Bulk electronic structure of non-centrosymmetric EuTGe3 (T= Co, Ni, Rh, Ir) studied by hard x-ray photoelectron spectroscopy

Non-centrosymmetric EuTGe3 (T=Co, Ni, Rh, and Ir) possesses magnetic Eu2+ ions and antiferromagnetic ordering appears at low temperatures. Transition metal substitution leads to changes of the unit cell volume and of the magnetic ordering. However, the magnetic ordering temperature does not scale with the volume change and the Eu valence is expected to remain divalent. Here we study the bulk electronic structure of non-centrosymmetric EuTGe3 (T=Co, Ni, Rh, and Ir) by hard x-ray photoelectron spectroscopy. The Eu 3d core level spectrum confirms the robust Eu2+ valence state against the transition metal substitution with a small contribution from Eu3+. The estimated Eu mean-valence is around 2.1 in these compounds as confirmed by multiplet calculations. In contrast, the Ge 2p spectrum shifts to higher binding energy upon changing the transition metal from 3d to 4d to 5d elements, hinting of a change in the Ge-T bonding strength. The valence bands of the different compounds are found to be well reproduced by ab initio band structure calculations

Valence Instability of YbCu2_2Si2_2 through its quantum critical point

We report Resonant inelastic x-ray scattering measurements (RIXS) in YbCu$_2$Si$_2$ at the Yb L$_{3}$ edge under high pressure (up to 22 GPa) and at low temperatures (down to 7 K) with emphasis on the vicinity of the transition to a magnetic ordered state. We find a continuous valence change towards the trivalent state with increasing pressure but with a pronounced change of slope close to the critical pressure. Even at 22 GPa the Yb$^{+3}$ state is not fully achieved. The pressure where this feature is observed decreases as the temperature is reduced to 9 GPa at 7K, a value close to the critical pressure (\itshape{p\normalfont{$_c$}}\normalfont $\approx$ 7.5 GPa) where magnetic order occurs. The decrease in the valence with decreasing temperature previously reported at ambient pressure is confirmed and is found to be enhanced at higher pressures. We also compare the f electron occupancy between YbCu$_2$Si$_2$ and its Ce-counterpart, CeCu$_2$Si$_2$

Metal-ligand interplay in strongly-correlated oxides: a parametrized phase diagram for pressure induced spin transitions

We investigate the magnetic properties of archetypal transition-metal oxides MnO, FeO, CoO and NiO under very high pressure by x-ray emission spectroscopy at the K\beta line. We observe a strong modification of the magnetism in the megabar range in all the samples except NiO. The results are analyzed within a multiplet approach including charge-transfer effects. The pressure dependence of the emission line is well accounted for by changes of the ligand field acting on the d electrons and allows us to extract parameters like local d-hybridization strength, O-2p bandwidth and ionic crystal field across the magnetic transition. This approach allows a first-hand insight into the mechanism of the pressure induced spin transition.Comment: 5 pages, 3 figure

Charge distribution across capped and uncapped infinite-layer neodymium nickelate thin films

Charge ordering (CO) phenomena have been widely debated in strongly-correlated electron systems mainly regarding their role in high-temperature superconductivity. Here, we elucidate the structural and charge distribution in NdNiO$_{2}$ thin films prepared with and without capping layers, and characterized by the absence and presence of CO. Our microstructural and spectroscopic analysis was done by scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) and hard x-ray photoemission spectroscopy (HAXPES). Capped samples show Ni$^{1+}$, with an out-of-plane (o-o-p) lattice parameter of around 3.30 angstroms indicating good stabilization of the infinite-layer structure. Bulk-sensitive HAXPES on Ni-2p shows weak satellite feature indicating large charge-transfer energy. The uncapped samples evidence an increase of the o-o-p parameter up to 3.65 angstroms on the thin-film top, and spectroscopies show signatures of higher valence in this region (towards Ni$^{2+}$). Here, 4D-STEM demonstrates (3,0,3) oriented stripes which emerge from partially occupied apical oxygen. Those stripes form quasi-2D coherent domains viewed as rods in the reciprocal space with $\Delta\text{q}_{z} \approx 0.24$ r.l.u. extension located at Q = ($\pm \frac{1}{3},0,\pm \frac{1}{3}$) r.l.u. and Q = ($\pm \frac{2}{3},0,\pm \frac{2}{3}$) r.l.u. The stripes associated with oxygen re-intercalation concomitant with hole doping suggests a possible link to the previously reported CO in infinite-layer nickelate thin films

Depth-resolving the redox compensation mechanism in LixNiO2

The performances of lithium-ion batteries are set by the electrodes materials capacity to exchange lithium ions and electrons faster and reversibly. To this goal Ni-rich layered metal oxides, especially LiNiO2, are attractive electrode candidate to achieve both high voltage and capacities. Despite its attractiveness, several drawbacks for its industrialization are related to different form of surface and bulk instabilities. These instabilities are due to redox process involving the charge transfer between cations and anions. Therefore, a fundamental understanding based on further experimental evidence is required to resolve of charge transfer between the cation and anion from the surface to the bulk in LiNiO2. Herein, we resolve the role of nickel and oxygen in the charge compensation process in LixNiO2 electrodes from the extreme surface down to 30 nm by energy-dependent core-level HAXPES supported by ab initio simulation. We emphasize the central role of oxygen in the bulk charge compensation mechanism from LiNiO2 to NiO2 due to the negative charge transfer and bond/charge-disproportionation characters of LiNiO2. This bulk behavior is in turn responsible for surface deoxygenation and nickel reduction upon delithiation

Temperature and pressure-induced spin-state transitions in LaCoO3

We report the continuous variation of the spin moment of cobalt in LaCoO3 across its temperature and pressure-induced spin transitions evidenced with K\beta emission spectra. The first thermal transition is best described by a transition to an orbitally nondegenerate intermediate spin (S=1) state. In parallel, continuous redistribution of the 3d electrons is also indicated by partial fluorescence yield X-ray absorption spectra. At high pressure, our study confirms that the material becomes low spin between 40 and 70 kbar at room temperature

a tool to disentangle overlapping core-excited states

We have measured resonant-Auger decay following Cl 1s−1 excitations in HCl and CH3Cl molecules, and extracted the pseudo-cross sections of different Cl 2p−2 final states. These cross sections show clear evidence of shake processes as well as contributions of electronic state-lifetime interference (ELI). To describe the spectra we developed a fit approach that takes into account ELI contributions and ultrafast nuclear dynamics in dissociative core-excited states. Using this approach we utilized the ELI contributions to obtain the intensity ratios of the overlapping states Cl 1s−14pπ/1s−14pσ in HCl and Cl 1s−14pe/1s−14pa1 in CH3Cl. The experimental value for HCl is compared with theoretical results showing satisfactory agreement