D. S. Reed

Vortex phases and dynamics in high-temperature and conventional amorphous superconductors

The vortex phases and dynamics of high-temperature and conventional amorphous superconductors are investigated over a broad frequency range. A second-order vortex-solid melting transition in both YBa2Cu3O7 single crystals with random point defects and amorphous Mo3Si films is manifested by a set of universal critical exponents (ν≈23 and z≈3) from both ac and dc transport as well as ac magnetic susceptibility measurements. In contrast, the melting transition in YBa2Cu3O7 single crystals with correlated columnar defects is found to be consistent with a Bose-glass phase transition characterized by a different set of critical exponents (ν⊥≈12 and z′≈4).

Experimental investigations of the critical vortex dynamics in extreme type-II superconductors with controlled static disorder

Experimental investigations of the vortex dynamics in high-temperature and conventional amorphous superconductors reveal universal critical phenomena which are associated with various phase transitions in the vortex systems with different types of static disorder. Evidence for second-order vortex-glass, Bose-glass and splayed-glass transitions are manifested by the universal critical exponents and scaling functions via measurements of ac and dc transport properties, ac magnetic susceptibility, ac third-harmonic transmissivity, and microwave surface impedance. In contrast, current-induced effects in the weak pinning limit result in novel non-equilibrium vortex dynamics which conceal the direct detection of the order of the thermodynamic vortex-solid melting transition with electrical transport measurements.

Vortex phases and dynamics of high-temperature superconductors with controlled defects

Abstract Vortex dynamics of YBa2Cu3O7 single crystals with controlled point and columnar defects are investigated near the vortex-solid to vortex-liquid phase transition. The critical exponents and the vortex phase boundaries for vortex-glass, Bose-glass, splayed-glass and vortex-liquid are determined.

ANISOTROPIC VORTEX DYNAMICS AND PHASE DIAGRAM OF YBa2Cu3O7 SINGLE CRYSTALS WITH CANTED COLUMNAR DEFECTS

The anisotropic vortex dynamics and phase diagram are determined for a YBa2Cu3O7 single crystal with columnar defects oriented at ±7.5° relative to the crystalline c-axis. A second-order splayed-glass to vortex-liquid transition is manifested for magnetic fields nearly parallel to the columns via the critical scaling of vortex AC and DC transport properties. In contrast, for magnetic fields aligned close to the ab-plane, an XY-like vortex-glass transition prevails. For magnetic fields at intermediate angles, there is no evidence of any vortex phase transition, and the vortex dynamics is described in terms of the thermally activated flux flow model.

High-Frequency Vortex Critical Dynamics of Superconducting Amorphous Mo3Si Films

The effects of static disorder on the vortex dynamics of type-II superconductors are investigated by comparing the high-frequency vortex response of superconducting amorphous Mo3Si (a-Mo3Si) films with that of the high-temperature superconductors. We find that for a-Mo3Si films in the three-dimensional limit, the microwave vortex response near the second-order vortex-solid to vortex-liquid glass transitions is consistent with vortex critical relaxation, in contrast to the diffusion vortex dynamics in high-temperature superconductors at the same frequencies. The observation of microwave vortex critical dynamics in a-Mo3Si is attributed to the extremely disordered nature of the amorphous superconductors, which results in a much shorter-range vortex correlation and therefore a faster critical relaxation.

Effects of Static Disorder on Vortex Phases and Dynamics of Type-II Superconductors — A Scaling Approach

The effects of static disorder on the vortex phases and dynamics of high-temperature and conventional amorphous superconductors are investigated with various experimental techniques from DC to microwave frequencies. It is found that the interaction between vortices and static defects results in symmetry-breaking and slower critical dynamics upon increasing disorder. Experimental evidence for second-order vortex-glass, Bose-glass and splayed-glass transitions are manifested by critical scaling of various physical quantities near the phase transitions via measurements of AC and DC transport properties, AC magnetic susceptibility, AC third-harmonic transmissivity, and microwave surface resistance. Effects of disorder on the Hall conductivity in the vortex-liquid state are also discussed.

Effects of Controlled Defects on the Vortex-Solid Melting Transition of Y-Ba-Cu-O Single Crystals

We report systematic studies of the dc transport properties in proton-irradiated Y-Ba-Cu-O single crystals. We find that the onset of vortex dissipation in moderately irradiated samples is associated with the occurrence of a second-order vortex-solid melting transition. In addition, the decreasing zero-field transition temperature and increasing critical current density with the increasing defects reveal the effects of disorder on reducing the electron mean-free-path and on increasing the pinning density.

Vortex Dynamics and Dissipation in High Temperature Superconductors — from DC to Microwave Frequencies

A second-order vortex-solid melting transition in twinned Y-Ba-Cu-O single crystals is manifested by two independent types of electrical transport measurements: the electric field (E) versus current density (J) isotherms, and the ac resistivity (ρ) vs current frequency (ω) isotherms. Universal static and dynamic exponents (ν ≈ 2/3 and Ζ ≈ 3, respectively) are found for magnetic fields ranging from 1 to 90 kOe, frequencies ranging from 0 to 2 MHz, magnetic directions parallel and perpendicular to the crystal c-axis, as well as samples with and without proton irradiations. At microwave frequencies, we find that the vortex dissipation in Nd-Ce-Cu-O epitaxial films is consistent with the viscous motion of individual vortices, due to the break down of the critical scaling theory in the high frequency limit.