Jonathan Pelliciari

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.

Doping dependence of the magnetic excitations in La2-xSrxCuO4

The magnetic correlations within the cuprates have undergone intense scrutiny as part of efforts to understand high temperature superconductivity. We explore the evolution of the magnetic correlations along the nodal direction of the Brillouin zone in La2-xSrxCuO4, spanning the doping phase diagram from the anti-ferromagnetic Mott insulator at x = 0 to the metallic phase at x = 0.26. Magnetic excitations along this direction are found to be systematically softened and broadened with doping, at a higher rate than the excitations along the anti-nodal direction. This phenomenology is discussed in terms of the nature of the magnetism in the doped cuprates. Survival of the high energy magnetic excitations, even in the overdoped regime, indicates that these excitations are marginal to pairing, while the influence of the low energy excitations remains ambiguous.

Local and collective magnetism ofEuFe2As2

We present an experimental study of the local and collective magnetism of EuFe2As2, that is isostructural with the high temperature superconductor parent compound BaFe2As2. In contrast to BaFe2As2, where only Fe spins order, EuFe2As2 has an additional magnetic transition below 20 K due to the ordering of the Eu2+ spins (J = 7/2, with L = 0 and S = 7/2) in an A-type antiferromagnetic texture (ferromagnetic layers stacked antiferromagnetically). This may potentially affect the FeAs layer and its local and correlated magnetism. Fe-Kβ x-ray emission experiments on EuFe2As2 single crystals reveal a local magnetic moment of 1.3±0.15 μB at 15 K that slightly increases to 1.45±0.15 μB at 300 K. Resonant inelastic x-ray scattering (RIXS) experiments performed on the same crystals show dispersive broad (in energy) magnetic excitations along (0, 0)→ (1, 0) and (0, 0)→ (1, 1) with a bandwidth on the order of 170-180 meV. These results on local and collective magnetism are in line with other parent compounds of the AFe2As2 series (A = Ba, Ca, and Sr), especially the well characterized BaFe2As2. Thus, our experiments lead us to the conclusion that the effect of the high magnetic moment of Eu on the magnitude of both Fe local magnetic moment and spin excitations is small and confined to low energy excitations.

Observation of momentum-dependent charge excitations in hole-doped cuprates using resonant inelastic x-ray scattering at the oxygen K edge

We investigate electronic excitations in La2-x(Br,Sr)xCuO4 using resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the dispersive mode is ascribed to intraband charge excitations which have been observed in electron-doped cuprates.

Quenched Magnon excitations by oxygen sublattice reconstruction in (SrCuO 2 ) n /(SrTiO 3 ) 2 superlattices

The recently discovered structural reconstruction in the cuprate superlattice (SrCuO2)n/(SrTiO3)2 has been investigated across the critical value of n = 5 using resonant inelastic x-ray scattering (RIXS). We find that at the critical value of n, the cuprate layer remains largely in the bulk-like two-dimensional structure with a minority of Cu plaquettes being reconstructed. The partial reconstruction leads to quenching of the magnons starting at the Γ-point due to the minority plaquettes acting as scattering points. Although comparable in relative abundance, the doped charge impurities in electron-doped cuprate superconductors do not show this quenching of magnetic excitations.

Intralayer doping effects on the high-energy magnetic correlations in NaFeAs

We have used resonant inelastic x-ray scattering (RIXS) and dynamical susceptibility calculations to study the magnetic excitations in NaFe1-x Co-x As (x = 0, 0.03, and 0.08). Despite a relatively low ordered magnetic moment, collective magnetic modes are observed in parent compounds (x = 0) and persist in optimally (x = 0.03) and overdoped (x = 0.08) samples. Their magnetic bandwidths are unaffected by doping within the range investigated. High-energy magnetic excitations in iron pnictides are robust against doping and present irrespectively of the ordered magnetic moment. Nevertheless, Co doping slightly reduces the overall magnetic spectral weight, differently from previous studies on hole-doped BaFe2As2, where it was observed constant. Finally, we demonstrate that the doping evolution of magnetic modes is different for the dopants being inside or outside the Fe-As layer.

Presence of magnetic excitations in SmFeAsO

We measured dispersive spin excitations in SmFeAsO, a parent compound of SmFeAsO1−xFx and one of the highest temperature superconductors of Fe pnictides (TC ≈ 55 K). We determine the magnetic excitations to disperse with a bandwidth energy of ca 170 meV at (0.47, 0) and (0.34, 0.34), which merges into the elastic line approaching the Γ point. Comparing our results with other parent Fe pnictides, we show the importance of structural parameters for the magnetic excitation spectrum, with small modifications of the tetrahedron angles and As height strongly affecting the magnetism.

Resonant inelastic X-ray scattering study of spin-wave excitations in the cuprate parent compound Ca2CuO2Cl2

By means of resonant inelastic x-ray scattering at the Cu L3 edge, we measured the spin wave dispersion along 100 and 110 in the undoped cuprate Ca2CuO2Cl2. The data yields a reliable estimate of the superexchange parameter J = 135 ± 4 meV using a classical spin-1/2 2D Heisenberg model with nearest-neighbor interactions and including quantum fluctuations. Including further exchange interactions increases the estimate to J = 141 meV. The 40 meV dispersion between the magnetic Brillouin zone boundary points (1/2, 0) and (1/4, 1/4) indicates that next-nearest neighbor interactions in this compound are intermediate between the values found in La2CuO4 and Sr2CuO2Cl2. Owing to the low-Z elements composing Ca2CuO2Cl2, the present results may enable a reliable comparison with the predictions of quantum many-body calculations, which would improve our understanding of the role of magnetic excitations and of electronic correlations in cuprates.

Charge crystallization in a Fermi liquid

Charge order has been established as a ubiquitous instability of the underdoped copper-oxide superconductors. New investigations reveal that it extends to the overdoped side of the phase diagram, a region otherwise known to host a conventional Fermi liquid state.

Magnetic moment evolution and spin freezing in doped BaFe 2 As 2

Fe-Kβ X-ray emission spectroscopy measurements reveal an asymmetric doping dependence of the magnetic moments μbare in electron- and hole-doped BaFe2As2. At low temperature, μbare is nearly constant in hole-doped samples, whereas it decreases upon electron doping. Increasing temperature substantially enhances μbare in the hole-doped region, which is naturally explained by the theoretically predicted crossover into a spin-frozen state. Our measurements demonstrate the importance of Hund’s-coupling and electronic correlations, especially for hole-doped BaFe2As2, and the inadequacy of a fully localized or fully itinerant description of the 122 family of Fe pnictides.