Coherence and Disorder in Arrays of Point Contacts

Abstract

Since the early work of London 1 supercurrents and the penetration depth of superconductors appear as manifestations of phase coherence, or order in momentum space, among the superconducting electrons. Similarly Josephson currents and penetration depth result from phase correlations between superconductors separated by a barrier. By the same token, one may expect intuitively that an array of a great number of junctions (such as a granular superconductor) will display supercurrents and static screening of external fields provided long-range correlations exist between superconducting phases in different electrodes. We report here on measurements of critical current and penetration depth of three-dimensional (3D) assemblies of bulk grains (diameter a ^ 50 jiim) weakly coupled through Josephson point contacts.Our samples are made by simply pressing together in an epoxy resin slightly oxidized superconducting Nb grains. We thus obtain arrays of about 10 6 point contacts which can be molded to the desired shape, in this case cylinders 3 mm in diameter and 13 mm long. Resistivities p" in the range 0.1-1 £2 cm are easily obtained by controlling the pressure on the system during hardening of the resin. On the basis of resistive measurements we have pointed out 2 * 3 that a phase transition from a high-temperature "paracoherent" state to a low-temperature coherent one takes place at a temperature T 0 <T C . This is due to the fact that superconducting phases in indi-By measuring the penetration depth and the critical current of three-dimensional assemblies of weakly coupled superconducting grains as a function of temperature, two critical exponents, j3 and v', were determined characterizing the transition to coherence of the system. As in calculations on disordered systems, strict universality does not hold, but the exponents are shown to be compatible with the hypothesis of weak universality. 1035

Effects of critical fluctuations and dimensionality on the jump in specific heat at the superconducting transition temperature: Application toYBa2Cu3O7−δ,Tsiaze1, Wirngo2, Tchouobiap3 et al. 2016Phys. Rev. E

“…It appears also that for d ≥ 4, υ dc = 0, which means that above dimension 4, the present SCA acts exactly like the MFT, whereas it predicts correctly the universal quantities for dimension 4 transitions, one at T c where the system becomes superconducting and locks to produce a state with very short-range phase coherence and the second at T * c < T c , where the superconducting phase locks to produce a state with long-range phase coherence. Only the lower transition is a true phase transition, with a divergent correlation length [36,37,38,39]. In 3D systems, the scaled width υ 3c = δTc Tc is of the order of 10 −14 to 10 −10 , much too small to be accessible experimentally.…”

Role of Flux Pinning in High Temperature Superconductors

“…A quasi-equilibrium (critical state) is established (Bean 1962) where the pinning force density is equal to the gradient of the local magnetic flux pressure such that at each point in the material (Kim et al 1962) 14~(CUrlH) XIlH I = O(H). (1) Here, H(r) is the local magnetic field obtained by averaging the meandering vortex lines over a volume containing a large number of grains, and 11 is the effective permeability of the medium due to the diamagnetism of the grains, representing the fraction of the sample which is in a non-Meissner state (Raboutou et al 1980).…”

Electronic transport transition at graphene/YBa2Cu3O7−δ junction

“…2(c). It is found that the critical current exhibits a linear relationship with respect to magnetic field, the phenomena may be related to the existence of the defects within the grains 17,18 connectivity 19,20 in YBCO film. The measurement was carried out several times, the results are reproducible.…”