The utility of resonant scattering for identifying electronic symmetries and density distributions changes dramatically as a function of photon energy. In the hard X-ray regime, strong core hole monopole potentials tend to produce X-ray absorption features with well defined electron number on the scattering site. By contrast, in the vacuum ultraviolet (VUV), resonant scattering from Mott insulators tends to reveal spectra that are characteristic of only the nominal valence, and are insensitive to deviations from nominal valence brought on by metal-ligand hybridization. Here, atomic multiplet simulations are used to investigate the interplay of monopolar and mulitpolar Coulomb interactions in the VUV and soft X-ray regimes, to identify how charge transfer thresholds and other signatures of mixed valence can manifest in this low photon energy regime. The study focuses on the Mott insulator NiO as a well characterized model system, and extrapolates interactions into non-physical regimes to identify principles that shape the spectral features.

The compound SmB6 is the best established realization of a topological Kondo insulator, in which a topological insulator state is obtained through Kondo coherence. Recent studies have found evidence that the surface of SmB6 hosts ferromagnetic domains, creating an intrinsic platform for unidirectional ballistic transport at the domain boundaries. Here, surface-sensitive X-ray absorption (XAS) and bulk-sensitive resonant inelastic X-ray scattering (RIXS) spectra are measured at the Sm N4,5-edge, and used to evaluate electronic symmetries, excitations and temperature dependence near the surface of cleaved samples. The XAS data show that the density of large-moment atomic multiplet states on a cleaved surface grows irreversibly over time, to a degree that likely exceeds a related change that has recently been observed in the surface 4f orbital occupation.The topological Kondo insulator (TKI) state is a variant of the topological insulator state [1][2][3][4], in which a topologically ordered insulating electronic band structure is obtained from Kondo physics. The realization of a TKI state in mixed-valent SmB 6 was strongly indicated by early theoretical investigations [4,5], and has now been established through direct measurement of the topological surface states via angle resolved photoemission [6][7][8][9][10][11] and transport studies [12,13]. Strong evidence has recently been found suggesting that the surface of polished SmB 6 samples can also host ferromagnetic domains [13], a property that is theoretically associated with exotic axion electrodynamics, an inverse spin-galvanic effect, and ballistic one dimensional transport channels at domain boundaries [2,[14][15][16]. Moreover, surface sensitive X-ray photoemission (XPS) measurements have shown that the surface 4f occupation evolves irreversibly towards 4f 5 as a function of time following cleavage in ultra high vacuum (UHV) [17]. Here, multiplet-dominated X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) measurements in the vacuum ultraviolet (VUV) regime are used as a symmetry-sensitive probe to map the Sm N 4,5 -edge excitations and show that a similarly large change in the density of large-moment samarium sites accompanies this time evolution. This evolution is consistent with expectations for the transition from a Kondo insulating state to magnetism, and represents a means for incrementally tuning the strength of the surface magnetic instability.Measurements were performed at the beamline 4.0.3 (MERLIN) RIXS endstation (MERIXS) [18,19] at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. Large single crystals of SmB 6 were grown by the Al flux method as in Ref. [17], cleaved at low temperature, and maintained at a UHV pressure of approximately 3×10 −10 Torr. The photon beam had a grazing 30 o or angle of incidence to the cleaved [001] sample face, and scattered photons were measured at 90 o to the incident beam trajectory. XAS was measured using the total electron yield (TEY) method, and the expecte...