Cryomagnetic STM imaging of the subsurface vortex lattice in the multiband superconductor $2H$-NbSe$_2$*

Using cryomagnetic scanning tunneling microscopy (STM) in a novel field geometry, we studied single crystals of the multiband superconductor $2H$-NbSe$_{2}$ under diamagnetically-induced superfluid momentum. Spectroscopy and conductance imaging were performed at 300 mK and in a field of up to 9 T, applied in the $ab$-plane.[1] Spatial maps of the Doppler effect on the quasiparticle tunneling spectrum revealed distinct stripe patterns that originate from in-plane vortices whose cores are buried in the bulk.[2] The stripe separation varies systematically as a function of the applied field. Our results are interpreted in terms of the interaction between vortical and screening currents, and demonstrate a general method for probing subsurface vortices, especially in emerging multiband superconductors such as the ferro-pnictides/chalcogenides.\\[4pt] [1] I. Fridman \emph{et al.}, arXiv:1110.6490 (2011) \\[0pt] [2] I. Fridman \emph{et al.}, Appl. Phys. Lett. \textbf{99}, 192505 (2011)