Raman Scattering fingerprints of the charge density wave state in one-dimensional NbTe4

We investigated charge density waves in NbTe4 bulk crystal. The available data include measurements of sample using Raman scattering and X-ray diffraction method. We also provide phonon dispersion calculations for NbTe4 obtained using the DFT method.Raman scatteringRaman scattering spectra were measured under laser excitation of λ=785 nm (1.58 eV). The excitation light in those experiments was focused by means of a 50x long-working distance objective with a 0.55 numerical aperture (NA) producing a spot of about 1 μm diameter. The signal was collected via the same microscope objective (the backscattering geometry), sent through a 0.75 m monochromator, and then detected using a liquid nitrogen-cooled charge-coupled device (CCD) camera. All measurements were performed with the samples placed on a cold finger in a continuous-flow cryostat mounted on x-y manual positioners. The excitation power focused on the sample was kept at 1 mW during all measurements. The polarization-resolved RS measurements were performed in the co-linear (XX) and cross-linear (XY) configurations, which correspond to the parallel orientation of the excitation and detection polarization axes. The analysis of the RS signal was done using a motorized half-wave plate mounted on top of the microscope objective, which provides simultaneous rotation of the polarization axis on excitation and detection.Theoretical calculationsStructural relaxations and force calculations were carried out within the density functional theory (DFT) using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional of the generalized gradient approximation (GGA), as implemented in VASP [2]. A plane-wave basis with an energy cutoff of 520 eV was employed throughout. Brillouin zone integrations for structural relaxations used Γ-centered Monkhorst–Pack meshes of 8×8×2 and 12×1×12 for the P4/ncc1 and P4/mcc structures, respectively. Electronic self-consistency was converged to 10−7 eV, while ionic relaxations were stopped when forces fell below 10−2 eV/AA. Phonon dispersions were computed using the finite displacement method implemented in Phonopy. For secondorder interatomic force constants, we employed a 2×2×1 supercell with a 4×4×2 Γ-centered k-mesh for the P4/ncc1 phase, and a 2 × 2 × 2 supercell with a 6 × 6 × 6 Γ-centered k-mesh for the P4/mcc phase.X-ray diffractionX-ray powder diffraction (XRD) was performed to confirm the crystal structure and phase purity of the synthesized NbTe4 crystals. Measurements were carried out at room temperature using a SmartLab 3 kW diffractometer equipped with a Cu anode source. The crushed bulk crystalline powder was sealed in a 0.5 mm glass capillary and examined over a 2θ range of 5* to 90* with Cu Ka radiation (L= 0.15418 nm, 8.04 keV). The obtained diffraction pattern verified the expected structure and indicated a phase-pure material.

本研究针对体相二碲化铌（NbTe₄）晶体中的电荷密度波展开了调研。所公开的数据集涵盖了基于拉曼散射（Raman scattering）与X射线衍射（X-ray diffraction）方法对样品的表征结果，同时还提供了采用密度泛函理论（DFT）计算得到的NbTe₄声子色散数据。

拉曼散射测试：
实验中采用波长λ=785 nm（1.58 eV）的激光作为激发源开展拉曼散射光谱测试。激发光通过数值孔径（NA）为0.55的50倍长工作距物镜聚焦，形成直径约1 μm的光斑。信号经由同一显微镜物镜以背散射几何构型收集，随后通过0.75 m单色仪分光，最终使用液氮冷却的电荷耦合器件（CCD）相机完成探测。所有测试均在置于连续流低温恒温器冷指上的样品上进行，该低温恒温器搭载于x-y手动位移平台之上。测试过程中，聚焦于样品的激发功率始终维持在1 mW。
本研究还开展了偏振分辨拉曼散射测试，测试分为共线（XX）与正交（XY）两种构型，分别对应激发与探测偏振轴的平行取向。拉曼信号的偏振调控通过安装于显微镜物镜上方的电动半波片实现，该器件可同时旋转激发与探测的偏振轴。

理论计算：
本研究基于广义梯度近似（GGA）下的Perdew–Burke–Ernzerhof（PBE）交换关联泛函，借助VASP所实现的密度泛函理论（DFT）框架，完成了结构弛豫与原子受力计算。计算全程采用能量截断值为520 eV的平面波基组。针对P4/ncc1与P4/mcc两种晶体结构，结构弛豫的布里渊区积分分别采用Γ中心型8×8×2与12×1×12的蒙霍斯特-帕克（Monkhorst-Pack）k点网格。电子自洽迭代的收敛阈值设置为10⁻⁷ eV，离子弛豫则在原子受力低于10⁻² eV/Å时终止。
声子色散曲线的计算采用Phonopy软件所实现的有限位移法。对于P4/ncc1相，我们采用2×2×1超胞搭配4×4×2 Γ中心k点网格；对于P4/mcc相，则采用2×2×2超胞搭配6×6×6 Γ中心k点网格。

X射线衍射测试：
本研究开展了X射线粉末衍射（XRD）测试，以验证合成的NbTe₄晶体的晶体结构与相纯度。测试在室温下进行，使用搭载Cu阳极靶源的SmartLab 3 kW衍射仪。将粉碎后的体相晶体粉末封装于0.5 mm内径的玻璃毛细管中，在2θ角范围5°至90°内以Cu Kα辐射（波长λ=0.15418 nm，能量8.04 keV）进行扫描。所得衍射图谱验证了预期的晶体结构，表明样品为纯相材料。