Quench-condensed indium-hydrogen films. I. Superconductivity enhancement and metastable state formation

The electrical and superconducting properties of indium films are studied. The films were condensed in H₂ atmosphere ( ranged from 4×10^–7 to 6×10^–5 Torr) onto a substrate cooled with liquid helium down to about 3 K. As the hydrogen concentration increased, the normal resistivity _n of the films continuously increased up to a point where the metal-insulator transition occurred. It was found for the first time that saturation of In films with hydrogen lead to a significant increase (up to 5.15 K) inT _c and to zero-dimensional paraconductivity aboveT _c . Possible mechanisms for an effect of dissolved hydrogen on _n andT _c are considered along with the influence of structural defects onT _c . At >2×10^–6 Torr the prepared films appear to be a metastable phase In(H) with high nonequilibrium hydrogen concentration, which decayed at 5 K. The destruction of In(H) was accompanied by a release of H₂ from the film, a sharp drop of resistance (by three and more orders of magnitude), and a rise inT _c . The properties and the formation and stabilization conditions of the In(H) phase are considered in detail and this phase is compared with the a-Bi(H) phase observed earlier.     

Superconductivity effects near the metal-insulator transition in granular indium films

The paper reports on an investigation of the electric resistance as a function of temperatureT, magnetic fieldH, and applied voltageV in granular indium films when approaching the metal-insulator transition (MIT) due to increased thickness of the oxide layers between granules. The dependences are shown to be governed atT5 K by competition of the hopping conductivity and the Josephson tunneling of electrons, sometimes giving rise to a resistance minimum inR(T) associated with superconductivity. It was also found that even when the intergranular Josephson tunneling is totally suppressed, transition of granules to the superconducting state influences essentially the dependencesR(T, H), changing the functional form ofR(T) and resulting in anomalously high negative magnetoresistance. This is shown to stem from the change of the character of activated electron tunneling as the granules become superconducting.     

A simple model for the superconducting phase diagram near the metal-insulator transition

We solve a simple model of competing superconductivity and disorder in a mean-field approximation with two order parameters. A phase diagram showing superconducting-normal metal-insulator as well as superconducting-insulator transitions is obtained. Comparison with experiments shows a good qualitative agreement.     

Heat capacities of granular aluminum films near the superconducting transition

The electrical conductivity and heat capacity were studied in the superconducting transition region of 1000-» thick, disordered, granular films that were evaporated onto cleaved mica substrates. The resistive transitions of films were used as thermometers in conjunction with optical heating in an ac calorimetry scheme to measure the temperature dependence of the 5 × 10^?9 J/K heat capacity of the films. Results obtained on nine samples indicated a rise in the heat capacity near the low-temperature limit of the transition region where film resistances became zero. This rise peaked in four films at values substantially greater than the estimated Bardeen-Cooper-Schrieffer jump in heat capacity and at temperatures 20–40 mK below the transition temperatures determined from a fit of the mean field theory to conductivity measurements. Quantitative comparison with theory cannot be made because transitions are broadened by film thickness variations resulting from irregular substrate topography. Results are in qualitative disagreement with the monotonic variation of the excess heat capacity calculated in the Hartree approximation. Observed peaks are also both wider and higher than those predicted by 1/n expansion and screening approximation calculations.     

Resistive transition in superconducting films

The consequences of a vortex unbinding picture of two-dimensional superconductivity are worked out. Although there is no true finite-temperature phase transition, dirty superconducting films should display anomalous behavior below the BCS transition temperature and above an effective Kosterlitz-Thouless vortex unbinding temperature. In particular, both the conductivity and fluctuation diamagnetism behave like ₊ ² in this regime, where ₊ is the correlation length calculated by Kosterlitz, _{+ c} exp (B/T – T _c)^1/2. We estimate _c, B, and the vortex unbinding temperature, and determine the nonlinear resistivity below T _c. A recent theory of vortex dynamics, together with a time-dependent Ginzburg-Landau theory, lead to a determination of the frequency-dependent conductivity.This work was supported by the National Science Foundation under Grant No. DMR 77-10210.     

Superconductivity and disorder-driven metal-insulator transition in quench-condensed tin films

We have studied the effects of disorder in a series of Sn films quench-condensed onto cryogenically cooled substrates. We find a simple scaling of the conductivity with film thickness ~ (d - d _c ), with 0.85, and a metal-insulator transition near d _c 40–50 ». The superconducting transition temperatures depend linearly on 1/d, provided d is not too small, and the films exhibit strong annealing effects. These features are explained using a model in which grain boundary scattering dominates intragrain scattering.     

Compositional and metal-insulator transition characteristics of sputtered vanadium oxide thin films on yttria-stabilized zirconia

Vanadium dioxide (VO₂) thin films have been shown to undergo a rapid electronic phase transition near 70 °C from a semiconductor to a metal, making it an interesting candidate for exploring potential application in high speed electronic devices such as optical switches, tunable capacitors, and field effect transistors. A critical aspect of lithographic fabrication in devices utilizing electric field effects in VO₂ is the ability to grow VO₂ over thin dielectric films. In this article, we study the properties of VO₂ grown on thin films of Yttria-Stabilized Zirconia (YSZ). Near room temperature, YSZ is a good insulator with a high dielectric constant ($\epsilon _{\rm r} > 25$\epsilon _{\rm r} > 25). We demonstrate the sputter growth of polycrystalline VO₂ on YSZ thin films, showing a three order resistivity transition near 70 °C with transition and hysteresis widths of approximately 7 °C each. We examine the relationship between chemical composition and transition characteristics of mixed phase vanadium oxide films. We investigate changes in composition induced by low temperature post-deposition annealing in oxidizing and reducing atmospheres, and report their effects on electronic properties.     

Lattice dynamics in VO2 near the metal-insulator transition

We present a comprehensive experimental study of the lattice dynamics in the vicinity of the metal-insulator transition in VO₂ by means of combined use of Raman spectroscopy and ultrasonic microscopy. Single crystalline samples of high quality particularly allow quite a complete determination of all Raman modes in the insulating phase at the Γ-point and the observation of an optical soft mode. In addition, the elastic behavior has been successfully investigated by measuring the propagation velocity of ultrasonic surface waves microscopically excited in various crystal directions. Our study reveals a striking coincidence of strong lattice softening attributable to certain acoustic branches and the occurrence of the optical soft mode, which precedes the metal-insulator transition over more than 100 K on approaching the critical temperature.     

Quantum effects and the resistive transition in superconducting granular films

Static and dynamic properties of an array of Josephson junctions shunted by Ohmic resistors are discussed within a quantum Ginzburg-Landau theory. The phase diagram at zero temperature is calculated in mean field approximation. It shows that global superconductivity atT=0 is possible only if the normal-state film resistanceR _n is smaller than a critical valueR _n ^c which depends only logarithmically on the Josephson coupling and charging energies. The particular valueR _n ^c =6.5 k found in recent experiments on granular films is in reasonable agreement with estimates for these parameters. A phenomenological order parameter relaxation mechanism is introduced and the associated fluctuation-induced conductivity and diamagnetic susceptibility aboveT _c are determined. The resulting precursor conductivity does not explain the observed exponential decrease withR _n-R _n ^c of the residual resistance at low temperature. However, a very simple model for the resistance due to vortex flow, generalizing the classical Kosterlitz-Thousless picture, is in good agreement with the experimental data.     

Tearing transition and plastic flow in superconducting thin films

A new class of artificial atoms, such as synthetic nanocrystals or vortices in superconductors, naturally self-assemble into ordered arrays. This property makes them applicable to the design of novel solids, and devices whose properties often depend on the response of such assemblies to the action of external forces. Here we study the transport properties of a vortex array in the Corbino disk geometry by numerical simulations. In response to an injected current in the superconductor, the global resistance associated to vortex motion exhibits sharp jumps at two threshold current values. The first corresponds to a tearing transition from rigid rotation to plastic flow, due to the reiterative nucleation around the disk centre of neutral dislocation pairs that unbind and glide across the entire disk. After the second jump, we observe a smoother plastic phase proceeding from the coherent glide of a larger number of dislocations arranged into radial grain boundaries.     

Specific-heat anomaly near superconducting transition in high temperature superconductors

A brief appraisal of the current state of understanding on the specific heat anomaly nearT _c in the oxide superconductors is presented. Spectacular specific heat double transitions are explained with plausible models.     

Metastable states of superconducting indium films. II

Supercooling fields of In films condensed at 295 and 77 K have been studied. Each series of films defines a different bulk material, whose Ginzburg-Landau (GL) parameter can be obtained from the measured supercooling fields. These bulk metals satisfy the Gor'kov-Goodman relation between the GL parameter and resistivity. Furthermore, this relation is also satisfied by GL parameters of individual type II films condensed at 77 K, while other layers condensed at 295 K show deviations ascribed to nonlocal effects. The importance of a careful study of transport properties of the films for a correct interpretation of experiments is stressed.     

The effect of electric field on metal-insulator phase transition in vanadium dioxide

The effect of a strong electric field on the metal-insulator phase transition in vanadium dioxide was studied. It was found that the field application to a silicon-silicon oxide-silicon nitride-vanadium dioxide (Si-SiO₂-Si₃N₄-VO₂) heterostructure shifts the critical temperature of this transition toward lower values under conditions when the thermal effects are minimized. Numerical modeling of the current-voltage characteristics of the vanadium dioxide-based sandwich-type switches measured at various temperatures in the range from 15 to 350 K showed that the applied electric field influences the critical concentration and temperature of the phase transition.     

Fluctuation conductivity of tin films below the superconducting transition temperature

Measurements have been carried out of the temperature dependence of the fluctuation-induced excess electrical conductivity of thin (thickness much less than the Ginzburg-Landau coherence length) superconducting films of tin below the transition temperature. Two types of specimen were investigated: (a) films deposited on a glass substrate held at 300 K and (b) films deposited on a glass substrate at 300 K and then covered by a protective layer of germanium. Special care was taken to ensure sample homogeneity. Analysis of the results shows that the fluctuation parameter, ε_c, is not affected by the germanium layer. Its value is, however, much larger than that predicted by the theory of Kajimura, Micoshiba, and Yamaji.     

On the threshold pinning force in type II superconducting films

The maximum pinning force of a two-dimensional vortex lattice in a random potential is calculated. A connection is established between this threshold pinning force and the potential energy discontinuities due to elastic and plastic instabilities of the vortex lattice. Inspired by recent computer simulations, we assume that the fluctuations in the commensurability between the random potential and the vortex potential breaks the vortex system up into a set of flowing channels in between trapped regions. Two instability mechanisms and their contribution to the threshold force are discussed within this channel-flow picture. We find that three different regimes exist depending on, w, the width of the channels;w=,a ₀w=a ₀ , wherea ₀ is the vortex lattice spacing. Weak pinning superconductors can pass through all three regimes as the reduced magnetic field is varied from 0 to 1, whereas strong pinning compounds can remain in the saturated region (w=a ₀ ) for all values of the field. We compare the expression for the threshold force with experimental results for both strong and weak pinning samples. A satisfactory qualitative agreement is obtained between theory and experiment.     

Using ion implantation to adjust the transition temperature of superconducting films

We summarize a continuing investigation into using ion implantation to alter the transition temperature of superconducting thin films. The primary motivation for the work presented here was to study the feasibility of using magnetic ion doping to replace the bi-layer T_c control process currently used for certain cryogenic detector applications at National Institute for Standards and Technology. The results from work with various ion species implanted into aluminum, molybdenum, titanium and tungsten host films are presented.     

He I-He II coexistence near the superfluid transition

We have made a numerical scheme of solving two fluid hydrodynamics near the point, which includes the mass density, momentum density, entropy density, and complex order parameter. In our model the normal fluid velocity vanishes at the boundaries due to finite viscosity. As preliminary examples we examine coexistence of a superfluid region and a normal fluid region in two space dimensions firstly in heat flow and secondly in gravity and heat flow. We observe formation of a vortex from a wall in the second case.     

Work function of vanadium dioxide thin films across the metal-insulator transition and the role of surface nonstoichiometry

Vanadium dioxide (VO(2)) undergoes a sharp metal-insulator transition (MIT) in the vicinity of room temperature and there is great interest in exploiting this effect in novel electronic and photonic devices. We have measured the work function of vanadium dioxide thin films across the phase transition using variable temperature Kelvin force microscopy (KFM). The work function is estimated to be ～5.15 eV in the insulating phase and increases by ～0.15 eV across the MIT. We further show that the work function change upon the phase transition is highly sensitive to near-surface stoichiometry studied by X-ray photoelectron spectroscopy. This change in work function is distinct from bulk resistance-versus temperature trends commonly used to evaluate synthesis protocols for such vanadium oxide films and optimize stoichiometry. The results are pertinent to understanding fundamental electronic properties of vanadium oxide as well as charge injection phenomena in solid-state devices incorporating complex oxides containing multivalence cations.     

Penetration depth measurements on type II superconducting films

A penetration depth (λ) measurement technique, which is based upon the λ-dependence of the propagation velocity of electromagnetic waves in superconducting transmission lines, is refined and used to accurately measure λ's in Pb films containing Au, In, or Bi additions. The results are applied to test two distinct predictions for λ's in London (or type II) superconductors. First, the measured dependence of λ upon low temperature electrical resistivity is compared with the theoretical prediction for a type II superconductor containing a dilute homogeneous impurity concentration. We find that the measured λ's follow this simple prediction reasonably well, irrespective of the species of impurity, with two anticipated exceptions. Namely, deviations occur for pure Pb, which is not a type II superconductor, and for the highest impurity concentration, in which case the Fermi surface may be appreciably distorted from that of pure Pb. Second, the London-theory prediction for the dependence of the effective penetration depth (λ^eff) upon film thickness (d), λ^eff= λ(T) coth d/λ(T), is tested, in a d/λ range in which coth d/λ, is not closely approximated by 1 or λ/d. Separate experiments in which either film thickness or temperature, T, are varied reveal excellent agreement with the above formula. To our knowledge this is the first detailed confirmation of this explicit hyperbolic cotangent dependency.     

Measurement of the transition temperature distribution and magneticfield profile of large superconducting films

A novel apparatus has been developed to measure the transition temperature (T_c) distribution and magnetic field profile of large superconducting films. Two techniques were used. The first one is an ac inductive method which provides a contactless measurement of T_c. The second technique employs a Hall device to measure magnetic field. Driven by two step motors, the measuring probe, either a two-coaxial-winding coil or a Hall sensor, can move over the sample on a polar plane. As the probe scans the sample step by step, various points on the sample are measured one by one. Experiments were carried out on a 30 mm diameter YBa₂Cu₃O_7-δ thin-film. The T_c distribution and the surface profile of trapped magnetic field were obtained