Memory formation and evolution of the vortex configuration associated with random organization

Abstract

We study the general phenomenon of random organization using a vortex system. When a periodic shear with a small shear amplitude d inp is applied to many-particle (vortex) assemblies with a random distribution, the particles (vortices) gradually self-organize to avoid future collisions and transform into an organized configuration. This is detected from the time-evolution of the voltage V t ( ) (average velocity) that increases towards a steady-state value. From the subsequent readout measurements of V t ( ) using various shear amplitudes, we find that the information of the input shear amplitude d inp is memorized in the configuration of the vortex distributions in the transient as well as the steady state, and that it is readable. We also find that the transient vortex configuration formed during random organization is not microscopically homogeneous but consists of disordered and organized regions.

Competition between dynamic ordering and disordering for vortices driven by superimposed ac and dc forces

“…After the vortex configuration of the plastic flow was frozen by turning off I, the vortices were then driven periodically by square I ac that was applied at t=0, and time-dependent V(t) induced by vortex motion was measured. The displacement was kept to be d inp =1 μm, which was immediately above d c ≈0.1 μm of RIT [30], and ¥ V was fixed to be 50 μV. After many cycles of the ac drive (n10 4 cycles), the system reached the steady state, which corresponds to the irreversible-flow state in the vicinity of the reversible-flow state, i.e.…”

“…It is noted that the height of the first voltage pulse, V 0 , is much smaller than ¥ V and that | ( )| V t increases monotonically with t or cycle numbers n, relaxing to the steady-state voltage ¥ V . | ( )| V t is reproduced by a relaxation function [4,30] using a single relaxation time τ:…”

“…the acceleration of current relaxation in the vortex state caused by ac field), which has been found experimentally [24][25][26] and explained theoretically [27][28][29]. To examine how random organization proceeds with increasing the number of shear cycles n, we have recently performed [30] two-step measurements [6,7] for the vortex system in the strip-shaped a-Mo x Ge 1−x film [5,22]. In the first experiment, which we name an 'input' experiment, a highly disordered initial vortex configuration [31] was subjected to ac shear with a small displacement d inp just above the critical one d c for RIT.…”

“…), and it is readable. From the analysis of readout V(t), it was found that the transient configuration of vortices created during the random-organization process is not uniform, at least in microscopic scale, but comprises organized and disordered regions (DRs) [30].…”

“…Then, interesting questions arise as to how random organization due to the ac drive is suppressed by superimposing the dc drive, and what configuration such vortex assemblies should take finally. To answer the fundamental and non-trivial questions, here we have performed two-step measurements of V(t) similar to those as mentioned above [30]. In the first input experiment, a highly disordered initial vortex assembly was sheared by an ac current I ac with a square waveform that yields a small displacement d inp and voltage amplitude ¥ V at  ¥ t .…”