Studies on microbridges of superconducting YBCO thin film

Critical current densities of the superconducting Y-Ba-Cu-O (YBCO) films have been observed to decrease with the increase of power of microwave radiation. Presence of Josephson type of junctions in the microbridges has been established from the microwave irradiation and magnetic field studies. BCS energy gap parameter (2/kT _c ) has been calculated from thedI/dV characteristics and found to be 3.7 at 13 K.     

钇钡铜氧超导薄膜光诱导弛豫特性研究

采用TFA-MOD法在LaAlO3(LAO)单晶基底上制备了钇钡铜氧(YBCO)超导薄膜,利用X射线衍射对YBCO超导薄膜的结构进行了表征,并对其最佳沉积工艺进行了分析;研究了YBCO薄膜在不同温度和不同光功率下的光诱导特性.在光诱导下,YBCO薄膜出现了明显与电子体系相关的弛豫现象,说明薄膜的光致电阻变化与激光的功率...     

The performance of thin film superconducting bolometers in magnetic fields

Thin films of pure superconducting metals and alloys can be used as rapid response bolometers over a wide temperature range if a small magnetic field is applied. The characteristics of various films are discussed together with a brief consideration of methods of use.     

Flux flow velocity in superconducting films by measurement of passage time

The velocity of flux tubes in thin superconducting films has been determined by measuring the time for their passing across the film. A legion of flux tubes has been pulsed in on the left side of a film carrying a current large enough to break the pinning. The arrival of the flux tubes on the right side is observed with a pick-up coil. Flow velocities of 3 to 50×10⁴ cm/sec have been observed with current densities up to 10⁵ A/cm².Supported in part by the National Science Foundation, in part by the Cryogenics Center through a grant from the DOD Themis program.     

Flux flow in type II superconducting wires in longitudinal magnetic fields

The resistive behavior of a type II superconducting wire in the presence of a longitudinal applied magnetic field is studied. The experimental data obtained are compared with the predictions of a phenomenological model based on viscous flux flow. For appropriate surface conditions the model is in satisfactory qualitative agreement with observations. The problems involved in obtaining a numerical value for the flux flow resistivity from longitudinal field I–V data are discussed. Estimates of the flux flow resistivity are made and are shown to be in order-of-magnitude agreement with accepted values.     

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.     

Nucleation field of a thin superconducting film on a thick normal layer

The nucleation fieldH ₀ of a normal-superconducting double layer is calculated when the normal layer is thick and the thickness of the superconducting film is varied and is of the order of magnitude of the coherence length in the superconducting metal. The results are general and are exact when the transition temperatures of the two metals are not too different. The above theory is extended to metal layers having very different transition temperatures. When both metal layers are thick the agreement with experiments by Fischer and Klein on lead-copper films is good.     

Effect of phase transitions in copper-germanium thin film alloys on their electrical resistivity

An investigation was carried out to study the phase transitions in Cu-Ge thin films (80–200 nm in thickness) containing 0, 5, 15, 20, 25, 30, 35, 40, 45 and 50 at% Ge, and the corresponding effects on electrical resistivity. For these films, the phase transitions were found to follow the sequence: -phase (disordered face centred cubic, fcc, solid solution); 5 at% Ge -phase (disordered hexagonal close packed, hcp); 15 at% Ge -phase + ₁-phase (ordered orthorhombic, Cu₃Ge); 20 at% Ge ₁-phase; 25 at% Ge (₁-phase + progressively increasing proportions of a disordered Ge-rich solid solution); 30–50 at% Ge. Germanium was found to have no marked effect on grain size of all films studied excluding grain boundaries as electron scattering centres. Transition of the -phase into the -phase was found to occur in a highly coherent manner, which could be related to the reduced stacking fault energy of Cu by the addition of Ge. Most evidence pointed out that the initial increase in resistivity within the -phase range was related to hcp scattering centres, which could be associated with a localized high concentration of Ge. At 15 at% Ge, the resistivity reached a maximum value of about 50 cm associated with the complete transformation of -phase into the -phase. With continued increase in Ge concentration, the resistivity was found to gradually decrease reaching a minimum value of about 10 at 25 at% Ge, which was correlated with complete transition of the -phase into the ordered ₁-phase (Cu₃Ge). It was shown that the superlattice of Cu₃Ge could directly be derived from the disordered -phase by minor atom rearrangement on the [0 0 0 1]_hcp plane. Even though, minor proportions of a Ge-rich solid solution containing a small concentration of Cu were formed at Ge concentrations above 25 at%, the minimum resistivity of 10 cm was maintained as the Ge concentration was increased to 35 at%. Subsequently, the resistivity was increased reaching about 46 cm at 50 at% Ge.     

Flux flow resistivity and upper critical field in ideal type II amorphous superconductors

Flux flow resistivity _f and upper critical fieldH _c2 of ideal type II amorphous bulk supercbnductors Zr₃Ni and Zr₃Rh on both as-quenched and thermally relaxed states have been studied. It is found that thermal annealing does not change the temperature dependence ofH _c2 in homogeneous superconductors. The temperature and field dependence of _f in all samples studied exhibits a universal scaling relation of the form _f / _n =f(h, t), where _n is the normal state resistivity, andh andt are the reduced field and reduced temperature, respectively. The results are compared with predictions of the time-dependent microscopic theories for bulk superconductors in the dirty limit. In the low-field region (HH _c2 ) the viscosity coefficient contains both the ordinary (Bardeen-Stephen, Tinkham) and anomalous (Gor'kov-Kopnin) terms. ForHH _c2 the results agree qualitatively with the theory of Imai with pair-breaking in the anomalous term. Implications of the present results are discussed.This work is sponsored by the National Science Foundation under Grant DMR-82-02624.     

Current distribution measurement in YBCO thin film for a superconducting fault current limiter

Current distribution in the superconducting film for a resistive fault current limiter is important, because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3 times of critical current (I_c). (2) The current in a superconducting film was uniform when the current was much higher than I_c. The current can be considered uniform when the film works as a fault current limiter, because the S/N transition starts about twice of I_c. (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.     

Electromagnetic wave attenuation by superconducting fluctuations in a thin,granular aluminum film

A sharp local minimum has been observed atT _c in the microwave attenuation by a thin, granular aluminum film which is believed to result from the imaginary component of the fluctuation conductivity and for which the particular geometry of the specimen appears to enhance the observation of the fluctuation effects.Research supported by the U.S. Air Force Office of Scientific Research under Grant No. AFOSR 71-2079.     

Modulation instability of electromagnetic excitations in the Josephson junction in a thin superconducting film

The modulation instability of plane nonlinear electromagnetic waves with finite amplitudes excited in the Josephson junction in a thin superconducting film has been studied within the framework of nonlocal electrodynamics. A dispersion equation for the increment of the wave amplitude perturbation is obtained, and the regions of the modulation instability development for the waves studied are determined. It is shown that, for the longwave amplitude perturbations, the modulation instability of these waves is developed within a finite region of wavevectors 0 <Q<Q _v , while the waves with Q≥Q _v are stable.     

Effects of thickness and composition on the resistivity of Cu-MgF2 cermet thin film resistors

Cermet composite thin film resistors of Cu and MgF₂ were prepared by conventional vacuum evaporation technique at a pressure of 5 × 10^–5 torr and at 302 ± 2 K. The composition by volume was varied from 10 vol% Cu and 90 vol% MgF₂ to 100% Cu for thicknesses in the range 110 to 300 nm. Starting materials were co-evaporated from separate molybdenum boats. An empirical formula has been proposed to describe the resistivity-thickness relation for the thin film resistors. The resistivity-thickness relation was found to be of Arrhenius type with ln _f being a linear function of inverse of film thickness in the entire thickness range investigated. A parameter S has been defined to be an estimate of the average separation of the metal islands in the composite thin films. It was observed that S increased with decrease in Cu content of the composite resistors. Another empirical formula has been proposed to describe the resistivity-composition relation for the same film resistors. The relation, obtained by regression analysis of the resistivity-composition data, was found to be exponential in terms of vol% Cu content of the Cu-MgF₂ cermets.     

High-Tc superconducting thin films

This review describes recent advances in thin film studies of high-T_c superconducting cuprates. Emphasis is placed on epitaxial thin film growth technologies for LnBa₂Cu₃O_x (Ln = Y, Eu) cuprates, and on microwave device applications. An exciting area of this research involves epitaxial multilayer structures, such as a superconductor/insulator/superconductor (SIS) combination, and so preliminary results on SIS fabrication are also described.     

Wave flow of a thin liquid film with reduced gravitation

The asymptotic small-parameter method is used to theoretically study steady regimes of flow of thin layers of a viscous liquid over an inclined surface at low film Reynolds numbers under conditions of reduced gravitation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 45, No. 3, pp. 419–423, September, 1983.     

Studies on laminar thin film flow along a vertical wall

Summary The problem of a laminar thin-film flow along a vertical wall is examined here. Using a fourth-degree polynomial velocity profile function inside the boundary layer, the momentum integral equation is solved analytically to determine the boundary layer thickness and the corresponding film-thickness.     

Electrical resistivity and peak effect in superconducting niobium

The flux flow resistivity _f has been measured in foils of high purity niobium as a function of magnetic field and temperature. The magnetic field dependence of _f just belowH _C2 is much stronger than predicted by the recent theory of Maki for the pure limit. NearH _C2 a peak in the critical current is observed that is independent of any voltage criterion.Based on work performed partly under the auspices of the U.S. Atomic Energy Commission.     

Flux penetration into a cylinder of superconducting indium

Magneto-optic techniques coupled with cinephotography have been used to observe the kinetics of flux penetration into a cylinder of superconducting indium.This work was supported by the Defence Research Board of Canada under Grant No. 9510-21.