Vortex states in de facto mesoscopic Mo80Ge20 pentagon plates

We present direct observations of vortex configurations in Mo80Ge20 pentagonal plates using a scanning SQUID microscope. The vortex distribution tends to adapt to one of five symmetric axes in a pentagon, as expected from the sample geometry. We found that, for a vorticity of six, the vortices form a two-shell structure in a small pentagon, and a single-shell structure in a larger pentagon. We revealed how the vortex configuration is strongly modified by the presence of a pinning site at the center of the pentagon. In particular, a two-shell structure is formed in the pinning-site sample with a vorticity of L = 4, while the second shell appeared in the pinning site-free sample with a vorticity up to L = 6. Furthermore, upon geometric confinement, the vortices attempt to form a symmetrical configuration reflecting the sample geometry even in the presence of a pinning site (or sites). The experimental results for pentagons are in good agreement with our theoretical predictions based on the nonlinear Ginzburg–Landau equation both for a pin-free pentagon and a pentagon with a pin at the center. (paper)