Penetration depth study of superconducting gap structure of 2\textit{H}-NbSe$_2

Phys. Rev. Lett. 98, 057003 (2007) We report measurements of the temperature dependence of both in-plane and out-of-plane penetration depths ($\lambda_a$ and $\lambda_c$ respectively) in 2\textit{H}-NbSe$_2$. Measurements were made with a radio-frequency tunnel diode oscillator circuit at temperatures down to 100 mK. Analysis of the anisotropic superfluid density shows that a reduced energy gap is located on one or more of the quasi-two-dimensional Nb Fermi surface sheets rather than on the Se sheet, in contrast to some previous reports. This result suggests that the gap structure is not simply related to the weak electron-phonon coupling on the Se sheet and is therefore important for microscopic models of anisotropic superconductivity in this compound.