Forthcoming Seminars at F-1

Seminars Archive

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19 Dec 2011
Vera M. O. BatistaShape shifters: Crystallization and packing of deformable colloids
Physics seminar room (106). Abstract:

The interactions between colloidal particles can often be described by simple potentials, making many experimentally observed colloids amenable to investigation through analytic theory and computer simulation. In this talk I will address the effect of internal flexibility which is relevant to several types of colloids including globular proteins, micelles and emulsions. I will introduce and characterize a model for a generic class of colloidal particles that have a preferred shape but can undergo deformations while always maintaining hard-body interactions. The model consists of hard spheres that can continuously change shape at fixed volume into prolate or oblate ellipsoids of revolution, subject to an energetic penalty. The severity of this penalty is specified by a single parameter that determines the flexibility of the particles. The deformable hard spheres crystallize at higher packing fractions than rigid hard spheres, have a narrower solid-fluid coexistence region and can reach high densities by a second transition to an orientationally ordered crystal. I will also address the effect of flexibility on the packing properties of deformable ellipsoids of revolution.

15 Nov 2011
Artem BadasyanPhysics of helix formation in biopolymers. What we know and what we dont.
Physics seminar room (106). Abstract:

Helical structures naturally appear in polymers of natural or artificial origin. Before going into dense polymer solutions, it is necessary to understand what is going on at the level of one single polymer (dilute solution regime), which is a statistical system itself. There are several necessary conditions a polymer model (be it analytical or numerical) should satisfy to reveal the helical state, namely: i) Left - right hand symmetry should be broken, otherwise appearance of racemate mixture will prohibit long and stable helices; ii) There should be a cancellation of long-range forces, so that the directional short-range attraction (hydrogen bonds, stacking interactions, fluctuating bond length) can play a significant role; iii) No significant long-range attraction should be present in the model, otherwise coil-globule transition will take place before the chain could afford for the helical structure, stabilised by short-range attractions.

I present to your attention my past advances and current acheivements regarding the studies of helical conformations in polymers using both spin-based analytical modelling and Monte Carlo simulations.

8 Nov 2011
Natasa PrzuljNetwork mining uncovers new biology
Physics seminar room (106).


Sequence-based computational approaches have revolutionized biological understanding.However, they can fail to explain some biological phenomena.Since proteins aggregate to perform a function instead of acting in isolation, the connectivity of a protein interaction network (PIN) will provide additional insight into the inner working on the cell, over and above sequences of individual proteins.We argue that sequence and network topology give insights into complementary slices of biological information, which sometimes corroborate each other, but sometimes do not.Hence, the advancement depends on the development of sophisticated graph-theoretic methods for extracting biological knowledge purely from network topology before being integrated with other types of biological data (e.g., sequence).However, dealing with large networks isnon-trivial, since many graph-theoretic problems are computationally intractable, so heuristic algorithms are sought.

Analogous to sequence alignments, alignments of biological networks will likely impact biomedical understanding.We introduce a family of topology-based network alignment (NA) algorithms, (that we call GRAAL algorithms), that produces by far the most complete alignments of biological networks to date: our alignment of yeast and human PINs demonstrates that even distant species share a surprising amount of PIN topology.We show that both species phylogeny and protein function can be extracted from our topological NA.Furtermore, we demonstrate that the NA quality improves with integration of additional data sources (including sequence) into the alignment algorithm: surprisingly, 77.7% of proteins in the bakers yeast PIN participate in a connected subnetwork that is fully contained in the human PIN suggesting broad similarities in internal cellular wiring across all life on Earth. Also, we demonstrate that topology around cancer and non-cancer genes is different and when integrated with functional genomics data, it successfully predicts new cancer genes in melanogenesis-related pathways.

25 Oct 2011
Soham BiswasNovel features and new universality class of a recently proposed opinion dynamics model
Tea room F1. Abstract:

A model for opinion dynamics (Model I) has been recently introduced in which the binary opinions of the individuals are determined according to the size of their neighboring domains (population having the same opinion) [1]. The coarsening dynamics of the equivalent Ising model shows power law behavior and has been found to belong to a new universality class with the dynamic exponent z = 1.0 ± 0.01 and persistence exponent 0.235 in one dimension. The critical behavior has been found to be robust for a large variety of annealed disorder that has been studied. A ballistic to diffussive crossover is obtained by introducing a size sensitivity parameter p [2]. Further, by mapping Model I to a system of random walkers in one dimension with a tendency to walk towards their nearest neighbour with probability , we find that for any > 0.5, the Model I dynamical behaviour is prevalent at long times [3].

[1] S. Biswas and P. Sen, Phys. Rev. E 80, 027101 (2009).
[2] S. Biswas and P. Sen, Journal of Physics A 44, 145003 (2011).
[3] S. Biswas, P. Sen and P. Ray, Journal of Physics : Conference series 297, 012003 (2011) more...

24 Oct 2011
Bogdan R. BulkaSpin Stark Effect in a Triangular Molecule
Tea room F1. Abstract:
The lecture will present our recent theoretical studies of strongly correlated electrons in a system of three coherently coupled quantum dots in the presence of an external electric field. In particular spin-spin correlations, a linear and quadratic Stark effect will be analyzed. In some situation one can find a spin dark state, which can be relevant for entanglement of three spin qubits as well as for spin dependent transport. This triangular qubit system is also interesting for studies of quantum dynamics (Landau-Zener passages) when one applies controlled electrical pulses.

18 Oct 2011
Veljko ZlaticThermal transport in strongly correlated multilayers
Tea Room F1. Abstract:
A theory of charge and heat transport in inhomogeneous multilayers with correlated electrons is presented. We consider a device which consists of several weakly correlated metallic mid-planes sandwiched between two semi-infinite Mott insulators. The electron dynamics is described by the Falicov-Kimball model which is solved by an inhomogeneous DMFT algorithm[1]. By increasing the correlation strength we bring the mid-section in the proximity of the metal-insulator transition and enhance the slope of the density of states close to the chemical potential; by varying the number of planes in the mid-section we can change the effective dimensionality of metallic electrons. Both effects are expected to have an impact on the thermoelectric properties and enhance the efficiency of the device.
For the macroscopic currents flowing parallel to the multilayer planes, we calculate the transport properties by linear response theory[2] and, thus, obtain the conductivity, the Seebeck coefficient, the power factor, and the figure-of-merit of the device.
The impact of the on-site correlations and the gate voltage on thermal transport is discussed.

30 Sep 2011
Dr. Antun BalazFaraday Waves in Binary Non-Miscible Bose-Einstein Condensates
Tea room F1. Abstract: Using the extensive numerical simulations and analytical variational approach, we show that binary non-miscible Bose-Einstein condensates subject to periodic modulations of the radial confinement exhibit a Faraday instability similar to that seen in one-component condensates. We first present two experimentally relevant stationary state configurations of the system: one in which the components form a dark-bright symbiotic pair (the ground state of the system), and one in which the components are segregated (first excited state). For each of these two configurations we show numerically that far from resonant regions the Faraday waves excited in the two components are of equal period, emerge simultaneously, and do not impact the dynamics of the bulk of the condensate. We derive analytically the period of the Faraday waves using a variational treatment of the coupled Gross-Pitaevskii equations combined with a Mathieu-type analysis for the selection mechanism of the excited waves. Finally, we show that for a modulation frequency close to twice that of the radial trapping, the emergent surface waves fade out in favor of a forceful collective mode that turns the two components miscible. more...

13 Sep 2011
prof. Takami TohyamaSpin and Charge Excitations in the Antiferromagnetic Metallic Phase of Iron Arsenides: Inelastic Neutron Scattering and Resonant Inelastic X-Ray Scattering
Tea Room F1.

22 Jun 2011
Andreas WeichselbaumQuantum impurity models and tensor networks
Tea room F1. Abstract: In this talk I will give a brief review on the intrinsic link between the numerical renormalization group (NRG) and the density matrix renormalization group (DMRG) in terms of tensor networks. For an efficient simulation, the inclusion of preserved symmetries is crucial which motivates our recent general framework on the implementation of non-abelian symmetries. Moreover within the NRG then, a complete many-body eigenbasis is available that is, in fact, fully tractable in practice. This allows textbook evaluations of physical properties of quantum impurity models including dynamical quantities. Examples will be given in terms of optical absorption spectra, or a clean numerical treatment of Anderson orthogonality in the thermodynamic limit.

21 Jun 2011
Jure KokaljModel self energy for the metallic phase of overdoped cuprate superconductors
Tea room F1. Abstract: I will present a model self energy motivated by Angle-Dependent Magneto-Resistance (ADMR) and consisting of an isotropic Fermi liquid term and a Marginal Fermi liquid term which is anisotropic over the Fermi surface, vanishing in the same directions as the superconducting gap and the pseudogap. This model self energy gives a consistent description of ADMR, specific heat, de Haas van Alphen, and photoemission experiments. In particular, it reconciles the strongly doping dependent anomalous scattering rate observed in ADMR with the almost doping independent specific heat. A comparison with microscopic theories will also be presented.

10 Jun 2011
Giniyat KhaliullinSpin excitations in cuprates and iridates probed by resonant x-ray scattering
Tea room F1. Abstract: Recently, resonant inelastic x-ray scattering (RIXS) experiments in superconducting cuprates have observed damped spin excitations with dispersions and spectral weights closely similar to those of magnons in undoped compounds over much of the Brillouin zone. We will discuss implications of this observation for the spin-fermion models for cuprates. Secondly, we will discuss recent RIXS data in iridium oxides where strong spin-orbit coupling drives unusual ground states and excitations.

2 Jun 2011
Marcin MierzejewskiNonlinear current response of interacting fermions in metallic and insulating regimes
Tea Room F1. Abstract: Understanding the differences between integrable and nonintegrable systems has been recognized as a mostly theoretical problem. However, recent advances in the physics of ultracold atoms enable direct experimental verification of the underlying theoretical concepts. The presence of conserved quantities (a hallmark of integrability) restricts thermalization processes, hence integrable systems may relax to the generalized Gibbs state instead of the thermal state. Qualitative differences between both system types show up also in their linear-response (LR) to an external field F. Although integrability is usually broken by F, this breaking is not visible in the LR theory, since LR susceptibility is calculated at F=0. The role of integrability at finite driving remains important but largely unexplored field. I will discuss studies on systems driven by electric field F and demonestrate how the integrability-related properties of the LR regime decay/change with increasing driving.

31 May 2011
Indu SatijaPhase Transitions, Entanglement and Quantum Noise Interferometry in Ultracold Atoms
Tea room F1. Abstract: Laboratory realization of Bose Einstein Condensation in 1995 and the subsequent creation of fermionic condensate in 2003 has opened door to new magic kingdom in the frontiers of quantum physics. These systems exhibit quantum effects at macroscopic level. Intensity interferometry, commonly referred as Quantum Noise Interferometry (based on bunching and anti-bunching properties of bosons and fermions) is emerging as a powerful tool to unveil complex quantum phases in ultracold atoms. In this talk, I will discuss the importance of this interferometry in study of quantum phase transitions in hard core bosonic systems that are closely related to quantum spin-1/2 systems. I will also discuss possible relationship between these noise correlations and entanglement measures as both are expressed in terms of four-point correlations. In view of the fact that entanglement measures as well as noise correlations exhibit characteristic interferometric peak at the onset to quantum phase transition, these experimentally accessible quantities can be used as a probe of entanglement in cold atomic systems.

10 May 2011
Vincent DemeryDiffusion in fluctuating fields: perturbative methods
Tea room F1. Abstract: Diffusion in quenched random potentials has been extensively studied. Here we investigate diffusion in time dependent fields where the diffusion can be coupled to the fluctuating field. Such diffusion can be active, if the particle affects the field, which is the case for a protein coupled to membrane curvature; it can also be passive, if the particle does not affect the field, as is seen for a tracer diffusing in a turbulent flow or a random potential. \ In the limit of a weak coupling between the particle and the field, we study how the field modifies the diffusion constant in these two cases. We present three different methods and discuss their domain of applicability. The general accuracy of these approaches are validated via numerical simulations. In general, for active diffusion the coupling with the field always slows down the particle, whereas for passive diffusion, the effect of the field depends on its speed of evolution: the particle slows down in a slow field, and speeds up in a fast field.

16 Mar 2011
Dr. Rok ZitkoEdge states of topological insulators
Tea room F1. Abstract: Topological insulators are materials which are insulating in the bulk, but conducting on their boundaries (edges in 2D, surfaces in 3D). The presence of these states is guaranteed by the topological properties of the bulk. The boundary states have properties which are interesting for possible applications in spintronics due to the absence of backscattering. By superconducting proximity effect, the topological insulators can be tuned to a phase which supports Majorana modes as its boundary states. I will discuss some schemes for the detection of these states.

24 Feb 2011
Dr. Mike FearnNegative bend resistance ballistic sensors for high spatial resolution magnetic field detection
Tea Room F1. Abstract:There is an ongoing drive to develop non-invasive magnetic field sensors with ultrahigh spatial resolution (<100nm) for biomedical and security applications. This talk will describe the development, optimisation and associated electron dynamics of nanoscale ballistic sensors utilizing the negative bend resistance of InSb/InAlSb quantum well cross junctions.

17 Feb 2011
Prof. Takami TohyamaSpin and orbital dynamics in AF metallic phase of iron arsenides
Tea room F1.

21 Jan 2011
Prof Dr Stefan ThurnerDeriving Human Behavioral Laws from a Large-Scale Computer Game Society
Main Lecture Hall. Abstract:The capacity to collect fingerprints of individuals in online media has revolutionized the way researchers explore social systems. Such systems can often be described in terms of large, complex networks that have a topology of interconnected nodes. Much emphasis has been put on the existence or absence of interactions between actors, as well as their possible strength or directionality, but the nature of these interactions has been overlooked in most empirical studies, mainly because it is usually unavailable in large-scale data. In this work, we perform an analysis of a large-scale social network of a computer game society where different types of one-to-one interactions can be identified: friendship, enmity, communication, trade, bounty and war. Based on these networks, we present a series of findings in regularities in human interaction patterns,and the role of interacting social networks in the organization of the social system. We present a first large-scale verification of structural balance theory and provide first results of the nature of streams of human actions - a human behavioral code. more...

11 Jan 2011
Robin SteinigewegProjection Operator Approach to Spin Diffusion in the Anisotropic Heisenberg Chain at High Temperatures
Seminar room F1 (106). Abstract: Spin transport in the anisotropic Heisenberg chain is typically investigated theoretically w.r.t. the finiteness of transport coefficients only. Assuming their finiteness at high temperatures, we develop a concrete quantitative picture of the diffusion constant/(dc-)conductivity as a function of both the anisotropy parameter $Delta$ and the spin quantum number $s$, going beyond the most commonly considered case $s=1/2$. Using this picture, we enable the comparison of finite transport coefficients from complementary theoretical methods on a quantitative level, having more significance than the finiteness alone. Our method is essentially based on an application of the so-called time-convolutionless (TCL) projection operator technique to current autocorrelations. This technique, although being a perturbation theory in $Delta$, is found to be applicable, even and especially if $Delta$ is not small. This finding supports the applicability to a wider class of strongly interacting many-particle quantum systems. First results in that direction are briefly discussed.

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