Grain Boundaries In High-temperature Superconductors Research Articles

Transport Properties of Low-Angle Grain Boundaries in High-Temperature Superconductors under the Model of Split Dislocations of the Boundary

Transport Properties of Low-Angle Grain Boundaries in High-Temperature Superconductors under the Model of Split Dislocations of the Boundary

Mechanisms for enhanced supercurrent across meandered grain boundaries in high-temperature superconductors

It has been well established that the critical current density Jc across grain boundaries (GBs) in high-temperature superconductors decreases exponentially with misorientation angle θ beyond ∼2°–3°. This rapid decrease is due to a suppression of the superconducting order parameter at the grain boundary, giving rise to weakly pinned Abrikosov-Josephson (AJ) vortices. Here we show that if the GB plane meanders, this exponential dependence no longer holds, permitting greatly enhanced Jc values: up to six times at 0T and four times at 1T at θ∼4°–6°. This enhancement is due to an increase in the current-carrying cross section of the GBs and the appearance of short AJ vortex segments in the GB plane, confined by the interaction with strongly pinned Abrikosov (A) vortices in the grains.

Self-enhanced flux penetration into small angle grain boundaries in YBCO thinfilms

Grain boundaries in high-temperature superconductors are well known to strongly decreasethe critical current. However, most information about their superconducting properties isbased on transport measurements which always give non-local, averaged data over thewhole sample size.In this paper, we report on the spatially resolved magneto-optical measurementsof the magnetic flux and current density distribution in bicrystallineY Ba2Cu3O7−x thin films with small angle grain boundaries. The current distributions are obtained by amodel-independent inversion of Biot–Savart’s law.In contrast to Bean-like models of the current pattern at the grain boundary,we find an inhomogeneous current density across the grain boundary, which isstrongly influenced by the magnetic stray fields of the currents in the grains. Theself-field effects are also responsible for a self-enhanced flux penetration wherethe penetration depth is significantly increased compared to a Bean-like model.

Flux pinning and critical currents at low-angle grain boundaries in high-temperature superconductors

Calculations of the pinning potential at low-angle grain boundaries in high-temperature superconductors are presented which fully incorporate the periodic nature of the low-angle boundary. A nonlocal kernel provides a smooth transition from an Abrikosov vortex far from the boundary to a Josephson vortex near the dislocations. We examine the angular dependence of critical current in the two idealized limits of pure strain and pure band bending. Recent data appear limited by band bending with significant potential for improvement through doping.

Flux flow of Abrikosov-Josephson vortices along grain boundaries in high-temperature superconductors.

Low-angle grain boundaries (GBs) in superconductors exhibit intermediate Abrikosov vortices with Josephson cores, whose length l along GB is smaller than the London penetration depth, but larger than the coherence length. We found an exact solution for a periodic vortex structure moving along GBs in a magnetic field H and calculated the flux flow resistivity R(F)(H), and the nonlinear voltage-current characteristics. The predicted R(F)(H) dependence describes well our experimental data on 7 unirradiated and irradiated YBa(2)Cu(3)O(7) bicrystals, from which the core size l(T), and the intrinsic depairing density J(b)(T) on nanoscales of a few GB dislocations were measured for the first time. The observed J(b)(T) = J(b0)(1-T/T(c))(2) indicates a significant order parameter suppression on GB.

Electric field distribution at low-angle grain boundaries in high-temperature superconductors

Electric fields $\mathbf{E}$ are created in type-II superconductors by thermally activated flux creep or flux flow, driven by electric currents through the specimen. Usually, an average value $〈E〉=V/L$ is determined in resistive four-terminal-transport measurements by measuring the voltage V between two contacts with distance L as a function of the applied current. However, this average value can deviate by orders of magnitude of the true local value $\mathbf{E}(x,y),$ if inhomogeneities like grain boundaries are present in the specimen. In this article, we show the spatial distribution of $\mathbf{E}(x,y)$ of high-temperature superconducting bicrystalline films with a low-angle grain boundary in the flux-creep state. Even in a somewhat ``relaxed state,'' the electric-field value in bicrystalline samples varies by about two orders of magnitude.

High-T c Bicrystal Grain Boundaries

Grain boundaries in high-temperature superconductors illustrate the interplay of physics, materials science, and applications in complex materials.

Erratum: Current transport through low-angle grain boundaries in high-temperature superconductors [Phys. Rev. B57, 13 878 (1998)

Erratum: Current transport through low-angle grain boundaries in high-temperature superconductors [Phys. Rev. B<b>57</b>, 13 878 (1998)

Current transport through low-angle grain boundaries in high-temperature superconductors

We consider mechanisms which can account for the observed rapid decrease of the critical current density ${J}_{c}(\ensuremath{\theta})$ with the misorientation angle $\ensuremath{\theta}$ through grain boundaries (GB's) in high-${T}_{c}$ superconductors (HTS's). We show that the ${J}_{c}(\ensuremath{\theta})$ dependence is mostly determined by the decrease of the current-carrying cross section by insulating dislocation cores and by progressive local suppression of the superconducting order parameter $\ensuremath{\psi}$ near GB's as $\ensuremath{\theta}$ increases. The insulating regions near the dislocation cores result from a strain-induced local transition to the insulating antiferromagnetic phase of HTS's. The structure of the nonsuperconducting core regions and current channels in GB's is strongly affected by the anisotropy of the strain dependence of ${T}_{c}$ which is essentially different for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ and Bi-based HTS's. We propose a mechanism of the progressive superconductivity suppression on GB's with $\ensuremath{\theta}$ due to an excess ion charge on the GB's which shifts the chemical potential in the layer of the order of screening length ${l}_{D}$ near the GB's. The local suppression of $\ensuremath{\psi}$ is amplified by the proximity of all HTS's to a metal-insulator transition, by their low carrier density and extended saddle point singularities in the electron density of states near the Fermi surface. Taking into account these mechanisms, we calculated ${J}_{c}(\ensuremath{\theta})$ analytically by solving the Ginzburg-Landau equation. The model well describes the observed quasiexponential decrease of ${J}_{c}(\ensuremath{\theta})$ with $\ensuremath{\theta}$ for many HTS's. The $d$-wave symmetry of the order parameter weakly affects ${J}_{c}(\ensuremath{\theta})$ in the region of small $\ensuremath{\theta}$ and cannot account for the observed drop of ${J}_{c}(\ensuremath{\theta})$ by several orders of magnitude as $\ensuremath{\theta}$ increases from 0 to $\ensuremath{\theta}\ensuremath{\simeq}20\ifmmode^\circ\else\textdegree\fi{}--40\ifmmode^\circ\else\textdegree\fi{}$.

Charged grain boundaries as pinning centers in high- Tc superconductors

The theory of the Abrikosov vortex pinning by charged grain boundaries in high-temperature superconductors is presented. We show that due to the small ratio ζ/ l D inhigh-temperature superconductors (where ζ is the coherence length and l D is the Debve screening length) the attraction of the Abrikosov vortices to the grain boundary is significantly greater than in conventional superconductors.