Andrew Boothroyd

Abstract:

We report strongly momentum-dependent local charge screening dynamics in CE-type charge, orbital, and spin ordered La$_{0.5}$Sr$_{1.5}$MnO$_4$, based on Mn K-edge resonant inelastic x-ray scattering data. Through a comparison with theoretical calculations, we show that the observed momentum dependence reflects highly localized, nearest neighbor screening of the transient local charge perturbation in this compound with an exciton-like screening cloud, rather than delocalized screening. The size of the screening cloud is estmated to be about 0.4-0.5 interatomic distances.

Abstract:

The low-energy electronic structure of the J eff=1/2 spinorbit insulator Sr 3Ir 2O 7 has been studied by means of angle-resolved photoemission spectroscopy. A comparison of the results for bilayer Sr 3Ir 2O 7 with available literature data for the related single-layer compound Sr2IrO4 reveals qualitative similarities and similar J eff=1/2 bandwidths for the two materials, but also pronounced differences in the distribution of the spectral weight. In particular, photoemission from the J eff=1/2 states appears to be suppressed. Yet, it is found that the Sr 3Ir 2O 7 data are in overall better agreement with band-structure calculations than the data for Sr 2IrO 4. © 2012 IOP Publishing Ltd.

Abstract:

The low-energy electronic structure of the J(eff) = 1/2 spin-orbit insulator Sr3Ir2O7 has been studied by means of angle-resolved photoemission spectroscopy. A comparison of the results for bilayer Sr3Ir2O7 with available literature data for the related single-layer compound Sr2IrO4 reveals qualitative similarities and similar J(eff) = 1/2 bandwidths for the two materials, but also pronounced differences in the distribution of the spectral weight. In particular, photoemission from J(eff) = 1/2 the states appears to be suppressed. Yet, it is found that the Sr3Ir2O7 data are in overall better agreement with band-structure calculations than the data for Sr2IrO4.

Abstract:

We report neutron inelastic scattering measurements on the normal and superconducting states of single-crystalline Cs 0.8Fe 1.9Se 2. Consistent with previous measurements on Rb xFe 2-ySe 2, we observe two distinct spin excitation signals: (i) spin-wave excitations characteristic of the block antiferromagnetic order found in insulating A xFe 2-ySe 2 compounds, and (ii) a resonance-like magnetic peak localized in energy at 11 meV and at an in-plane wave-vector of (0.25,0.5). The resonance peak increases below T c=27 K, and has a similar absolute intensity to the resonance peaks observed in other Fe-based superconductors. The existence of a magnetic resonance in the spectrum of Rb xFe 2-ySe 2 and now of Cs xFe 2-ySe 2 suggests that this is a common feature of superconductivity in this family. The low-energy spin-wave excitations in Cs 0.8Fe 1.9Se 2 show no measurable response to superconductivity, consistent with the notion of spatially separate magnetic and superconducting phases. © 2012 American Physical Society.