Search Talks

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.

This talk will be about non-equilibrium many-body physics in integer quantum Hall edge states far from equilibrium. Recent experiments have generated a highly non-thermal electron distribution by bringing together at a point contact two quantum Hall edge states originating from sources at different potentials. The relaxation of this distribution to a stationary form is observed as a function of distance downstream from the contact [Phys. Rev. Lett. 105, 056803 (2010)]. I will discuss the broader context for the experiments and a physical picture of the equilibration process. I will also present an exact treatment of a minimal model for the experiment with results that account well for the observations. [Joint work with Dmitry Kovrizhin]

The space of convex polyhedra can be given a dynamical structure. Exploiting this dynamics we have performed a Bohr-Sommerfeld quantization of the volume of a tetrahedral grain of space, which is in excellent agreement with loop gravity. Here we present investigations of the volume of a 5-faced convex polyhedron. We give for the first time a constructive method for finding these polyhedra given their face areas and normals to the faces and find an explicit formula for the volume. This results
in new information about cylindrical consistency in loop gravity and a couple of surprises about polyhedra. In particular, we are interested in discovering whether the evolution generated by this volume is chaotic or integrable as this will impact the interpretation of the spin network basis in loop gravity. 

We discuss the extension of the smooth entropy formalism to arbitrary physical systems with no bound on the number of degrees of freedom, comparing them with already existing notions of entropy for infinite-dimensional systems. Our analysis is both conceptual as well as based on operational primitives, for example we ask for the ability to perform privacy amplification against any kind of quantum side information.  As an application, we show how to employ a version of the entropic uncertainty relation to provide a security analysis for continuous variable quantum key distribution protocols.   based on arXiv:1107.5460, 1112.2179. This is joint work with Mario Berta, Fabian Furrer as well as with Torsten Franz, Marco Tomamichel, Reinhard Werner

There is strong theoretical evidence that black holes have a finite thermodynamic entropy equal to one quarter the area A of the horizon. Providing a microscopic derivation of the entropy of the horizon is a major task for a candidate theory of quantum gravity. Loop quantum gravity has been shown to provide a geometric explanation of the finiteness of the entropy and of the proportionality to the area of the horizon. The microstates are quantum geometries of the horizon. What has been missing until recently is the identification of the near-horizon quantum dynamics and a derivation of the universal form of the Bekenstein-Hawking entropy with its 1/4 prefactor. I report recent progress in this direction. In particular, I discuss the covariant spin foam dynamics and and show that the entropy of the quantum horizon reproduces the Bekenstein-Hawking entropy S=A/4 with the proper one-fourth coefficient for all values of the Immirzi parameter.

It is certainly possible to express ordinary quantum mechanics in the framework of a real vector space: by adopting a suitable restriction on all operators--Stueckelberg’s rule--one can make the real-vector-space theory exactly equivalent to the standard complex theory.  But can we achieve a similar effect without invoking such a restriction?  In this talk I explore a model within real-vector-space quantum theory in which the role of the complex phase is played by a separate physical system called the ubit (for “universal rebit”).  The ubit is a single binary real-vector-space quantum object that is allowed to interact with everything in the world.  It also rotates in its two-dimensional state space.  In the limit of infinitely fast rotation, one recovers standard quantum theory.  When the rotation rate is large but not infinite, one finds small deviations from the standard theory.  Here I describe a few such deviations that we have seen numerically and explained analytically.

We add a gravitational background lattice to the simplest holographic model of matter at finite density and calculate the optical conductivity. With the lattice, the zero frequency delta function found in previous calculations (resulting from translation invariance) is broadened and the DC conductivity is finite. The optical conductivity exhibits a Drude peak with a cross-over to power-law behavior at higher frequencies. Surprisingly, these results bear a strong resemblance to the properties of some of the cuprates.