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Surface-acoustic-wave propagation on high-quality AlGaAs/GaAs heterostructures containing a twodimensional electron system has been examined in the fractional quantum Hall effect (FQHE) regime. The response of the electron system to the sound wave is found to be similar in the FQHE states to that previously studied in the integral quantum Hall states. However, a striking disparity is observed at Landau-level filling v** j, where sound propagation is distinctly different from that in the neighboring filling-factor range. We propose that phase separation of the 2DEG may be responsible for the feature at v-j.

Temperature-and magnetic-field-dependent measurements of the longitudinal Rxx and transverse Rxy resistance of amorphous indium oxide thin-film superconductors reveal the presence of distinct insulating phases for sufficiently high disorder and/or magnetic field. For field-swept transitions at fixed disorder and low temperatures there is a critical field where Rxx diverges and the superconductor is transformed into a Bose-glass insulator with localized Cooper pairs. At higher fields there is a second critical field where Rxy appears to diverge, the pairs unbind, and localized single electrons characterizing a Fermiglass insulator dominate. PACS numbers: 74.65.+n, 73.60.Hy, 74.70.Mq, 74.75.+t Numerous studies, both theoretical and experimental, have been concerned with the competition between disorder and superconductivity [1]. Disorder induces localized eigenstates and enhanced Coulomb repulsions which jointly act to reduce the transition temperature and ultimately quench all remnants of superconductivity. This competition between disorder and superconductivity is particularly interesting in two dimensions (2D) where the delicate and marginally attractive pairing interaction is derived from electronic states which are localized for arbitrary weak disorder. Experimentally, systematics of the destruction of superconductivity with increasing disorder in thin films is studied by varying the thickness [2-7], composition [3,8], or resistivity for fixed thickness [9]. Convergence between experimental results and theoretical description has been somewhat hampered by the wide variation in microstructural and compositional properties of these thin-film systems. Recent scaling theories [10-12] of the superconducting-insulating (S-I) transition have avoided these details by taking a broad view and formulating a description which is conveniently insensitive to microscopic material properties. In this description the paired electrons and vortices are treated as quantum-mechanical objects which obey Bose statistics and hence can be treated within a purely "bosonic" formulation. There is experimental evidence for the predicted S-I transitions, driven either by increasing disorder in zero magnetic field [6,7,9] or by increasing field for fixed disorder [13,14]. In these experiments the putative S-I transition takes place at r=0 sheet resistances which vary within a factor of 2 of the predicted universal value, /z/4e^=6450 n/D. Measurements of the field-tuned S-I transition of InO;c films [13], however, confirm detailed predictions [11] of the theory such as the unity value of the dynamical exponent and the scaling collapse of the temperature-and field-dependent resistance data onto a single curve.An implicit assumption of the scaling treatment is that Cooper pairs exist and are finite in extent at the S-I transition. Thus the magnitude ^o of the order parameter is finite on both sides of the S-I transition and the true superconducting phase transition, where long-range order is established and the dc resistance is zero, is dom...

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We have measured the structure of the field cooled flux line lattice (FLL) in single crystal Nb using small angle neutron scattering. Augmented by transport and thermodynamic data, a scenario for the dramatic disordering of the FLL near the peak effect emerges. A precursor to the peak effect is an hexatic FLL as positional order is lost. As the critical current rises, the orientational and longitudinal correlation lengths also fall, leading to an amorphous array of lines at the critical current maximum. [S0031-9007(97)05112-0]