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Register a new userInelastic X-ray scattering from valence electrons near absorption edges of FeTe and TiSe$_2$
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Added by
Bernardo Barbiellini
Publication date
2014
fields
Physics
and research's language is
English
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We study resonant inelastic x-ray scattering (RIXS) peaks corresponding to low energy particle-hole excited states of metallic FeTe and semi-metallic TiSe$_2$ for photon incident energy tuned near the $L_{3}$ absorption edge of Fe and Ti respectively. We show that the cross section amplitudes are well described within a renormalization group theory where the effect of the core electrons is captured by effective dielectric functions expressed in terms of the the atomic scattering parameters $f_1$ of Fe and Ti. This method can be used to extract the dynamical structure factor from experimental RIXS spectra in metallic systems.
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Motivated by the recent synthesis of Ba$_2$CuO$_{3+delta}$ (BCO), a high temperature superconducting cuprate with putative $d_{3z^2-r^2}$ ground state symmetry, we investigated its electronic structure by means of Cu $L_3$ x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge on a polycrystalline sample. We show that the XAS profile of BCO is characterised by two peaks associated to inequivalent Cu sites, and that its RIXS response features a single, sharp peak associated to crystal-field excitations. We argue that these observations are only partially compatible with the previously proposed crystal structure of BCO. Based on our spectroscopic results and on previously published powder diffraction measurements, we propose a crystalline structure characterized by two inequivalent Cu sites located at alternated planes along the $c$ axis: nominally trivalent Cu(1) belonging to very short Cu-O chains, and divalent Cu(2) in the oxygen deficient CuO$_ {1.5}$ planes. We also analyze the low-energy region of the RIXS spectra to estimate the magnitude of the magnetic interactions in BCO and find that in-plane nearest neighbor superexchange exceeds 120~meV, similarly to that of other layered cuprates. Although these results do not support the pure $d_{3z^2-r^2}$ ground state scenario, they hint at a significant departure from the common quasi-2D electronic structure of superconducting cuprates of pure $d_{x^2-y^2}$ symmetry.
The photoinduced valence dynamics of EuNi$_2$(Si$_{0.21}$Ge$_{0.79}$)$_2$ are investigated using time-resolved X-ray absorption spectroscopy for Eu $M_5$-edge. Through the pump-probe technique with synchrotron X-ray and Ti:sapphire laser pulse, a photoinduced valence transition is observed from Eu$^{3+}$ to Eu$^{2+}$. Because the lifetime of a photoinduced state can be up to 3 ns, a metastable state is considered to be realized. By comparing the experimental results with the theoretical calculations, the photoinduced valence transition between Eu 4$f$ and conduction electrons is quantitatively evaluated.
We present an x-ray absorption study of the oxidation states of transition-metal-ions of LiMnO2 and its related materials, widely used as cathodes in Li-ion batteries. The comparison between the obtained spectrum and the configuration-interaction cluster-model calculations showed that the Mn3+ in LiMnO2 is a mixture of the high-spin and low-spin states. We found that Li deficiencies occur in the case of Cr substitution, whereas there are no Li deficiencies in the case of Ni substitution. We conclude that the substitution of charge-transfer-type Ni or Cu is effective for LiMnO2 battery materials.
We report the observation and the theoretical explanation of the parametric down-conversion nonlinear susceptibility at the $K$-absorption edge of diamond and at the $L_{23}$-absorption edge of a silicon crystal. Using arguments similar to those invoked to successfully predict resonant inelastic x-ray spectra, we derive an expression for the renormalization term of the non-linear susceptibility at the x-ray edges, which can be evaluated by using first-principles calculations of the atomic scattering factor $f_1$. Our model is shown to reproduce the observed enhancement of the parametric down-conversion at the diamond $K$ and the Si $L_{23}$ edges rather than the suppression previously claimed.
The valence and spin state evolution of Mn and Co on TbMn$_{rm 1-x}$Co$_{rm x}$O$_3$ series is precisely determined by means of soft and hard x-ray absorption spectroscopy (XAS) and K$beta$ x-ray emission spectroscopy (XES). Our results show the change from Mn$^{3+}$ to Mn$^{4+}$ both high-spin (HS) together with the evolution from Co$^{2+}$ HS to Co$^{3+}$ low-spin (LS) with increasing $rm x$. In addition, high energy resolution XAS spectra on the K pre-edge region are interpreted in terms of the strong charge transfer and hybridization effects along the series. These results correlate well with the spin values of Mn and Co atoms obtained from the K$beta$ XES data. From this study, we determine that Co enters into the transition metal sublattice of TbMnO$_3$ as a divalent ion in HS state, destabilizing the Mn long range magnetic order since very low doping compositions (${rm x} le 0.1$). Samples in the intermediate composition range ($0.4 le {rm x} le 0.6$) adopt the crystal structure of a double perovskite with long range ferromagnetic ordering which is due to Mn$^{4+}$-O-Co$^{2+}$ superexchange interactions with both cations in HS configuration. Ferromagnetism vanishes for ${rm x} ge 0.7$ due to the structural disorder that collapses the double perovskite structure. The spectroscopic techniques reveal the occurrence of Mn$^{4+}$ HS and a fluctuating valence state Co$^{2+}$ HS/Co$^{3+}$ LS in this composition range. Disorder and competitive interactions lead to a magnetic glassy behaviour in these samples.
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