Talks

The growing fascination with unconventional pairing is driven in part by continuing discoveries of exotic superconductors. The first of these, superfluid 3He, was found by Osheroff, Richardson, and Lee in 1971. This was followed soon thereafter by superconductivity in the heavy fermion compound, UPt3. And then an explosion of interest accompanied the observation of superconductivity in cuprates, Sr2RuO4, and organic materials. The newest discoveries are sperconducting compounds of FeAs. These systems have been demonstrated (or in some cases it is just suspected) that they have pairing condensates with non-zero angular momentum, L= 1, 2, and even 3. But all of them have the common hallmark of a high degree of sensitivity to impurities. In this talk I will discuss impurity effects in the best known of these unconventionally paired systems, 3He, a paradigm for the other unconventional superconductors. Impurity scattering is deftly controlled in superfluid 3He by imbibing it into high porosity silica aerogel. We can understand the suppression of its superfluid state (the transition temperature), the effect on its order parameter (the pairing energy), the appearance of quasiparticle bound states (gaplessness), and possibly new phases, in the context of current theory. I will discuss experiments from many laboratories and their theoretical interpretation leading to the topical question of the day, “Can anisotropic scattering stabilize new anisotropic states?”

'The discovery in 1996 of superconductivity at 0.2K near a magnetic quantum phase transition in CeIn3 opened a new dynasty of superconducting heavy electron materials, with many peculiar parallels to cuprate superconductors. In 2000, the introduction of additional layers of XIn_2, led to the discovery of the so-called ''115'' superconductors, with a tenfold increase in Tc[1]. By 2002, the replacement of Ce by Pu, drove the Tc up by an additional order of magnitude to 18.5K[2]. The recent discovery of a second material in this family has further deepened the mystery. In this talk I'll discuss the two newest ''high temperature'' heavy fermion superconductors in this series: PuCoGa5 and NpPd_2Al_5. These materials radically challenge the way we think about strongly correlated superconductivity. The way these materials directly transition from Curie paramagnets into anisotropic superconductors suggests a central role of spin as a driver for heavy electron superconductors - not just as the pairing glue - but as the basic fabric of the condensate. Motivated by these new materials, I'll discuss a model for superconductivity in the highest temperature superconductors in which the superconducting condensate involves formation of composite pairs between spins and conduction electrons[3]. Using this idea, we'll discuss how the physics of superconductivity and the Kondo effect can be combined, giving rise to a composite pairing model for the new superconductors. [1]}H. Hegger, C. Petrovic, E. G. Moshopoulou, M. F. Hundley, J. L. Sarrao, Z. Fisk, and J. D. Thompson, ''Pressure-Induced Superconductivity in Quasi-2D $CeRhIn_{5}$'' Phys. Rev. Lett. 84, 4986-4989 (2000). [2]J. L. Sarrao et al. , ``Plutonium-based superconductivity with a transition temperature above 18 K'', Nature (London) {bf 420}, 297-299 (2002). [3] Rebecca Flint, M. Dzero, P. Coleman, ''Heavy electrons and the symplectic symmetry of spin.'', Nature Physics 4, 643 - 648 (2008).Nature Physics, '

Physicists are often so awestruck by the lofty achievements of the past, we end up thinking all the big stuff is done, which blinds us to the revolutions ahead. We are still firmly in the throes of the quantum revolution that began a hundred years ago. Quantum gravity, quantum computers, qu-bits and quantum phase transitions, are manifestations of this ongoing revolution. Nowhere is this more so, than in the evolution of our understanding of the collective properties of quantum matter. Fifty years ago, physicists were profoundly shaken by the discovery of universal power-law correlations at classical second-order phase transitions. Today, interest has shifted to Quantum Phase Transitions: phase transitions at absolute zero driven by the violent jigglings of quantum zero-point motion. Quantum, or Qu-transitions have been observed in ferromagnets, helium-3, ferro-electrics, heavy electron and high temperature superconductors. Unlike its classical counterpart, a quantum critical point is a kind of 'black hole' in the materials phase diagram: a singularity at absolute zero that profoundly influences wide swaths of the material phase diagram at finite temperature. I'll talk about some of the novel ideas in this field including 'avoided criticality' - the idea that high temperature superconductivity nucleates about quantum critical points - and the growing indications that electron quasiparticles break up at a quantum critical point.

Particle physics, cosmology, and the study of a wide variety of theoretical models – most notably ones involving extra dimensions of space. Randall works on several of the competing models of string theory in the quest to explain the fabric of the universe.