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We are developing 1s3p resonant inelastic X-ray scattering (RIXS) as a versatile spectroscopic tool to probe active systems under working conditions. In order to establish detailed reference materials for cobalt-based systems, we have applied 1s X-ray absorption spectroscopy (XAS) and 1s3p RIXS to cobalt oxides (CoO, Co3O4) and sulfides (CoS and CoS2). We show that the Co3+ ions in Co3O4 contain a large nonlocal peak intensity in the pre-edge and that the resonant 3p final state of Co3O4 is dominated by the two exchange-split peaks of the tetrahedral Co2+ site. In contrast, the 3p final state of CoS and CoS2 shows a single asymmetric peak due to the large screening of the 3p-3d exchange interaction. While CoS has a single pre-edge peak, CoS2 contains an additional large nonlocal peak. This indicates that although CoS2 is often indicated as Co2+(S2)2-, it has a very strong sulfur-mediated cobalt-cobalt bonding, similar to the low-spin Co3+ sites in oxides. These electronic structure details reveal the significance of 1s3p RIXS spectroscopy for determining the symmetry, spin and covalence of transition metal compounds, information that is largely inaccessible in conventional K edge XAS. The experimental results are simulated with multiplet calculations that reveal the partial screening of the 3p3d exchange interaction and the presence of nonlocal transitions. We conclude that 1s3p RIXS combined with spectral simulations presents a valuable tool to unravel the electronic properties of functional materials under working conditions.

This study shows that a terahertz (THz) wave can be generated from the (001) surface of cleaved Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ and Cu-doped Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ single crystals using 800 nm femtosecond pulses. The generated THz power is strongly dependent on the carrier concentration of the crystals. An examination of the dependence reveals the two-channel free carrier absorption to which Dirac fermions are indispensable. Dirac fermions in Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ are significantly better absorbers of THz radiation than bulk carriers at room temperature. Moreover, the characteristics of THz emission confirm the existence of a recently proposed surface phonon branch that is normalized by Dirac fermions., Comment: 5 pages, 4 figures, 1 table

We have examined the electronic structure of the heavy fermion Yb2Pd2Sn in comparison with the superconductor YbPd2Sn by means of high-resolution photoelectron spectroscopy (PES) and resonant x-ray emission spectroscopy (RXES). The PES data are interpreted in light of the density of states calculated within the local density approximation plus the dynamical mean-field theory. Our analysis of the RXES data shows that the valence of Yb in Yb2Pd2Sn is weakly fluctuating, indicating weak but finite c–f hybridization, and decreases slightly with temperature. In contrast, the Yb valence in YbPd2Sn is nearly 3+ and does not show any temperature dependence, suggesting that the Yb 4f electrons are localized and the superconductivity is due to the Pd conduction electrons. The constant initial state spectra at the Yb 4d–4f resonance for Yb2Pd2Sn clearly show resonant behavior for Yb3+ and antiresonance for Yb2+, indicating that the Yb3+ and Yb2+ states are intimately correlated.