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 T-N = 155 K. It is also a rare Ni-based negative charge-transfer-type insulator with Ni valence in a linear superposition state psi= alpha d(8)+ beta d(9)L-+ gamma d(10)L(-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) (P-2), associated with the vibrations of Ni ions, exhibits a distinct Fano asymmetry. The Fano resonance is particularly pronounced in the paramagnetic phase above T-N, 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 d(9) and d(10) 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.