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“ If I were again beginning my studies, I would follow the advice of Plato and start with mathematics. ”
- Galileo Galilei


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Physics Conference Room, SB B326
Coffee starts at 12:00 PM and talk starts at 12:15 PM
Aug '15
Benoît Gérardin  -  Monday, August 3, 2015

Multiple scattering of waves in disordered media is often seen as a nightmare whether it be for communication, imaging or focusing purposes. The ability to control wave propagation through scattering media is thus of fundamental interest in many domains of wave physics, ranging from optics or acoustics to medical imaging or electromagnetism. Thirty years ago, it was shown theoretically that a properly designed combination of incident waves could be fully transmitted through (or reflected by) a disordered medium. Although this remarkable prediction has attracted a great deal of attention, open and closed channels have never been accessed experimentally.

Here, we study the propagation of elastic waves through a disordered elastic waveguide. Thereby, we present experimental measurements of the full S-matrix across a disordered elastic wave guide. To that aim, laser-ultrasonic techniques have been used in order to obtain a satisfying spatial sampling of the field at the input and output of the scattering medium. The unitarity of the S−matrix is investigated and the eigenvalues of the transmission matrix are shown to follow the expected bimodal distribution. Full transmission and reflection of waves propagating through disorder are obtained for the first time experimentally. The wave-fields associated to these open and closed channels are imaged within the scattering medium to highlight the interference effects operating in each case.

In the second part of the talk, we study beam-like states which can be seen as spatio-temporal open / closed channels. To that aim, the eigenstates of the Wigner-Smith time-delay matrix are considered in a regular cavity and a weakly disordered medium. The propagation of the wave-packets associated to these transmitted trajectory-like states is investigated.

Sep '15
Dmitriy Polyansky  -  Monday, September 21, 2015
Sep '15
Valter Zazubovich  -  Monday, September 28, 2015
ABSTRACT: In the past several years our group has been exploring protein energy landscapes in pigment-protein complexes involved in photosynthesis. These complexes offer a unique opportunity to explore native protein environments using optical spectroscopy methods, as chromophores are built into them by nature, without any extraneous manipulations that could potentially alter the structure or dynamics of the protein. Single Molecule (or singe complex) Spectroscopy has recently been a technique of choice for studying spectral dynamics in photosynthetic complexes. However, here I am going to demonstrate that Spectral Hole Burning (SHB) is capable of providing additional or competing information. In particular, most of the spectral shifts observed in single complex experiments are in fact light-induced (and not occurring anyway whether one observes them or not) and thus constitute SHB on a single-molecule level [1]. Analysis of the hole broadening allows us to claim that fast-small shift spectral dynamics in the LH2 complex is specific only to single-complex scenarios and not to the bulk sample. And so on.
Inspired by these results we undertook a detailed SHB study of spectral dynamics in several photosynthetic complexes, with the main focus on the CP43 antenna complex [2] of Photosystem II and dimeric Cytochrome b6f [3]. We also developed a unified approach to modeling SHB and spectral hole recovery, at fixed (burn) temperature and upon thermocycling. This approach relies on the argument that in the presence of “spectral memory” (holes recovering mostly due to burnt systems returning to the pre-burn configuration) the barrier distributions encoded into the non-saturated spectral holes and manifesting during the hole recovery differ from the full true barrier distributions. These partial barrier distributions are vastly different for different shapes of the true full distributions, and one can easily distinguish their manifestations. Quantitatively, all complexes we have explored so far exhibit similar barrier distribution parameters, distinct from those of some simple organic glasses. Qualitatively, however, barrier distribution shapes show great variability. Unlike in CP43 [2], the distributions of barriers between protein sub-states involved in light-induced conformational changes (SHB) in Cytochrome b6f are more likely glass-like ~V^(0.5) (V is the barrier height), and not Gaussian.
There is a high degree of correlation between the heights of the barriers in the ground and excited states in the individual pigment-protein systems, as well as nearly perfect spectral memory. Both spectral hole burning and recovery are due to phonon-assisted tunneling associated with the increase of the energy of a scattered phonon. As the latter is unlikely for simultaneously both the hole burning and hole recovery, proteins must exhibit a NPHB mechanism involving diffusion of the free volume towards the pigment. Entities involved in the light-induced conformational changes are characterized by md2 value of about 1.0.10-46 kg.m2. Thus, these entities are protons or, alternatively, small groups of atoms experiencing sub-Å shifts. However, explaining all SHB and recovery data simultaneously, employing just one barrier distribution, requires a drastic decrease in the attempt frequency to about 100 MHz. This decrease may occur due to cooperative effects.
1. Grozdanov, D.; Herascu, N.; Reinot, T.; Jankowiak, R.; Zazubovich, V. J. Phys. Chem. 2010, 114, 3426-3438.
2. Najafi, M., Herascu, N., Seibert, M., Picorel, R., Jankowiak, R., Zazubovich, V., J. Phys. Chem. B 2012, 116, 11780.
3. Najafi, M.; Herascu, N.; Shafiei, G.; Picorel, R.; Zazubovich, V.; J. Phys. Chem. B 2015, 10.1021/acs.jpcb.5b02845
Oct '15
Mikael Rechtsman  -  Monday, October 26, 2015
Aspects of photonic topological insulators
Pennsylvania State University
Nov '15
Victor Yakovenko  -  Monday, November 2, 2015
Economic inequality from a statistical physics point of view
Department of Physics, University of Maryland, College Park
ABSTRACT: Similarly to the probability distribution of energy in physics, the probability distribution of money among the agents in a closed economic system is also expected to follow the exponential Boltzmann-Gibbs law, as a consequence of entropy maximization.  Analysis of empirical data shows that income distributions in the USA, European Union, and other countries exhibit a well-defined two-class structure.  The majority of the population (about 97%) belongs to the lower class characterized by the exponential ("thermal") distribution.  The upper class (about 3% of the population) is characterized by the Pareto power-law ("superthermal") distribution, and its share of the total income expands and contracts dramatically during booms and busts in financial markets.  Globally, data analysis of energy consumption per capita around the world shows decreasing inequality in the last 30 years and convergence toward the exponential probability distribution, in agreement with the maximal entropy principle. Similar results are found for the global probability distribution of CO2 emissions per capita.  All papers are available at http://physics.umd.edu/~yakovenk/econophysics/.  For recent coverage in Science magazine, see http://www.sciencemag.org/content/344/6186/828
Nov '15
Allyson Sheffield  -  Monday, November 16, 2015
Dec '15
Pier Mello  -  Monday, December 7, 2015