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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Reentrant first order and second order transitions are observed in thin TiN films under the paramagnetic limitation of superconductivity, where the temperature gradients of critical fields are positive. Then, the superconducting condensate has to have higher entropy than the normal state immediately below the critical fields. The observations of reentrant transition in thin TiN films above the tricritical point reveal that the renormalized Fermi liquid effect plays an important role for the superconducting transition. The antisymmetric Fermi liquid parameters obtained from the temperature dependence of critical field have positive definite values of 0.23. Spin orbit scattering parameters almost inversely increase with decreasing film thickness. Orbital depairing parameters are almost linear to the square of film thickness.We have investigated the reentrant transition and the excitation spectra of superconductivity under the spin paramagnetic limitation of critical fields in thin films. 1-6) It is recognized that the spin paramagnetic energy limits the superconducting critical field H c and gives rise to the first order transition at absolute zero temperature T = 0. 7, 8) The paramagnetically limited critical field H p is given by μ B H p = 0.707 Δ 0 , which is called Chandrasekhar-Clogston limit, where μ B is the Bohr magneton and Δ 0 the order parameter at T = 0 in the absence of external field. Sarma has shown that the tricritical point exists at a finite temperature that is higher than the supercooling reentrant temperature in a uniform exchange field. 9) Then, an energy gap opens at the first order transition point in a clean superconductor.On the other hand, Maki discussed the gapless state near the second order transition of dirty superconductors under the existence of external magnetic field. 10) For thin films under parallel magnetic fields, it has been implicitly shown that the total density of states at the Fermi energy is enhanced above that of the normal states due to the spin paramagnetic splitting of the gapless spectra for up and down spins. For this gapless state, Fulde's group examined the stability of superconducting state on the basis of Ginzburg-Landau theory and predicted the existence of a reentrant temperature where critical fields of the second order transition became locally maximal under the spin paramagnetic limitation. 11) This prediction has been confirmed in thin Al 1-4) and TiN 5, 6) films under exactly parallel alignment of external magnetic through the measurements on the temperature dependence of resistance and the bias dependence of tunnel conductance. The former clearly showed the reentrant second order transition, which indicated the enhancement of entropy in the superconducting state compared fields 1462 with that of the normal state. The latter revealed that the density of states at the Fermi energy was enhanced above that of the normal state, which confirmed the existence of the high entropy superconducting state. However, a comparison with the theory revealed an important ...

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Calcination of an ytterbium-O-phenylene hybrid copolymer under an argon atmosphere gave material composed of nanosized Yb2O3 particles and carbon clusters. ESR spectral examinations showed that material had a photo-responsive oxidationreduction function though an electron transfer from Yb2O3 particles to carbon clusters .