Probing electron-phonon coupling in magnetic van der Waals material NiPS3: a non-magnetic site-dilution study

NiPS3 is a Van der Waals antiferromagnet which has been shown to exhibit spin-phonon and spin-charge coupling in the antiferromagnetically ordered state below TN = 155 K. It is also a rare Ni-based negative charge-transfer-type (NCT) insulator with Ni valence in a linear superposition state ψ= αd8+ βd9L_+ γ d10L_2, where L_ is the ligand hole. Here, we study high-quality single-crystals of Ni1-xZnxPS3 (0 < x < 0.2) using temperature-dependent specific heat and Raman spectroscopy probes. We show that in pristine NiPS3, the phonon mode at 176 cm-1 (P2), associated with the vibrations of Ni, exhibits a distinct Fano asymmetry. The Fano resonance is found to be particularly pronounced in the paramagnetic phase above TN, which was further confirmed by temperature dependent Raman data on the Zn-doped crystals. In the Zn-doped crystals, while the magnetism weakens following the mean-field prediction for site-dilution in a honeycomb lattice, the Fano coupling 1/q strengthens, increasing monotonically with increasing Zn-doping. The X-ray photoemission spectra suggest an increase in the weight of the d9 and d10 components in the Zn-doped crystals. These observations indicate the presence of strong electron-phonon coupling in Ni1-xZnxPS3, in addition to the spin-phonon, and spin-charge coupling previously reported.