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 −...

Practical high-temperature superconductors must be textured to minimize the reduction of the critical current density J(gb) at misoriented grain boundaries. Partial substitution of Ca for Y in YBa(2)Cu(3)O(7-delta) has shown significant improvement in J(gb) but the mechanisms are still not well understood. Here we report atomic-scale, structural and analytical electron microscopy combined with transport measurements on 7 degrees [001]-tilt Y(0.7)Ca(0.3)Ba(2)Cu(3)O(7-delta) and YBa(2)Cu(3)O(7-delta) grain boundaries, where the dislocation cores are well separated. We show that the enhanced carrier density, higher J(gb) and weaker superconductivity depression at the Ca-doped boundary result from a strong, non-monotonic Ca segregation and structural rearrangements on a scale of approximately 1 nm near the dislocation cores. We propose a model of the formation of Ca(2+) solute atmospheres in the strain and electric fields of the grain boundary and show that Ca doping expands the dislocation cores yet enhances J(gb) by improving the screening and local hole concentration.