Speaker
Description
We report polarization-dependent resonant inelastic X-ray scattering (RIXS) measurements that reveal the order–disorder nature of the antiferroelectric (AFE)–paraelectric (PE) transition in the van der Waals multiferroic CuCrP$_2$S$_6$. The RIXS transition from the high-spin $^4A_{2g}$ ground state of the Cr$^{3+}$ ion to a state with $^{4}T_{1g}$ symmetry splits into two $dd$ excitations at 2.0 and 2.15 eV, arising from the splitting of $^{4}T_{1g}$ into $^{4}E$ and $^{4}A_2$ due to trigonal distortion. The excitation energies remain nearly unchanged across the AFE–PE transition, indicating that the average Cr-centered trigonal crystal-field strength is insensitive to the Cu positions within RIXS resolution. However, in the AFE phase, these two peaks exhibit a distinct polarization contrast between RIXS excitations with $\pi$ and $\sigma$ polarizations, which evolves strongly with temperature. This polarization contrast vanishes in the PE phase, indicating that while the local trigonal splitting persists, Cu$^{+}$-ion distribution becomes dynamically disordered, thereby washing out the RIXS polarization anisotropy. These observations support the Cu-driven order–disorder mechanism for the AFE–PE transition, rather than a displacive transition.