Fractional Josephson vortices at YBa$_2$Cu$3$O${7-x}$ grain boundaries (original) (raw)
Fractional ac Josephson effect in unconventional superconductors
2004
For certain orientations of Josephson junctions between two px-wave or two d-wave superconductors, the subgap Andreev bound states produce a 4π-periodic relation between the Josephson current I and the phase difference φ: I ∝ sin(φ/2). Consequently, the ac Josephson current has the fractional frequency eV /h, where V is the dc voltage. In the tunneling limit, the Josephson current is proportional to the first power (not square) of the electron tunneling amplitude. Thus, the Josephson current between unconventional superconductors is carried by single electrons, rather than by Cooper pairs. The fractional ac Josephson effect can be observed experimentally by measuring frequency spectrum of microwave radiation from the junction.
Underlying physical aspects of fluctuations in YBa2Cu3O7−δ grain boundary Josephson junctions
Physica C: Superconductivity, 2008
High quality YBa 2 Cu 3 O 7Àd biepitaxial grain boundary junctions allowed one of the first evidences of macroscopic quantum behavior and mesoscopic transport in high critical temperature superconductors (HTS) systems. Both these studies touch delicate issues on fluctuations and give important feedback on the understanding of coherent transport of quasiparticles and of the dissipation mechanisms in HTS.
Flux Flow of Abrikosov-Josephson Vortices along Grain Boundaries in High-Temperature Superconductors
Physical Review Letters, 2002
We show that low-angle grain boundaries (GB) in high-temperature superconductors exhibit intermediate Abrikosov vortices with Josephson cores, whose length l along GB is smaller that the London penetration depth, but larger than the coherence length. We found an exact solution for a periodic vortex structure moving along GB in a magnetic field H and calculated the flux flow resistivity RF (H), and the nonlinear voltage-current characteristics. The predicted RF (H) dependence describes well our experimental data on 7 • unirradiated and irradiated Y Ba2Cu3O7 bicrystals, from which the core size l(T ), and the intrinsic depairing density J b (T ) on nanoscales of few GB dislocations were measured for the first time. The observed temperature dependence of J b (T ) = J b0 (1 − T /Tc) 2 indicates a significant order parameter suppression in current channels between GB dislocation cores.
Modelling of high-Tc superconductors by means of disordered Josephson-junction arrays
Journal of the Less Common Metals, 1990
A simple model is presented for attempting to reproduce the magnetic response of high-T, granular superconductors to an applied magnetic field. A "sample" is assumed to be composed by 2D array of superconducting grains interacting via Josephson or proximity-effect coupling. We studied the influence of an external magnetic field acting upon the sample. Its ground-state properties, that is, the zero-temperature properties, are examined. In particular we report on the behaviour of the magnetic moment with the field, for different "disordered" samples. We considered both the disorder due to a random distribution of the grain positions and to the bond dilution (always beyond the percolation threshold). This latter is by far more effective, and causes a shrinkage to occur in the butterfly-like hystheresis cycle. It allows, furthermore, to approach through a different way the question of whether a percolative transition of the junction ensemble takes place. The hystheretic effects are only seen when the field is increased beyond a certain critical value, because of vortices in the wave-function phase penetrating into the system. Such vortices are pinned near the defects.
Observation of mesoscopic conductance fluctuations in YBa2Cu3O7−δ grain boundary Josephson junctions
Physical Review B - PHYS REV B, 2007
Magneto-fluctuations of the normal resistance R_N have been reproducibly observed in high critical temp erature superconductor (HTS) grain boundary junctions, at low temperatures. We attribute them to mesoscopic transport in narrow channels across the grain boundary line. The Thouless energy appears to be the relevant energy scale. Our findings have significant implications on quasiparticle relaxation and coherent transport in HTS grain boundaries.
Physical Review B, 1999
The dynamical vortex response of a two-dimensional array of the resistively shunted Josephson junctions in a perpendicular magnetic field is inferred from simulations. It is found that, as the magnetic field is increased at a fixed temperature, the response crosses over from normal to anomalous, and that this crossover can be characterized by a single dimensionless parameter. It is described how this crossover should be reflected in measurements of the complex impedance for Josephson junction arrays and superconducting films.
Quasi-One-Dimensional Intermittent Flux Behavior in Superconducting Films
Physical Review X, 2012
Intermittent filamentary dynamics of the vortex matter in superconductors is found in films of YBa 2 Cu 3 O 7-deposited on tilted substrates. Deposition of this material on such substrates creates parallel channels of easy flux penetration when a magnetic field is applied perpendicular to the film. As the applied field is gradually increased, magneto-optical imaging reveals that flux penetrates via numerous quasi-one-dimensional jumps. The distribution of flux avalanche sizes follows a power law, and data collapse is obtained by finite-size scaling, with the depth of the flux front used as crossover length. The intermittent behavior shows no threshold value in the applied field, in contrast to conventional flux jumping. The results strongly suggest that the quasi-one-dimensional flux jumps are of a different nature than the thermomagnetic dendritic (branching) avalanches that are commonly found in superconducting films.
Self-generated flux in Josephson junctions with alternating critical current density
Physical Review B
We study an interesting state with a self-generated magnetic flux in a long Josephson junction. The critical current density j c is assumed to alternate randomly along the tunnel contact on a length scale much less than the local Josephson penetration depth. The phase difference is then a sum of an alternating term and a smooth term which self-consistently contribute to the formation of a stationary state with self-generated flux. Two types of Josephson vortices are found in this state, one with magnetic flux ⌽ 1 Ͻ⌽ 0 /2 and another one with magnetic flux ⌽ 2 ϭ⌽ 0 Ϫ⌽ 1 Ͼ⌽ 0 /2. ͓S0163-1829͑98͒50306-2͔
Vortex Lattice Structure and Josephson Plasma in High Tc Superconductors
Chinese Journal of Physics- Taipei-
The nature of the Josephson vortices induced by an external magnetic field applied parallel to the CuO2 layers in a high T, superconductor end it ofthe Josephson plasma in the presence ofthe vortices are theoretically investigated. The ordered structure ofthe vortices is obtained by solving the nonlinear differential equation for the interlayer phase difference in a stacked system of Josephson junctions. The current distribution in the Josephson vortex is highly anisotropic and the vortices are arranged in straight chains, dissimilar to hexagonal lattices of Abrikosov and pancake vortices. The appearance of the chain structure in the system is also confirmedby a Monte Carlo simulation. The Josephson plasma is a coupled oscillation of the phase difference between the layers and of the electromagnetic field. The vortices modify the phase difference and thus strongly influence the Josephson plasma oscillation. Therefore, the dispersion spectra of the longitudinal and transverse plasma waves in the presence of the vortices are strongly -dependent on the strength of an external magnetic field and temperature.
A two dimensional model for magnetic flux fractionalization in high Tc superconductors
arXiv (Cornell University), 2004
We show how the recently proposed effective theory for a Quantum Hall system at "paired states" filling ν = 1 [1][2], the twisted model (TM), well adapts to describe the phenomenology of Josephson Junction ladders (JJL) in the presence of defects. In particular it is shown how naturally the phenomenon of flux fractionalization takes place in such a description and its relation with the discrete symmetries present in the TM. Furthermore we focus on "closed" geometries, which enable us to analyze the topological properties of the ground state of the system in relation to the presence of half flux quanta.