Talks

The nature of the pairing mechanism in the recently discoverediron-pnictide family of superconductors remains an outstanding issue. To answer this question, it is instructive to know the symmetry of the superconducting energy gap. Low temperature thermal conductivity measurements provide a robust test of the presence or absence of low energy electronic quasiparticles that in turn can be used to characterise the symmetry of the gap function. In this talk, I will review what has been learnt so far from such measurements across several families of iron-pnictide superconductors, including our own recent results on the stoichiometric material LaFePO.

Direct visualization of the electronic structure within each crystalline unit cell of a solid is a new frontier in condensed matter physics (M.J. Lawler et al, Nature 466, 347 (2010)). In this talk, I will introduce the techniques of spectroscopic imaging scanning tunneling microscopy (SI-­‐STM) and then explain how our new application of this technique allows visualization of the intra-­‐unit-­‐cell electronic structure. We use this approach to study the pseudogap phase of cuprate high temperature superconductors. Recent experiments provide evidence that this phase may be associated with spontaneously broken electronic symmetries. By studying the Bragg peaks in Fourier transforms of SI-­-STM images, and in particular by resolving both the real and imaginary components of these Bragg amplitudes (as opposed to the Bragg intensities without phase information which are the observables in scattering experiments), we reveal the intra-­‐unit-­‐cell broken electronic symmetries of the cuprate pseudogap phase (J.P.Hinton et al, Science (2011)).

Recently resonant elastic soft x-ray scattering (RSXS) has emerged as a powerful new tool to study electronic ordering in materials like cuprates and manganites. The power of this technique is to combine xray scattering, which is sensitive to spatial order, with x-ray spectroscopy, which is sensitive to the valence, spin and orbital symmetry of specific atoms. This combination allows one to probe very directly and considerable detail a variety of exotic spin, charge, orbital or structural ordering phenomena. I will discuss the application of this technique to an investigation of charge density wave (CDW) order in the cuprate superconductor La1.475Nd0.4Sr0.125CuO4. Analysis of the photon energy dependence of the resonant scattering intensity provides unique insight into the microscopic details of the CDW ordering. New results from the REIXS beamline at the Canadian Light Source will be presented.

Many of the most interesting electronic behaviors arise in materials with strong electron-electron correlations. Many of these same materials are disordered either intrinsically or due to doping. The combination of disorder and interactions generally gives rise to a feature in the density of states at the Fermi level, with two of the most influential examples being the Altshuler-Aronov anomaly and the Efros-Shklovskii Coulomb gap. Experiments on strongly correlated materials and recent numerical results on the Anderson-Hubbard model, however, show behavior which is inconsistent with both of these frameworks. This talk will present some of the features of the zero bias anomaly in strongly correlated systems, both in the case of a purely on-site interaction and in the presence of nearest-neighbor interactions, and it will describe the physical origin of some of these features.

Mass, a concept familiar to all of us, is also one of the deepest mysteries in nature. Almost all of the mass in the visible universe, you, me and any other stuff that we see around us, emerges from QCD, a theory with a negligible microscopic mass content. How does QCD and the family of gauge theories it belongs to generate a mass? This class of non-perturbative problems remained largely elusive despite much effort over the years. Recently, new ideas based on compactification have been shown useful to address some of these. Two such inter-related ideas are circle compactifications, which avoid phase transitions and large-N volume independence. Through the first one, we realized the existence of a large-class of "topological molecules", e.g. magnetic bions, which generate mass gap in a class of compactified gauge theories. The inception of the second, the idea of large-N volume independence is old. The new progress is the realization of its first working examples. This property allows us to map a four dimensional gauge theory (including pure Yang-Mills) to a quantum mechanics at large-N.

It is a generic feature of strongly correlated electronic systems that several mechanisms (broadly represented by interactions) compete with each other. This competition often leads to the phenomenon of frustration. In strongly correlated systems such as doped Mott insulators kinetic energy and Coulomb repulsion frustrate the tendency of doped holes to phase separate. The result is the onset of a set of novel phases, which we dubbed Electronic Liquid Crystal (ELC) states, with varying degrees of complexity. Much like classical liquid crystals, electronic liquid crystal phases break translation and rotational invariance to varying degrees. In this talk I will focus on the experimental evidence and theory of two these phases, stripes and nematics, in several different systems including two-dimensional electron gases in large magnetic fields, ruthenate oxides, heavy fermions, and cuprate and pnictide superconductors.

Using the framework of deconstruction, we construct simple, weakly-coupled supersymmetric models that explain the Standard Model flavor hierarchy and produce a flavorful soft spectrum compatible with precision limits. Electroweak symmetry breaking is fully natural/ the mu-term is dynamically generated with no B mu-problem and the Higgs mass is easily raised above LEP limits without reliance on large radiative corrections. These models possess the distinctive spectrum of superpartners characteristic of 'effective supersymmetry': the third generation superpartners tend to be light, while the rest of the scalars are heavy.

I will describe the current state of our attempts to characterize the nature of the Dark Energy, the name given to the unknown phenomenology that is driving the observed accelerating cosmic expansion. There is a historical analogy between our current situation and the days of confusion before the advent of quantum mechanics. But while quantum physics emerged in a single academic generation, I fear that our attaining a deeper understanding of Dark Energy may not be as rapid. I will outline some steps we can take to try to avoid an extended period of sophisticated confusion and intellectual stagnation.