X-ray and neutron scattering

Abstract:

We report on a detailed x-ray resonant scattering study of the bilayer iridate compound Sr 3Ir 2O 7 at the Ir L 2 and L 3 edges. Resonant scattering at the Ir L 3 edge has been used to determine that Sr 3Ir 2O 7 is a long-range ordered antiferromagnet below T N 230K with an ordering wave vector q=(12,12,0). The energy resonance at the L 3 edge was found to be a factor of ∼30 times larger than that at the L 2 edge. This remarkable effect has been seen in the single-layer compound Sr 2IrO 4 and has been linked to the observation of a J eff=12 spin-orbit insulator. Our result shows that despite the modified electronic structure of the bilayer compound, caused by the larger bandwidth, the effect of strong spin-orbit coupling on the resonant magnetic scattering persists. Using the program sarah, we have determined that the magnetic order consists of two domains with propagation vectors k 1=(12,12,0) and k 2=(12,-12,0), respectively. A raster measurement of a focused x-ray beam across the surface of the sample yielded images of domains of the order of 100μm, with odd and even L components, respectively. Fully relativistic, monoelectronic calculations using the Green's function technique for a muffin-tin potential have been employed to calculate the relative intensities of the L 2,3 edge resonances, comparing the effects of including spin-orbit coupling and the Hubbard U term. A large L 3 to L 2 edge intensity ratio (∼5) was found for calculations including spin-orbit coupling. Adding the Hubbard U term had no significant effect on the calculated spectra. © 2012 American Physical Society.

Abstract:

The solid state exhibits a fascinating variety of phases, which can be stabilized by the variation of external parameters such as temperature, magnetic field and pressure. Until recently, numerical analysis of magnetic and/or orbital phases with collective excitations on a periodic lattice tended to be done on a case-by-case basis. Nowadays dynamical matrix diagonalization (DMD) has become an important and powerful standard method for the calculation of dispersive modes. The application of DMD to the interpretation of inelastic neutron scattering (INS) data on dispersive magnetic excitations is reviewed. A methodical survey of calculations employing spin-orbit and intermediate coupling schemes is illustrated by examples. These are taken from recent work on rare earth, actinide and transition metal compounds and demonstrate the application of the formalism developed.