Talks

New developments in time-of-flight neutron spectroscopy allow a remarkably comprehensive determination of the full spin and phonon excitation spectrum in many materials. I will discuss these new techniques and show results from the "214" family of layered quantum magnets - which are also the La(2-x)Ba/Sr(x)CuO4 high Tc superconductors. We observe the expected highly dispersive spin excitations emanating from the "pi-pi" magnetic zone centres, as well as a host of optic and acoustic phonons, and roughly speaking, the spin and phonon excitations separate as a function of momentum. However, a very interesting resonant phenomena is observed which is coincident with the low energy crossings of the dispersive spin excitations with the relatively dispersion less phonons.

Recently three-dimensional versions of honeycomb-lattice iridates, beta and gamma phases of Li2IrO3, have been discovered. It has been theoretically suggested that these hyperhoneycomb iridates may hold promise for the realization of the Kitaev spin liquid or the exactly solvable model for the elusive quantum spin liquid phase. Recent scattering experiments on these systems have, however, revealed highly non-trivial magnetic spiral phases. We propose a theory that may explain the emergence of this unusual magnetic order and discuss why the current experimental data may suggest that the hyperhoneycomb iridates are not too far from the Kitaev limit.

In this talk, I will discuss some of the recent progress made on low energy effective field theories for non-Fermi liquids. Based on a dimensional regularization scheme, physical properties of various non-Fermi liquid states can be computed in controlled ways. I will emphasize novel features that arise due to the interplay between interaction and the presence of extensive gapless modes near Fermi surface. The examples include non-analytic expansion in coupling, emergent locality and UV/IR mixing.