# physics & astronomy

## Colloquium: Organic (Plastic!) Semiconductors

Organic materials are finding an increasing number of possible niches in electronic devices. I will give a very brief overview of some of the present and anticipated applications of organic semiconductors and issues in understanding their application. I will then discuss some of our recent experiments on using infrared electro-optics to measure the mobility of charges in organic thin-film transistors and conclude by describing our ongoing experiments to measure the thermal conductivities of organic semiconductors, important for possible applications as thermoelectric power generators.

Speaker / Presenter: Joseph Brill, Dept of Physics & Astronomy, University of Kentucky

## Spin and Pseudo-Spin in Graphene

Graphene, a single atomic layer of graphite, has been provided physicists opportunities to explore an interesting analogy to the relativistic quantum mechanics. The unique electronic band structure of graphene lattice yields a linear energy dispersion relation where the Fermi velocity replaces the role of the speed of light and pseudo spin degree of freedom for the orbital wavefunction replaces the role of real spin in usual Dirac Fermion spectrum. The exotic quantum transport behavior discovered in these materials, such as unusual half-integer quantum Hall effect and Klein tunneling effect, are a direct consequence of the pseudo-spin rotation in graphene. Interacting systems with internal symmetries will tend to break those symmetries in order to lower their energy. In graphene, the strong Coulomb interactions and approximate spin-pseudo spin symmetry are predicted to lead to a variety of quantum Hall ferromagnetic ground states and excitations which manifest as integer quantum Hall plateaus appearing within a graphene. In this presentation I will discuss various experimental evidence support the importance of spin and pseudo-spin structures in graphene at the strong quantum limit.

## The story of the Black Hole Information Paradox

Some 40 years ago Bekenstein argued that black holes should have an enormous entropy. Shortly thereafter, Hawking showed that black holes evaporate in a way that violates quantum mechanics. The latter result has been a long standing problem, known as the black hole information paradox. Recent results in string theory has shown that the microstates corresponding to Bekenstein's entropy are 'fuzzballs' that do not have a regular horizon; the horizon of the hole arises as an emergent statistical concept. Further, the large entropy of these states leads to a violation of the semiclassical approximation at the horizon of a black hole; this alters Hawking's computation and provides a resolution of the information paradox.

## Colloquium: Topological Surface States in Topological Insulators, Superconductors and Beyond

Topological Surface States in Topological Insulators, Superconductors and Beyond

M Zahid Hasan, Dept of Physics, Princeton University

Bulk Topological Insulators are a new phase of electronic matter which realizes a non-quantum-Hall-like topological state in the bulk matter and unlike the quantum Hall liquids can be turned into superconductors. In this talk, I will first review the basic theory of topological matter and experimental probes that reveal topological order. I will discuss experimental results that demonstrate the fundamental properties of topological insulators such as spin-momentum locking, non-trivial Berry’s phases, mirror Chern number, absence of backscattering or no U-turn rule, protection by time-reversal symmetry and the existence of room temperature topological order (at the level of M.Z.H and C.L. Kane, Rev. of Mod. Phys., 82, 3045 (2010)). I will then discuss the possible exotic roles of broken symmetry phases such as superconductivity and magnetism in doped topological insulators and their potential device applications in connection to our recent results as well as outline the emerging research frontiers of the field as a whole. Time permitting, I will also present experimental results on a new class of topological insulators beyond the Kane-Mele Z2 theory.

## Observation of the thermal Casimir effect and new limits on non-Newtonian forces in the micrometer range

Quantum theory predicts the existence of the Casimir force between macroscopic bodies, a force arising from the zero-point energy of electromagnetic field modes around them. I will report the experimental observation of the thermal Casimir force between two gold plates, due to thermal rather than quantum fluctuations of the electromagnetic field at room-temperature. The thermal Casimir force dominates over the quantum force for separations greater than a micrometer. We use our measurements to place new upper bounds on short-range exotic forces, arising, for example, in quantum gravity theories with extra dimensions.

## Colloquium: Discovery of a North-South Asymmetry in the Distribution of Local Stars and its Implications

Studies which would try to fix the local dark matter density through observations of the nearby stars invariably assume that the stars are in gravitational equilibrium. In recent years, however, it has become clear that the Milky Way displays many transient phenomena; it is a much more violent place than earlier thought. I will show how observations of the local solar neighborhood from the Sloan Digitial Sky Survey can probe the vertical equilibrium of the Galactic disk, by testing the symmetries present in the stars' distribution. Our analysis reveals a failure of north-south symmetry with respect to the galactic plane and thus that vertical equilibrium is wanting. I will consider our result in light of other observational studies, suggest further observational tests, and finally offer a perspective on its implications for dark-matter direct detection experiments.

## UK Physics Professor Discovers Important Experimental Result

## Cosmos and Computers: Gary Ferland discusses infrastructure upgrades for studying space.

The University of Kentucky recently announced big upgrades to its supercomputing infrastructure. This means more power for researchers across the campus working on some of the questions that have puzzled us the longest.

This podcast was produced by Patrick O'Dowd.

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

## A New Path Forward: UK Proposes to Self-fund Transformation of Campus

## Colloquium: Viscosity, Quark Gluon Plasma, and String Theory

Viscosity, quark gluon plasma, and string theory

Viscosity is a very old concept which was introduced to physics by Navier in the 19th century. However, in strongly coupled systems, viscosity is difficult to compute from first principle. In this talk I will describe some recent surprising developments in string theory which allow one to compute the viscosity for a class of strongly interacting quantum fluids not too dissimilar to the quark gluon plasma. I will describe efforts to measure the viscosity and other physical properties of the quark gluon plasma created in relativistic heavy ion collisions.