Physics Colloquia

Fall 2016 (usually Fridays 2:00 PM in SE 319)

Date Speaker Title
Jan 13 Jim McGuire Numbers That come from Nowhere
Jan 27 Maurizio Giannotti  Astrophysical Anomalies and Axions: the physics potential of the International Axion Observatory
Mar 10 J. B. Sokoloff Theory of the Thermal Diffusion of Microgel Particles in Highly Compressed Suspensions
Mar 13 Lingzhen Guo The giant acoustic atom --- a single quantum system with a deterministic time delay
Mar 24 Waseem Asghar Micro and Nanoscale Devices for Disease Management
Mar 31 Antonia Zipfel How to derive predictions from LQG
Apr 7 G. M. Pastor Energy landscapes and relaxation dynamics in disordered ensembles of magnetic nanoparticles: Transition from good folding to trapping
Apr 24 Janet Hung Tensor Network and the AdS/CFT correspondence


Colloquium Abstracts

Numbers That come from Nowhere

James McGuire  (FAU), Jan 13

In QED Feynman wrote: "There is a most profound and beautiful question associated with the observed coupling constant, e, the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won't recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with about an uncertainty of about 2 in the last decimal place. It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it.)" I have never seen it written on anvone's wall except Feynman's. This talk will be a discussion of this and other numbers that come from nowhere,


Astrophysical Anomalies and Axions: the physics potential of the International Axion Observatory

Maurizio Giannotti (Barry U.), Jan 27

Iwill give an update on the anomalies observed in the cooling of several stellar systems and on the interpretation in terms of axions and axion like particles (ALPs). I will show that the relevant region in the axion and ALP parameter space hinted by these anomalies can be probed by the next generation of axion detectors, in particular by the International Axion Observatory (IAXO).


Theory of the Thermal Diffusion of Microgel Particles in Highly Compressed Suspensions

J. B. Sokoloff (Northeastern U.), Mar 10

One amazing property of microgel colloids is the ability of the particles to thermally diffuse, even when they are compressed to a volume well below their swollen state volume, despite the fact that they are surrounded by other particles. A glass transition is expected to occur when the colloid is sufficiently compressed for microgel diffusion to cease. It is proposed that the diffusion is due to the ability of the highly compressed microgel particles to change shape with little cost in free energy. It will be shown that most of the free energy required to compress both polyelectolyte and neutral microgel particles is due to osmotic pressure resulting from either counterions or monomers inside of the gel, which depends on the particle's volume. There is still, however, a cost in free energy due to polymer elasticity when   microgel particles undergo the distortions necessary for them to move around each other as they diffuse through the compressed colloid, even if it occurs at constant volume. Using a scaling theory based on simple models for the linking of microgel particles, we examine the conditions under which the cost in free energy needed for a particle to diffuse is smaller than thermal energy, which is a necessary condition for microgel particle diffusion. Based on our scaling theory, we predict that thermally activated diffusion should be possible when the mean number of links along the axis along which a distortion occurs is much larger than N 1/5, where N is the mean number of monomers in a polymer chain connecting two links in the gel. We have also performed simulations based on this model to demonstrate how such diffusion comes about.


The giant acoustic atom --- a single quantum system with a deterministic time delay

Lingzhen Guo (TFP, Karlsruhe Institute of Technology (KIT), Germany), Mar 13

We investigate the quantum dynamics of a single two-level artificial atom (transmon qubit) coupled to surface acoustic waves (SAWs) via two distant connection points. Since the acoustic speed is five orders of magnitude slower than the speed of light, the travelling time between the two connection points needs to be taken into account. Therefore, we treat the transmon qubit as a giant atom with a deterministic time delay. We find that the spontaneous emission of the system, formed by the giant atom and the SAWs between its connection points, initially follows a polynomial decay law instead of an exponential one, as would be the case for a small atom. We obtain exact analytical results for the scattering properties of the giant atom up to two-phonon processes by using a diagrammatic approach. The time delay gives rise to novel features in the reflection, transmission, power spectra, and second-order correlation functions of the system. 


Micro and Nanoscale Devices for Disease Management

Waseem Asghar (FAU), Mar 24

The integration of medicine and technology at nano- and micro-scales offers tremendous opportunities for solving important problems in bioengineering, and enables a wide range of applications in diagnostics, therapeutics, and tissue engineering. Recently, a number of disease diagnostic assays have entered into the production phase. In this talk, I will cover the challenges and approaches to design smart biosensors with potentials applications in HIV disease management, cancer diagnostics, in vitro fertilization and infectious disease diagnostics. Novel cell phone based point-of-care (POC) devices and flexible microfluidic platforms will be discussed. Such platforms are used to explore optical, electrical, biophysical, and mechanical properties of biological targets. 


How to derive predictions from LQG

Antonia Zipfel (FAU), Mar 31

Quantizing gravity is one of the most challenging tasks of modern day physics and a prerequisite for understanding our fascinating universe. A promising candidate for a theory of quantum gravity is Loop Quantum Gravity (LQG), which has experienced a tremendous development in recent years. By now, the foundations of the theory are well-understood and many magnificent predictions, e.g. the replacement of the big bang by a big bounce, have been made. Due to the complexity of the theory, most of these results have, however, been derived from simplified toy models, whose relation to the full theory is not sufficiently understood. I will discuss some ideas how to overcome this situation and make LQG more predictive and thus falsifiable. Hereby, I will focus especially on the following two aspects: a) How can we build explicit physical states in canonical LQG? and b) How can we identify the physically interesting subsectors? 


Energy landscapes and relaxation dynamics in disordered ensembles of magnetic nanoparticles: Transition from good folding to trapping

G. M. Pastor (University of Kassel), Apr 7

The collective properties of two-dimensional ensembles of dipole-coupled magnetic nanoparticles (NPs) are investigated theoretically. Extended nanostructures are modeled by considering various distributions of the NPs in finite square unit cells with periodic boundary conditions. The elementary relaxation processes connecting neighboring metastable states across first-order saddle points are determined, together with the corresponding energy barriers and Hamming distances between the critical points. Results are given for the statistical distributions of these properties as a function of particle density, size dispersion, and degree of disorder. The changes in the magnetic configuration of the nanostructure along the elementary transitions are analyzed from a local perspective. The relaxation region, in which the orientations of the local magnetic moments change significantly, is found to rapidly localize as a result of disorder. Thus, the minimum distance between independent bits of information is significantly reduced, which should be relevant for high-density data storage media. The topology of the ergodic network of critical points is characterized from both local and energy perspectives by using connectivity and disconnectivity graphs. One observes that increasing the degree of disorder changes the nature of the energy landscape in a very profound way. Weakly disordered ensembles are good folders, which are funneled towards the global minima, whereas strongly disordered systems show very rough landscapes with complex relaxation pathways. Kinetic Monte Carlo simulations demonstrate that when disorder is weak the evolution towards equilibrium is fast, exponential-like, with a single time scale. In contrast, if disorder is strong, the relaxation involves multiple time scales, so that the dynamics is significantly slowed down as time goes by, which is reminiscent of spin glasses. Finally, we conclude with a brief perspective on current open problems.


Tensor Network and the AdS/CFT correspondence

Janet Hung (Fudan Univ.), Apr 24

We will give a review of the journey that unites tensor network and the AdS/CFT correspondence. We will discuss new insights into the program of understanding the AdS/CFT, and more generally a (quantum) gravity theory inspired by the tensor network.