Search Talks

Frustrated pyrochlore magnets with Ising-like moments have attracted much attention due to the spin ice and spin liquid disordered states these materials display at low temperatures. We recently focused attention on Er2Ti2O7 and Yb2Ti2O7 which possess local XY, or planar, moments on the pyrochlore lattice - a network of corner sharing tetrahedra. I will describe our neutron scattering experiments in which magnetic long range order is destroyed by application of a (110) magnetic field in the XY antiferromagnetic pyrochlore Er2Ti2O7, and experiments in which unexpected field-induced magnetic order is observed in its ferromagnetic counterpart, Yb2Ti2O7.

In correlated electron systems, electrons can organize themselves in states that are analogous to classical liquid crystal phases. The search for such phases in solid state systems, in particular for the quantum version of an anisotropic liquid crystal state, dubbed electronic nematic phase, has been of great interest. For example, anisotropic metal bounded by two consecutive meta-magnetic transitions was reported in bilayer Ruthenates, and anisotropic neutron scattering patterns were observed in high temperature Cuprates In this talk, I will review theoretical development of the nematic phase and present relevant experimental issues. The interplay with other competing orders and avoided quantum criticality will be also discussed.

I will discuss NMR study of two types of iron based superconductors, electron doped Ba(Fe,Co)2As2 and stoichiometric FeSe. The primary focus will be on normal state spin fluctuations and its possible relation with the superconducting mechanism, and the pairing symmetry as probed by NMR.

Recent theory and experiment have revealed that strong spin-orbit coupling (SOC) can have dramatic qualitative effects on weakly interacting electrons. For instance, it leads to a distinct phase of matter, the topological band insulator. I will discuss the combined effects of SOC and strong electron correlation. For a ''strong'' Mott insulator, in which the electrons are well localized, SOC can compete with exchange interactions, leading to quenching of orbital degeneracy and even an instance of quantum criticality. For intermediate correlations, SOC has both quantitative and qualitative effects upon the Mott transition. An illustrative example of Ir-based pyrochlores will be presented, suggesting a rich interplay of correlations and SOC, and the possibility of distinct new electronic phases such a ''topological Mott insulator''.

Frustrated magnets are materials in which localized magnetic moments, or spins, interact through competing exchange interactions that cannot be simultaneously satisfied, giving rise to a large degeneracy of the system ground state. Under certain conditions, this can lead to the formation of fluid-like states of matter, so-called spin liquids, in which the constituent spins are highly correlated but still fluctuate strongly down to a temperature of absolute zero. The fluctuations of the spins in a spin liquid can be classical or quantum and show remarkable collective phenomena such as emergent gauge fields and fractional particle excitations. This exotic behaviour is now being uncovered in the laboratory, providing insight into the properties of spin liquids and challenges to the theoretical description of these materials.

We present a holographic description of four-dimensional single-scalar inflationary universes in terms of a three-dimensional quantum field theory. The holographic description correctly reproduces standard inflationary predictions in their regime of applicability. In the opposite case, wherein gravity is strongly coupled at early times, we propose a holographic description in terms of perturbative QFT and present models capable of satisfying the current observational constraints while exhibiting a phenomenology distinct from standard inflation. This provides a qualitatively new method for generating a nearly scale-invariant spectrum of primordial cosmological perturbations.

WIMPless dark matter offers an attractive framework in which dark matter can be very light. We investigate the implications of such scenarios on invisible decays of bottomonium states for dark matter with a mass less than around $5 {\rm GeV}$. We relate these decays to measurements of nucleon-dark matter elastic scattering. We also investigate the effect that a coupling to $s$ quarks has on flavor changing $b\to s$ processes involving missing energy.