Fractional Josephson vortices at YBa$_2$Cu$3$O${7-x}$ grain boundaries (original) (raw)

Nonlocal electrodynamics of Josephson junctions in thin films and fractional vortices

Superconductor Science & Technology, 2002

The phase difference ϕ across a Josephson junction is considered for a film with a thickness d λ, where λ is the London penetration depth in the superconducting banks. Special attention is given to the case of a critical current density j c varying along the junction. It is shown that a nonlinear integro-differential equation determines the spatial distribution of ϕ for d λ. Josephson properties of grain boundaries in thin-YBCO films are treated for the case of j c alternating along these boundaries. It is shown that if the typical amplitude of alternations of j c is high compared to the average value of j c , then a spontaneous flux and two types of fractional Josephson vortices can be observed. The fractional Josephson vortices keep magnetic fluxes φ 1 and φ 2 , where φ 1 + φ 2 = φ 0 , φ 0 is flux quantum, and φ 1 < φ 0 /2, φ 2 > φ 0 /2. We demonstrate that these fractional vortices can be observed in thin-YBCO films under conditions of appearance of the spontaneous magnetic flux. A method is proposed to extract the fractional vortices from the experimental flux patterns. Propagation of an electromagnetic wave along a grain boundary with an alternating critical current density is treated as an example of an application of the integro-differential equation for the phase difference ϕ.

Observation of Splintered Josephson Vortices at Grain Boundaries in YBa2Cu3O7-δ

Physical Review Letters, 2002

We have directly observed well-separated Josephson vortex splinters with unquantized magnetic flux at asymmetric 45 grain boundaries in YBa 2 Cu 3 O 7ÿ films by imaging magnetic flux with scanning SQUID microscopy. The existence of these splinter vortices has been predicted and is well described by a model based on d x 2 ÿy 2 pairing symmetry and facetting of the grain boundary on a length scale shorter than the Josephson penetration depth.

Observation of splintered Josephson vortices at grain boundaries in YBa2Cu3O7-d

Physical Review Letters, 2002

We have directly observed well-separated Josephson vortex splinters with unquantized magnetic flux at asymmetric 45 grain boundaries in YBa 2 Cu 3 O 7ÿ films by imaging magnetic flux with scanning SQUID microscopy. The existence of these splinter vortices has been predicted and is well described by a model based on d x 2 ÿy 2 pairing symmetry and facetting of the grain boundary on a length scale shorter than the Josephson penetration depth.

Observation of Splintered Josephson Vortices at Grain Boundaries in YBa2Cu3O7-delta

Physical Review Letters, 2002

We have directly observed well-separated Josephson vortex splinters with unquantized magnetic flux at asymmetric 45° grain boundaries in YBa2Cu3O7-δ films by imaging magnetic flux with scanning SQUID microscopy. The existence of these splinter vortices has been predicted and is well described by a model based on dx2-y2 pairing symmetry and facetting of the grain boundary on a length scale shorter than the Josephson penetration depth.

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.