N. E. Shaik

Spin-orbit-induced orbital excitations in Sr2RuO4 and Ca2RuO4: A resonant inelastic x-ray scattering study

High-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen K edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism x-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λso=200 meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.

Damped spin excitations in a doped cuprate superconductor with orbital hybridization

A resonant inelastic x-ray scattering study of overdamped spin excitations in slightly underdoped La2−x Srx CuO4 (LSCO) with x = 0.12 and 0.145 is presented. Three high-symmetry directions have been investigated: (1) the antinodal (0,0) → ( 1 ,0), (2) the nodal (0,0) → ( 1 , 1 ), and (3) the zone-boundary direction 2 4 4 ( 1 1 1 2 ,0) → ( 4 ,4 ) connecting these two. The overdamped excitations exhibit strong dispersions along (1) and (3), whereas a much more modest dispersion is found along (2). This is in strong contrast to the undoped compound La2CuO4 (LCO) for which the strongest dispersions are found along (1) and (2). The t − t i − t ii − U Hubbard model used to explain the excitation spectrum of LCO predicts—for constant U/t —that the dispersion along (3) scales with (t i/t )2. However, the diagonal hopping t i extracted on LSCO using single-band models is low (t i/t ∼ −0.16) and decreasing with doping. We therefore invoked a two-orbital (dx2 −y2 and dz2 ) model which implies that t i is enhanced. This effect acts to enhance the zone-boundary dispersion within the Hubbard model. We thus conclude that hybridization of dx2 −y2 and dz2 states has a significant impact on the zone-boundary dispersion in LSCO.

Intrachain antiferromagnetic exchange in a 1D branched-chain built of two different copper(II) centres interlinked by end-on azido and phenoxo bridges: electron density map, electrochemical and magnetic properties

A 1D, end-on azido- and phenoxo-bridged, coordination polymer, [Cu2(μ-NAPPR)(μ1,1-N3)2]n (1), (NAPPR2− = 1,3-bis(naphthylideneimino)propane dianion), was synthesized and characterized. The 1D branched-chain is built of two types of slightly different, square-pyramidal, Cu(II) ions with different sets of donor atoms. Temperature- and field-dependent magnetic analyses reveal an antiferromagnetic interaction between copper centres in the main chain (Cu2) mediated by EO-azide bridges along the chain. The interaction between the copper centres Cu1 (in the branches of the main chain) and Cu2 is very weak and is included by means of an own Weiss constant θ′. The electron density map of 1 from synchrotron data shows that the dx2−y2 orbitals of Cu2 are not well described by the procrystal model, a difference which might be meaningful for the magnetic properties. Cyclic voltammetry studies show two different redox waves with a large difference between the redox potentials of the two types of copper centres (1.21 V). The resulting data are consistent with the crystal structure of this complex determined by single crystal X-ray diffraction.

J1-J2 square lattice antiferromagnetism in the orbitally quenched insulator MoOPO4

We report magnetic and thermodynamic properties of a 4d1 (Mo5+) magnetic insulator MoOPO4 single crystal, which realizes a J1−J2 Heisenberg spin-1/2 model on a stacked square lattice. The specific-heat measurements show a magnetic transition at 16 K which is also confirmed by magnetic susceptibility, ESR, and neutron diffraction measurements. Magnetic entropy deduced from the specific heat corresponds to a two-level degree of freedom per Mo5+ ion, and the effective moment from the susceptibility corresponds to the spin-only value. Using ab initio quantum chemistry calculations, we demonstrate that the Mo5+ ion hosts a purely spin-1/2 magnetic moment, indicating negligible effects of spin-orbit interaction. The quenched orbital moments originate from the large displacement of Mo ions inside the MoO6 octahedra along the apical direction. The ground state is shown by neutron diffraction to support a collinear Neel-type magnetic order, and a spin-flop transition is observed around an applied magnetic field of 3.5 T. The magnetic phase diagram is reproduced by a mean-field calculation assuming a small easy-axis anisotropy in the exchange interactions. Our results suggest 4d molybdates as an alternative playground to search for model quantum magnets.

Electronic and magnetic excitations in the half-stuffed Cu-O planes of Ba2Cu3O4Cl2 measured by resonant inelastic x-ray scattering

We use resonant inelastic x-ray scattering (RIXS) at the Cu L3 edge to measure the charge and spin excitations in the “half-stuffed” Cu-O planes of the cuprate antiferromagnet Ba2Cu3O4Cl2. The RIXS line shape reveals distinct contributions to the dd excitations from the two structurally inequivalent Cu sites, which have different out-of-plane coordinations. The low-energy response exhibits magnetic excitations. We find a spin-wave branch whose dispersion follows the symmetry of a CuO2 sublattice, similar to the case of the “fully stuffed” planes of tetragonal CuO (T-CuO). Its bandwidth is closer to that of a typical cuprate material, such as Sr2CuO2Cl2, than it is to that of T-CuO. We interpret this result as arising from the absence of the effective four-spin inter-sublattice interactions that act to reduce the bandwidth in T-CuO.

Magnetic excitations from the two-dimensional interpenetrating Cu framework in Ba2Cu3O4Cl2

We report detailed neutron scattering studies on Ba

Possibility of an unconventional spin state of Ir4+ in Ba21Ir9O43 single crystal

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Magnons in tetragonal CuO

Cu

Homo- and Heterodinuclear Iron Clathrochelate Complexes with Functional Groups in the Ligand Periphery

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Publisher's Note: Spin-orbit-induced orbital excitations inSr2RuO4andCa2RuO4: A resonant inelastic x-ray scattering study [Phys. Rev. B91, 155104 (2015)]

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