The superconductor-insulator transition of ultrathin films of bismuth, grown on liquid-helium-cooled substrates, has been studied. The transition was tuned by changing both film thickness and perpendicular magnetic field. Assuming that the transition is controlled by a Tϭ0 critical point, a finite-size scaling analysis was carried out to determine the correlation length exponent and the dynamical critical exponent z. The phase diagram and the critical resistance have been studied as a function of film thickness and magnetic field. The results are discussed in terms of bosonic models of the superconductor-insulator transition, as well as the percolation models which predict finite dissipation at Tϭ0.

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A field effect conductance modulation experiment has been performed on a series of nominally homogeneous ultrathin films of metals. The thicknesses of the films were varied over a range such that their properties traversed the insulator-to-superconductor transition. At low gate voltages V G the conductance G͑V G ͒ increased with either polarity for films on the insulating side of the transition and decreased at temperatures in the transition region for films which were just thick enough to be superconducting. A qualitative interpretation of these results suggests the consideration of Cooper pairing even for insulating films. [S0031-9007(97)02374-0] PACS numbers: 74.40.+ k, 73.40.Rw, The superconductor-insulator (S-I) transition in ultrathin films of metals, either as a consequence of disorder [1] or applied magnetic field [2], has been described by the boson Hubbard model and its variants which highlight the role of order parameter phase fluctuations [3]. In this approach, the superconducting state is considered to be a Cooper pair condensate with localized vortices, and the insulating state is a vortex condensate with localized Cooper pairs. The issue of the relevance of this theory to experiment has been challenged by tunneling investigations which have been interpreted as evidence that S-I transitions are dominated by order parameter amplitude fluctuations. Because the boson Hubbard model implies that there are Cooper pairs even on the insulating side of the transition, it is of interest to study the insulating state and the S-I transition in other ways, going beyond conductance and tunneling measurements, to determine whether there is evidence of behavior different from the usual picture of a strongly localized disordered system. This has been done through investigations of the thickness and temperature dependence of the field effect modulation of the conductance [5] of incrementally quench-deposited films of metal above and below the S-I transition. In this Letter we describe our findings which suggest the existence of a symmetry between the insulating and superconducting states implying that insulating films may be other than Coulomb glasses [6] of interacting localized electrons.Investigations were carried out on ultrathin films of Bi or Pb ranging in thickness from 3 to 20 Å, formed on a 10 Å thick predeposited layer of a-Ge, with all films being grown in situ under UHV conditions ͑ϳ10 210 to 10 29 Torr͒ onto single-crystal SrTiO 3 (100) substrates which were 0.75 mm thick. Substrate temperatures during all depositions were held at 9 K, and UHV conditions were sustained over an extended period so that sequential depositions to increase the film thickness could be carried out without contamination. It has been found that films prepared in such a manner become continuous at an average metal thickness on the order of one monolayer and, because of this, are generally considered to be homogeneous [7]. The quartz crystal monitor used to determine nominal film thickness was calibrated using Rutherford backscatte...

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The magnetoresistance of ultrathin insulating films of Bi has been studied with magnetic fields applied parallel and perpendicular to the plane of the sample. Deep in the strongly localized regime, the magnetoresistance is negative and independent of field orientation. As film thicknesses increase, the magnetoresistance becomes positive, and a difference between values measured in perpendicular and parallel fields appears, which is a linear function of the magnetic field and is positive. This is not consistent with the quantum interference picture. We suggest that it is due to vortices present on the insulating side of the superconductor-insulator transition.

The temperature dependence of the resistance R(T ) of ultrathin quench-condensed films of Ag, Bi, Pb and Pd has been investigated. In the most resistive films films, R(T ) = R0 exp (T0/T ) x , where x = 0.75 ± 0.05. Surprisingly, the exponent x was found to be constant for a wide range of R0 and T0 in all four materials, possibly implying a consistent underlying conduction mechanism. The results are discussed in terms of several different models of hopping conduction.

Inelastic proton scattering has been used to measure the der/dO and A~a ngular distributions for several low-lying collective states in ' ' Pt using 647 MeV polarized protons with special emphasis on the excitation of the 4+ states below 2 MeV. The data have been analyzed in the framework of the coupled-channels scheme using the program EcIS. Large E4 transition strengths to the 4+ states in both nuclei are found in sharp contrast with the predictions of the interacting boson model with only s (L =0) and d (L =2) bosons. It is found that although the E2 properties of these nuclei are consistent with the interacting boson model (sd) predictions, higher degrees of freedom, e.g. , g (L =4) bosons, need to be invoked in order to get a reasonable agreement with the measured E4 properties.

Exchange anisotropy in NiFe/NiMn and NiFe/IrMn exchange coupled films was studied as a function of temperature using vibrating sample magnetometry. The exchange field was measured using three different methods: (1) as a shift of the hysteresis loop measured in an external field applied parallel to the exchange field direction; (2) calculated from the initial susceptibility in the field applied perpendicular to the exchange field; and (3) calculated from the shift of minor reversible hysteresis loops measured in external fields applied in a few different directions close to the perpendicular to the exchange field. The values of the exchange field in NiFe/NiMn samples measured using methods 2 and 3 were similar and approximately twice as high as the values measured using method 1. For the NiFe/IrMn samples methods 2 and 3 gave exchange field values slightly exceeding the values obtained using method 1. The results are explained using a model in which it is assumed that the interfacial interactions between antiferromagnetic and ferromagnetic layers induce unidirectional and uniaxial anisotropy in the ferromagnetic layer. The temperature dependence of induced interfacial uniaxial anisotropy was calculated from the experimental data. For the NiFe/NiMn samples, the temperature dependence of the induced uniaxial anisotropy was significantly different from that of the unidirectional anisotropy.