Superconducting energy gap of $2H$-NbSe$_2$ in phonon spectroscopy

TL;DR

This study uses high-resolution inelastic x-ray scattering to investigate the interplay between charge-density-wave order and superconductivity in 2H-NbSe2, revealing a localized CDW gap and a superconducting gap with momentum dependence.

Contribution

It provides new insights into the momentum dependence of the superconducting gap and the localization of the CDW gap in 2H-NbSe2 using phonon spectroscopy.

Findings

Superconducting gap shows l-dependence along the c* direction.

CDW energy gap is highly localized along the c* direction.

The CDW transition is driven by electron-phonon coupling rather than the energy gain from the CDW gap.

Abstract

We present a high energy-resolution inelastic x-ray scattering data investigation of the charge-density-wave (CDW) soft phonon mode upon entering the superconducting state in $2H$-NbSe$_2$. Measurements were done close to the CDW ordering wavevector $\mathbf{q}_{CDW}$ at $\mathbf{q}=\mathbf{q}_{CDW}+(0,0,l)$,$0.15\leq l \leq 0.5$, for $T=10\,\rm{K}$ (CDW order) and $3.8\,\rm{K}$ (CDW order + superconductivity). We observe changes of the phonon lineshape that are characteristic for systems with strong electron-phonon coupling in the presence of a superconducting energy gap $2\Delta_c$ and from which we can demonstrate an $l$-dependence of the superconducting gap. Reversely, our data imply that the CDW energy gap is strongly localized along the $c^*$ direction. The confinement of the CDW gap to a very small momentum region explains the rather low competition and easy coexistence of CDW order and superconductivity in $2H$-NbSe$_2$. However, the energy gained by opening $\Delta_{CDW}$ seems to be too small to be the driving force of the phase transition at $T_{CDW}=33\,\rm{K}$ , which is better described as an electron-phonon coupling driven structural phase transition.