Wydział Fizyki

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 scattering

Raman 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 calculations

Structural 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 diffraction

X-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.