Extended x-ray absorption fine-structure measurements at the Zn, Mn and Te K edges of the diluted magnetic semiconductors have been performed to investigate the local coordination around each of the atoms. The nearest-neighbour and next-nearest-neighbour interatomic distances are found to be almost unchanged on changing the Mn concentration (x). These results indicate that the tetrahedral and clusters are embedded in the alloys with a well-preserved form, in agreement with the x-independent Mn 3d partial density of states derived from resonant photoemission measurements.

We study the mixed valence transition (T v ∼80 K) in EuNi2(Si0.2Ge0.8)2 using Eu 3d-4f X-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The Eu 2+and Eu 3+ main peaks show a giant resonance and the spectral features match very well with atomic multiplet calculations. The spectra show dramatic temperature (T )-dependent changes over large energies (∼10 eV) in RESPES and XAS. The observed non-integral mean valencies of ∼2.35 ± 0.03 (T = 120 K) and ∼2. 7 In contrast, a static or inhomogeneous MV state is one in which electron configurations are different at different sites, representing one specific electron configuration at a site. While the MV in f -electron systems are often homogeneous, there do exist exceptions. 1,8Many f -electron systems exhibit a MV transition induced by temperature (T ), magnetic field and/or pressure.These include the α-γ transition in Ce metal, 1 the pressure-induced transitions in SmS, 6 and TmTe, 9 the T -dependent transitions in YbInCu 4 , 10 Tm-monochalcogenides, 11 as well as Eubased intermetallics, EuPd 2 Si 2 , 12 Eu(Pd 1−x Au x ) 2 Si 2 , 13and EuNi 2 (Si 1−x Ge x ) 2 . 14 Among T -induced transitions, the Eu systems exhibit the largest change in valency, ∆v ∼ 0.3-0.5. 12,13,14 And of these, EuNi 2 (Si 1−x Ge x ) 2 has been extensively studied to show T -, 14,15 magnetic field-, 15 and pressure-16 induced valence transitions. By tuning composition [x in EuNi 2 (Si 1−x Ge x ) 2 ], the transition is observed to be first-order like for compositions close to x = 0.8, with a hysterisis as a function of T , pressure and magnetic field. 15,16,17 The MV transition in EuNi 2 (Si 0.2 Ge 0.8 ) 2 has thus been investigated across the critical T (T v ) of ∼80 K by magnetic susceptibility, high-energy bulk-sensitive Eu L-edge X-ray absorption spectroscopy (XAS), and X-ray diffraction to show that the transition is accompanied by a Kondo-like volume collapse across T v . 14,15,16,17 XAS and resonant photoemission spectroscopy (RE-SPES) are important techniques for studying the electronic structure (ES) of f -electron systems. 18,19,20 In XAS applied to a solid, a core electron of a particular site or element is excited to an empty state, and hence, it probes site-specific angular momentum projected unoccupied states of a solid.21 RESPES is a complementary technique which probes the resonantly enhanced partial occupied density of states (DOS) of a solid.20 These techniques provide important insights into the physical properties of strongly correlated materials, including MV, Kondo effect, heavy fermion behavior, etc. However, recent studies using ultraviolet photoemission spectroscopy (PES) of MV systems revealed modifications of the surface ES compared to the bulk. 22,23,24 While signatures of T -dependent MV are observed, the mean valence estimated from these measurements are incompatible with bulk thermodynamic studies. Significantly, the important role of hard X-ray (HX: hν ∼ 3 to 8 keV) PES in general, 25,26,27,28,29,30,31 as well as soft X-ray (SX: hν ∼ ...

We have performed hard x-ray photoemission spectroscopy (HAXPES) for Yb-based Kondo lattice compounds; an antiferromagnetic heavy-fermion system YbNi 3 Al 9 and a valence fluctuation system YbNi 3 Ga 9. The Yb 3d 5/2 spectra of YbNi 3 Ga 9 showed both Yb 2+ and Yb 3+-derived structures indicating strong valence fluctuation, and the intensity of Yb 2+ (Yb 3+) structures gradually increased (decreased) on cooling. The Yb 3d 5/2 spectra of YbNi 3 Al 9 mostly consisted of Yb 3+-derived structures and showed little temperature dependence. The Yb valences of YbNi 3 Ga 9 and YbNi 3 Al 9 at 22 K were evaluated to be 2.43 and 2.97, respectively. Based on the results of the Ni 2p and valence-band HAXPES spectra together with soft x-ray valence-band spectra, we described that the difference of physical properties of YbNi 3 X 9 (X = Al, Ga) is derived from the differences of the 4f-hole level relative to the Fermi level (E F) and Ni 3d density of states at E F. The HAXPES results on the Yb valences were consistent with those obtained by x-ray absorption spectroscopy using the partial fluorescence yield mode and resonant x-ray emission spectroscopy at the Yb L 3 edge.

Due to the strain gradient near each surface of a BaTiO3 nanocube in their ordered assembly, electric polarization appears due to flexoelectric effect. The magnitude of the flexoelectric polarization could be one order of magnitude larger than that of ferroelectric spontaneous polarization of BaTiO3. Thus, dielectric response of an assembly could be dominated by that of the flexoelectric polarization if there is no ferroelectric domain-wall motion. Numerical simulations of the dielectric response of a BaTiO3 nanocube in an ordered assembly are performed from kHz to MHz range based on a dynamic model of flexoelectric polarization assuming anharmonic potential. The calculated temperature dependence of the dielectric constant is consistent with the experimental data of high dielectric constant with nearly-flat temperature dependence. It is suggested that high dielectric constant with nearly-flat temperature dependence is not originated in ferroelectric nature of BaTiO3 nanocubes but originated in flexoelectric polarization in nanocubes which is also seen in non-ferroelectric materials.

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