Andrew Boothroyd

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.

Abstract:

This report presents azimuthal dependent and polarization dependent x-ray resonant magnetic scattering at the Ir L(3) edge for the bilayered iridate compound Sr(3)Ir(2)O(7). The two magnetic wave vectors, k1 = (1/2, 1/2, 0) and k2 = (1/2, -1/2, 0), result in domains of two symmetry-related G-type antiferromagnetic structures, denoted A and B, respectively. These domains are approximately 0.02 mm(2) and are independent of the thermal history. An understanding of this key aspect of the magnetism is necessary for an overall picture of the magnetic behaviour in this compound. The azimuthal and polarization dependence of the magnetic reflections, relating to both magnetic wavevectors, show that the Ir magnetic moments in the bilayer compound are oriented along the c axis. This contrasts with single layer Sr(2)IrO(4) where the moments are confined to the ab plane.