Resonant X-ray scattering studies of magnetic order and excitations in pyrochlore iridates.

The rare-earth pyrochlore iridates (R$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$, R$=$rare earth) have been proposed to host a number of exotic electronic states as a consequence of the existence of strong spin-orbit coupling of the Ir$^{\mathrm{4+}}$ ion in the presence of significant electron correlations. Of crucial importance to understanding whether any of these states can be realized in practice is to determine the effective low-energy Hamiltonian describing the system. Here we report a comprehensive series of resonant X-ray experiments, both elastic (REXS) and inelastic (RIXS), which reveal the nature of the magnetic order and excitations in single crystals of Sm$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7\thinspace }}$and Nd$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$.