Flux Flow of Abrikosov-Josephson Vortices along Grain Boundaries in High-Temperature Superconductors

Abstract

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.PACS numbers: PACS numbers: 74.20. De, 74.20.Hi, Mechanisms of current transport through grain boundaries (GB) in high-temperature superconductors (HTS) have attracted much attention, because a GB is a convenient tool to probe the pairing symmetry of HTS by varying the misorientation angle ϑ between the neighboring crystallites [1,2]. As ϑ increases, the spacing between the GB dislocations decreases, becoming comparable to the coherence length ξ(T ) at the angle ϑ 0 ≃ 4 − 6 • . The exponential decrease of the GB critical current density, makes GBs one of the principal factors limiting critical currents of HTS [3]. Atomic structure of GBs revealed by high-resolution electron microscopy have been used to determine local underdoped states of GB, defect-induced suppression of superconducting properties at the nanoscale and controlled increase of J b by overdoping of GB [2,4]. Much progress has been made in understanding the microscopic factors controlling J b (ϑ) at zero magnetic field, but the behavior of vortices on GBs is known to much lesser extent.The extreme sensitivity of J b (ϑ) to the misorientation angle makes GB a unique tool to trace the fundamental transition between Abrikosov (A) and Josephson (J) vortices[5] in a magnetic field H above the lower critical field H c1 . For ϑ ≪ ϑ 0 , vortices on a GB are A vortices with normal cores pinned by GB dislocations [6]. For ϑ > ϑ 0 , the maximum vortex current density circulating across the GB is limited to its intrinsic J b (ϑ), much smaller then the bulk depairing current density J d . Because vortex currents must cross the GB which can only sustain J b ≪ J d , the normal core of an A vortex turns into a J core, whose length l ≃ ξJ d /J b along the GB is greater then ξ, but smaller then the London penetration The AJ structures have two length scales: the core size l > ξ and the intervortex spacing a = (φ 0 /B) 1/2 . The larger core of AJ vortices leads to their weaker pinning along a GB, which thus becomes a channel for motion of AJ vortices between pinned A vortices in the grains [5,7] (Fig. 1). The percolative motion of AJ vortices along GBs gives rise to a linear region in the V −...

Microwave properties of DyBCO monodomain in the mixed state and comparison with other RE-BCO systems

“…Independently from the adopted model, the main signature of GB in the in-field microwave surface impedance consists in an abrupt, quasi-step-like increase of the surface resistance R s with increasing field, followed by a flat plateau [14,18]. The field scale over which the step actually extends varies from a few mT [14,18] for weak-links and Josephson vortices up to 0.1 T or larger for AJ vortices [13,19].…”

Scanning laser imaging of dissipation in YBa2Cu3O7−δ-coated conductors

“…Such ␦V patterns are consistent with vortices channeling along GBs. 11,14 It is striking that the major dissipation at 227 mA, approximately twice I c , is still associated with the initially dissipative channels and is highly nonuniform. This has important implications for thermal stability of CC in applications.…”

“…(3) we measured ␦V maps on a YBCO epitaxial films grown by pulsed laser deposition on SrTiO 3 single crystals. 11 The films were 250 nm thick, W = 200 m wide, and 2 mm long, a cut of 1 m width and 0.1 W length was patterned perpendicular to the film edge. The steadystate distribution of E͑x , y͒ for this case was calculated by solving the nonlinear Maxwell equations by the hodograph method 5 for the power-law E-j characteristics, E = ͑j / j c ͒ n E c with n = 6.…”

Critical Current Oscillations in the Intrinsic Hybrid Vortex State of SmFeAs(O,F)

“…In between these extreme cases, the situation of partial suppression of ψ may result in the formation of intermediate vortices characterized by deformed core regions. Such intermediate vortices, called hybrid or A-J, have been observed in isolated, strongly coupled junctions such as YBCO low-angle grain boundaries [1] or artificial layered structures [2,3], but so far no indications of intrinsic hybrid vortices within the unit cell have been observed.…”

“…Hence, the two key ingredients theoretically expected in hybrid vortices [1] are essential to the observation of this phenomenon in SmFeAs(O,F): (1) A core region small enough to gain energy from aligning with the intrinsic potential, and (2) an incomplete Josephson nature to allow vortex lattice relaxation along the c-axis. This occurs exactly at the T ⋆ transition, as illustrated in Fig 4 (middle panel).…”