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.

It has been recently established that a magnetized relativistic plasma yields an interesting example of a Weyl metal. I discuss the properties of magnetized relativistic plasma and its possible role in some astrophysics phenomena.

Our search for new materials of (hopeful) relevance to materials physics is wide ranging. One of our primary interests is in finding new geometrically frustrated magnets and working on their structure-property relations. In this context in recent years we have found and grown crystals of a new class of pyrochlore magnets based on fluorine instead of oxygen. This chemical difference allows pyrochlores to be made with magnetic transition metals on the “B sites”, and non-magnetic ions on the A sites, which results in stronger magnetic coupling than is seen in the rare earth pyrochlores. This therefore provides the ability to probe magnetic frustration on the pyrochlore lattice at more easily accessible temperatures. Some of our initial work on these emergent materials, NaCaCo2F7 is an example of one of them, will be described. We have also been exploring the edges of the Dirac semimetal materials space and have recently gotten some interesting results on the Dirac semimetal superconductor Au2Pb. I will describe this material and a recent finding in Ca3P2, a material most often encountered in rat poison. Finally, if time permits, I will describe recent results in our search for the perfect topological insulator.

Double perovskites, a class of oxide materials with 3d and 5d transition metal ions, can realize a wide variety of interesting phases. Here we focus on our recent theoretical work suggesting the appearance of Chern insulators, and possible emergent nematic phases at Chern transitions in such systems. We will also discuss Mott insulating double perovskites with iridium moments which appear to host unusual magnetic interactions on the 3-dimensional fcc lattice, and present comparisons with experimental data.

Muon spin rotation/relaxation is a powerful technique for studying unconventional superconductors, whose order parameter doesn't have the same symmetry as its host material's crystal structure. I will describe our work on Sr2RuO4, UPt3, CuxBi2Se3 and other topical systems, some of which exhibit broken time reversal symmetry in their superconducting states.

A series of fundamental discoveries over the past thirty years has dramatically improved our ability to read, write, and process magnetically stored information. I will briefly review some of these advances before focusing on the recently discovered and particularly promising spin-orbit torques which act on the collective spin of thin film magnetic conductors when they are placed on a substrate with strong spin-orbit interactions. Spin-orbit torques are normally interpreted in terms of the spin Hall effect, spin-current that flows perpendicular to charge current in any conductor. The spin-Hall effect in the best spin-orbit torque materials is thought to have a large contribution from states away from the Fermi energy that is insensitive to disorder and therefore referred to as intrinsic. I will argue that the physics of the intrinsic spin Hall effect is quite closely related to the physics of the quantum Hall effect, and on this basis speculate on strategies to find the material combinations that optimize the spin-orbit torque effect.

The spatially-indirect exciton condensates (SIXC) is an interesting ordered electronic state in which coherence is spontaneously established between particles localized in separate two-dimensional layers. I will discuss some of the properties SIXCs, commenting on their counterflow superfluidity, their collective excitations, and on similarities and differences relative to superconductors, easy-plane ferromagnets and anti-ferromagnets, and the standard model of particle physics.  Finally I will discuss the prospects of achieving SIXC states at elevated temperatures in multi-layer 2D material systems.