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

机译:声子光谱中的超导能量差为2H $ -Nbse $ _2 $

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摘要

We present a high energy-resolution inelastic x-ray scattering datainvestigation of the charge-density-wave (CDW) soft phonon mode upon enteringthe superconducting state in $2H$-NbSe$_2$. Measurements were done close to theCDW 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 observechanges of the phonon lineshape that are characteristic for systems with strongelectron-phonon coupling in the presence of a superconducting energy gap$2\Delta_c$ and from which we can demonstrate an $l$-dependence of thesuperconducting gap. Reversely, our data imply that the CDW energy gap isstrongly localized along the $c^*$ direction. The confinement of the CDW gap toa very small momentum region explains the rather low competition and easycoexistence of CDW order and superconductivity in $2H$-NbSe$_2$. However, theenergy gained by opening $\Delta_{CDW}$ seems to be too small to be the drivingforce of the phase transition at $T_{CDW}=33\,\rm{K}$ , which is betterdescribed as an electron-phonon coupling driven structural phase transition.
机译:我们提出了在$ 2H $ -NbSe $ _2 $进入超导状态时电荷密度波(CDW)软声子模式的高能分辨率非弹性X射线散射数据研究。在接近CDW排序波矢$ \ mathbf {q} _ {CDW} $ at $ \ mathbf {q} = \ mathbf {q} _ {{CDW} +(0,0,l)$,$ 0.15 \ leq l \ leq 0.5 $,用于$ T = 10 \,\ rm {K} $(CDW阶)和$ 3.8 \,\ rm {K} $(CDW阶+超导)。我们观察到在存在超导能隙$ 2 \ Delta_c $的情况下,具有强电子-声子耦合的系统所特有的声子线形变化,从中我们可以证明超导能隙具有$ l $依赖性。相反,我们的数据表明CDW能隙沿$ c ^ * $方向强烈定位。 CDW缝隙限制在很小的动量区域,这说明在$ 2H $ -NbSe $ _2 $中CDW顺序和超导电性的竞争程度较低,容易共存。但是,打开$ \ Delta_ {CDW} $所获得的能量似乎太小,不足以成为$ T_ {CDW} = 33 \,\ rm {K} $时相变的驱动力,这可以更好地描述为电子-声子耦合驱动结构相变。

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