Direct spatial imaging of vortices in a superconducting wire network

We report direct observations of vortices in a square superconducting wire grid imaged using scanning Hall probe microscopy. Real space images of vortex configurations are obtained as a function of the flux per unit cell f by measuring the local magnetic field just above the sample. At f=1/2 we observe domains of the checkerboard ground state. As f is reduced from 1/2 to 1/3 vacancies first penerate the grain boundaries and then the checkerboard domains. Near f=1/3 we observe domains of the 1/3 staircase ground state. Heating the sample close to Tc produces correlated vortex hopping.