Supercurrent density above 106A∕cm2 at 77K in a single-crystal film conductor of the cuprate high-Tc superconductor YBa2Cu3O7−δ—dream or reality?

Abstract

Supercurrent transport phenomena in c-oriented epitaxial thin films of the the high-Tc superconducting (HTS) cuprate YBa2Cu3O7−δ (YBCO) with a high critical current density Jc(77K)⩾2×106A∕cm2 are investigated by four-probe transport measurements, low-frequency magnetic susceptibility studies, and SQUID magnetometry. The film samples are deposited on a single-crystal sapphire (r-cut) substrates with a CeO2 or LaAlO3(100) buffer layer by off-axis dc magnetron sputtering or pulsed laser ablation. A model of the mechanisms of Abrikosov vortex pinning and supercurrent limitation is developed and discussed by comparing its predictions with the results of measurements of the critical current and its dependence on applied magnetic fields of different strength and orientation and also with nanostructure data obtained by high-resolution transmission electron microscopy and electron diffraction in a backscattering geometry. It is shown that the low-angle subboundaries (LABs) formed between domains with a slight azimuthal misorientation during the epitaxial growth of the film play a key role in the phenomena observed in the transport of supercurrent. The tilt LABs form equidistant ordered rows of edge dislocations with nonsuperconducting cores about 3–4nm in diameter. The dislocation lines in the LABs are parallel to each other and perpendicular to the film plane. The average density of dislocations over the area of the film depends on the real statistics of the random system of LABs and can reach 1011cm−2. Since the diameter of the “normal” core of a dislocation is close to the diameter of the core of an Abrikosov vortex, the elementary pinning force of the vortex to the core of the dislocation is close to the maximum possible. The pinning on dislocation subboundaries has the following characteristics: 1) the achievement of high values Jc(77K)⩾2×106A∕cm2 and Jc(20K)>107A∕cm2 in epitaxial films and conductors; 2) the existence of a “plateau” on the Jc(H) curve, i.e., Jc(H)=const for H<Hm; 3) a logarithmic decline of Jc(H) for H>Hm, i.e., at the transition from the single-particle pinning regime to the collective pinning of the vortex lattice on the statistical ensemble of randomly distributed dislocation subboundaries; 4) the existence of a threshold field Hp that determines the limit up to which the vortices in a thin film (d⩽λ) remain rectilinear and perpendicular to the film even in a field inclined at a large angle; 5) the evolution of the angle dependence of Jc(θ) with a change of field strength is in complete agreement with the model of dominant pinning on “threading” edge dislocations. A new “peak effect”—an increase of Jc(H‖ab) with increasing longitudinal field—is observed for the first time for H>Hm, i.e, after the end of the “plateau” Jc(H‖ab)=const.