Quasiparticle recombination time and multiplication coefficient in superconducting Pb films

The quasiparticle recombination time and the multiplication coefficient in superconducting Pb films have been determined independently from measurements of the energy gap changes under optical pumping.     

Flux nucleation in the current-induced resistive state of superconducting Pb films

A method is demonstrated whereby the rate of magnetic flux-tube nucleation in type I superconducting Pb films is measured with minimal disturbance to the current-induced resistive state.     

Harmonic generation in superconducting Bi(Pb)SrCaCuO thick films

We present a detailed study of harmonic generation in Bi(Pb)SrCaCuO superconducting thick film prepared by screen printing technique. A comparative study of amplitude of harmonics of two films of different J_c have been carried out. The variation of amplitudes of the harmonics are studied as a function of magnitude of ac and dc field. The temperature dependence of amplitude of third harmonic (V³) is studied with increasing amplitude as well as frequency of ac field. These results are analyzed in the frame work of critical state model. V₃ -T curve also indicates the presence of two phases in Bi(Pb)SrCaCuO film.     

Staircase magnetization anomalies in superconducting Pb/Au bilayer films patterned with antidot lattices

Superconducting Pb(x)/Au(25 nm) bilayers (x = 50, 100 nm) patterned with antidot lattices exhibit various matching field anomalies depending on experimental conditions. Magnetization peaks at applied fields H = n[20 Oe] (n = integer) resemble superconducting wire network data; cusps are also observed, consistent with predictions of “giant” vortices in low-kappa films. Sharp “staircase” anomalies spaced by 1–3 Oe are observed in AC magnetization, possibly a result of depinning of intermediate state domains, or macroscopic quantum tunneling between reproducible states of different quantized flux.     

Energy-gap dynamics of superconducting NbN thin films studied by time-resolved terahertz spectroscopy

Using time-domain terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ=1.1±0.1, which is in excellent agreement with theoretical estimates.     

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.Supported by the Swedish Natural Science Research Board.     

Superconductivity of the Pb/Ag superlattice

Layered Pb/Ag thin films were produced by vacuum evaporation on cooled glass substrates. Measured x-ray diffraction spectra showed peaks characteristic of a superlattice. The superconducting energy gap and the transition temperature of the Pb/Ag superlattice were measured by tunneling experiments. BCS-reduced tunneling conductance revealed the dependence the phonon structure on the superlattice period value. It was also observed that the order of the crystal structure has an influence on the superconducting parameters.     

The temperature dependence of the resistive response of superconducting Pb films upon picosecond optical excitation

We report a study of the temperature dependence of the transient resistivity of superconducting Pb thin films excited by short duration (70 ps) laser pulses of varying irradiances. The results are analyzed in terms of a model proposed by Elesin which features the existence of a nonstationary, inhomogeneous, intermediate state.     

Superconductivity dependent friction of water, nitrogen, and superheated He films adsorbed on Pb(111)

We report a quartz crystal microbalance study of sliding friction levels in N2, H2O, and superheated He films adsorbed on Pb(111) substrates alternating in and out of the superconducting state. Reductions in friction upon entry into the superconducting state are greater for N2 than He, consistent with a recent theory that linked electronic friction to adsorbate polarizability. Our work also reveals that repetitive cycling of an externally applied magnetic field may impact friction.     

Superconductivity of quench-condensed beryllium and beryllium-germanium films

Tunneling experiments on Be films condensed onto helium cooled substrates show that these films are homogeneously disordered with a uniform transition temperatureT _c when Ge is codeposited, whereas thick films of pure Be do not grow homogeneously. For films of Be+10 at.% Ge a ratio 2Δ/k _B T _c =3.7 is found. Phonon induced structure in the tunneling density of states is not observed. The metastable phase obtained by quench-condensation is considered to be a disordered high-temperature phase of Be which transforms to the room temperature phase at about 60 K.     

Paraconductivity of superconducting tin films

Experimental results on paraconductivity of superconducting thin tin films are compared with theoretical predictions of the Maki-Thompson theory which includes a small correction term proposed by Imamura et al. It is found that the corrected Maki-Thompson expression for paraconductivity of thin superconducting films improves the fit between theory and experimental data at relatively higher temperatures. It also gives a smaller value of δ, a fitting variable in the M-T theory which is known as the pair- breaking interaction parameter, than the value obtained in the unmodified M-T expression for the paraconductivity.     

MMW detection by superconducting films-applications of high temperature superconducting films

Conclusions Superconductors exhibit two basic modes of radiation detection —a thermal or bolometer mode and a rectification mode which depends on a nonlinear voltage-current characteristic. The SIS detector has been shown to have a quantum limited current responsivity of e/hf. By analogy, weak link detectors may have a quantum limited voltage responsivity of about 2eR/hf. Granular films behave like arrays of weak links.Both thermal and rectification modes have been observed in HTOS films. They can be characterized by the dependence of film resistance on temperature and current. Analysis of the temperature dependence of film resistance indicates that bolometer responsivities are limited by the broader resistive transition observed in HTOS films. HTOS films show a strong nonlinear dependence of resistance on current at lower temperatures. In this region, we expect voltage responsivities greater than 1 KV/W. This is a very promising mode for detection and mixing of far infrared and millimeter waves.     

Superconductivity and normal state magnetoresistance in superconducting FeSe:Sb

We prepared a series of β-FeSe samples with a nominal composition of Fe1.11Se1-xSbx(0≤x≤0.5).The X-ray diffraction,transport and magnetic measurements were performed on these samples to investigate the structure,the superconducting properties and the normal state transport and magnetic properties.Although the X-ray diffraction data suggested that Sb atoms were not incorporated into the β-FeSe phase,the transport data showed observable changes of superconductivity,normal state resistivity and magnetoresistance.This was represented by the increase in the superconducting transition temperature and the upper critical field.Also,for the samples with a low level of Sb content,a clear decrease of the normal state resistivity and a substantial increase of the residual resistance ratio were observed.Furthermore,the samples showed a significant increase of the normal state magnetoresistance that appeared not to follow the Kohler's rule.The results were discussed in the frame of reduction of excess Fe at interstitial sites of β-FeSe.     

Superimposed normal and superconducting films

Tunneling characteristicsdI/dV have been taken on both sides of “dirty”In - Bi/Zn superimposed films, the temperatureT being such thatIn - Bi was superconducting and Zn normal. Most of the attention has been devoted to the regionT～T _c where the experimental curves can be compared with a reasonably accurate microscopic theory. We find reasonable agreement for the voltage scale and, more important, for theamplitude of the effects in the superconducting phase.     

Gauge fluctuations in superconducting films

In this paper we consider a type-I superconducting film modeled by the Ginzburg-Landau model, confined between two parallel planes a distance L apart from one another. Our approach is based on the Gaussian effective potential in the transverse unitarity gauge, which allows to treat gauge contributions in a compact form. Using techniques from dimensional and -function regularizations, modified by the confinement conditions, we investigate the critical temperature as a function of the film thickness L. The contributions from the scalar self-interaction and from the gauge fluctuations are clearly identified. The model suggests the existence of a minimal critical thickness below which superconductivity is suppressed. A comparison with present experimental observations is done.Received: 19 December 2003, Published online: 9 April 2004PACS: 74.20.-z Theories and models of superconducting state - 11.15.Ex Spontaneous breaking of gauge symmetries - 05.10.Cc Renormalization group methods     

Superconductivity in thin films of boron-doped carbon nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. However, its correlation with carrier doping has not been reported. We report on the Meissner effect found in thin films consisting of assembled boron (B)-doped single-walled CNTs (B-SWNTs). We find that only B-SWNT films consisting of low boron concentration leads to evident Meissner effect with Tc=12 K and also that a highly homogeneous ensemble of the B-SWNTs is crucial. The first-principles electronic-structure study of the B-SWNTs strongly supports these results.