Forthcoming Seminars at F-1

Seminars Archive

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30 Nov 2010
16:00
prof. dr. Olav Syljuaa{}senConductance from quantum Monte Carlo
In physics seminar room 106. Abstract: Numerical methods for studying quantum many-body systems have improved steadily in the last decades. In this talk I will give a brief presentation of the Stochastic Series Expansion, a quantum Monte Carlo method, that has become a leading tool for calculating properties of several classes of quantum systems on lattices. In particular I will explain how it can be used to calculate the conductance of one-dimensional lattice fermion systems. Some results for thin wires coupled to leads will also be presented. more...

23 Nov 2010
15:00
David GarciaModeling Collective Emotions in Online Communities
As part of the EU Cyberemotions project, we have developed an agent-based framework to model the emergence of collective emotions in online communities. The concept of Brownian agents is used to describe the eigendynamics and the emotional feedback present in online communication. We derive conditions for the emergence of collective emotional states and identify scenarios where such states appear only once or in a repeated manner. The analytical results are illustrated by agent-based computer simulations. Our framework provides testable hypotheses about the emergence of collective emotions, which can be verified by data from online communities and psychology research. (These results are published in: F. Schweitzer, D. Garcia, 2010, The European Physical Journal B 77, 4, 533-545, DOI: 10.1140/epjb/e2010-00292-1)

14 Oct 2010
12:00
Jernej F. KamenikElectroWeak Precision Observables at One-Loop in Higgsless models
more...

13 Sep 2010
15:00
Avadh Saxena
Los Alamos National Lab
Mesoscopic Modeling of Multiferroic Materials and Color Symmetry
In physics seminar room (106).
Materials exhibiting ferroic phase transitions are ubiquitous in nature. Ferroic materials are those which possess two or more orientation states (domains) that can be switched by an external field and show hysteresis. Typical examples include ferromagnets, ferroelectrics and ferroelastics which occur as a result of a phase transition with the onset of spontaneous magnetization (M), polarization (P) and strain (e), respectively. A material that displays two or more ferroic properties simultaneously is called a multiferroic, e.g. magnetoelectrics (simultaneous P and M). Another novel class of ferroic materials called ferrotoroidics has been recently found. These materials find widespread applications as actuators, transducers, memory devices and shape memory elements in biomedical technology. First I will provide a historical perspective on this technologically important class of materials and then briefly illustrate the relevant concepts. I will discuss their properties, model the transitions at mesoscale and describe their microstructure. To this end I will introduce the notion of symmetry breaking: Broken rotational symmetry in a crystal leads to ferroelasticity, broken inversion symmetry leads to ferroelectricity and broken time reversal symmetry results in ferromagnetism. For the latter class and multiferroics, I will introduce the concept of color symmetry in order to describe various magnetic phases. Next, I will emphasize the role of long-range, anisotropic forces such as those arising from either the elastic compatibility constraints or the (polar and magnetic) dipolar interactions in determining the microstructure. Much of the excitement in this field stems from the unusual optical, spin and lattice properties of these materials which renders them as truly viable candidates for future metamaterials, e.g. as negative refractive index materials (NIM).

29 Jun 2010
15:00
prof. dr. Chaouqi Misbah
CNRS and Université de Grenoble
Red blood cells and vesicles under flow: towards a bottom-up approach to blood rheology
V seminarski sobi odsekov za fiziko (106).

A general introduction to blood flow both in macro and microcirculation will be provided. Understanding blood flow is essential both at the fundamental and practical levels. Blood is a complex fluid and the description of its flow properties escapes the traditional Navier-Stokes law known for simple fluids (e.g. water). Studying blood flow from microscopic considerations (bottom-up approach) is essential in order to derive effective constitutive laws for blood rheology. We present results on red blood cell (RBC) dynamics and on their biomimetic counterpart, represented by vesicles. We discuss various rich dynamics under linear and nonlinear flows. We shall show results on tank-treading motion, tumbling, vacillating-breathing, kayaking, etc. Link between microscopic dynamics and rheology will be discussed. Among other results, we shall focus on a recently solved problem, i.e., why do RBCs adopt a non symmetric shape (called slipper) in small blood vessels. A key result of our study is that the parachute symmetric shape is shown to be unstable, while the slipper shape is stable. That is, small flow disturbances which are always present in real blood flow cause RBCs to assume slipper shapes. It is further shown that the slipper shape offers a better transport efficiency to RBCs. In addition the slipper shape favors hemoglobin mixing in the cell, and thus enhances oxygen transport efficiency.

24 Jun 2010
17:15
Milan RajkovicSimplicial complexes and Combinatorial Laplacian of Complex Networks
In this exposition we focus on simplicial complexes (obtained from random, scale- free networks and networks with exponential connectivity distributions) and their homological and persistent homological properties. Simplicial complexes may be constructed from undirected or directed graphs (digraphs) in several different ways. Here we consider two of them: the neighborhood and the clique complex. We show how a new branch of topology may be introduced which we call statistical mechanics of simplicial complexes. Homology and persistent homology are discussed in light of applications beyond complex networks. Mathematical properties of normalized higher order combinatorial Laplacian of simplicial complex are presented and illustrated on several complex networks.

22 Jun 2010
15:00
Dr. Georgios PaltoglouA study of Information Retrieval weighting schemes for sentiment analysis
V seminarski sobi odsekov za fiziko (106).
Most sentiment analysis approaches use as baseline a support vector machines (SVM) classifier with binary unigram weights. In this paper, we explore whether more sophisticated feature weighting schemes from Information Retrieval can enhance classification accuracy. We show that variants of the classic tf.idf scheme adapted to sentiment analysis provide significant increases in accuracy, especially when using a sublinear function for term frequency weights and document frequency smoothing. The techniques are tested on a wide selection of data sets and produce the best accuracy to our knowledge.

15 Jun 2010
15:00
Prof. Dr. Takami TohyamaElectronic and Magnetic Properties in the Antiferromagnetic Metallic Phase of Iron Pnictide Superconductors
V seminarski sobi odsekov za fiziko (106).
We examine electronic states and magnetic properties of antiferromagnetic (AF) metallic phase in iron pnictides by mean-field calculations for a five-band Hubbard model. Collective and continuum excitations in the spin channel are investigated by RPA calculations for both the AF and paramagnetic phases. We found that the particle-hole gap opening in the bare susceptibility is found to be crucial to obtain spin excitations fully consistent with inelastic neutron scattering data. We also analyze the charge dynamics to make predictions for resonant inelastic x-ray scattering spectra [1]. Furthermore, we investigate Dirac fermions in the AF metallic phase. Deriving an effective Hamiltonian for the Dirac fermions, we reveal that there exist two Dirac cones carrying the same chirality, contrary to graphene, compensated by a Fermi surface with a quadratic energy dispersion as a consequence of a non-trivial topological property inherent in the band structure. The presence of the Dirac fermions gives the difference of sign-change temperatures between the Hall coefficient and the thermopower, being consistent with available experimental data [2].
[1] E. Kaneshita and T. Tohyama, arXiv:1002.2701.
[2] T. Morinari, E. Kaneshita, and T. Tohyama, arXiv: 1003.5469

8 Jun 2010
15:00
Daniele Coslovich
Technische Universitaet Wien
Structural motifs, heterogeneity and dynamics in glassy systems
V seminarski sobi odsekov za fiziko (106).
The relation between structure and dynamics in glass-forming liquids is the subject of a lively debate within the glass community. Recent simulation and experimental results indicate a deep connection between the outstanding dynamical properties of glassy systems and the so-called locally preferred structures [1-3]. I will review these findings and report molecular dynamics simulations of glass-forming liquids displaying different types of local order (icosahedral, prismatic, tetrahedral). Quite generally, locally preferred structures form ordered, slow domains whose size grows by decreasing temperature. The rate of growth and the spatial correlations of such structures are related to super-Arrhenius behavior of relaxation times and dynamic heterogeneity - two hallmarks of the glassy dynamics. Finally, I will complement my analysis with the study of the potential energy surface of these models, and show how the properties of localized and soft unstable modes encode relevant information about the dynamics.

[1] F. Sausset and G. Tarjus, Phys. Rev. Lett. 104, 065701 (2010)
[2] U.R. Pedersen et al., Phys. Rev. Lett. 104, 105701 (2010)
[3] C.P. Royall et al., Nature Mater. 7, 556 (2008)

20 May 2010
12:00
Sasa PrelovsekLattice searches for tetraquarks

18 May 2010
15:00
Dr. Raša PircBirelaxors
V seminarski sobi odsekov za fiziko (106).

Birelaxors are novel materials which possess both the properties of relaxor ferroelectrics and relaxor ferromagnets. Recent experiments on thin films of PFW/PZT revealed a strong magnetic field dependence of the dielectric polarization at room temperature. In particular, the dielectric relaxation time was found to satisfy a modified Vogel-Fulcher relation. These phenomena will be discussed in the framework of a semi-phenomenological model of birelaxors by applying the mechanism of growth and percolation of polar nanoregions in relaxor ferroelectrics.

11 May 2010
15:00
Lye-Hock Ong
School of Physics, Universiti Sains Malaysia
Switching Properties of Ferroelectric Thin Films and Superlattices
V seminarski sobi odsekov za fiziko (106).
The well-established one dimensional Tilley-Zeks model of Landau Devonshire (LD) free energy expansion is used to elucidate the effects of size and surface on switching phenomena on a one dimensional FE thin film. In dynamic field, the hysteresis loops obtained reflect the effects of extrapolation length $delta$ and film thickness; and the results are compared with those from the experiments. The effect of surfaces on switching speed and coercive field of the FE films is significant from the results of applied static field. These data also fit a formalism which gives a function of switching time in applied field and coercive field. A preliminary calculation on polarization reversal in FE bilayer is based on a simplified formed of Landau-type free energy model for a FE bilayer and Landau-Khalatnikov equation of motion and future work on FE superlattices will take the full form of the model.

4 May 2010
15:00
Dr. Fabian Heidrich-Meisner
LMU Munich, Germany
Nonequilibrium transport and the steady state in interacting nanostructures
V seminarski sobi odsekov za fiziko (106).

While the equilibrium properties of interacting nano-structures such as quantum dots are well understood, the nonequilibrium regime poses viable challenges to theorists. Most notably, methods developed for the equilibrium cannot necessarily be extended to the realm of nonequilibrium beyond perturbative limits.
In this talk, I will discuss the application of the time-dependent density matrix renormalization group (DMRG) method to the problem of non-equilibrium transport in quantum dots. The focus will be on the single-impurity Anderson model, yet I shall also mention more complex geometries. Using real-time simulations, we are able to access both the transient and the steady-state regime [1]. As a main result, we obtain the current-voltage characteristics for several models and I will comment on an extensive comparison with several other methods, both on the qualitative and the quantitative level [2]. One of the challenges for time- dependent DMRG is reaching the Kondo regime, due to the exponentially long time- and length scales [3]. I will demonstrate that DMRG simulations performed on Wilson leads with a logarithmic discretization of the density of states provide a solution to this issue [4].
Besides the numerical calculation of steady-state currents, it is of great conceptual interest to characterize the steady-state in terms of correlations and fluctuations. I will discuss our recent progress on this question.

[1] Heidrich-Meisner, Feiguin, Dagotto, Phys. Rev B 79, 235336 (2009).
[2] Eckel, Heidrich-Meisner, Jakobs, Thorwart, Pletyukhov, and Egger, New J. Phys., in press; preprint arXiv:1001.3773.
[3] Holzner, McCulloch, Schollwöck, von Delft, Heidrich-Meisner, Phys. Rev. B 80, 205114 (2009)
[4] Dias da Silva, Heidrich-Meisner, Feiguin, Büsser, Martins, Anda, and Dagotto, Phys. Rev. B 78, 195317 (2008)

23 Apr 2010
11:00
Dr. Abolfazl BayatQuantum State Transferring, Entanglement Generation and Transportation and Entanglement in Many Body-Systems
V cajni sobi odseka za teoreticno fiziko.

9 Apr 2010
11:00
Prof. Dr. Elzbieta Zipper,
University of Silesia in Katowice
Quantum computing with ring - shaped quantum dots
V cajni sobi odseka F1.

30 Mar 2010
15:00
Dr. Jalal Sarabadani
University of Isfahan, Iran
What is the Casimir effect?
V seminarski sobi odsekov za fiziko (106).

In this Seminar I will briefly introduce the static Casimir effect, and then I will go through the dynamic Casimir effect. What happen if you put two flat parallel perfect conductors in the quantum vacuum of electromagnetic field? The answer is that they attract each other. This is called the Casimir effect and people suggest that the contribution of this effect should be taken onto account in the designing of the microelectromechanical systems. When the boundaries (parallel conductors) move (oscillate) then in some oscillation frequencies the vacuum behaves like viscous medium and you feel that the amount of energy you are giving to the system are wasting by the friction of the vacuum. This is called the dynamic Casimir effect.

23 Feb 2010
14:00
Shigeru Ajisaka
Waseda University
Nonequilibrium Peierls Transition
We study nonequilibrium phase transition of the open Takayama Lin-Liu Maki model (the TLM model) embedded between two infinitely extended reservoirs by combining a mean field approximation and a result of the algebraic approach to nonequilibrium statistical mechanics. The TLM model[1] describes charge-density-wave order commensurate with the lattice. For the mean-field Hamiltonian, a natural nonequilibrium steady state (NESS) of Ruelle[2] is characterized as a state where the incoming field of reservoir operators distributes according to the Fermi distribution function of the reservoirs.
We report on nonequilibrium phases with a uniform lattice dimerization[3] as well as those with localized excitations[4]. For the former case, the phase diagram and the possibility of the negative differential conductance will be reported. For the latter case, we show that the polaron excitation is possible {it only when} the system is far from equilibrium, and report its features (temperature and applied-voltage dependence of its size and amplitude, etc.). This aspect is in contrast with the equilibrium case where the spinless TLM model is known to admit no polaron solutions [see Refs.[5] and references therein]. This observation suggests that the new polaron is an example of microscopic dissipative structure.
References:
[1] H. Takayama, Y.-R. Lin-Liu, and K. Maki: Phys. Rev. B 21, 2388 (1980).
[2] D. Ruelle: J. Stat. Phys. 98, 57 (2000); Comm. Math. Phys. 224, 3 (2001).
[3] S. Ajisaka, H. Nishimura, S. Tasaki and I. Terasaki: Prog. Theo. Phys. 121, 1289 (2009).
[4] S. Ajisaka, H. Nishimura, S. Tasaki and I. Terasaki: arXiv:0906.5337; arXiv:0907.0218.
[5] D.K. Campbell and A.R. Bishop: Nuclear Physics B 200, 297 (1982)

12 Jan 2010
15:00
Marcin Mierzejewski, Institute of Physics, University of SilesiaHubbard rings: current induced by change of magnetic flux
V seminarski sobi odsekov za fiziko (106)

The equilibrium state of a small mesoscopic ring threaded by a constant magnetic flux contains dissipationless current despite the ring is made out of resistive material. The properties of this current have been investigated for almost 30 years. However, important controversies concerning the magnitude and the flux--periodicity of the currents have been resolved only recently by means of novel experimental techniques. Temporal changes of magnetic field may drive the system out of equilibrium. In that case one can study far-from-equilibrium behavior a quantum system that is disconnected from macroscopic reservoirs. In particular, we investigate currents in a quantum ring threaded by a magnetic flux which is varied in an arbitrary way from an initial value $phi_i$ at time $t_i$ to a final value$phi_f$ at time $t_f$. Dynamics of electrons in the ring is described by the Hubbard and the extended Hubbard models. We demonstrate that time dependence of the induced current bears information on electron correlations. In the case of the Hubbard model with infinite on--site repulsion we present a simple proof that the current for $t>t_f$ is independent of the flux variation before $t_f$. Apart from mesoscopic rings our results pose some implications for designing of quantum motors built out as the ring--shaped optical lattice.

5 Jan 2010
15:00
Andrej KosmrljEffects of thymic selection of the T cell repertoire on HLA-class I associated control of HIV infection
V seminarski sobi odsekov za fiziko (106).

Without therapy, most persons infected with the human immunodeficiency virus (HIV) ultimately progress to AIDS. Rare individuals (elite controllers) maintain very low HIV levels without therapy, thereby making disease progression and transmission unlikely. Certain HLA Class I alleles are markedly enriched in elite controllers, with the highest correlation observed for HLA-B57. Since HLA molecules present viral peptides that activate CD8+ T cells, an immune mediated mechanism is likely responsible for superior control of HIV. We report that the peptide binding characteristics of HLA-B57 molecules impact thymic development such that, compared to other HLA-restricted T cells, a larger fraction of the naïve repertoire of B57-restricted clones recognizes a viral epitope and these T cells are more cross- reactive to mutants of targeted epitopes. Our calculations predict that such a T cell repertoire imposes strong immune pressure on immunodominant HIV epitopes and emergent mutants, thereby promoting efficient control of virus. Supporting these predictions, in a large cohort of HLA-typed individuals, our experiments show that the relative ability of HLA-B alleles to control HIV correlates with their peptide binding characteristics that impact thymic development. Our computational and experimental data provide a conceptual framework that unifies diverse empirical observations, and has implications for vaccination strategies.

 
 
 
 
 
 
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