Transport properties of the multiple vortices in superconductors with square pinning arrays

This work mainly examines the anisotropic transport phenomenon in the presence of square arrays of the mesoscopic pinning sites in superconducting samples. Our results were obtained from a series of numerical simulations as functions of the magnetic field and the pinning radius. Dynamic behaviors of the multi and interstitial vortex states were studied by means of the current-voltage V (I) curves which illustrate anisotropic behavior as a function of the driving force applied in two perpendicular directions. At the matching magnetic fields, it was found that V (I) curves are mainly isotropic for a large pinning radius, but also anisotropic when at least two or more vortices are present at the interstitial regions. The results indicated that the anisotropic effects took place at low driving forces and the critical depinning force in the x-direction is mostly lower than that obtained in the y-direction. In the presence of multiply occupied pinning sites and interstitial vortices, the initial stable configuration and orientational orderings of the entire vortices determined the degree of transport anisotropy.