We have used high-resolution resonant inelastic x-ray scattering (RIXS) to study a thin film of NdNiO3, a compound whose unusual spin-and bond-ordered electronic ground state has been of long-standing interest. Below the magnetic ordering temperature, we observe well-defined collective magnon excitations along different high-symmetry directions in momentum space. The magnetic spectra depend strongly on the incident photon energy, which we attribute to RIXS coupling to different local electronic configurations of the expanded and compressed NiO6 octahedra in the bond-ordered state. Both the noncollinear magnetic ground state and the observed site-dependent magnon excitations are well described by a model that assumes strong competition between the antiferromagnetic superexchange and ferromagnetic double-exchange interactions. Our study provides direct insight into the magnetic dynamics and exchange interactions of the rare-earth nickelates, and demonstrates that RIXS can serve as a site-selective probe of magnetism in these and other materials.arXiv:1806.10499v1 [cond-mat.str-el]

Abstractα-RuCl3 is a major candidate for the realization of the Kitaev quantum spin liquid, but its zigzag antiferromagnetic order at low temperatures indicates deviations from the Kitaev model. We have quantified the spin Hamiltonian of α-RuCl3 by a resonant inelastic x-ray scattering study at the Ru L3 absorption edge. In the paramagnetic state, the quasi-elastic intensity of magnetic excitations has a broad maximum around the zone center without any local maxima at the zigzag magnetic Bragg wavevectors. This finding implies that the zigzag order is fragile and readily destabilized by competing ferromagnetic correlations. The classical ground state of the experimentally determined Hamiltonian is actually ferromagnetic. The zigzag state is stabilized by quantum fluctuations, leaving ferromagnetism – along with the Kitaev spin liquid – as energetically proximate metastable states. The three closely competing states and their collective excitations hold the key to the theoretical understanding of the unusual properties of α-RuCl3 in magnetic fields.

Abstract. Ammonia interactions and competition with water at the interface of nanoporous metal organic framework thin films of HKUST-1 (Cu 3 Btc, Btc=1,3,5-benzenedicarboxylate) are investigated with Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS). In the absence of water, ammonia adsorption at the Cu 2+ metal center weakens the metal-linker bond of the framework. In the presence of water, due to the higher binding energy (adsorption strength) of ammonia compared to water, ammonia replaces water at the unsaturated Cu 2+ metal centers. The water molecules remaining in the pores are stabilized by hydrogen bonding to ammonia.Hydrogen bonding between the water and ammonia strengthens the metal-ammonia interaction due to cooperative interactions. Cooperative interactions result in a reduction in the metal center oxidation state facilitating linker replacement by other species explaining the previously reported structure degradation. 2 1. Introduction.

The layer-by-layer epitaxial growth of HKUST-1 (Cu 3 (btc) 2 where btc = 1,3,5-benzenetricarboxylate) thin films is measured by quartz crystal microbalance with dissipation monitoring (QCM-D), X-ray diffraction (XRD), and scanning electron microscopy (SEM) as a function of the number of layers (20−80 layers) for −OH and −COOH functionalized surfaces. Up to approximately 40 layers, the film growth proceeds by a layer-by-layer mode controlled by the chemical functionalization of the surface. For example, on hydroxylated SiO 2 , film growth is in the preferred [222] direction. Beyond 40 layers, for both −COOH and −OH functionalized surfaces, the crystallite grain size increases and ∼50−100 nm octahedral crystals are formed. Independent of the surface functional groups (−COOH and −OH), the octahedral crystals form with the {200} planes oriented parallel to the surface. By monitoring changes in mass and dissipation, the QCM data provides evidence for the change in growth behavior. The stability of the films, determined by measuring CO 2 adsorption isotherms, depends on film properties (morphology and grain size) as well as film age. For films deposited on hydroxylated SiO 2 surfaces, CO 2 uptake decreases rapidly within a few days after film synthesis with the 40 layer films ({222} planes) exhibiting a more pronounced decrease than the 80 layer films ({200} planes, octahedral crystals). The decrease in CO 2 uptake is attributed to the differing propensities for water uptake in thin films of different morphologies as evidenced by water vapor adsorption isotherms and Raman spectral changes.