In-gap state and effect of light illumination inCuIr2S4probed by photoemission spectroscopy

Abstract

We have studied disorder-induced in-gap states and effect of light illumination in the insulating phase of spinel-type CuIr 2 S 4 using ultraviolet photoemission spectroscopy ͑UPS͒. The Ir 3+ / Ir 4+ charge-ordered gap appears below the metal-insulator transition temperature. However, in the insulating phase, in-gap spectral features with softgap are observed in UPS just below the Fermi level ͑E F ͒, corresponding to the variable range hopping transport observed in resistivity. The spectral weight at E F is not increased by light illumination, indicating that the Ir 4+ -Ir 4+ dimer is very robust, although the long-range octamer order would be destructed by the photoexcitation. Present results suggest that the Ir 4+ -Ir 4+ bipolaronic hopping and disorder effects are responsible for the conductivity of CuIr 2 S 4 .

X-ray-induced electronic structure change in CuIr2S4

“…In addition, the resistivity change occurs quite slowly, over a period of several minutes, and this effect could be reversed by heating the sample above 100 K. Detailed single crystal x-ray scattering studies showed that the long-range CO was destroyed and a short-range incommensurate CO appeared in this irradiation-induced low-temperature (IILT) phase [17]. Although spectroscopic data suggest that substantial electronic structure modification occurs at the MIT, no spectroscopic changes in this IILT phase below 50 K have been reported to date [18,19]. For example, opening of an insulating gap of 0.15 eV below the MIT temperature was observed in both optics [21] and photoemission experiments [11,18], but the photoemission spectra did not show any change in the IILT phase [18,19].…”

“…Although spectroscopic data suggest that substantial electronic structure modification occurs at the MIT, no spectroscopic changes in this IILT phase below 50 K have been reported to date [18,19]. For example, opening of an insulating gap of 0.15 eV below the MIT temperature was observed in both optics [21] and photoemission experiments [11,18], but the photoemission spectra did not show any change in the IILT phase [18,19].…”

“…However, detailed investigation of electronic structure using core level x-ray spectroscopy has been limited. Although several Ir 4f photoemission experiments have reported that Ir 3+ /Ir 4+ charge disproportion does exist in the insulating phase, due to its surface sensitivity, the interpretation has not been straightforward [18][19][20].While x-ray absorption spectroscopy (XAS) at the S K-edge has been valuable in detecting redistribution of S 3p states across the MIT [12], the Ir L 3 -edge XAS has not been very useful due to broad spectral features arising from the short core-hole lifetime [12,13]. In this work, we report bulksensitive x-ray spectroscopic studies which allow us to obtain direct information regarding the Ir 5d electronic states across the MIT and also in the IILT phase.…”

Broadening of the orbitally-induced Peierls phase transition in Cu 1−x Na x Ir 2 S 4

“…The spinel CuIr 2 S 4 exhibits a first-order metallic-insulating phase transition at $230 K with an opening energy gap, which accompanies with a magnetic transition from Pauli paramagnetic to diamagnetic state [16][17][18][19]. Meanwhile, the structure changes from cubic (space group Fd3m) to triclinic ðP1Þ cell with lattice shrinkage of 0.7% [20,21].…”

Substitution effect on metal–insulator transition in Cu(Ir1−xMx)2S4 (M=Sn, Hf)

“…This CuIr 2 S 4 has the mixed valence as Cu + Ir 3+ Ir 4+ S 4 2− and the intensive investigations have been carried out [2][3][4]. The M-I transition in CuIr 2 S 4 is demonstrated as Fig.…”