Evidence for quantum tunneling of vortices in superconductors

Flux creep in disordered superconductors may be governed by quantum tunneling of Abrikosov vortices rather than by thermal activation processes. The expectation is that in the quantum tunneling regime the creep rate would be temperature independent. This assumes that the parameters describing the pinning potential and other aspects of the superconducting films are temperature independent. In the case of extremely thin superconducting films the coherence length retains its temperature dependence well into the quantum tunneling regime, leading to an unusual temperature dependence of the electrical resistance in this regime. This has been observed in ultrathin superconducting films of Pb, Al, and Bi. In low magnetic fields, at low temperatures, sheet resistances vary with temperature as RR₀ exp(T/T₀), where T₀ and R₀ are constants.     

Critical data of some A15 type superconductors in transverse fields up to 230 kOe

Critical field curves and quenching curves for the A15 (-W) type compounds Nb₃Sn, V₃Si, and V₃Ga have been measured. The critical field curves have been extrapolated to zero temperature, and the following zero upper critical fields have been found: 245 kOe for Nb₃Sn, 235 kOe for V₃Si, and 208 kOe for V₃Ga. These values lie between the upper and lower limits given by theories for critical fields not taking into account spin-orbit scattering. Critical currents for Nb₃Sn samples with small admixtures of zirconium are higher than for corresponding samples without zirconium. Special samples of Nb₃Sn multiwires 522 have still high critical currents in the high-field region up to 230 kOe.On leave of absence from the University of Giessen.Supported by the U.S. Air Force Office of Scientific Research.     

Cryogenic scanning tunneling spectroscopy studies of inhomogeneous superconductors

We applied scanning tunneling microscopy and spectroscopy, combined with magnetic susceptibility measurements and Mossbauer spectroscopy, in studies of three superconducting materials. Two belong to a new class of high T_c magnetic-superconductor systems, R_1.4Ce_0.6RuSr₂Cu₂O_10-δ (R=Eu and Gd), which are magnetically ordered at T_N≪T_c. The third is a granular normal-superconductor system, where the superconducting phase is YNi₂B₂C. In this paper we focus on the tunneling measurements, which exhibit large spatial variations of the local electronic properties for all three materials. Albeit, these local measurements also reveal pronounced differences between the third and the first two systems, which will be discussed in view of their macroscopic properties.     

Quantum tunneling of vortices in cuprate and heavy fermion superconductors

In high-T_c and in heavy fermion superconductors at low enough temperatures, pinned vortices move from one potential well to another mainly by quantum tunneling. In the superconducting cuprates, the measured creep rates at low magnetic induction and in the T 0 limit are in excellent agreement with the values calculated within the framework of the Quantum Collective Creep theory. In the heavy fermion superconductor UPt₃, the measured rates at T 0 are much higher than the theoretical values, giving evidence that the low temperature motion of vortices in this material is possibly related to its unconventional character.We thank the Swiss National Foundation for financial support.     

Review on the fabrication techniques of A-15 superconductors

This Paper reviews the present state-of-the-art of preparing multifilamentary A-15 superconductors. The most common types, Nb₃Sn and V₃Ga, are presently produced by the so-called bronze process. The highest J_c (overall) = 3.5 × 10⁴ cm⁻² (at 15 T and 4.2 K), obtained for bronze processed Nb₃Sn composites through Ti addition, has pushed the useful limit of this material from 12 to 16 T. Similarly a J_c of 1 × 10⁵ A cm⁻² (at 20 T and 4.2 K) for the A-15 V₃Ga has been attained through elemental additions to the core and the bronze matrix. To circumvent the problem of work-hardening of the bronze, several variations of the bronze process such as the internal tin method, the Nb tube method, the ECN method and jelly roll method have also been upgraded to commercial scale. Composites of Nb₃Sn and V₃Ga have been recently produced successfully on a laboratory scale following the so called in situ technique. These composites not only have a superior J_c value but display improved strain tolerance due to the ultrafine nature of the filaments formed in situ. In situ filamentary A-15 composites with high J_c values have also been produced by following the powder metallurgy technique. The infiltration technique has been found useful for producing high field Nb₃(Al, Ge), Nb₃(Al, Si) and Nb₃Sn composite conductors with high ε_irr. Superior materials such as Nb₃Al, Nb₃Ga and Nb₃(Al,Ge) with high J_c performance have been synthesized using the laser beam technique. Nb₃Ge tapes with T_c = 21 K and J_c = 10⁵ A cm⁻² (at 18 T and 4.2 K) have been successfully produced on a laboratory scale by following the CVD technique. Thus, there are several available options from which to choose a technique for fabricating filamentary composites of ubiquitous Nb₃Sn and V₃Ga. New techniques for fabricating superior materials like Nb₃Al, Nb₃Ga, Nb₃Ge and Nb₃(Al, Ge) also seem to be at an advanced stage of development.     

On the anomalous transparency of superconductors

The penetration of an electromagnetic wave through a thick superconducting film (with a thickness much larger than the London depth or skin depth) is considered. It is shown that such penetration is possible due to the transformation of any incident eddy electromagnetic wave into a potential mode decaying at much larger distances. The amplitude of the wave transmitted through the sample is calculated. The possibilities for the experimental observation of such an effect are discussed.     

On the analysis of field failure data for repairable systems

Reliability assessments of repairable (electronic) equipment are often based on failure data recorded under field conditions. The main objective in the analyses is to provide information that can be used in improving the reliability through design changes. For this purpose it is of particular interest to be able to locate ‘trouble-makers’, i.e. components that are particular likely to fail. In the present context, reliability is measured in terms of the mean cumulative number of failures as a function of time. This function may be considered for the system as a whole, or for stratified data. The stratification is obtained by sorting data according to different factors, such as component positions, production series, etc. The mean cumulative number of failures can then be estimated either nonparametrically as an average of the observed failures, or parametrically, if a certain model for the lifetimes of the components involved is assumed. As an example we here consider a simple component lifetime model based on the assumption that components are ‘drawn’ randomly from a heterogeneous population, where a small proportion of the components are weak (with a small mean lifetime), and the remaining are standard components (with a large mean lifetime). This model enables formulation of an analytical expression for the mean cumulative number of failures. In both the nonparametric and the parametric case the uncertainty of the estimation may be assessed by computing a confidence interval for the estimated values (a confidence band for the estimated time functions). The determination of confidence bands provides a basis for assessing the significance of the factors underlying the stratification. The methods are illustrated through an industrial case study using field failure data.     

Electron-magnon interaction effect in antiferromagnetic superconductors. Proposal of a tunneling experiment

It is shown that in an antiferromagnetic superconductor the electronic density of states is modified to exhibit a deviation from the BCS theory due to the second-order process of the electron-magnon interaction. An electron tunneling experiment is proposed to detect this effect in existing antiferromagnetic superconductors. A close analogy is pointed out between the present electron-magnon system and the electron-phonon system of strong coupling superconductors.     

On the critical temperature of anisotropic superconductors

An approximate analytic formula for the increase in the superconducting critical temperature caused by anisotropy is derived and compared with both the results of exact numerical solutions of the Eliashberg equations and with a previous approximate formula due to Allen and Butler. In all the cases considered our new formula is found to be more accurate than the previous one and to agree reasonably well with the exact results for the intermediate and weak coupling superconductors considered.Research supported by the Natural Sciences and Engineering Research Council of Canada.     

A numerical model for the analysis of stability and quench characteristics of forced-flow cooled superconductors

A numerical model for the analysis of stability and quench characteristics of forced-flow cooled superconductors was developed at SIN. From this model, which includes transients in the heat transfer coefficient and in the fluid flow, and introducing only a limited number of specific modules, two computer codes were generated. Since 1979 they have been further developed and used for the stability and quench analysis of the Swiss conductors for the LCT, INTOR/NET and SULTAN projects. These applications have shown that they are powerful numerical tools available to the designer of superconducting magnets. Moreover, thanks to their modular structure, they are flexible enough to be used for different conductor configurations and to be further developed. Verification of these two codes is expected to be completed by comparison with experiments in SULTAN, which is at present being modified, and in the LCT test facility, which will start operation in the near future.     

Tip-induced excitation of a single vortex in nano-size superconductors using scanning tunneling microscopy

A single vortex is excited into nano-size Pb superconducting island structures by a local current injection from a probe tip of a scanning tunneling microscope. For the excitation, a sufficient amount and duration of the pulsed current are required. Injecting the current at peripheral sites is more effective than the center for the vortex excitation. Time-dependent Ginzburg-Landau calculations suggest that a current-induced normal-state area, which can be nucleated by the tunneling current exceeding the critical current and expanded by the Joule heating, reduces the required magnetic field for the vortex penetration and excites a vortex into the islands.     

On the proximity effect in unconventional superconductors

Boundary conditions are derived that link the quasiclassical Green's functions of a singlet and triplet superconductor across a proximity contact. The boundary conditions are used to calculate the order parameter and the tunneling spectra of proximity junctions specifically arranged to study unconventional superconductors for possible triplet pairing.     

On the characteristic voltage of high-T c superconductors

The critical currents and normal resistances of the small bridges from yttrium-based high-T _c superconducting ceramics have been measured. The characteristic voltage of these bridges was found to be approximately 20 μV. This effect can be explained if, between the ceramic grains, there are contacts of an order of one crystalline cell in size.     

On the theory of resistive states in superconductors

The contribution R _s of a superconductor S to the resistance of a superconductor-normal metal (S-N) system is calculated. The case when the characteristics of metals S and N are different (different mean free path, density of states, etc.) is considered. It is shown that the value and temperature dependence of R _s differ from those obtained earlier. The resistance of the S-N-S system and of a system consisting of alternating S-N layers is calculated. The excess current I _exc in contacts S₁-c-S₂ (S_1,2 are different superconductors, c denotes a narrow constriction) is determined. The influence of magnetic field and of paramagnetic impurities on the value of the I _exc is found.     

Approximating the electron-phonon coupling constant in superconductors from thermodynamic data

A method is discussed for estimating the electron-phonon coupling constant _ep from the jump in heat capacity at the superconducting transition which can be enhanced above the BCS value by strong coupling effects. Although based on a simple model of the superconducting state, reasonable agreement is found with published experimental data for a variety of crystalline superconductors, none of which is strongly anisotropic.     

A data structure for finite element analysis

The philosophy of a simple data structure for handling data for finite element analyses is described. The technique is advocated as an efficient way of handling finite element data and of communicating between programs using the data.     

A two-fluid model for the normal-state properties of cuprate superconductors

We consider a two-dimensional metal with metallic layers of widthd separated by thick layers with a large dielectric constant ₂. Electronic states near the Fermi surface are renormalized so that their velocity increases significantly, as in the Lindhard model. In contrast, states removed from the FS are renormalized so that their velocity decreases, as described by the Gutzwiller theory. We suggest that these two types of states are decoupled, and can be described by a two-fluid model. The resistivity of the FS fluid, due to elastic scattering, becomes temperature-dependent, decreasing significantly atT=0 and actually vanishing as we approach the Mott transition. The increased velocity accounts for the small London penetration depth found in the superconducting state. This model can also account for the zero bias anomaly observed in some exotic superconductors.     

A computer system for analysis of chromatographic data

Marshall, R.J., Bleasby, A.J., Turner, R. and Cooper, E.H., 1987. A computer system for analysis of chromatographic data. Chemometrics and Intelligent Laboratory Systems, 1: 285–295.An interactive computer program (CHAS) for chromatogram processing is described. CHAS is a FORTRAN program which has three basic functions: (a) for data management, (b) for graphical display, and (c) for chromatogram analysis. The program is designed to run off-line by accessing data from a library of chromatograms. Various types of graphical displays are available and its analytical procedures incorporate new algorithms to detect chromatogram peaks, to remove baseline drift and to compute similarities between chromatograms. We present some illustrative uses of the program for data generated by high-pressure liquid chromatography.     

Pair tunneling as a probe of order-parameter fluctuations in superconductors: Zero magnetic field effects

Thermal fluctuations of the order parameter in a superconductor above its transition temperature result in manifestations of the ordered state which appears below the transition. In a Josephson junction consisting of two metals with different transition temperatures there is associated with the incipient superconductivity an enhanced conductivity across the junction at temperatures between the transition temperatures of the two metals. The enhanced conductivity is observed as a current which flows in excess of the junction quasiparticle current. The excess current has been investigated using Sn-Sn _x O _y -Pb and Al-Al _x O _y -Pb junctions masked to eliminate effects of graded film edges. Details of the variation of the excess current with voltage and temperature are in quantitative agreement with calculations in the Al-Al _x O _y -Pb junctions. The excess current-voltage characteristic is a direct measure of the imaginary part of the generalized susceptibility above T _c . The pair relaxation frequency in aluminum based on the data is consistent with the theoretical value.Supported in part by the U.S. Atomic Energy Commission under contract AT(11-1)-1569.NSF Trainee.