


Forthcoming Seminars at F1
Tuesday 11 Dec 2018 11:15  Lucas Hackl (MPQ Garching)  Low energy spectra from variational tangent planes 

Variational methods are extensively used to study properties of ground states and low excited states. By focusing on a suitable submanifold of states, we can overcome the exponential growth of the Hilbert space with the system size. In this talk, I will introduce a geometric framework to approximate the low lying energy spectrum by studying the linearized Hamiltonian flow on the tangent plane at the ground state. The spectrum is then given by the eigenvalues of the covariant derivative on the variational manifold. Using the paradigmatic BoseHubbard model as an example, I will discuss advantages compared to methods based on directly diagonalizing the Hamiltonian restricted to the tangent plane.
Seminar room for physics (JSI main building) 
Seminars Archive
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 30 Nov 2010 16:00  prof. dr. Olav Syljuaa{}sen  Conductance from quantum Monte Carlo 

In physics seminar room 106.
Abstract:
Numerical methods for studying quantum manybody 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 onedimensional lattice fermion systems. Some results for thin wires coupled to leads will also be presented. more... 
23 Nov 2010 15:00  David Garcia  Modeling Collective Emotions in Online Communities 

As part of the EU Cyberemotions project, we have developed an
agentbased 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 agentbased 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, 533545, DOI: 10.1140/epjb/e2010002921)

14 Oct 2010 12:00  Jernej F. Kamenik  ElectroWeak Precision Observables at OneLoop 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 longrange, 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 bottomup 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 NavierStokes law known for simple fluids
(e.g. water). Studying blood flow from microscopic considerations
(bottomup 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 tanktreading motion,
tumbling, vacillatingbreathing, 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 Rajkovic  Simplicial 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 Paltoglou  A 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 Tohyama  Electronic 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 meanfield calculations for a
fiveband 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 particlehole 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 xray
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 nontrivial topological
property inherent in the band structure. The presence of the Dirac
fermions gives the difference of signchange 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 glassforming 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
socalled locally preferred structures [13]. I will review these
findings and report molecular dynamics simulations of glassforming
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 superArrhenius 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 Prelovsek  Lattice searches for tetraquarks 


18 May 2010 15:00  Dr. Raša Pirc  Birelaxors 

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
VogelFulcher relation. These phenomena will be discussed in the
framework of a semiphenomenological model of birelaxors by applying
the mechanism of growth and percolation of polar nanoregions in
relaxor ferroelectrics. 
11 May 2010 15:00  LyeHock Ong School of Physics, Universiti Sains Malaysia  Switching Properties of Ferroelectric Thin Films and Superlattices 

V seminarski sobi odsekov za fiziko (106).
The wellestablished one dimensional TilleyZeks 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 Landautype free energy model for a FE bilayer and LandauKhalatnikov equation of motion and future work on FE superlattices will take the full form of the model. 
4 May 2010 15:00  Dr. Fabian HeidrichMeisner 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 nanostructures 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 timedependent
density matrix renormalization group (DMRG) method to the problem of
nonequilibrium transport in quantum dots. The focus will be on the
singleimpurity Anderson model, yet I shall also mention more complex
geometries.
Using realtime simulations, we are able to access both the transient
and the steadystate regime [1]. As a main result, we obtain the
currentvoltage 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 steadystate currents, it is of
great conceptual interest to characterize the steadystate in terms of
correlations and fluctuations. I will discuss our recent progress on
this question.
[1] HeidrichMeisner, Feiguin, Dagotto, Phys. Rev B 79, 235336 (2009).
[2] Eckel, HeidrichMeisner, Jakobs, Thorwart, Pletyukhov, and Egger,
New J. Phys., in press; preprint arXiv:1001.3773.
[3] Holzner, McCulloch, Schollwöck, von Delft, HeidrichMeisner,
Phys. Rev. B 80, 205114 (2009)
[4] Dias da Silva, HeidrichMeisner, Feiguin, Büsser, Martins, Anda,
and Dagotto, Phys. Rev. B 78, 195317 (2008)

23 Apr 2010 11:00  Dr. Abolfazl Bayat  Quantum State Transferring, Entanglement Generation and Transportation and Entanglement in Many BodySystems 

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 LinLiu
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 chargedensitywave order commensurate with the lattice.
For the meanfield 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 appliedvoltage 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. LinLiu, 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 Silesia  Hubbard 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 fluxperiodicity
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 farfromequilibrium 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 onsite 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 ringshaped optical lattice. 
5 Jan 2010 15:00  Andrej Kosmrlj  Effects of thymic selection of the T cell repertoire on HLAclass 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 HLAB57. 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 HLAB57 molecules impact
thymic development such that, compared to other HLArestricted T
cells, a larger fraction of the naïve repertoire of B57restricted
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 HLAtyped individuals, our experiments show that the
relative ability of HLAB 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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