Pressure-induced anomalous insulating behavior in frustrated iridate La_(3)Ir_(3)O_(11)

The geometrically frustrated iridate La_(3)Ir_(3)O_(11) with strong spin–orbit coupling and fractional valence was recently predicted to be a quantum spin liquid candidate at ambient conditions. Here, we systematically investigate the evolution of structural and electronic properties of La_(3)Ir_(3)O_(11) under high pressure. Electrical transport measurements reveal an abnormal insulating behavior rather than metallization above a critical pressure P_(c) ~ 38.7 GPa. Synchrotron x-ray diffraction(XRD)experiments indicate the stability of the pristine cubic KSbO_(3)-type structure up to 73.1 GPa. Nevertheless, when the pressure gradually increases across P_(c), the bulk modulus gets enhanced and the pressure dependence of bond length d_(Ir-Ir) undergoes a slope change. Consistent with the XRD data, detailed analyses of Raman spectra reveal an abnormal redshift of Raman mode and a change of Raman intensity around P_(c). Our results demonstrate that the pressure-induced insulating behavior in La_(3)Ir_(3)O_(11) can be assigned to the structural modification, such as the distortion of IrO_6 octahedra. These findings will shed light on the emergent abnormal insulating behavior in other 5 d iridates reported recently.

Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS2

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.

Pressure-induced enhancement of optoelectronic properties in PtS2

PtS2, which is one of the group-10 transition metal dichalcogenides, attracts increasing attention due to its extraordinary properties under external modulations as predicted by theory, such as tunable bandgap and indirect-to-direct gap transition under strain; however, these properties have not been verified experimentally. Here we report the first experimental exploration of its optoelectronic properties under external pressure. We find that the photocurrent is weakly pressuredependent below 3 GPa but increases significantly in the pressure range of 3 GPa–4 GPa, with a maximum ～ 6 times higher than that at ambient pressure. X-ray diffraction data shows that no structural phase transition can be observed up to26.8 GPa, which indicates a stable lattice structure of PtS2 under high pressure. This is further supported by our Raman measurements with an observation of linear blue-shifts of the two Raman-active modes to 6.4 GPa. The pressure-enhanced photocurrent is related to the indirect-to-direct/quasi-direct bandgap transition under pressure, resembling the gap behavior under compression strain as predicted theoretically.