Hysteretic Phonons and Quasielastic Response: A Raman Study of Thermal Memory in Two-dimensional CuCrP2S6

Abstract: We present a comprehensive temperature and polarization dependent inelastic light scattering (Raman) study on single crystals of two-dimensional CuCrP2S6, a layered van der Waals material exhibiting coupled magnetic and electric degrees of freedom. Raman measurements were performed from 5 to 300 K to probe phonon dynamics across multiple structural and magnetic phase transitions. Our analysis reveals pronounced thermal hysteresis in phonon self-energy parameters and dynamic Raman susceptibility, confirming the first-order nature of the antipolar transition near TC1 ~ 145 K and a second-order transition near TC2 ~ 190 K. Low-frequency modes associated with Cu+ and Cr3+ ions exhibit softening and anomalous linewidth behaviour, in particular phonon mode P2 (~ 37 cm-1) showing non-monotonic temperature dependence and intensity enhancement near 60 K suggesting persistent off-centre Cu+ dynamics in the quasi-antipolar phase. The coexistence and coupling of soft phonon modes and central peaks indicate a crossover from displacive to order-disorder type transition mechanisms. Additionally, phonon anomalies below the Néel temperature (TN ~ 32 K) reflect spin-phonon coupling, linking lattice vibrations to long-range magnetic correlations. Our findings provide critical insight into the lattice instabilities, symmetry evolution, and quasiparticle interactions in CuCrP2S6, offering a deeper understanding of phase transition dynamics in two-dimensional multiferroic systems and guiding future design of magnetoelectric and spintronic devices.