  We report real-space evidence for an in-plane vortex lattice (VL) transition
in the multiband superconductor 2H-NbSe2, observed by laterally imaging
subsurface vortices with scanning tunneling spectroscopy. With magnetic field
applied || ab-surface up to 6.5 T at 300 mK, spatial maps of the zero-bias
conductance revealed a stripe pattern whose dominant periodicity showed a
discrete shift near 0.7 T. The stripes can be interpreted as the surface
projection of a distorted hexagonal VL that undergoes a first-order
reorientation transition. This transition occurs for field || [100] but not
[110], and is correlated with the multigap characteristics seen in the heat
capacity and thermal transport data. These results implicate the multiband
pairing of 2H-NbSe2 in the VL transition we observed.
