^/ma^, which corresponds to the energy of confinement in the Wigner-Seitz radius, a, defined by Ka^^X/ris. As the density is reduced the Coulomb interaction Vc^e^lea in the medium of dielectric constant e gains in importance, ultimately ordering the electrons into a crystal. Wigner * predicted this quantum phase transition long ago, but it has never been seen for want of a suitable physical system.In a magnetic field, however, each single-particle state is already confined by the Lorentz force to an area --(po/H'^lKli occupied by a flux quantum ^o^hc/e Un'^ihc/eH)^^^ is the zero-point cyclotron radius] and takes on discrete energy values (n+ j)hcoc where h(Oc -h^/ml^ -heH/mc is the cyclotron energy. Each of these Landau cyclotron oscillators has an average share of v=2(////a)^ electrons. Increasing the field decreases this filling factor v, successive Landau levels empty, and, in the presence of moderate disorder, one observes the integrally quantized Hall resistance.^ When V < 1 there is spatial freedom for all the electrons for the same zero-point energy. This greatly favors Coulomb-induced correlation. The many-body ground state is a correlated liquid giving rise to the fractional quantum Hall effect^ until, below a critical filling factor estimated between y and j,^ a new ground state with long-range crystallike correlation is expected. To date, this transition, too, has eluded unambiguous observation.The present experiment gives positive evidence of this magnetically induced Wigner solid. Advances in the technology of modulation-dopedGaAs/GaAlAs heterojunctions have made available very high-quality 2D quantum electron systems. Some time ago,^ it was already suggested that the abnormal behavior observed in the dc transport for v < j could be a result of a magnetically induced Wigner solid. But to be affirmative required a much more specific type of experiment. Our experiment tests for a defining property of a solid-its rigidity to shear. An unbounded charged 2D liquid has only compressional (plasmon) excitations in zero field whose frequency-wave-vector relation is (ol^lKe^risqlem. A perpendicular magnetic field i...

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The 2D quantum system of electrons at a GaAs/GaAlAs heterojunction in high magnetic field at low temperature is shown to exhibit conduction typical of pinned charge-density waves. Crossover from Ohmic conduction occurs on the same boundary at which radio-frequency resonances signal the onset of transverse elasticity. A further small non-Ohmic region is isolated from the main area by a v-j quantum-Hall-effect phase. The relationship found between the threshold conduction field and the resonance frequency is well accounted for by a model of a pinned electron crystal.