**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 |

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,

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).

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.

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.