Y Kawamura

Irreversibility and Dislocation in Periodically Sheared Vortex Matter

Recently, we have provided evidence for a reversible-to-irreversible flow transition, as well as a depinning transition, in periodically sheared vortices in a Corbino-disk (CD) superconductor [S. Okuma, Y. Tsugawa, and A. Motohashi: Phys. Rev. B 83 (2011) 012503]. Here, to obtain a possible microscopic picture of reversible and irreversible flow, we measure the time evolution of voltage V ( t ) generated by ac-driven vortices in a CD, focusing on how the initial vortex configuration affects the transient vortex dynamics. When the initial vortex configuration is disordered or prepared by freezing the irreversible flow, the increase in | V ( t )| toward the t →∞ unique state, indicative of random organization, is observable. By contrast, when the initial vortex configuration is ordered or prepared by freezing the reversible flow, only a fast decay in V ( t ) resulting from the dynamic pinning of driven vortices is observed. The results are consistent with the view that the irreversible vortex motion occurs ...

Random organization of vortices under an anisotropic condition

Many colliding particles that are periodically sheared by ac drive self-organize to avoid future collisions, which is known as random organization. Recently, we have observed the random organization in the vortex system of a strip-shaped amorphous Mo x Ge1−x film, where the vortices experience periodic local shear from ac drive and the random pinning potential. In this work, we study how random organization changes in the vortex system under the tilted field, where an anisotropic vortex-vortex interaction is introduced. We find that characteristic times of random organization for the vortices driven in the tilted direction are significantly smaller than those in the untilted field.

Dynamic Melting of Driven Abrikosov Lattices in an Amorphous MoxGe1−x Film in Tilted Field

We report a comparative study of the dynamic melting of driven vortex lattices in magnetic field tilted (by θ = 36°) from the normal to the film surface and that of a driven Abrikosov lattice in untilted field (θ = 0). From the mode-locking (ML) resonance, we confirm that vortex lattices in tilted field are stretched in the tilt direction and that, with increasing dc velocity at ML, the shape and orientation of the driven lattice change. Associated with this structural change, the dynamic melting field at which the driven lattice melts also changes. Our results show that, regardless of the lattice shape and orientation, dynamic melting occurs as the shorter side of the distorted lattices reaches close to the side at which the isotropic lattice melts dynamically.

Unusual current–voltage characteristics and dynamic correlation of rotating vortex-lattice rings

We report the unusual dc current–voltage (I–V) characteristics for the vortex-lattice rings rotated by a dc current I superimposed with ac currents of different amplitudes Irf in a Corbino disk of an amorphous MoxGe1−x film. As Irf is increased from zero, the I–V curve in the large I region shows a trend to decrease. This is in contrast to the usual behavior of vortex flow in which ac-induced depinning leads to an increase in the flow voltage. We attribute the finding to the Irf-induced growth in the transverse dynamic correlation of the rotating lattice rings.

Dynamics of Superconducting Vortices Driven by Periodic Shear in the Plastic-Flow and Flux-Flow Regimes

We measure the time-dependent voltage V(t) (velocity) of periodically sheared vortex matter by an ac current Iac in a Corbino-disk superconductor with random pinning. The amplitude of the voltage |V(t)| grows toward a ‘steady-state’ value |V(∞)|(≡ V∞) indicative of dynamic ordering. Here, to clarify how the disorder in the moving vortex lattice influences on the dynamic ordering, we systematically reduce the number of dislocations in the driven lattice by increasing |Iac|, thus increasing V∞, thereby transforming plastic flow (PF) with many dislocations to flux flow (FF) with few dislocations. With increasing V∞ in PF, the relaxation time to reach the steady state decreases steeply: after showing a peak at around the transition region between PF and FF, it falls to near zero in the FF state. Our results support the notion that the vortex configuration of FF corresponds to the reversible flow and that the onset of irreversibility corresponds to the emergence of PF in FF.