Possible Tricritical Behavior and Anomalous Lattice Softening in van der Waals Itinerant Ferromagnet Fe_3GeTe_2 under High Pressure

We present a high-pressure study of van der Waals ferromagnetic metal Fe₃GeTe₂ through electrical transport and Raman scattering measurements in diamond anvil cells at pressures up to 22.4 GPa.Upon compression,the ferromagnetic transition temperature P_c manifested by a kink in resistance curve decreases monotonically and becomes undiscernable around P_c = 10 GPa,indicative of suppression of the itinerant ferromagnetism.Meanwhile,by fitting the low tempe...     

Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS_2

The insulator-metal transition triggered by pressure in charge transfer insulator NiS2 is investigated by combining high-pressure electrical transport,synchrotron x-ray diffraction and Raman spectroscopy measurements up to40-50 GPa.Upon compression,we show that the metallization firstly appears in the low temperature region at～3.2 GPa and then extends to room temperature at～8.0 GPa.During the insulator-metal transition,the bond length of S-S dimer extracted from the synchrotron x-ray diffraction increases with pressure,which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa.Considering the decreasing bonding-antibonding splitting due to the expansion of S-S dimer,the charge gap between the S-ppπ* band and the upper Hubbard band of Ni-3 d eg state is remarkabl.y decreased.These results consistently indicate that the elongated S-S dimer plays a predominant role in the insulator-metal transition under high pressure,even though the p-d hybridization is enhanced simultaneously,in accordance with a scenario of charge-gap-controlled type.