Multiband superconductivity in $2H$-NbSe$_2$ probed by Doppler-modulated scanning tunneling spectroscopy

Cooper pairing in multiband superconductors can involve carriers from bands having different dimensionalities, and the interband coupling can provide for novel pairing interactions. In addition to MgB$_2$, recent experiments on $2H$-NbSe$_2$ have studied the Fermi surface topology using angle- and temperature-dependent scanning tunneling spectroscopy.[1] We present another novel method for probing multiband pairing: using a field-induced diamagnetic supercurrent, applied along different crystal axes, to perturb the quasiparticle density-of-states spectrum. By measuring the evolution of the quasiparticle spectrum under finite superfluid momentum, we characterize the pairing gaps and gap anisotropies. This approach is demonstrated on $2H$-NbSe$_2$ at 300 mK with a magnetic field of up to 9 T applied in the $ab$-plane.[2] The STM measurements revealed unambiguous evidence for multiband pairing, and evidence for a novel transition of the in-plane vortex lattice. We discuss the characteristics of this transition in light of data from other probes. [1] Y. Noat \textit{et al.}, Phys. Rev. B 92, 134510 (2015). [2] I. Fridman \textit{et al.}, Applied Physics Letters 99, 192505 (2011).