文章来源： npj计算材料学
手性电荷密度波(chiral CDW)是一种通常出现于低温下的关联电子物态，其表现为电荷密度在空间中的周期性调制，并伴随着破缺镜面以及中心对称性的晶格结构畸变。这种物态被认为与许多新奇的物理现象有关，例如非局域霍尔效应，手性库伯对（chiral Copper pairs），轴子绝缘体（insulator states）等等。
但是，到目前为止，人们对手性电密度波形成的机理还没有一个足够的认识，目前发现的具有手性电荷密度波的材料也少之又少。理解手性密度波形成的理论机制不仅仅有利于发现合成更多的举要该物态的材料，更有利于深入研究相关的物性。
Fig. 2 | First-principles calculations of 2D 1T-TiSe2.
中国科学院理论物理研究所的张田田副研究员团队，结合理论分析以及第一性原理计算，提出在通过电声耦合诱导的电荷密度波相变体系中，手性声子（chiral phonon）的软化是一种诱导手性电荷密度波的可能机制，如图1所示。同时，他们通过计算考虑多声子热扩散效应的X射衍，首次提出了X射线衍射峰的各向异性可作为实验上探测手性电荷密度波的有效标记。
手性声子是一种特殊的晶格激发，其实空间图像对应原子的圆极化运动（如图1）。这种声子模式可存在于不具备镜面（2D）以及中心对称性（3D）的系统中。当该声子模式对应的电声耦合强度足以使得该模式发生软化后，便会导致一种螺旋式的晶格畸变以及电荷密度分布，即手性电荷密度波态。该团队计算了单层TiSe2的声子谱，发现了其中非手性声子的软化，会诱导形成非手性电荷密度波态，而手性声子的软化则会导致手性电荷密度波的形成，这在数值计算上验证了该理论的正确性（如图2所示）。结合这项理论，该团队提出非线性电/磁致伸缩效应（nonlinear electro-/magnetostrictive effect）可作为调控手性电荷密度波的有效途径。此外，当前探测手性电荷密度波的实验方法仍有待拓展。通常，手性电荷密度波可以利用镜面对称的动量点上衍射峰的强度不同来表征。然而衍射峰的强度区别往往取决与很多因素，并且这种区别亦可能不甚明显。该团队在考虑了多声子的热散射效应后，计算了更符合实际情况的X射线衍射。他们发现，对于手性电荷密度波态，X射线衍射斑点的各向异性也破缺了镜面对称性（图3），相较于非手性结构呈现出明显的偏转。这证明了衍射峰的各项异性亦可作为探测手性电荷密度波的实验手段。
作者的研究给出了一种手性电荷密度波相变的机制、探测和调控方法、以及新的实验观测手段，为关于手性电荷密度波以及相关物性的研究奠定了坚实的基础。该文近期发表于npj Computational Materials 10, 264 (2024)。手机阅读原文，请点击本文底部左下角“阅读原文”，进入后亦可下载全文PDF文件。
Editorial Summary
Chiral charge density wave (chiral CDW) is a correlated electronic state that typically emerges at low temperatures, characterized by periodic modulation of charge density in space, accompanied by lattice structural distortions that break mirror and inversion symmetries. This state is believed to be associated with various intriguing physical phenomena, such as the nonlocal Hall effect, chiral Cooper pairs, and axion insulator states. However, to date, materials exhibiting chiral charge density wave states are exceedingly rare, and the underlying theoretical mechanisms remain poorly understood. This has significantly limited the in-depth exploration of this intriguing state of matter.
A research team led by Associate Professor Zhang Tiantian from the Institute of Theoretical Physics, Chinese Academy of Sciences, combined theoretical analysis and first-principles calculations to propose that the softening of chiral phonons, induced by electro-acoustic coupling interactions, could be a potential mechanism for triggering chiral charge density waves (chiral CDWs). Additionally, by incorporating the multi-phonon thermal diffusion effect into their calculations of X-ray diffraction, they suggested that the anisotropy of X-ray diffraction peaks could serve as a possible method for detecting chiral CDWs.Chiral phonons are a unique type of lattice excitation characterized by the helical vibration of atoms propagating in a specific direction. These phonon modes lack both mirror and inversion symmetry. When the electron-phonon coupling strength associated with such modes becomes sufficiently strong to induce their softening, it results in helical lattice distortions and charge density modulations, leading to the formation of a chiral charge density wave (chiral CDW) state. The team computed the phonon spectrum of monolayer TiSe₂ and observed that the softening of non-chiral phonons does not lead to the formation of a chiral CDW state. In contrast, the softening of chiral phonons results in the emergence of chiral CDWs, providing numerical confirmation of the theoretical predictions. Building on this theory, the team proposed that the nonlinear electro-/magnetostrictive effect could serve as an effective mechanism for regulating chiral CDWs. Additionally, by incorporating multi-phonon thermal scattering effects, the team conducted more realistic X-ray diffraction simulations. They discovered that for chiral CDW states, the anisotropy of X-ray diffraction spots also breaks mirror symmetry. This finding demonstrates that diffraction peak anisotropy can serve as a reliable method for detecting chiral CDWs.
The authors provide a mechanism for the chiral charge density wave phase transition and a novel detection method, laying a solid foundation for the exploration of chiral charge density waves and related phenomena.. This article was recently published in npj Computational Materials 10, 264 (2024).
原文Abstract及其翻译
Understanding chiral charge-density wave by frozen chiral phonon (通过软化手性声子理解手性电荷密度波)
Shuai Zhang,Kaifa Luo & Tiantian Zhang
Abstract Charge density wave (CDW) is discovered within a wide interval in solids, however, its microscopic nature is still not transparent in most realistic materials, and the recently studied chiral ones with chiral structural distortion remain unclear. In this paper, we try to understand the driving forces of chiral CDW transition by chiral phonons from the electron-phonon coupling scenario. We use the prototypal monolayer 1T-TiSe2as a case study to unveil the absence of chirality in the CDW transition and propose a general approach, i.e., symmetry-breaking stimuli, to engineer the chirality of CDW in experiments. Inelastic scattering patterns are also studied as a benchmark of chiral CDW (CCDW, which breaks the mirror/inversion symmetry in 2D/3D systems). We notice that the anisotropy changing of Bragg peak profiles, which is contributed by the soft chiral phonons, can show a remarkable signature for CCDW. Our findings pave a path to understanding the CCDW from the chiral phonon perspective, especially in van der Waals materials, and provides a powerful way to manipulate the chirality of CDW.
摘要电荷密度波（CDW）在固体中被发现具有广泛的分布范围，然而，在大多数实际材料中，其微观机制仍不清晰，特别是最近研究的具有手性结构畸变的手性电荷密度波仍然未被完全理解。本文尝试通过电子-声子耦合机制，从手性声子的角度理解手性电荷密度波（CCDW）转变的驱动力。我们以原型单层1T-TiSe₂为研究案例，揭示了CDW转变中手性的缺失，并提出通过对称性破缺的外界刺激，可在实验中实现对CDW手性的定向调控。对于手性电荷密度波（CCDW，打破二维/三维系统中的镜像/反演对称性），我们首次提出非弹性散射图案的形状可以作为实验标志。相应的第一性原理计算也验证了，由软手性声子引起的布拉格峰轮廓各向异性的变化，可以作为CCDW的显著特征。我们的研究从手性声子视角为理解CCDW提供了新思路，尤其是在范德华材料中，并提供了一种强有力的方法来调控、观测CDW的手性。
