Observation of an in-plane vortex lattice transition in the multiband superconductor 2H-NbSe2 using scanning tunneling spectroscopy

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.