VSe $_2$三维电荷密度波的声子崩塌和范德华熔化
Phonon collapse and van der Waals melting of the 3D charge density wave of VSe$_2$
摘要：
在过渡金属二硫属化物（TMD）中，VSe $_2$被认为在T $_{CDW}$ =110 K处产生纯三维（3D）电荷密度波（CDW）。在这里，通过高分辨率的非弹性X射线散射（IXS），我们表明，CDW转变驱动的临界波矢\textit{q} $_{CDW}$ =（2.25 0 0.7）r.l.u.临界温度T $_{CDW}$ =110 K。这种模式的软化开始显着的温度低于2 $\times$ T $_{CDW}$，并扩展到一个相当宽的区域的布里渊区，这表明了很大的贡献的电子-声子相互作用的CDW形成。这种解释是支持我们的第一性原理计算，确定一个大的动量依赖的电子-声子相互作用，峰值在CDW波矢，在嵌套的存在。完全非谐{ \it ab initio}计算证实了在\textit{q} $_{CDW}$处的一个声学分支的软化是CDW形成的原因，并表明范德华相互作用对熔化CDW至关重要。我们的工作还强调了面外相互作用在TMD中描述3D CDW的重要作用。
英文摘要：
Among transition metal dichalcogenides (TMDs), VSe$_2$ is considered to develop a purely 3-dimensional (3D) charge-density wave (CDW) at T$_{CDW}$=110 K. Here, by means of high resolution inelastic x-ray scattering (IXS), we show that the CDW transition is driven by the collapse of an acoustic mode at the critical wavevector \textit{q}$_{CDW}$= (2.25 0 0.7) r.l.u. and critical temperature T$_{CDW}$=110 K. The softening of this mode starts to be pronounced for temperatures below 2$\times$ T$_{CDW}$ and expands over a rather wide region of the Brillouin zone, suggesting a large contribution of the electron-phonon interaction to the CDW formation. This interpretation is supported by our first principles calculations that determine a large momentum-dependence of the electron-phonon interaction, peaking at the CDW wavevector, in the presence of nesting. Fully anharmonic {\it ab initio} calculations confirm the softening of one acoustic branch at \textit{q}$_{CDW}$ as responsible for the CDW formation and show that van der Waals interactions are crucial to melt the CDW. Our work also highlights the important role of out-of-plane interactions to describe 3D CDWs in TMDs.
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