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Register a new userInsulator-to-metal transition in the pyrochlore iridates series (Eu_1-xBi_x)2Ir2O7 probed using Hard X-ray Photoemission Spectroscopy
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Eu2Ir2O7, a candidate Weyl semimetal, shows an insulator-to-metal transition as a function of Bi substitution at the Eu site. In this work, we investigate the (Eu_1-xBi_x)2Ir2O7 series via Hard X-ray Photoemission Spectroscopy (HAXPES), where substitution of larger Bi3+ for Eu3+ is reported to result in an anomalous lattice contraction (20 %) for low Bi doping (3.5 %). Using HAXPES, we confirm that all the cations retain their nominal valence state throughout the series. The asymmetric nature of Bi core-level spectra for compositions in the metallic region indicates that Bi contributes to the density of states at the Fermi energy in this doping range. The valence band spectra shows that the Bi 6s peak is unaltered throughout the series and is situated deep within the valence band. based on these observations we argue that Bi 6p - Ir 5d hybridization drives the insulator-to-metal transition.
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GdNi is a ferrimagnetic material with a Curie temperature Tc = 69 K which exhibits a large magnetocaloric effect, making it useful for magnetic refrigerator applications. We investigate the electronic structure of GdNi by carrying out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at T = 25 K in the ferrimagnetic phase. We analyze the Gd M$_{4,5}$-edge ($3d$ - $4f$) and Ni L$_{2,3}$-edge ($2p$ - $3d$) spectra using atomic multiplet and cluster model calculations, respectively. The atomic multiplet calculation for Gd M$_{4,5}$-edge XAS indicates that Gd is trivalent in GdNi, consistent with localized $4f$ states. On the other hand, a model cluster calculation for Ni L$_{2,3}$-edge XAS shows that Ni is effectively divalent in GdNi and strongly hybridized with nearest neighbour Gd states, resulting in a $d$-electron count of 8.57. The Gd M$_{4,5}$-edge XMCD spectrum is consistent with a ground state configuration of S = 7/2 and L=0. The Ni L$_{2,3}$-edge XMCD results indicate that the antiferromagnetically aligned Ni moments exhibit a small but finite magnetic moment ( $m_{tot}$ $sim$ 0.12 $mu_B$ ) with the ratio $m_{o}/m_{s}$ $sim$ 0.11. Valence band hard x-ray photoemission spectroscopy shows Ni $3d$ features at the Fermi level, confirming a partially filled $3d$ band, while the Gd $4f$ states are at high binding energies away from the Fermi level. The results indicate that the Ni $3d$ band is not fully occupied and contradicts the charge-transfer model for rare-earth based alloys. The obtained electronic parameters indicate that GdNi is a strongly correlated charge transfer metal with the Ni on-site Coulomb energy being much larger than the effective charge-transfer energy between the Ni $3d$ and Gd $4f$ states.
The rich physics manifested by 5d oxides falls outside the Mott-Hubbard paradigm used to successfully explain the electronic and magnetic properties of 3d oxides. Much consideration has been given to the extent to which strong spin-orbit coupling (SOC), in the limit of increased bandwidth and reduced electron correlation, drives the formation of novel electronic states, as manifested through the existence of metal-insulator transitions (MITs). SOC is believed to play a dominant role in 5d5 systems such as iridates (Ir4+), undergoing MITs which may or may not be intimately connected to magnetic order, with pyrochlore and perovksite systems being examples of the former and latter, respectively. However, the role of SOC for other 5d configurations is less clear. For example, 5d3 (e.g Os5+) systems are expected to have an orbital singlet and consequently a reduced effect of SOC in the groundstate. The pyrochlore osmate Cd2Os2O7 nonetheless exhibits a MIT intimately entwined with magnetic order with phenomena similar to pyrochlore iridates. Here we report the first resonant inelastic X-ray scattering (RIXS) measurements on an osmium compound, allowing us to determine the salient electronic and magnetic energy scales controlling the MIT in Cd2Os2O7, which we benchmark against detailed quantum chemistry calculations. In particular, we reveal the emergence at the MIT of a magnetic excitation corresponding to a superposition of multiple spin-flip processes from an Ising-like all-in/all-out magnetic groundstate. We discuss our results with respect to the role of SOC in magnetically mediated MITs in 5d systems
Electronic properties of V$_2$OPO$_4$ have been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) and subsequent theoretical calculations. The V 1$s$ and 2$p$ HAXPES spectra are consistent with the charge ordering of V$^{2+}$ and V$^{3+}$. The binding energy difference between the V$^{2+}$ and V$^{3+}$ components is unexpectedly large indicating large bonding-antibonding splitting between them in the final states of core level photoemission. The V 1$s$ HAXPES spectrum exhibits a charge transfer satellite which can be analyzed by configuration interaction calculations on a V$_2$O$_9$ cluster. The V 3$d$ spectral weight near the Fermi level is assigned to the 3$d$ $t_{2g}$ orbitals of the V$^{2+}$ site. The broad V 3$d$ spectral distribution is consistent with the strong hybridization between V$^{2+}$ and V$^{3+}$ in the ground state. The core level and valence band HAXPES results indicate substantial charge transfer from the V$^{2+}$ site to the V$^{3+}$ site.7 figure
We present a detailed infrared study of the insulator-to-metal transition (IMT) in vanadium dioxide (VO2) thin films. Conventional infrared spectroscopy was employed to investigate the IMT in the far-field. Scanning near-field infrared microscopy directly revealed the percolative IMT with increasing temperature. We confirmed that the phase transition is also percolative with cooling across the IMT. We present extensive near-field infrared images of phase coexistence in the IMT regime in VO2. We find that the coexisting insulating and metallic regions at a fixed temperature are static on the time scale of our measurements. A novel approach for analyzing the far-field and near-field infrared data within the Bruggeman effective medium theory was employed to extract the optical constants of the incipient metallic puddles at the onset of the IMT. We found divergent effective carrier mass in the metallic puddles that demonstrates the importance of electronic correlations to the IMT in VO2. We employ the extended dipole model for a quantitative analysis of the observed near-field infrared amplitude contrast and compare the results with those obtained with the basic dipole model.
Oxides RNiO3 (R = rare-earth, R # La) exhibit a metal-insulator (MI) transition at a temperature TMI and an antiferromagnetic (AF) transition at TN. Specific heat (CP) and anelastic spectroscopy measurements were performed in samples of Nd1-xEuxNiO3, 0 <= x <= 0.35. For x = 0, a peak in CP is observed upon cooling and warming at essentially the same temperature TMI = TN ~ 195 K, although the cooling peak is much smaller. For x >= 0.25, differences between cooling and warming curves are negligible, and two well defined peaks are clearly observed: one at lower temperatures, that define TN, and the other one at TMI. An external magnetic field of 9 T had no significant effect on these results. The elastic compliance (s) and the reciprocal of the mechanical quality factor (Q^-1) of NdNiO3, measured upon warming, showed a very sharp peak at essentially the same temperature obtained from CP, and no peak is observed upon cooling. The elastic modulus hardens below TMI much more sharply upon warming, while the cooling and warming curves are reproducible above TMI. On the other hand, for the sample with x = 0.35, s and Q^-1 curves are very similar upon warming and cooling. The results presented here give credence to the proposition that the MI phase transition changes from first to second order with increasing Eu doping.
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