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Seminars Archive
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 9 Dec 2016 11:00  Marija Mitrovic Dankulov  How random are complex networks? 

Every complex system can be represented as a complex network, where constituent units are represented with nodes and interactions between them are expressed by network links. These networks are neither of regular or random structure, but rather an intricate combination of order and disorder. Scientists have developed large set of different topological measures for characterization and description of different structural properties of real networks. It turns out that these statistical measures are not independent, i.e., many properties appear as a statistical consequence of relatively small number of fixed topological proper ties in real network. Here we explore this dependence by applying the method of dkseries to six real networks representing different complex systems [1]. We find that many important local and global topological properties of networks are closely reproduced by dkrandom graphs with the same degree distribution, degree correlations, and clustering as in the studied real network. We discuss important conceptual, methodological, and practical applications of this evaluation of network randomness. [1] C. Orsini, M. Mitrovic Dankulov, P. ColomerdeSimon, A. Jamakovic, P. Mahade van, A. Vahdat, K. E. Bassler, Z. Toroczkai, M. Boguna, G. Caldarelli, S. Fortunato, D. Krioukov, Nature Communications 6 (2015) 8627.

8 Nov 2016 15:00  Tilen Cadez  Dynamical correlation functions of the 1D Hubbard model 

Cajna soba F1. 
11 Oct 2016 15:00  Jacek Herbrych  Dynamical structure factor in disordered model of interacting fermions 

I will present the behavior of the dynamical structure factor S(q,w) in the whole range of wavevectors q within the prototype onedimensional model of manybody localization (MBL). Extracted effective dynamical conductivities and currentrelaxation rates confirm strong dependence on disorder but modest variation with q. Furthermore, I will present an analytical selfconsistent approximation based on the perturbation theory to qualitatively account for the nontrivial features of dynamical quantities at all q: the emergence of the maximum in dynamical conductivities, nonanalytical lowomega variation in the ergodic phase, and the transition to the nonergodic (MBL) phase. Finitesize scaling also reveals the possibility of the subdiffusive behavior in the ergodic regime. more... 
25 Aug 2016 14:00  Banasri Basu  Dynamics of Optical and Electron Vortex Beams: some interesting features 

A unified framework has been proposed towards the dynamics of optical and electron vortex beams from the perspective of the geometric phase and the associated Hall effects. The unification is attributed to the notion that the spin degrees of freedom of a relativistic particle, either massive or massless, are associated with a vortex line. It has been shown that propagation of paraxial electron vortex beams in an external electric field gives rise to an orbital angular momentum (OAM) Hall effect, whereas that for nonparaxial beams with tilted vortices initiates a spin Hall effect. On the other hand, the paraxial optical vortex beams in an inhomogeneous medium induce an OAM Hall effect and nonparaxial beams with tilted vortices drive the spin Hall effect. Furthermore, for the electron vortex beam in a laser field, our analysis provides a possible physical mechanism responsible for the shift of the center of the beam with respect to the center of the fieldfree electron vortex beam.

23 Aug 2016 15:00  Ross McKenzie  Absence of a quantum limit to the shear viscosity of strongly interacting fermion systems 

Are there fundamental limits to how small the shear viscosity of a macroscopic fluid can be? Could Planck constant and the Heisenberg uncertainty principle determine that lower bound? In 2005 mathematical techniques from string theory and black hole physics were used to conjecture a lower bound for the ratio of the shear viscosity to the entropy of all fluids. From both theory and experiment, this bound appears to be respected in ultracould atoms and the quarkgluon plasma. However, we have shown that this bound is strongly violated in the bad metal regime that occurs near a Mott insulator, and described by a Hubbard model [1]. I will give a basic introduction to shear viscosity, the conjectured bounds, bad metals, and our results.
[1] N. Pakhira and R.H. McKenzie, Phys. Rev. B 92, 125103 (2015). 




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