Condensed Matter Physics Seminar
2006/2007 | 2007/2008 | 2008/2009 | 2009/2010 | 2010/2011 | 2011/2012 | 2012/2013 | 2013/2014 | 2014/2015 | 2015/2016 | 2016/2017 | 2017/2018 | 2018/2019 | 2019/2020 | 2020/2021 | 2021/2022 | 2022/2023 | 2023/2024 | 2024/2025
2013-05-31 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15 
dr Federico Benitez (Max Plack Stuttgart, and Univ. Montevideo, Uruguay)Branching and annihilating random walks: exact results at low branching rate
I will present some exact results on the behavior of Branching andAnnihilating Random Walks, which are some simple out of equilibriumclassical systems which can suffer continuous phase transitions.Contrary to usual perturbation theory, we perform an expansion in thebranching rate around the non trivial Pure Annihilation model, whosecorrelation and response function we have computed exactly. With this,the non-universal threshold value for having a phase transition in thesimplest such system (belonging to the Directed Percolationuniversality class) is found to coincide with previous NonPerturbative Renormalization Group approximate results. We also showthat the Parity Conserving universality class has an unexpected RG fixed point structure, with a PA fixed point which is unstable inall dimensions of physical interest.
2013-05-24 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
dr hab. Ryszard Buczko (IFPAN)Topological crystalline insulators
In normal topological insulators, time-reversal symmetry and strongrelativistic (spin-orbit) effects require that the bulk insulating statesare accompanied by metallic helical Dirac-like electronic states on thesurface of the crystal. These surface states are encoded in topologicallynon-trivial wave functions of valence electrons and robustly resistnon-magnetic disorder. Recent theoretical results have suggested theexistence of a new class of topological insulators, namely topologicalcrystalline insulators (TCIs), in which crystalline symmetry replaces therole of time-reversal in ensuring topological protection of the surfacestates.I will discuss the theoretical predictions of the properties of surfaceDirac-like electrons in IV-VI TCIs and describe the experiments confirmingthe existence of TCI surface states in PbSnSe and PbSnTe mixed crystals. Iwill show that these topologically nontrivial states are spin polarized andthat their spin texture depends on the surface orientation.
2013-05-17 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
Łukasz Cywiński (IFPAN)Qubit as a spectrometer of noise
Quantum states of a microscopic two-level system are subject to decoherence due to the interaction of the system with its environment. In many cases one can assume that the environment is in fact a source of classical noise. I will attempt to describe how the measurement of qubit's decoherence can be used in order to gain information about statistical properties of the environmental noise. Specifically, I will describe how one can reconstruct the spectral density of noise from measurements of coherence dynamics of a qubit driven by a sequence of fast rotations (so-called dynamical decoupling from the environmental influence. Simple theoretical ideas will be illustrated with examples of measurements on semiconductor spin qubits, superconducting qubits, and trapped ions.
2013-04-26 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
prof. Magdalena A. Załuska-Kotur (IFPAN oraz Wydział Matematyczno-Przyrodniczy UKSW)Step patterns formation in the process of crystal surface dynamics - Kinetic Monte Carlo simulations
Formation of various geometric patterns during crystal growth remains asubject of continuous interests of many researchers. It is important as aninteresting example of far from equilibrium, stationary process andbecause of practical applications in the crystal growth technology. Kinetic Monte Carlo simulation method of analysis is based on a simplemodel of particles jumping between well-defined crystal site positions.The main aim of the extended kMC study is to find proper model parameters– coupling constants between particles, energy barriers for each type ofparticle jump, such that describe the main characteristics of the studiedsystem. The simplicity of the model and small number of control parametersallows studying systems of large particle numbers, performing long timesimulations and describing the model behavior in various conditions. Twodifferent systems were analyzed: GaN(0001) surface model in N-richconditions represented by lattice of Ga atoms only and full, two componentmodel of 4H SiC(0001). For both surfaces we can find such regimes ofparameters where straight steps move uniformly during crystal growth orsublimation processes. However within wide parameter range differentinstability phenomena are observed. Step bunching and step meandering arethe most characteristic ones. Both processes were studied as a functionof time, temperature or misorientation angle.
2013-04-19 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
Wojciech Grochala (Faculty of Chemistry & CENT, University of Warsaw)Fascinating magnetic properties of the compounds of Ag(2+), a heavier congener of Cu(2+)
Compounds of Cu(2+) are common in chemistry and best characterized among chemical connections of all transition metals; their crystal structuresand magnetic properties are well studied, indeed.However, physicochemical properties of rare and exotic compounds ofdivalent silver, Ag(2+), are less known. The lecturer will review thecrystal structures and magnetic properties for a handful of importantcompounds of Ag(2+) with emphasis on unusually strong magneticsuperexchange in selected systems.
2013-04-12 (Friday)
Sali Duża Teoretyczna, Hoża 69 at 12:15
Anna Kauch (Institute of Physics, Academy of Sciences of the Czech Republic, Prague)Correlated bosons in optical lattices -- dynamical mean-field approximation for inhomogeneous systems
With the development of optical lattices a direct experimentalrealization of the quantum many-body theory models has become possible.One of the models realized with cold atomic lattices is the bosonicHubbard model. It describes a genuine competition between the itinerantbehavior of bosons, which leads to the formation of a Bose-Einsteincondensate, and a localizing effect of the interactionbetween particles. The dynamical mean-field approximation to the bosonic Hubbard model gives a comprehensive theoretical description ofcorrelated bosons in the superfluid and Mott-insulating phases. In thedynamical mean-field theory (DMFT), the lattice problem is replaced byan impurity coupled to dynamical self-consistent baths. The DMFT can beextended to treat inhomogeneous systems, which allows for studyingrealistic models of bosons in optically generated potentials. In theseminar I will present the theoretical framework of the DMFTapproximation for inhomogeneous systems and some preliminary results ofcomputations performed on Graphical Processing Units (GPUs).
2013-03-22 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
dr Magdalena Gruziel (IFT UW)A simulation study of the aggregation of simple, chiral polymers
In the seminar I will present the results of the Langevin dynamicssimulations of chiral polymers represented by a simple, coarse-grainedmodel. These studies, inspired by a chiral bifurcation phenomenonobserved in the experiments of vortex-induced or pH-dependent aggregationof insulin, show cluster configurations, in terms of their handedness,dependent on the twist and handedness of constituting polymers.Whether the possible explanation of this behaviour helps inunderstanding the experimental results still remains a question.
2013-03-15 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
Prof. dr hab. Andrzej Sobolewski (IFPAN)Photophysics of Hydrogen Bonded Molecular Systems: from theory to Applications
2013-01-25 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
dr Cezary Śliwa (IF PAN)Bulk uniaxial [110] anisotropy in (Ga,Mn)As
The virtual crystal approximation is a building block of condensed matterphysics which allows to apply the methods of group theory to randomalloys. In the case of (Ga,Mn)As, however, even a qualitative descriptionof the experimental magnetic anisotropy is not possible within the grouptheory for zinc-blend crystals.In our work [1] we demonstrate that this apparent failure of the grouptheory is associated with a non-random distribution of the magneticconstituent of the alloy. We show that the effect of the non-randomnesscan be parameterized by effective strains, introduced earlier asphenomenological parameters. We determine the magnitude of these effectivestrains either ab initio or from second order perturbation theory in theeffective mass approach. The effect of a finite size of supercellsintroduced commonly to make ab initio computations feasible is discussed.[1] M. Birowska, C. Śliwa, J. A. Majewski, T. Dietl, Origin of BulkUniaxial Anisotropy in Zinc-Blende Dilute Magnetic Semiconductors, Phys.Rev. Lett. 108, 237203 (2012).
2013-01-18 (Friday)
Sali Duża Teoretyczna (229), Hoża 69 at 12:15
dr hab. Ryszard Buczko (IFPAN)Topological crystalline insulators
In normal topological insulators, time-reversal symmetry and strongrelativistic (spin-orbit) effects require that the bulk insulating statesare accompanied by metallic helical Dirac-like electronic states on thesurface of the crystal. These surface states are encoded in topologicallynon-trivial wave functions of valence electrons and robustly resistnon-magnetic disorder. Recent theoretical results have suggested theexistence of a new class of topological insulators, namely topologicalcrystalline insulators (TCIs), in which crystalline symmetry replaces therole of time-reversal in ensuring topological protection of the surfacestates.I will discuss the theoretical predictions of the properties of surfaceDirac-like electrons in IV-VI TCIs and describe the experiments confirmingthe existence of TCI surface states in PbSnSe and PbSnTe mixed crystals. I will show that these topologically nontrivial states are spin polarized andthat their spin texture depends on the surface orientation.