- 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.
- Monday, September 21, 2015
ABSTRACT: Artificial photosynthetic systems exploit a variety of photochemical transformations with the ultimate result of efficient conversion of the photon energy into chemical bonds. The efficiency of these transformations strongly depends on how successfully proton-coupled electron transfer (PCET) processes are implemented. In our research program we focus on a mechanistic understanding of the role of PCET in reactions such as: (1) photochemical formation and reactivity of NADPH-like transition metal complexes; (2) hydrogen atom transfer (HAT) in the excited states of transition metal systems; (3) transition-metal complexes as photo- and electro-catalysts for proton reduction; and (4) light-driven water oxidation catalyzed by transition metal complexes.
In my presentation I will cover the basic principles of artificial photosynthesis and will explain how simple synthetic models may be used to mimic the action of natural photosystems. I will follow with several examples of catalytic transformations relevant to the production of solar fuels such as water oxidation and the reduction of protons and carbon dioxide. In each of these examples the importance of coupling the movement of multiple equivalents of electrons with proton transfer will be emphasized. Also, the modern time-resolved spectroscopy techniques such as pulse radiolysis and laser flash photolysis will be presented as experimental tools for unraveling complex mechanisms of PCET transformations which take place during catalytic cycles.
- 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 . 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  of Photosystem II and dimeric Cytochrome b6f . 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 , 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
- Monday, October 19, 2015
ABSTRACT: Recently it has been understood that the effect of random fields on the long-range order in systems with continuous-symmetry order parameter is controlled by topology. The n-component order parameter in d dimensions, interacting with the random field, exhibits glassy behavior at n < d + 1 due to the pinning of singularities. Nonsingular topological objects at n = d + 1 provide weak metastability. At n > d + 1 topological defects are absent and the behavior of the system is fully reversible, characterized by the exponential decay of correlations. Topological arguments have been confirmed numerically on lattices of up to one billion sites. Along these lines the effects of magnetic impurities on the ferromagnetic order and random anisotropy effects in amorphous and sintered magnets have been studied. Our findings also shed new light on properties of pinned charge density waves and properties of pinned flux lattices in superconductors. This research is supported by the U.S. Department of Energy through grant No. DE-FG02-93ER45487.
1. D. A. Garanin, E. M. Chudnovsky, and T. C. Proctor, The Role of Vortices in the Three-Dimensional Random-Field XY Model, Europhysics Letters 103, 67009 (2013).
2. D. A. Garanin, E. M. Chudnovsky, and T. C. Proctor, Random-Field XY Model in Three Dimensions, Physical Review B 88, 224418 (2013).
3. T. C. Proctor, D. A. Garanin, and E. M. Chudnovsky, Random Fields, Topology, and the Imry-Ma Argument, Physical Review Letters 112, 097201 (2014).
4. D. A. Garanin and E. M. Chudnovsky, Ordered vs Disordered States of the Random-Field Model in Three Dimensions, European Physics Journal B 88, 81 (2015).
5. T. C. Proctor, E. M. Chudnovsky, and D. A. Garanin, Scaling of Coercivity in a 3d Random Anisotropy Magnet, Journal of Magnetism and Magnetic Materials 384, 181 (2015).
6. T. C. Proctor and E. M. Chudnovsky, Effect of Dilute Random Field on Continuous Symmetry Order Parameter, Physical Review B 91, 140201(R) (2015).
- Monday, November 2, 2015
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