Superconductivity In Ultrathin Films Research Articles

The role of ion-beam cleaning in the growth of strained-layer epitaxial thin transition metal films

Low-energy ion-beam cleaning of the substrates prior to a deposition greatly enhances the quality of ultrathin (< 100 Å) refractory superconducting (Nb, V) films. Using this technique Nb films as thin as 7 Å have been grown, from which good tunnel junctions have been fabricated. Both the native films and the tunnel junctions are sturdy and can be thermally recycled without any degradation. In-situ surface study along with transmission electron microscopy (TEM) results suggest the removal of the carbon atoms from the surface of the substrate without an apparent surface damage as the causes of the improvement. The TEM results indicate that the Nb films grow perfectly lattice matched to the sapphire substrate when the substrate is ion-beam cleaned. This strained-layer epitaxy is observed up to 40 Å, the maximum thickness investigated through TEM.

Zero-Point Quantum Fluctuations in Ultra-Thin Superconducting Films

Investigations of the onset of superconductivity in ultra-thin films of Ga and Pb, deposited onto substrates held at low temperatures have been carried out. The measurements have revealed the existence of a threshold for superconductivity in which the normal-state sheet resistance is the only relevant parameter. It is found that only when the normal-state sheet resistance falls below h/4e2 does a film exhibit zero resistance at low temperatures. This value for the threshold is in agreement with recent theoretical models predicting a dissipation driven phase transition. At values greater than this threshold, but nevertheless close to it, a systematic and rapid variation of the low-temperature resistance with normal-state resistance is observed. We interpret this behavior as due to strong zero-point quantum fluctuations and speculate that it can be modelled as a 2-D XY transition in which dissipation rather than temperature is the control -parameter.

Competition between superconductivity and localization in two-dimensional ultrathin a-MoGe films

The competition between superconductivity and localization in two dimensions has been systematically studied in ultrathin homogeneous superconducting films of amorphous MoGe. These films were model 2D systems, with essetially constant equivalent bulk properties down to film thicknesses of ∼10 Å. A strong and systematic reduction in the superconducting transition temperature T c with increasing 2D disorder (i.e., sheet resistance R □) has been observed. Experimental evidence argues aginst “proximity effects” as the primary cause of this behavior.Furthermore,results of superconductive tunneling indicate that this reduction does not arise from simple pairbreaking effects alone. Superconductivity appears particularly sensitive to disorder in 2D, far more so than the normal state properties. For example, by sheet resistances of R p[≅2000Ω , T c has been suppressed by 97% from the bulk value, whereas the normal state resistivity has only increased 40%. The results are found to be consistent with recent formulations of the theory, which not only take into account the effects of localization and the related enhancement in the Coulomb interaction, but also the dynamical nature of the screening.

Localization and interaction effects in ultrathin amorphous superconducting films

Ultrathin superconducting films of amorphous Mo-Ge were made that remain homogeneous down to \ensuremath{\sim}20 \AA{}. The expected localization behavior was observed in their resistivity-versus-temperature and sheet resistance (${R}_{\ensuremath{\square}}$) data. A systematic reduction of the transition temperature with increasing ${R}_{\ensuremath{\square}}$ was observed for several compositions, consistent with recent theories of localization and Coulomb interaction effects on superconductivity. Critical-field data provide further tests of these theories.

Bakeable, ultrahigh-vacuum compatible, retractable contacting mechanism for i n s i t u thin film resistance and superconductivity measurements

A bakeable, ultrahigh-vacuum compatible, retractable four-point resistance probe has been constructed to make in situ measurements of superconductivity in ultrathin films. A bellows actuation mechanism allows the very compact contacting mechanism to be withdrawn from the substrate area for cleaning and characterization of the substrate surface, then returned for film resistance measurements. The device has been used to make contact to Pb films less than 10 Å thick deposited on high-resistivity Si substrates, which require a cleaning step that would be destructive to permanent or predeposited contacts.

A simple two-layer model for ultrathin superconducting films

Abstract A simple two-layer model for ultrathin superconducting films is formulated. The main part of the film, having bulk properties, is in metallic contact with a layer with depressed superconductivity representing the electron depleted surfaces. The model accounts satisfactorily for the experimentally observed monotonie depression of T c with decreasing thickness of ultrathin amorphous films. The model can also explain an enhanced pairbreaking mainfested in the fluctuation conductivity as well as quasi-particle tunneling effects.

Experimental search for excitonic superconductivity

A search for excitonic superconductivity in a thin-film structure of the type proposed by Ginsberg and Allender, Bray, and Bardeen was undertaken. Ultrathin layers of Pb, In, and Tl were deposited on PbTe, and layers of Pb and In were deposited on Te. Substrate preparation and film deposition were accomplished in an ultrahigh-vacuum system with integral cryostat, and were monitored by low-energy electron diffraction and Auger electron spectroscopy. In situ measurement of crystalline, epitaxial Pb films as thin as 10-15 \AA{} deposited on PbTe showed a strong film-substrate interaction; however, no excitonic enhancement of ${T}_{c}$ was observed. A simple proximity-effect model is presented to explain the observed decrease in ${T}_{c}$ for the thinner films, and the influence of the substrate on the superconductivity of ultrathin films is discussed. Several aspects of these experiments involving the transition temperatures of In and Tl films are unexplained.

Superconductivity in ultrathin films

It is shown that the pair breaking parameter of the Maki-Thompson contributions to the fluctuation conductivity above the superconducting transition temperature has the same thickness dependence as the transition temperature depression in very thin amorphous Be-Al films. Both can be ascribed to an extremely thin surface sheath with suppressed superconductivity.

Superconductivity in ultrathin films I. Transition temperatures of amorphous Bi and Ga

Superconducting transition temperatures (T c) and electron tunneling characteristics have been measured on amorphous, vapour quenched films of Ga and Bi in the thickness range 3 to 50 nm. TheT c's are depressed and the tunnel densities of states are smeared as compared to bulk superconductors. A model considering the proximity effect between an extremely thin non-superconducting sheath (presumably due to the decrease of electron concentration at the surfaces) and a remaining bulk superconducting part gives good agreement withT c measurements, while the predicted tunnel density of states is only in qualitative correspondence with experiments.

Superconductivity in ultrathin films II. Structure in tunneling curves

Additional structure is seen in conductance versus voltage curves when electrons are tunneled into very thin superconducting films. This peak like structure has been detected in several materials; it is observed at multiples of the energy gap above the main conductance maximum. The peak amplitude is very thickness dependent. At roughly half the transition temperature of the ultrathin film the amplitude goes to zero within a narrow temperature range. Effects of magnetic field, an overlay of a non-superconductor, and annealing are less pronounced. The most probable explanation seems to be a model where the tunneling rate is stimulated whenever the relaxation and recombination phonon frequencies equal.

Energy gaps and transition temperatures of ultrathin superconducting films

Energy gaps and transition temperatures of extremely thin aluminum, indium and lead films, deposited on SiO or naphthalene, depend on the film thickness up to about 300 Å. This dependence is due to quantum size effects and to surface interactions.

Ratio between energy gap and transition temperature in ultrathin superconducting films

The ratio 2Δ(0) k B T C increases with decreasing film thickness (down to 5 nm) for vapour-quenched micro-crystalline Pb, while it is almost independent of thickness for amorphous Ga and Bi.