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

TL;DR

This Raman study of CuCrP2S6 reveals complex phonon behaviour, hysteresis, and spin-phonon coupling across multiple phase transitions, advancing understanding of lattice dynamics in 2D multiferroics.

Contribution

It provides the first detailed Raman analysis of phase transition mechanisms and phonon anomalies in CuCrP2S6, highlighting hysteresis and spin-lattice interactions.

Findings

Pronounced thermal hysteresis in phonon parameters.

Identification of soft phonon modes and their temperature dependence.

Evidence of spin-phonon coupling below the Néel temperature.

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 N\'eel 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.