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 YBa2Cu3O7−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.

The microwave vortex dissipation of superconducting Nd, 8, Ceo»CuO epitaxial films is investigated by use of a sapphire dielectric-ring resonator in the whispering gallery modes, and in dc magnetic fields ranging from 0 to 25 kG. By extending the CoFey-Clem model to the thin-film and finite-grain-size limit, we infer important material parameters, including the irreversibility line, vortex pinning potential, pinning force constant and viscosity, through a direct least-squares fit to the microwave-absorption data.The complex physical properties of the vortex states of high-temperature superconductors (HTS's) have stimulated intense research effort. ' ' However, most experimental work in high magnetic fields has mainly focused on the low-frequency transport' and magnetic ' properties of vortices. Because of practical concerns of HTS device applications, most high-frequency (radio and microwave) measurements concentrate on the properties in dc magnetic fields smaller than the lower critical field 8"(T). ' Recent theoretical advances in the vortex electrodynamics of HTS systems" ' have prompted new experimental efforts' ' to investigate the microwave properties of type-II superconductors in large dc magnetic fields. In this paper we report microwave studies of Ndi &&Ceo &5CuO (Nd-Ce-Cu-0) epitaxial films in large dc magnetic fields with the use of a sapphire dielectricring resonator. By extending the Coffey-Clem model' ' of forced-damped harmonic oscillators in the presence of a Langevin force to the thin-film and finite-grain-size limit, we show that important material parameters can be determined through a direct least-squares fit to the microwave absorption data. Finally, we compare the vortex dynamics at microwave frequencies to the critical dynamics near the vortex-solid melting transition.The technique employed for measuring the vortex dissipation involves the use of the whispering gallery modes of a sapphire dielectric-ring resonator. ' This unconventional technique was originally developed for a more stable maser clock. ' In this work we show the application of the sapphire ring resonator to superconductivity research. The following advantages associated with the sapphire ring resonator make the technique more favorable over conventional metallic and superconducting resonators. First, the resonators show high-Q values (~10 ) over a broad temperature range. ' Second, there is no additional loss in the presence of high magnetic fields. Third, each resonator provides several high-Q resonant modes, so that the frequency dependence can be conveniently investigated.The sapphire resonator is machined from a monocrystal, with the high-symmetry axis along the crystalline optic axis. The surface of the resonator is polished to optical Aatness and has been annealed at high temperatures after the machining. The ring resonator is suspended by a sapphire rod inside a cylindrical, thin-walled, and gold-plated copper shield. The shield is to reduce the radiation loss, and the combination of the low dielectric loss of the sa...

The anisotropic vortex transport properties of c-axis-oriented Nd& 85Ceo»Cu04 "superconducting epitaxial films are found consistent with a dislocation-mediated second-order vortex-solid melting transition. The anisotropic critical exponents and the electronic mass ratio are determined from the scaling of the current-voltage characteristics. The nucleation energy of vortex dislocations due to both thermal and disorder fluctuations and its relation to the vortex elastic moduli is inferred from our experiments.

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