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Register a new userHigh multipole transitions in NIXS: valence and hybridization in 4f systems
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Momentum-transfer (q) dependent non-resonant inelastic x-ray scattering measurements were made at the N4,5 edges for several rare earth compounds. With increasing q, giant dipole resonances diminish, to be replaced by strong multiplet lines at lower energy transfer. These multiplets result from two different orders of multipole scattering and are distinct for systems with simple 4f^0 and 4f^1 initial states. A many-body theoretical treatment of the multiplets agrees well with the experimental data on ionic La and Ce phosphate reference compounds. Comparing measurements on CeO2 and CeRh3 to the theory and the phosphates indicates sensitivity to hybridization as observed by a broadening of 4f^0-related multiplet features. We expect such strong, nondipole features to be generic for NIXS from f-electron systems.
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Recent discoveries of a new type of quantum criticality arising from Yb-valence fluctuations in Yb-based metal in periodic crystal and quasicrystal have opened a new class of quantum critical phenomena in correlated electron systems. To clarify whether this new concept can be generalized to other rare-earth-based semimetal and insulator, we study SmS which exhibits golden-black phase transition under pressure. By constructing the model for SmS, we show that Coulomb repulsion between 4f and 5d orbitals at Sm drives first-order valence transition (FOVT) and semimetal-to-insulator transition (MIT) simultaneously, which explains the golden-black phase transition. We clarify the ground-state phase diagram for the FOVT and MIT by identifying the quantum critical point of the FOVT. We find that exciton condensates in both semimetal and insulator phases. Our result explains measured peak anomalies in the specific heat and compressibility in pressurized golden SmS and provides a cue to clarify recently-observed anomalies in black SmS.
Ce 3d-4f resonant angle-resolved photoemission measurements on CeCoGe$_{1.2}$Si$_{0.8}$ and CeCoSi$_{2}$ have been performed to understand the Fermi surface topology as a function of hybridization strength between Ce 4$f$- and conduction electrons in heavy-fermion systems. We directly observe that the hole-like Ce 4$f$-Fermi surfaces of CeCoSi$_{2}$ is smaller than that of CeCoGe$_{1.2}$Si$_{0.8}$, indicating the evolution of the Ce 4$f$-Fermi surface with the increase of the hybridization strength. In comparision with LDA calculation, the Fermi surface variation cannot be understood even though the overall electronic structure are roughly explained, indicating the importance of strong correlation effects. We also discuss the relation between the Ce 4$f$-Fermi surface variation and the Kondo peaks.
The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at $approx$ 45 GPa is accompanied by a reentrant Eu valence transition into a $emph{lower}$ valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.
Magnetic excitations in the isostructural spin-dimer systems Sr3Cr2O8 and Ba3Cr2O8 are probed by means of high-field electron spin resonance at sub-terahertz frequencies. Three types of magnetic modes were observed. One mode is gapless and corresponds to transitions within excited states, while two other sets of modes are gapped and correspond to transitions from the ground to the first excited states. The selection rules of the gapped modes are analyzed in terms of a dynamical Dzyaloshinskii-Moriya interaction, suggesting the presence of phonon-assisted effects in the low-temperature spin dynamics of Sr3Cr2O8 and Ba3Cr2O8
In the series R2PdSi3, Nd2PdSi3 is an anomalous compound in the sense that it exhibits ferromagnetic order unlike other members in this family. The magnetic ordering temperature is also unusually high compared to the expected value for a Nd-based system, assuming 4f localization. Here, we have studied the electronic structure of single crystalline Nd2PdSi3 employing high resolution photoemission spectroscopy and ab initio band structure calculations. Theoretical results obtained for the effective electron correlation strength of 6 eV corroborate well with the experimental valence band spectra. While there is significant Pd 4d-Nd 4f hybridization, the states near the Fermi level are found to be dominated by hybridized Nd 4f-Si 3p states. Nd 3d core level spectrum exhibits multiple features manifesting strong final state effects due to electron correlation, charge transfer and collective excitations. These results serve as one of the rare demonstrations of hybridization of Nd 4$f$ states with the conduction electrons possibly responsible for the exoticity of this compound.
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