講演題目：　Intermittently Flowing Rivers of Quantized Magnetic Flux: Vortex Motion, Noisy Fractal Networks, and Flux Avalanches Superconductors.

講　師　：　Prof. Franco Nori
　　　　　 （米国ミシガン大学物理学科）

日　時　：　平成１０年６月２９日（月）　１０：３０〜１１：３０
場　所　：　北海道大学工学部　Ａ１０８教室（１階）

要　旨　：
This general talk will be mostly about vortex dynamics and a bit about granular media. After a pedagogical introduction, the main ideas and several results will be presented, sometimes using videos. A more technical description follows below. Vortices in superconductors exhibit a variety of equilibrium phases, including liquid, lattice, and glassy states. In addition, vortices might be driven, producing several dynamical phases with steady states of plastic and elastic motion. We characterize the dynamical instabilities (i.e., flux avalanches or cascades producing voltage bursts), as well as the evolution of the topological order and vortex flow paths ("vortex streets" surrounded by regions of pinned flux). Our analysis of the microscopic spatio-temporal dynamics of individual flux-lines in superconductors leads insight to commonly measured bulk macroscopic quantities, such as magnetization and critical currents. We have studied [1] flux-gradient-driven flux lines (i.e., there is no artificial uniform external force on the vortices) as an external field H(t) is quasi-statically ramped up and down. We explore a wide variety of relevant parameters which are difficult to continuously tune experimentally, such as the density, strength, radius, and location of the pinning sites. Our predictions (e.g., magnetization hysteresis loops) can be directly compared with commonly-measured experimental quantities. We analyze both global (e.g., M(H), J_c(H)) and local (e.g.,B(x,y,H(t)), M(x,y,H(t)), J_c(x,y,B)) measurable quantities. Our results elucidate the topological order dynamics of a driven plastic lattice interacting with a rigid disordered substrate, a problem that has recently attracted considerable attention [2].

[1] C. Reichhardt et al, Phys. Rev. B 52, 10441 (1995); 53, R8898 (1996); 54, 16108 (1996); Phys. Rev. Lett. 78, 2648 (1997); J. Groth et al, Phys. Rev. Lett. 77, 3625 (1996); Olson et al, Phys. Rev. B 56, 6175 (1997); Phys. Rev. Lett. 80, 2197 (1998); and preprints.
[2] F. Nori, Science 271, 1373 (1996).

　　　　　　　　　　　　　　　　
世話人　 田 村 信 一 朗
北海道大学・大学院工学研究科・量子物理工学専攻
（TEL 706-6714）