Cryomagnetic STM spectroscopy study of multiband pairing in layered superconductors*

Cooper pairing in layered superconductors can involve multiple bands and give rise to complex gap structures in momentum space. Using scanning tunneling microscopy (STM) with a magnetic field applied parallel to the $ab$-plane, we investigate multiband pairing under diamagnetically-induced superfluid momentum. STM spectroscopy and conductance imaging were performed down to 300 mK and up to 9 T, on single-crystals of the Nb-chalcogenide $2H$-NbSe$_2$ and the Fe-pnictides LiFeAs and electron-doped BaFe$_2$As$_2$. Spectroscopy data taken on $2H$-NbSe$_2$ at 300 mK showed a distinctly two-sloped field evolution of the zero-bias conductance, consistent with Doppler-induced depairing on parts of the Fermi surface [1]. Spatial conductance maps revealed stripe patterns that originate from in-plane vortices whose cores are buried in the bulk [2] and which undergo a transition as pairing on one of the bands is suppressed. Our results demonstrate a general method for probing multiband superconductors, especially ones whose band structures host coexisting orders and also play a direct role in the pairing mechanism. \\[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)