论文标题
$ 2H $ -NBSE $ _2 $的非常规电荷密度波转换的晶格贡献：一种非平衡光学方法
Lattice contribution to the unconventional charge density wave transition in $2H$-NbSe$_2$: a non-equilibrium optical approach
论文作者
论文摘要
过渡金属二盐元素（TMDC）的复杂费米表面挑战了标准的PEIERLS-INSCORNICANIC驱动的电荷密度波（CDW）的形成。最近，证据已经积累了离子热波动的重要作用，该作用冻结了$ t_ {cdw} $，引起了定期的晶格失真（PLD）。我们专注于$ 2H $ -NBSE $ _2 $，在$ t_ {cdw} $ $ \ $ 33 k以下显示准均匀的CDW，并使用时间分解的光谱（TR-OS）来检测和删除自由的电子和lattice naterange。我们在h $ν$ $ \ sim $ 1 ev中揭示了有序阶段的指纹：在$ t_ {cdw} $中，一个发散的放松时间表，差分反射率的签名变化表明CDW间隙开口和PLD形成发生在同一温度下。但是，我们表明这些效果可以在中等的光激发下解耦，形成了一个长寿状态，在该状态下，电子秩序被破坏了，但晶格失真却没有。我们的结果和计算表明，非常规CDW机制为$ 2H $ -NBSE $ _2 $，强调了晶格在驱动有序的相形组中的主要作用。
The complex Fermi surfaces of transition-metal dichalcogenides (TMDCs) challenge the standard Peierls-instability-driven charge-density-wave (CDW) formation. Recently, evidence has been accumulating of a prominent role of ionic thermal fluctuations, which frozen out below $T_{CDW}$ inducing a periodic lattice distortion (PLD). We focus on $2H$-NbSe$_2$, displaying a quasi-commensurate CDW below $T_{CDW}$ $\approx$33 K, and use time-resolved optical spectroscopy (TR-OS) to detect and disentangle the electronic and lattice degrees of freedom. We reveal a fingerprint of the ordered phase at h$ν$ $\sim$1 eV: at $T_{CDW}$, a divergent relaxation timescale and a sign-change of the differential reflectivity indicate that CDW gap opening and PLD formation occur at the same temperature. However, we show that these effects can be decoupled under moderate photoexcitation, forming a long-lived state in which the electronic order is destroyed, but the lattice distortion is not. Our results and computations suggest an unconventional CDW mechanims in $2H$-NbSe$_2$, highlighting the dominant role of the lattice in driving the ordered-phase formation.
