Flux Behavior in the Bose Glass Regime near the Mott Transition

We present molecular dynamics simulations of superconducting vortices interacting with columnar pinning sites as an external field is quasi-statically swept through the commensurate field B_{phi}. We analyze the local flux profile and magnetization and find a sharp transition in the flux profile as the local flux density passes B_{phi} when the vortices pass from the strong pinning at individual pinning sites to a weaker interstitial pinning. We also analyze the voltage fluctuations and coordination number as functions of H which characterizes the change over from strong to weak pinning regimes.

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Flux density profiles
Fig. 1 - Magnetic flux density profiles. Here, B_[\phi]=1.0. For the initial rampup phase in (1), a total of 2600 vortices are added so the external field is 2.0. In (2), the field is ramped down and reversed to a final value of -2.0. Finally, in (3), the field is brought back up to 2.0. A large gradient in B can be seen for |B|<B_{\phi}.

Vortices and pinning sites
Fig. 2 - An x8 region of a 24*36 sample studied in Fig. 1. The vortices enter the left edge of each frame (which corresponds to the sample edge. Panels A, B, and C correspond to B/B_{phi}=.31 and 1.6 respectively, of the ramp-up phase in Fig. 1. Pinning sites are indicated by open circles, while vortices are shown as filled dots.

Magnetization loops
Fig. 3 - Magnetization loops (top panels) and the corresponding critical currents (bottom panels) for several samples. The J(B)'s are taken directly from the B(x) during the "ramp-down" (e.g., stage (2) in Fig. 1). In (a,b), f_{p} is held fixed at 2.5 and the density of pinning sites n is varied: B_{phi}=0.5, 0.75, 1.0. In (c,d), n remains fixed, (B_{phi}=1.0) and the pinning strength f_p remains changed. In (e,f), B_{phi}=1.0, f_p=2.5, and the location of the pinning sites is varied. (f) shows the significant enhancement of J(B) that results from defects placed in a regular triangular array, as opposed to random placement. The results show that even a distorted triangular array of pinning sites significantly enhances J(B) over the case with a random location of sites.

Plastic flow
Fig. 4 - (a,b) show the trajectories of vortices while the external field is raised from 0.95 to 1.4. (a) A x10 region of the sample used in Fig. 1. The strength of the pinning force is 2.5 for the strong pinning case (a), and 0.3, for the weak pinning case (b). In (a), the vortex transport is characterized by vortex trails of interstitial vortices which move around regions with flux lines that are strongly pinned at defects. In (b), vortex transport proceeds in a different manner: pin/to pin vortex motion is possible, the previously narrow vortex trails become considerably broader.

C. Reichhardt, C. J. Olson, J. Groth, Stuart Field, and Franco Nori. Published in Phys. Rev. B 53, R8898 (1996) (Rapid Communications)

A copy of the paper is available here.

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Created by: Jared Groth and Bartholomew Hsu
Last modified: 1/5/97