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We have investigated the behavior of the superconducting energy gap A in ultrathin films of quench condensed Bi near the superconductor-insulator (SI) transition. From electron tunneling measurements on these films, we conclude that A becomes very small and approaches zero at the SI transition. We studied high-sheet-resistance films with TVo' s as low as 0.19 K. This is a factor of 40 lower than the low-sheet-resistance film Tco of 6.4 K. PACS numbers: 74.65.+n, 74.75.+t, 74.50.+r As the amount of disorder is increased in a metallic system the conduction-electron states begin to localize, and the strength of the effective electron-electron interactions increases. These two effects hinder the formation of the normally superconducting state in a disordered material. The localization of the electrons opposes the formation of a coherent state over distances longer than the localization length. The enhanced repulsive electronelectron interactions will reduce the net attractive interaction that is required for Cooper pairing. In twodimensional systems the effects of increasing disorder on the superconducting state are dramatic and eventually drive a transition from a superconducting to an insulating state [1][2][3][4][5][6].To qualitatively describe this transition it is helpful to consider the superconducting order parameter ^ oc Ao^^e'^ where the amplitude of the order parameter, Ao, is the zero-temperature energy gap and 0 is its phase. If a film is composed of islands that are large enough to have independent superconducting properties then the superconducting state can be weakened by reducing the (Josephson) coupling between the islands. This is accomplished by reducing the tunneling probability between the islands which results in an increase in the normal-state sheet resistance, Ru, of the film. Fluctuations in the phase of the order parameter ensue, which when strong enough destroy the long-range phase coherence across the film [2,6,7]. The situation has been shown to be diflferent for the case where the morphology of the film is more homogeneous, i.e., for disorder on shorter length scales. In systems that are not too close to the superconducting to insulating transition, increases in Ru have been shown to lead to well-defined decreases in the amplitude of the order parameter [7,8] and the mean-field transition temperature, Tco [3][4][5][6][7][8][9][10][11], in direct contrast with the islanded film case. One might expect that very near the superconducting to insulating transition, fluctuations in the amplitude of the order parameter would play a dominant role in driving the transition. Recently, however, it has been argued that even in these systems, phase fluctuations dominate the physics of the superconducting to insulating transition [6,12]. In this paper we present tunneling and transport results on uniformly disordered Bi films that are very close to the superconducting to insulating transition. We find that the energy gap in these films is reduced significantly from its bulk value, showing that the ...

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%'e have performed systematic tunneling measurements on two-dimensional quench-condensed films of Sn and Pb to investigate the destruction of superconductivity by localization effects. With increasing sheet resistance of the films, the energy gaps and T, 's decrease only slightly, but the gap edges broaden until the width becomes comparable to the gap. Associated with this broadening are low-temperature finiteresistance tails in the resistive transitions.Near the metal-insulator transition, both localization and interaction effects have a strong influence on the nature of conduction. ' These effects become even more important as the dimensionality of the system is reduced. Extensive transport, magnetotransport, and tunneling measurements in a variety of three-dimensional (3D), 2D, and 1D samples have resulted in a comprehensive description of these phenomena in the normal state. Recently, there has been a great deal of interest in the role these effects play in the destruction of the superconducting state of a disordered metal. Tunneling studies in 3D granular aluminum samples close to the metal-insulator transition' showed that the superconducting energy-gap edge broadens, possibly due to lifetime effects, as the resistivity of the sample increases.%hen the broadening is comparable to the gap, superconductivity disappears. In this paper, we report our results on a corresponding. 2D experiment using very thin quenchcondensed films of tin and lead.Two-dimensional systems are particularly appealing for studying localization and interaction effects because it is now understood that all the electronic states are localized and that these effects are some~hat independent of the material studied, ' depending only on the sheet resistance (Ro) of the sample. However, this assumption does not seem to hold for studies of the competition between these effects and superconductivity.Differences between our results on quench-condensed films and those of other experimental investigations on several diverse systems4 have led us to recognize the distinction between reducing the pair amplitude and reducing the phase coherence in the destruction of superconductivity. This experiment addresses the latter. Our earlier tunneling measurements5 on quenchcondensed tin films concentrated on the modification of the normal-metal density of states due to Coulomb interactions.In this experiment, we have focused on the changes in the superconducting excitation spectrum. The films were fabricated as previously described: A film of 99.99% Sn or Pb was evaporated onto fire-polished glass substrates held between 1.5 and 8 K. This film straddled four gold contacts and the aluminum (Al) counterelectrode which was previously deposited at room temperature and oxidized in air.Since we were interested in studying films of very high Rp, it was necessary to work with relatively high-resistance junctions. This was accomplished by oxidizing the Al in air for 20 min to obtain resistances &10000 0 for a 0.25 mm' junction area. The films were deposited in a low...

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We have measured the superconducting and normal-state properties of ultrathin quenchcondensed Pb and Sn films by electron tunneling and transport. The normal-state measurements illustrate a substantial reduction in the electronic density of states about the Fermi surface. Good agreement with theory is obtained for this reduction. The collapse of T, can be accounted for by the reduced density of states but some problems stand in the way of a full description of the superconducting behavior.For a highly disordered metal, it is known that superconductivity disappears in the vicinity of the metalinsulator transition. For a pair wave function /= foe either amplitude reduction (go) or phase breaking (P) will result in loss of superconductivity, and these two modes manifest themselves differently. In the case of amplitude reduction, T, and the energy gap 60 both remain well defined and decrease. ' In the case of phase breaking, T, (onset of superconductivity) reinains unchanged, ' but the current carrying capacity disappears, the transition width increases until the material has no region of zero resistance, and the energy gap 60 broadens substantially.These two limiting cases can be accessed in real materials by controlling the material morphology (either homogeneous or inhomogeneous on a length scale less than the coherence length). In two dimensions both limits have been identified, and it has been determined that in the homogeneous case, a simple Eliashberg description of superconductivity is inadequate. ' Why Ao and T, decrease continuously in the homogeneous case in ultrathin films remains an issue, and it is this problem we address. Previous work has suggested Couloumb interactions as a possible cause of these effects.By tunneling and transport measurements we directly measure the electronic density of states for ultrathin films and show that it is profoundly altered, presumably by Coulomb interactions.This change in the density of states is compared with recent theories and found to be in good agreement. Using this density of states, we And that the superconducting transition temperature should indeed be reduced an amount which is consistent with that observed experimentally. Some problems remain, however, and these will be discussed.The measurements described here were performed on films evaporated at low temperature (4 -7 K) in a cryogenic evaporator that has been described previously.The tunnel junctions used were of the configuration Al/oxide/disordered superconductor.Prior to cooling down the apparatus, gold contact pads and a narrow Al strip, which was allowed to oxidize for a short time in air, were evaporated onto a fire-polished glass substrate. At low temperatures, a 1 -2 monolayer Ge Alm was eva-porated onto the substrate followed by a series of Sn or Pb Alm evaporations. The Sn and Pb films became electrically continuous at a thickness to, -6 and -2 atomic layers, respectively. The conductivity grew nearly linearly with film thickness beyond to. The deviations from linearity in the regime where thes...

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