A magnetic field is used to tune through a new superconducting-insulating transition of amorphouscomposite indium oxide films at various stages of disorder. The results are in accord with scaling theory which identifies a universal sheet resistance separating a superconducting phase of localized vortices and Bose-condensed electron pairs from an insulating phase of Bose-condensed vortices and localized electron pairs. A unity dynamical exponent is confirmed and scaling behavior of the resistance over a wide range of temperatures and magnetic fields is found. PACS numbers: 74.65.+n, 74.70.Mq, 74.75.+t Superconductivity in two dimensions (2D) provides a unique arena in which a wide variety of novel and fundamental physical phenomena occur. There is now, for example, almost unanimous agreement that the seminal ideas of Berezinskii 1 and Kosterlitz and Thouless 2 are applicable both to the melting of the Abrikosov vortex lattice and to the zero-magnetic-field vortex-antivortex phase transition. In addition, the systematic introduction and control of disorder provides an opportunity to explore the validity of predictions 3,4 and the suggestions of experiment 5 that a superconducting-insulating transition should have universal properties, including a universal metallic sheet resistance near A/4e 2=ss 6450 ft/D at the transition.In experimental systems, the presence of disorder can introduce complications related to specific materials properties such as microstructure, contamination, chemical composition, and defect concentration. If the microscopic disorder is sufficiently homogeneous in the sense that the length scale characterizing the uniformity of disorder is longer than the length scale used by the theory to model the superconducting behavior, then there is an opportunity for a realistic comparison of experimental behavior and theoretical prediction. In this Letter, we present such a comparison using scaling theory of the magnetic-field-tuned superconductor to insulating transition, discussed in a companion paper, 6 to model the temperature-and field-dependent behavior of the resistance of amorphous-composite indium oxide (aInO*) films fabricated at different stages of disorder.In this theory, disorder is measured by a variable A which is assumed to have continuous behavior through the transition. The superconducting coherence length £, introduced in the theory, diverges as | A c -A | ~~w when disorder approaches criticality, A-• A c . The exponent v, not measured in our experiments, is predicted to have a lower bound of unity in 2D. The identification of the Kosterlitz-Thouless transition temperature T c with a characteristic frequency (energy) which scales as £~z together with the expectation that the critical field B c will vanish as £ ~2 implies the relation B c~~T } lz , where z is the dynamical exponent. Our experimental determination of T c and B c for films with varying amounts of disorder confirms the predicted value z = l. Furthermore, we verify the scaling dependence of the sheet resistance R on both...

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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.

Millikelvin measurements of the conductivity as a function of donor density and uniaxial stress in bulk samples of phosphorus-doped silicon establish that the transition from metal to insulator is continuous, but sharper than predicted by scaling theories of localization. The divergence of the dielectric susceptibility as the transition is approached from below also points out problems in current scaling theories. The temperature dependence of the conductivity and the magnetoresistance in the metal indicate the importance of Coulomb interactions in describing the behavior of disordered systems.