Signature of phonon anharmonicity in highly in-plane anisotropic ternary HfGeTe₄ single crystals

Ternary layered single crystals of HfGeTe₄ have been synthesized and systematically investigated to study their electronic and vibrational properties using angle-resolved photoemission spectroscopy and Raman scattering (RS) spectroscopy. The experimental results were supported through density functional theory by calculating the electronic band structures, phonons dispersion, lattice thermal conductivity (κ_L), and phonon lifetimes. The measured Fermi surfaces represent integrated intensities at different binding energies along the opposite symmetry directions, revealing the anisotropic Fermi surfaces. Their band structure shows different band curvatures in the high symmetry direction with anisotropic band dispersions. Moreover, the change in magnetoresistance under different orientations reveals anisotropic electrical conductivity, which further supports our results of 2D anisotropic crystals. HfGeTe₄ shows a strong in-plane anisotropic response under polarized RS with phonon intensities changing with the rotation angle, manifesting twofold and fourfold symmetry orientations. Furthermore, the Raman scattering spectra show phonon softening observed with increasing temperature, which could be attributed to the phonon anharmonicity estimated from both the frequency shift and linewidth. The results were further supported by lattice-dynamics calculations. We have found very low lattice thermal conductivity (∼0.31 Wm⁻¹ K⁻¹ at 300 K), which decays as the temperature is increased due to enhanced phonon-phonon scattering, as confirmed by the decreased phonon lifetimes at higher temperatures. The unique structure of this material coupled with the anisotropy and strong phonon anharmonicity depicts significant potential in 2D nanoelectronic applications.