Talks

I will describe a new numerical effort to solve Einstein gravity in 5-dimensional asymptotically Anti de Sitter spacetimes (AdS). The motivation is the gauge/gravity duality of string theory, with application to scenarios that on the gravity side are described by dynamical, strong-field solutions. For example, it has been argued that certain properties of the quark-gluon plasma formed in heavy-ion collisions can be modeled by a conformal field theory, with the dual description on the gravity side provided by the collision of black holes. As a first step towards modeling such more general phenomena, we initially focus on spacetimes with SO(3) symmetry in the bulk; i.e., axisymmetric gravity, dual to states with spherical or special conformal symmetry on the boundary. For a first application we study quasi-normal ringdown of highly deformed black holes in the bulk. Even though the initial states are far from equilibrium, the boundary state is remarkably well described as a hydrodynamic flow from early times. The code is based on the generalized harmonic formulation of the field equations, and though this method has been shown to work well in many asymptotically flat scenarios, there are unique challenges that arise in obtaining regular, stable solutions in asymptotically AdS spacetimes. I will describe these challenges, and the way we have addressed them.

We study the isotropization of a homogeneous, strongly coupled, non-Abelian plasma by means of its gravity dual. We compare the time evolution of a large number of initially anisotropic states as determined, on the one hand, by the full non-linear Einstein's equations and, on the other, by the Einstein's equations linearized around the final equilibrium state. The linear approximation works remarkably well even for states that exhibit large anisotropies. For example, it predicts with a 20% accuracy the isotropization time, which is of order 1/T, with T the final equilibrium temperature. We comment on possible extensions to less symmetric situations.

Neutron scattering can provide unique atomic scale information about structural and dynamical properties of frustrated magnetism. In this lecture I will give a brief description of theoretical and practical aspects of the technique. Experiments on frustrated magnets that illustrate the capabilities of new instrumentation at the Spallation Neutron Source and NIST will serve as examples. The target audience is scientists who are interested in using neutrons in their research, collaborating with a neutron scattering group, or who seek a deeper understanding of the corresponding experimental
literature [1].

[1] See http://jins.tennessee.edu/course2012/ for information about an online graduate course to be offered in the fall of 2012 on the use of neutron scattering in quantum condensed matter physics.

*Work at IQM was supported by the US DoE, office of Basic Energy Sciences, Division of
Material Sciences and Engineering under DE-FG02-08ER46544.

We provide a brief introduction to quantum spin liquid and review current status of theoretical and experimental progresses on this subject. Spin liquid phases that arise in different situations are examined in the light of both theoretical models and experimental systems.

Artificial spin ice was invented in 2006 by Peter Schiffer's group as a large-scale model of pyrochlore spin ice. Their system, an array of mesoscopic magnets with submicron dimensions, offered a way to observe the physics of ice rules at the level of individual, albeit large, spins. Today we have artificial magnetic arrays that follow the ice rules almost flawlessly, which makes them better ice models than their natural counterparts. We will highlight both similarities and differences between natural and artificial versions of spin ice. Large differences are expected in the dynamics of these systems. Mesoscopic spins easily transfer energy to internal degrees of freedom. Putting them in motion typically requires driving the system far from equilibrium. Thus, in some aspects, artificial spin ice is closer to granular matter and random magnets than to a regular magnetic solid. 

In the last few decades, there has been a marked rise in the diversity of compounds studied with frustrated networks of spins. This was clearly not the case in the early days of this field, where only a handful of “model” systems were being studied (ie. in two dimensions, the triangular or kagome lattices, and in three dimensions, the pyrochlore lattice).  Solid state chemists have played a major role in not only the identification of new geometrically frustrated materials, but also in the synthesis of high quality crystals with low degrees of chemical disorder that are essential for direct comparisons to theory. In this tutorial, strategies for the discovery of new frustrated materials and their synthesis will be reviewed, with an emphasis on recent advances in the literature.