Vortex pinning in ion-irradiated NbSe2 studied by scanning tunneling microscopy.

Vortex pinning by columnar defects in a ${\mathrm{NbSe}}_{2}$ crystal irradiated with 2.2 GeV ${\mathrm{Au}}^{+24}$ ions is studied by scanning tunneling microscopy (STM) at 3 K. The vortex arrangement is clearly resolved in magnetic fields up to 500 mT and compared to the defect locations, also extracted from STM data. At vortex densities much larger than the defect density, the triangular vortex lattice is perturbed close to the defects. At vortex densities comparable or smaller, only some defects trap a vortex and the whole vortex arrangement is disordered, although less random than the defect distribution.