We have investigated the low temperature phase of a Nd 0.5 Sr 0.5 MnO 3 single crystal by x-ray resonant scattering at the Mn K-edge of the (3 0 0

The optical design of the BOREAS beamline operating at the ALBA synchrotron radiation facility is described. BOREAS is dedicated to resonant X-ray absorption and scattering experiments using soft X-rays, in an unusually extended photon energy range from 80 to above 4000 eV, and with full polarization control. Its optical scheme includes a fixed-included-angle, variable-line-spacing grating monochromator and a pair of refocusing mirrors, equipped with benders, in a Kirkpatrick-Baez arrangement. It is equipped with two end-stations, one for X-ray magnetic circular dichroism and the other for resonant magnetic scattering. The commissioning results show that the expected beamline performance is achieved both in terms of energy resolution and of photon flux at the sample position.

How magnetism emerges in low-dimensional materials such as transition metal dichalcogenides at the monolayer limit is still an open question. Herein, we present a comprehensive study of the magnetic properties of single crystal and monolayer VSe2.0, both experimentally and ab initio. Magnetometry, X-ray magnetic circular dichrosim (XMCD) and ab initio calculations demonstrate that the charge density wave in bulk stoichiometric VSe2.0 causes a structural distortion with a strong reduction in the density of sates at the Fermi level, prompting the system towards a nonmagnetic state but on the verge of a ferromagnetic instability. In the monolayer limit, the structural rearrangement induces a Peierls distortion with the opening of an energy gap at the Fermi level and the absence of magnetic order. Control experiments on defect-induced VSe 2−δ single crystals show a breakdown of magnetism, discarding vacancies as a possible origin of magnetic order in VSe2.0. arXiv:1907.02034v1 [cond-mat.mes-hall]

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The crystal and local structures of La 1−x Sr x FeO 3−␦ ͑0 ഛ x ഛ 1͒ samples have been studied by x-ray diffraction and x-ray absorption spectroscopy techniques. The Fe-O bond length decreases with increasing x. Accordingly, the x-ray absorption near edge spectroscopy ͑XANES͒ spectra reveal a chemical shift of the iron K edge to higher energies. Both results agree with an Fe valence increase as La is substituted with Sr. Extended x-ray absorption fine structure spectroscopy and XANES show that the chemical state of Fe atoms in intermediate compositions can be described either by a bimodal distribution of formal Fe 3+ and Fe 4+ ions or by an Fe 3.x+ intermediate valence. The large value of the Debye-Waller factors obtained for intermediate compositions indicates that hole doping produces local disorder around the Fe ions. These factors show unusually large values below the metal-insulator ͑MI͒ transition for x =2/ 3 or 3/ 4. We show that a significant charge disproportionation of the type 2Fe 4+ → Fe 3+ +Fe 5+ cannot account for the local structure observed below the MI transition temperature of these samples. We suggest that an electronic localization arises from an order-disorder transition between dynamic and static distortions, resulting in the opening of a gap at the Fermi level.

We present a study of the Co spin-state changes in Pr 0.5 Ca 0.5 CoO 3 across the metal-insulator transition by means of the analysis of two complementary synchrotron x-ray spectroscopic techniques. Recorded experimental and charge transfer multiplet-based calculated Co L 2,3 absorption spectra, in combination with Co Kβ main emission lines, indicate that Co 3+ ion spin state evolves from low in the insulating phase to a low:high mixed state in the metallic regime, close to a 1:1 ratio at ambient temperature. A pure high spin state can be discarded, while the occurrence of an intermediate state is also questioned.

Recent findings show the emergence of two-dimensional electron gases (2DEGs) at LaAlO 3 /SrTiO 3 interfaces along different orientations, yet details on band reconstructions have remained unknown so far. Via x-ray linear dichroism spectroscopy we demonstrate that crystal symmetry imposes distinctive 2DEG orbital hierarchies on (001)-and (110)-oriented quantum wells, allowing selective occupancy of states of different symmetry. Such orientational tuning expands the possibilities for electronic engineering of 2DEGs and opens up enticing opportunities to understand the link between orbital symmetry and complex correlated states at LaAlO 3 /SrTiO 3 quantum wells.Two-dimensional electron gases at LaAlO 3 /SrTiO 3 interfaces: orbital symmetry and hierarchy engineered by crystal orientation 2The electronic structure of solids is deeply modified whenever the dimensionality of the system is reduced. One particular case occurs when the electron motion is confined within a plane in quantum well structures. For instance, in the realm of III-V or II-VI semiconductors, selecting a particular quantization direction for the quantum well growth is fundamental to achieve optimum efficiency for optoelectronic applications [1][2][3]. This is attained by a judicious selection of the crystal orientation that confines the electron motion, so that the effective masses or the internal polarization fields can be largely modulated to values that optimize the device performance.Beyond these more conventional systems, the recent discovery of quantum well structures based on the d-band SrTiO 3 oxide semiconductor has broken new ground [4][5][6][7][8][9][10][11]. The basic reasons are (i) the extremely confined character of the oxide quantum wells (on the order of a few nanometers) and (ii) a sheet carrier density above one order of magnitude higher than in conventional semiconductors. In addition, the much narrower bandwidth of d-derived electronic levels of transition metals -as compared to the wide s or p bands -promotes the emergence of complex electronically correlated states not present in the traditional semiconductors. Epitomizing this complexity, both magnetism and superconductivity have been reported to emerge at the LaAlO 3 /SrTiO 3 interface [12][13][14][15], which is the most intensively investigated system of this kind. The possibility of applying electrostatic gate voltages to these extremely narrow quantum wells [16][17][18] provides a unique opportunity to explore fundamental questions in the field of quantum fluids [19,20]. One of these aspects is related to the possible multiband character of superconductivity at the SrTiO 3 -quantum wells and its connexion with the detailed orbital structure of the t 2g -states [21][22]. The microscopic nature of the interface magnetism has also been linked to the orbital energy hierarchy of t 2g and e g levels [23][24] and even spectroscopic investigations emphasize the specific role of d xystates regarding the emergence of magnetism [25].While the vast majority of these studies have been...

X-ray absorption spectroscopy measurements in Pr 0.5 Ca 0.5 CoO 3 were performed at the Pr M 4,5 , Pr L 3 , and Ca L 2,3 absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple scattering simulations, which provide quantitative details on an electron loss at Pr 4f orbitals. We conclude that in the insulating phase a fraction [15(±5)%] of Pr 3+ undergoes a further oxidation to adopt a hybridized configuration composed of an admixture of atomiclike 4f 1 states (Pr 4+ ) and f-symmetry states on the O 2p valence band (Pr 3+ L states) indicative of a strong 4f-2p interaction.