Vortex structure and dynamics in kagomé and triangular pinning potentials

Abstract

We study the dynamics of thermally driven superconducting vortices in two types of periodic pinning potentials: kagom\'e and triangular. For the first, second, and third matching fields, we obtain the corresponding ground-state vortex configurations and their phase diagrams. We analyze the system properties by looking at the vortex trajectories and the structure factor, as well as the linear and angular diffusion. The temperature versus pinning force phase diagram is analyzed in detail for each matching field. When the temperature is varied, we observe several stages of lattice pinning and melting. In most of the cases we find, for decreasing temperature, first a pinning of vortices and afterwards a freezing transition of the interstitial vortices. The intermediate regime corresponds to interstitial vortices in a confined liquidlike state and pinned vortices. The kagom\'e pinning potential shows interesting behavior at low temperatures: there is a phase with rotating vortex triangles caged by kagom\'e hexagons (``cooperative ring elementary excitations''), and there is geometric frustration for $\stackrel{\ensuremath{\rightarrow}}{T}0$ with a nearly degenerate ground state.