I study dynamical instabilities in Ta$_2$NiSe$_5$ using density functional theory based calculations. The calculated phonon dispersions show two unstable optical branches. All the acoustic branches are stable, which shows that an elastic instability is not the primary cause of the experimentally observed orthorhombic-to-monoclinic structural transition in this material. The largest instability of the optical branches occurs at the zone center, consistent with the experimental observation that the size of the unit cell does not multiply across the phase transition. The unstable modes have the irreps $B_{1g}$ and $B_{2g}$. Full structural relaxations minimizing both the forces and stresses find that the monoclinic $C2/c$ structure corresponding to the $B_{2g}$ instability has the lowest energy. Electronic structure calculations show that this low-symmetry structure has a sizable band gap. This suggest that a $B_{2g}$ zone-center optical phonon instability is the primary cause of the phase transition. An observation of a softening of a $B_{2g}$ zone-center phonon mode as the transition is approached from above would confirm the mechanism proposed here. If none of the $B_{2g}$ modes present in the material soften, this would imply that the transition is caused by electronic or elastic instability.
Unveiling Excitonic Insulator Signatures in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$
Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院量子材料与技术研究所）；Karlsruhe Nano Micro Facility (KNMFi), Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院卡尔斯鲁厄纳米微设施）；ESRF, The European Synchrotron（欧洲同步辐射装置）
LaCu$_{6-x}$Ag$_{x}$: A promising host of an elastic quantum critical point
采用中子衍射、X射线衍射和共振超声谱（RUS）研究了LaCu $_{6-x}$ Ag $_{x}$的结构性质。衍射测量表明从正交（$Pnma$）到单斜（$P2_1/c$）结构的连续结构转变。RUS测量结果表明，结构过渡时固有频率软化，与结构基态的弹性性质一致。LaCu $_{6-x}$ Ag $_{x}$的结构转变温度随着Ag组分的增加而降低，直到单斜相被完全抑制在$x_c$ = 0.225。所有的证据都与LaCu $_{6-x}$ Ag $_{x}$中弹性量子临界点的存在相一致。
Electron-Phonon Coupling Mode in Excitonic Insulator
Ta 2NiSe 5被认为是一个有前途的激子绝缘体（EI）的候选人与轻微的声子贡献，因为它表现出微小的正交单斜结构畸变在328 K没有任何超晶格结构。我们的同步加速器非弹性X射线散射测量揭示了强的电子光学声子耦合发生在高于转变温度的温度。密度泛函理论计算表明，两个耦合的光学模式出现由于Ta和Se离子的振动。此外，这两种模式被冻结，使得Ta和Se彼此接近，在单斜相中形成原子位移型电偶极子。反铁电偶极子排列形成电子环矩的特点是EI在Ta_2NiSe_5和1 T-TiSe_2之间的普适性。
Pressure-Driven Structural Transitions without a Displacive Charge-Density Wave in La$_2$SmNi$_2$O$_7$
Université Paris-Saclay（巴黎萨克雷大学）；CNRS, Laboratoire de Physique des Solides（法国国家科学研究中心，固体物理实验室）；Synchrotron SOLEIL（索雷尔同步辐射中心）；CNRS, Université Grenoble Alpes, Institut Néel（法国国家科学研究中心，格勒诺布尔阿尔卑斯大学，奈耳研究所）；Institut universitaire de France (IUF)（法国高等研究院）；Institut de Chimie de la matière condensée de Bordeaux, Université Bordeaux（波尔多物质化学研究所，波尔多大学）；Thapar Institute of Engineering and Technology（塔帕工程技术学院）
