Souliou S M, Lacmann T, Heid R, Meingast C, Frachet M, Paolasini L, Haghighirad A-A, Merz M, Bosak A, Le Tacon M
Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany.
ESRF The European Synchrotron, 71 avenue des Martyrs, CS 40220 F-38043 Grenoble, Cedex 9, France.
Phys Rev Lett. 2022 Dec 9;129(24):247602. doi: 10.1103/PhysRevLett.129.247602.
We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave (I-CDW) in BaNi_{2}As_{2}, a candidate system for charge-driven electronic nematicity. Intense diffuse scattering is observed around the modulation vector of the I-CDW, Q_{I-CDW}. It is already visible at room temperature and collapses into superstructure reflections in the long-range ordered state where a small orthorhombic distortion occurs. A clear dip in the dispersion of a low-energy transverse optical phonon mode is observed around Q_{I-CDW}. The phonon continuously softens upon cooling, ultimately driving the transition to the I-CDW state. The transverse character of the soft-phonon branch elucidates the complex pattern of the I-CDW satellites observed in the current and earlier studies and settles the debated unidirectional nature of the I-CDW. The phonon instability and its reciprocal space position are well captured by our ab initio calculations. These, however, indicate that neither Fermi surface nesting, nor enhanced momentum-dependent electron-phonon coupling can account for the I-CDW formation, demonstrating its unconventional nature.
我们使用漫散射和非弹性X射线散射来研究BaNi₂As₂中非公度电荷密度波（I-CDW）的形成，BaNi₂As₂是电荷驱动电子向列相的候选体系。在I-CDW的调制矢量Q₁-CDW周围观察到强烈的漫散射。它在室温下就已可见，并在出现小正交畸变的长程有序状态下坍缩为超结构反射。在Q₁-CDW附近观察到低能横向光学声子模式色散的明显下降。随着冷却，声子持续软化，最终驱动向I-CDW态的转变。软声子分支的横向特性阐明了在当前和早期研究中观察到的I-CDW卫星的复杂模式，并解决了关于I-CDW单向性质的争论。我们的第一性原理计算很好地捕捉到了声子不稳定性及其倒易空间位置。然而，这些计算表明，费米面嵌套和增强的动量依赖电子-声子耦合都不能解释I-CDW的形成，这证明了其非传统性质。
