Resonance-Induced Anomalies in Temperature-Dependent Raman Scattering of PdSe_2

We report a comprehensive Raman study of the phonon behaviour in PdSe_2 in the temperature range of 5 K to 300 K. A remarkable change in the Raman spectrum is observed at 120 K: a significant enhancement of the out-of-plane phonon A^1_g mode, accompanied by a suppression of the in-plane A^2_g and B^2_1g modes. This intriguing behavior is attributed to a temperature-dependent resonant excitation effect. The results are supported by density functional theory (DFT) calculations, which demonstrate that the electron-phonon coupling for the phonon modes varies and is strongly associated with the relevant electronic states. Furthermore, nonlinear frequency shifts are identified in all modes, indicating the decay of an optical phonon into multiple acoustic phonons. The study of Raman emission reported here, complemented by linear optical spectroscopy, reveals an unexpected scenario for the vibrational properties of PdSe_2 that holds substantial promise for future applications in PdSe_2-based optoelectronics.