We have studied single‐grain‐boundary junctions in the neodymium‐, bismuth‐, and thallium‐based cuprate superconductors and find that they behave as weak links, qualitatively similar to the YBa₂Cu₃O_7−δ superconductor. In general, the grain boundary critical current is determined by flux flow for small misorientation angles and by Josephson junctionlike coupling for large angles. The latter is verified by the observation of voltage oscillations with an external magnetic field in superconducting quantum interference devices built using single‐grain‐boundary junctions of these materials. The commonality of behavior of grain boundaries in all of the high temperature cuprate superconductors suggests that the weak link is most likely associated with the structure of the grain boundary and the evidence points increasingly to dislocations, which describe the topology of the boundary.

Measurements of the in-plane magnetic-field penetration depth for the n-doped high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ are reported. The measurements were performed at 4 GHz by a microstrip resonator technique on thin films obtained by laser ablation. The temperature dependence of the penetration depth is carefully analyzed and the energy gap-ratio is estimated. The results confirm that ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ behaves as a BCS-like superconductor.

We have recently reported on the persistence of a significant zero voltage current across bicrystal grain boundary junctions of YBa₂Cu₃O_7−δ in an external magnetic field of several Tesla. In this note we present data on the dependence of this residual current on temperature and orientation of the grain boundary. A description of the grain boundary as being comprised of superconducting microbridges separated by normal regions is consistent with the experimental data.

We have investigated the effect of midgap bound states on the Josephson properties of symmetric 45° [001] tilt bicrystal grain-boundary junctions (GBJ’s) characterized by different junction widths. GBJ’s, $1\mu \mathrm{m}$ wide, show a monotonic temperature dependence of the Josephson current with a slight upwards curvature, the result of the competition between midgap states and the opposite-sign continuous current. In some junctions, an increasing contribution of the midgap-state-mediated current is observed at low temperatures, leading to an anomalous nonmonotonic temperature dependence of the critical current. This behavior is more evident for junctions smaller than $500\mathrm{nm}$, where, in some cases, a 0- to $\pi$-junction transition is clearly observed. Experimental results agree qualitatively with theoretical models taking into account midgap states in strongly nonuniform barrier interfaces.

We report on the magnetic field and angular dependence of the critical current density of epitaxial FeTe_0.5Se_0.5 thin films. The films exhibit high critical current values and weak dependence on the applied magnetic field. The J_c is larger for field parallel to the c-axis, which is the opposite behavior of what expected from the critical field anisotropy. The analysis of the activation energy for vortex motion indicates that the single pinning regime holds up to 9 T, suggesting that correlated pinning centers are more effective than the vortex-vortex interaction even at the largest applied fields. Scanning tunneling microscope analysis indicates threading dislocations as possible pinning centers.

The critical current (I_c) of YBa₂Cu₃O_7−δ grain boundary Josephson junctions was measured up to magnetic fields of 5 T. Magnetic field history dependent I_c values were observed even after correction for self‐field effects stemming from hysteretic shielding currents in the grain adjacent to the boundary. A novel feature observed is an anomalous increase in I_c in high magnetic fields of several Telsa.