Physics Colloquia Spring 2013

(usually Fridays 2:00 PM in SE 319)

Titles link to the abstracts.

Date Speaker Title
Jan 25
Chris Beetle
Feb 8
Shen Li Qiu
Mar 15
Mingzhou Ding
Mar 22
Efthymios Liarokapis
(NTU Athens)
Mar 29
Petr Tsatsin
Apr 5
Shawn Wilder
Apr 19
Bernd Bruegmann
(Univ. of Jena)

Colloquium Abstracts

Photons with Orbital Angular Momentum and Quantum Cryptography
Chris Beetle (FAU), Jan 25
The spin angular momentum of photons, their polarization, provides a useful "quantum bit" for optical quantum computing and cryptography applications. It has been understood since at least the 90's, however, that beams in classical optics can also have a definite orbital angular momentum associated with the shape of the phase front in space. The corresponding orbital angular momentum quantum number of single photons offers a natural way to develop optical quantum computing in Hilbert spaces of higher dimension. This talk will review orbital angular momentum in classical optics, examine how to adapt the idea to particle states in quantum field theory, and discuss possible applications to quantum cryptography.
Crystal phases and phase transitions under pressure from first-principles calculations
Shen Li Qiu (FAU), Feb 8
We have recently introduced an important new concept in the first-principles description of crystal phases and their transitions under pressure: an equilibrium line of quantum states which are saddle points (sp) of Gibbs free energy (G) in structure space, called the saddle-point equilibrium line (denoted sp-eq line) of states. This line is in addition to the two phase equilibrium lines (denoted ph-eq lines), which the sp-eq line joins. The sp-eq line has the following features: (1) it merges with one ph-eq line at low p and with the other ph-eq line at high p, hence terminates the metastable range of the two interacting phases of a crystal; (2) it gives the finite ranges of pressure and structure in which two stable phases exist and out of which one branch of each phase emerges; (3) it gives the barrier height at the transition pressure which is the barrier that keeps the metastable ranges from transitioning. These features imply that the sp-eq line along with the two interacting ph-eq lines form a complete description of crystal phases and their transitions under pressure.
Normal and Pathological Oscillations of the Brain
Mingzhou Ding (UFL)
Oscillatory phenomena are ubiquitous in the physical world. It turns out that there are a variety of rhythmic or repetitive neural activities in the central nervous system as well. These neural oscillations play important roles in normal brain functions. Abnormal oscillations are indicative of neurological and psychiatric disorders. To study the network mechanisms of neural oscillations we perform multielectrode neurophysiological recordings. Such recordings produce massive quantities of data. Multivariate time series analysis provides the basic framework for identifying the patterns of network interactions in these data. In this talk, I will introduce the commonly used measures for analyzing oscillatory neural activity and discuss applications to cognitive neuroscience experiments
Lattice anomalies in pnictides
Efthymios Liarokapis (National Technical University of Athens)
Iron pnictides and cuprates have similar phase diagrams with antiferromagnetic ordering with a spin density wave and a structural phase transition at low doping and superconductivity in a range of doping level. The RFeAsO series (R is a rare earth) is usually doped by F substitution for O or oxygen deficiency with about the same maximum transition temperature (Tc). In oxygen deficient NdFeAsO0.85 compound with the maximum Tc we have observed slight modifications in the infrared spectra at a temperature well above Tc, which have been verified by low temperature synchrotron xrd measurements on the same compound. These xrd results indicate that a slight structural modification sets-in at Tc to disappear at the temperature of the SDW or structural phase transition, which should not be present in the optimally doped sample. We have also studied by Raman spectroscopy and synchrotron powder xrd the effect of hydrostatic pressures on RFeAsO1-xFx compounds (R=Sm, Nd) of various doping levels. We have found slight lattice distortion, which disappear at low doping levels, as in cuprates. All data indicate some role for the lattice in superconductivity.
Initial data for binary neutron stars with arbitrary spins
Petr Tsatsin (FAU)
The starting point of any general relativistic numerical simulation is a solution of the Hamiltonian and momentum constraints that (ideally) represents an astrophysically realistic scenario. We present a new method to produce initial data sets for binary neutron stars with arbitrary spins and orbital eccentricities. The method only provides approximate solutions to the constraints. However, we show that the corresponding constraint violations subside after a couple of orbits, becoming comparable to those found in evolutions of standard conformally flat, helically symmetric binary initial data. We evolve in time three data sets, corresponding to binaries with spins aligned, zero and anti-aligned with the orbital angular momentum. These simulations show the orbital "hang-up" effect previously seen in binary black holes. Additionally, all three show orbital eccentricities up to one order of magnitude smaller than those found in helically symmetric initial sets evolutions.
Quasilocal Observables of the Gravitational Field and Stability of Approximate Killing fields
Shawn Wilder (FAU)
In relativistic physics, a precise definition of a black hole's angular momentum is possible only when its horizon possesses an axial symmetry. Unfortunately most black hole horizons have no such symmetry. However, it is possible to pose an eigenvalue problem that has solutions corresponding to any manifold's ``approximate Killing fields.'' This allows one to generalize formulae requiring symmetry to cases where no symmetry is present and thus define, for example, the spin of an arbitrary black hole. This talk will discuss work using perturbation theory of a horizon to quantify the stability of quantities generalized in this way. We will present precise conditions for the stability of solutions to the eigenvalue problem.
Black holes and gravitational waves - what Einstein could not know
Bernd Bruegmann (University of Jena)
Among the fascinating predictions of Einstein's theory of general relativity are black holes and gravitational waves. Recently it has become possible to simulate several inspiral orbits and the merger phase of two black holes and the waves they generate using the methods of numerical relativity on high performance parallel computers. These simulations are of interest for the theoretical and technical challenges they represent, but more importantly for their use in the larger context of gravitational wave astronomy. We will review some basic theoretical and experimental facts of black holes and gravitational waves and describe recent numerical results.