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Physical review research 4(3), L032030 (2022). doi:10.1103/PhysRevResearch.4.L032030, A nontrivial balance between Coulomb repulsion and kinematic effects determines the electronic structure of correlated electron materials. The use of electromagnetic fields strong enough to rival these native microscopic interactions allows us to study the electronic response as well as the time scales and energies involved in using quantum effects for possible applications. We use element-specific transient x-ray absorption spectroscopy and high-harmonic generation to measure the response to ultrashort off-resonant optical fields in the prototypical correlated electron insulator NiO. Surprisingly, fields of up to 0.22 V/Å lead to no detectable changes in the correlated Ni $3d$ orbitals contrary to previous predictions. A transient directional charge transfer is uncovered, a behavior that is captured by first-principles theory. Our results highlight the importance of retardation effects in electronic screening and pinpoints a key challenge in functionalizing correlated materials for ultrafast device operation., Published by APS, College Park, MD

Rare-earth (RE) 4f elements play a key role for technological applications. The strong magneto-crystalline anisotropy (MCA) and large magnetic moment of the 4f electron shell makes these materials important ingredients for magnetic devices. Being able to control magnetism in 4f materials on ultrafast timescale is one of the major challenges in ultrafast spin physics. Required for an ultrafast, non-thermal control of magnetic alignment are pathways to directly act on the magnetic moment. With the spin angular momentum being a conserved quantity, it is the orbital degrees of freedom that provides an ultrafast handle on the atomic magnetic moment, but mechanisms for light-induced changes of 4f orbital angular momenta have not been discussed, yet. Here we show for Tb metal that after near-infrared laser excitation, 5d-4f electron-electron scattering leads to selective excitations in the 4f shell, which modify the orbital state and locally flip the MCA within femtoseconds. Our study provides a new level of understanding of optically controlled magnetism in 4f metals and shows a path towards ultrafast, non-thermal modification of the 4f-atomic magnetic moments., Comment: Manuscript (20 pages, 2 figures) and Supplementary Information (26 pages, 9 figures)

X-ray diffraction with photon energies near the Ru L$_2$-absorption edge was used to detect resonant reflections characteristic of a G-type superstructure in RuSr$_2$GdCu$_2$O$_8$ single crystals. A polarization analysis confirms that these reflections are due to magnetic order of Ru moments, and the azimuthal-angle dependence of the scattering amplitude reveals that the moments lie along a low-symmetry axis with substantial components parallel and perpendicular to the RuO$_2$ layers. Complemented by susceptibility data and a symmetry analysis of the magnetic structure, these results reconcile many of the apparently contradictory findings reported in the literature., Comment: to appear in PRL

Utilizing a sum rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO3 is strongly reduced from its ionic value, both below and above the Ne el temperature. Using Ti L2,3 x-ray absorption spectroscopy as a local probe, we found that the crystal-field splitting in the t2g subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates., Comment: 4 pages, 3 figures