Forthcoming Seminars at F-1

Seminars Archive

2007  2008  2009  2010  2011  2012  2013  2014  2015  2016  2017  
23 Dec 2013
Matej KandučWater-mediated forces between hydrophilic and hydrophobic surfaces
Tea room F1. Abstract:

Water is a natural solvent, which mediates complex interactions between various surface structures that are even nowadays poorly understood. Using the novel thermodynamic extrapolation technique in all-atom molecular dynamics simulations, I investigate water-mediated interactions between surfaces of various polarities in order to elucidate the relation between the repulsive hydration and attractive hydrophobic forces. Based on the simulations at prescribed chemical potential and the free energy analysis, I determine the crossover which delimits the repulsive and attractive regimes. The surface attraction appears as a result of water cavitation, since the liquid water between the membranes becomes metastable with respect to vapor phase. It can be shown that the attraction–repulsion regimes highly correlate with the formation and breaking of overall hydrogen-bond network upon bringing the surfaces into close-contact state.

23 Dec 2013
Žiga OsolinIntroduction to Unix, F-1 clusters, Python and parallelization
Tea room, F1. Abstract:

With the upcoming software and hardware upgrade on F-1 cluster we felt the need to introduce hardware and software tools available. In this seminar I shall present the basics of cluster usage (connecting to cluster, preparing and submitting jobs to queue, ... ) and Unix tips and tricks. Additionally I shall make a brief introduction to a powerful computer language for scientists - Python. If time permits, I will conclude with a tutorial on parallelization techniques with the emphasis on OpenMP.

5 Dec 2013
Marco BerrittaInterplay of interference and geometrical effect in molecular break junction
Tea room F1. Abstract:

We theoretically study the effects of functionalized molecular structures on the electronic transport of single-molecule junctions between Au electrodes. The molecular structures are functionalized, in para and meta configuration, with methylsulfide anchor group on both or only one of their phenyl termini. We compared the results of transport calculations for molecular structures at zero and non-zero temperature. The molecular structures at zero temperature are obtained using the software SIESTA for Density Functional Theory. Molecular structures at non-zero temperature are obtained using the software for molecular dynamics lammps. The Hamiltonians of the molecular structures of both zero and non-zero temperature are obtained using SIESTA. The transport calculations are performed in the wide band limit. Such approach is computationally efficient and allowed us to make a significant statistical study to account the effect of the temperature. We found that the thermal fluctuations enhance the effect of quantum destructive interference on the transport through the molecules functionalized in meta position.

25 Nov 2013
Matteo RauziEmbryo scale mechanism and symmetry controlling tissue morphogenesis
Tea room F1. Abstract:

Cell shape change and movement drive tissue morphogenesis during embryo development. The mechanism responsible for cell behavior can be explained by dissecting the subcellular machinery working in each cell in a tissue but this is not sufficient for a complete understanding. Cells forming a tissue are cells that interact among each other. To have a deeper understanding of tissue morphogenesis, cell shape change needs also to be integrated in a tissue and ultimately in an embryo scale context. During early Drosophila embryo development, a furrow forms in the ventral most region. Furrow initialization has been shown to be dependent on local shape changes of ventral cells but what other processes are necessary for furrow internalization this still is not clear. Using a multyview imaging technique and local laser-based tissue immobilization, we show that the movement of the lateral tissue towards the ventral side (tissue epiboly) is a process partially independent from ventral furrow formation and necessary for the furrow to be internalized. We show that the synchronic movement of the two symmetric lateral tissues define the position of the ventral midline. Our analysis suggests that a polarized acto-myosin meshwork at the embryo-scale is responsible for the lateral tissue to move ventrally.

15 Nov 2013
Markus AichhornIron-based superconductors: Strongly-correlated materials far from Mott
Tea room F1. Abstract:

Combining density-functional theory and dynamical mean-field theory we investigate the ground-state of a variety of iron-based superconductors. We will see that a common feature to all these compounds is that Hunds rule coupling plays a dominant role for the correlations of the conduction electrons. We will compare pnictide and chalcogenide materials, where electronic correlations range from moderate in pnictide, to strong correlations in chalcogenide compounds such as FeSe, KFe2Se2, or K2Fe4Se5. Nevertheless, we will demonstrate, that the systems are not close to a Mott insulator, and that insulating behavior found in experiments has to be explained by a different mechanism. Furthermore, we will discuss the electronic structure and fermiology of so-called perovskite pnictides, where large blocking layers are inserted in the compound to separate the iron-layers. Again, Hunds physics can turn these rather complicated materials into a rather standard pnictide superconductor with known fermiology.

7 Oct 2013
Ross H. McKenzieTutorial: Effective Hamiltonians for quantum dynamics for complex chemical systems
Tea room F1.

This informal tutorial will introduce some of the key concepts and approaches associated with modelling and understanding quantum dynamical processes in complex molecular materials. This will provide background and motivation for understanding some of my work [1-4].

1. Examples of functional materials: optically active biomolecules, organic light emitting diodes and solar cells, enzymes, ...
2. Examples of dynamical processes: charge separation, proton transfer, exciton transport,
3. Partition: discrete quantum system + environment (solvent or protein)
4. Form of Model Hamiltonians
Example: spin boson model
5. Diabatic states and potential energy surfaces
6. Outstanding questions: quantum coherence, sequential vs. co-operative, breakdown of Born-Oppenheimer, ...

[1] J. Gilmore and R.H. McKenzie, J. Phys. Chem. A 112, 2162 (2008).
[2] J. Bothma, J. Gilmore, and R.H. McKenzie, New. J. Phys. 12, 055002 (2010).
[3] S.C. Olsen and R.H. McKenzie, J. Chem. Phys. 130, 184302 (2009).
[4] R.H. McKenzie, Chem. Phys. Lett. 535, 196 (2012).

5 Sep 2013
Jacek HerbrychLocal spin relaxation within the random Heisenberg chain
Tea room F1. Abstract:

Finite-temperature local dynamical spin correlations are studied numerically within the random spin-1/2 antiferromagnetic Heisenberg chain. The aim is to explain measured NMR spin-lattice relaxation times in BaCu2(Si0.5Ge0.5)2O7, which is the realization of a random spin chain. In agreement with experiments we find that the distribution of relaxation times within the model shows a very large span similar to the stretched-exponential form. The distribution is strongly reduced with increasing T, but stays finite also in the high-T limit. Anomalous dynamical correlations can be associated to the random singlet concept but not directly to static quantities. Our results also reveal the crucial role of the spin anisotropy (interaction), since the behavior is in contrast with the ones for XX model, where we do not find any significant T dependence of the distribution.

2 Jul 2013
Rodolfo A. JalabertScattering Phase of Quantum Dots: Emergence of Universal Behavior
Tea room F1. Abstract:

In a coherent conductor like a quantum dot, the transmission phase is of key importance in determining the transport properties. Unlike the conductance, the transmission phase is not directly measurable. Nevertheless, phase-sensitive experiments have been performed by embedding a quantum dot in one of the arms of an Aharonov-Bohm interferometer. These experiments, observing in-phase behavior between consecutive Coulomb-blockade resonances, have remained unexplained for more than a decade. We provide a solution of this puzzle by showing that wave-function correlations existing in chaotic ballistic quantum dots are responsible for the emergence of large universal sequences of in-phase resonances in the short wavelength limit [1].

In quantum dots with at least one hundred electrons the description of Coulomb blockade physics within a constant charging energy model allows for an effective one-particle description. Smaller dots require going beyond mean-field approaches by including the effect of electronic correlations. We develop a numerical method, using the Density Matrix Renormalization Group algorithm, in order to obtain the transmission phase of a strongly interacting system. We demonstrate that electronic correlations do not generically lead to the in-phase behavior and that small dots are always in the mesoscopic regime of random relative transmission phases [2].

[1] R.A. Molina, R.A. Jalabert, D. Weinmann, and Ph. Jacquod; Phys. Rev. Lett. 108, 076803 (2012).
[2] R.A. Molina, P. Schmitteckert, D. Weinmann, R.A. Jalabert, and Ph. Jacquod; arXiv:1212.2114 (2012).

17 Jun 2013
Zala LenarčičUltrafast charge recombination in photoexcited Mott-Hubbard insulator
Tea room F1. Abtract:

Recent femtosecond pump-probe experiments on Mott-Hubbard insulators reveal charge recombination, which is in picosecond range, much faster than in clean band-gap semiconductors although the excitation gaps in Mott-Hubbard insulators are larger. I will present a calculation of the recombination rate of the excited holon-doublon pairs, based on two-dimensional model relevant for undoped cuprates, which shows that such fast processes can be explained even quantitatively with the multi-magnon emission. We find that the recombination rate is exponentially dependent on the Mott-Hubbard gap and on the magnon energy, with a small prefactor which can be traced back to strong correlations and consequently large charge-spin coupling. Moreover, in the frame of spin degrees of freedom different stages and timescales of thermalization will be identified and interpreted.

11 Jun 2013
Andrej KošmrljMechanical Properties of Warped Membranes
Tea room F1. Abstract: In this talk I will demonstrate how geometrical shape affects the mechanical properties of thin membranes with a shear modulus. We focus on nearly flat warped membranes with a height profile characterized by random Gaussian variables h(q) in Fourier space with zero mean and variance <|h(q)|^2> ~ q^(-s) and show that in the linear response regime mechanical properties depend dramatically on the system size L for s>2. Membranes with s=4 could be produced by flash polymerization of lyotropic smectic liquid crystals. Using methods rooted in statistical physics we find that the renormalized bending rigidity increases as L^x, while the Young and shear modulii decrease as L^(-x) resulting in a universal Poisson ration, where x=(s-2)/2. Numerical results show good agreement with analytically determined exponents.

10 Jun 2013
Oleg P. SushkovSpin glass behavior of underdoped cuprates due to freezing of skyrmions in the spin spiral state
Tea room F1. Abstract: YBa2Cu3Oy is the least disordered cuprate and hence it is the best testing ground for understanding of generic properties of the entire cuprate family. Due to the low disorder and unlike other cuprates the compound demonstrates usual metallic conductivity in the antiferromagnetic phase at doping below x1 ≈ 6%. This is the critical concentration for onset of superconductivity and simultaneously this is the Lifshitz Point (LP) for onset of incommensurate static magnetic order (spin spiral). Moving further up in doping the static incommensurate magnetic order propagates well inside the superconducting phase and disappears at the magnetic Quantum Critical Point (QCP) at x2 = 9%. At doping above the QCP the spin spiral order is fully dynamic. In the present work we consider magnetic properties of the static spin spiral superconducting state at doping slightly above the magnetic LP where period of the spin spiral is about 30 lattice spacing. Our analysis is based on Monte Carlo simulations of the effective field theory developed in Refs. [1-4]. The aim of the present work is to make predictions for polarized neutron scattering. Unlike La2-xSrxCuO4 [1] influence of the quenched charge disorder on the spin structure of YBa2Cu3Oy is minimum. We demonstrate that noncoplanar topological defects, skyrmions, significantly influence the spin configuration. The defects develop when cooling the system. Skyrmions lead to the spin freezing. An example of the obtained magnetic structure and the corresponding map of the spin structure factor are shown in the Figure. The obtained spin configuration is metastable, nevertheless, because of the freezing, this is the configuration observable experimentally. [1] A. Luscher, A. I. Milstein, and O. P. Sushkov, Phys. Rev. Lett. 98, 037001 (2007). [2] A. I. Milstein and O. P. Sushkov, Phys. Rev. B 78, 014501 (2008). [3] O. P. Sushkov, Phys. Rev. B 79, 174519 (2009). [4]A. I. Milstein and O. P. Sushkov, Phys. Rev. B 84, 195138 (2011). more...

2 Apr 2013
Frank MarsiglioEliashberg Theory of Superconductivity: do we have it right?
Tea room F1. Abstract:

Superconductivity is more than 100 years old; however, the established theory of conventional superconductors is barely more than half as old. But how established is it, and how conventional are the old superconductors? This talk will discuss some of the successes of this established framework, generally known as BCS-Eliashberg theory, and how some of these ideas fit in with more modern, high-Tc superconductors. But I will also highlight some of the uncertain aspects of the conventional theory, and our recent attempts to shed more light on this uncertainty.

27 Mar 2013
Prof. Emeritus Guenther Meissner2 D Wigner Crystal and Quantum Phenomena
A Wigner crystal is the solid phase predicted by E. Wigner in 1934 to be the ground state of a system of electrons, where the Coulomb interaction dominates the kinetic energy [1]. In 1976 we found the extremely low-density electrons on the surface of liquid helium to crystallize in such a two-dimensional (2D) triangular lattice [2], which was confirmed experimentally 1979 [3]. Strong external magnetic fields applied perpendicular to 2D systems, as semi-conductor hetero-structures, allow for crystallization at higher densities, due to localization of the electrons in Landau levels (LL) with cyclotron motion around their guiding centers. It will be indicated, that in our many-body approach [4] with non-commuting density fluctuations of non-commuting guiding centers, various effects can be related to a formation of composites of fractional charges and magnetic flux quanta. An example is the recent observation [5] of pinned Wigner solid phases near fractional filling factor 1/3 and integer filling factor 1, exhibiting the fractional respectively integer quantum Hall effect. Meanwhile, non-commutativity became a hot subject in various other fields. [1] E. Wigner, PR 46, 1002 (1934) [2] G. Meissner, H. Namaizawa, and M. Voss, PR B 13, 1370 (1976) [3] C.C. Grimes and G. Adams PRL, 22, 11 (1979) [4] G. Meissner, Physica B 184, 66 (1993) [5] Han Zhu, Yong P. Chen, P. Jiang, L.W. Engel, D.C. Tsui, L.N. Pfeiffer, and K.W West, PRL 105, 126803 (2010)

25 Mar 2013
Marco BerrittaCoherent nanostructures: dynamics control and noise
Tea room F1. Abstract:

In the last three decades the scientific community has been attracted by the possibility of controlling quantum system, for example for quantum computing or quantum simulators. Initially this idea was exploited only in microscopic quantum systems as atoms and molecules. However these systems present difficulties on the large scale application due to the extreme laboratory condition that they need, for example ultra low temperature $Tsim 1 mu K$. On the other hand, great progress has been made with superconducting nanodevices, that can be more easily scaled and coupled. The rapid technological progress in quantum-state engineering in superconducting nanodevices demands for the implementation of new advanced techniques of quantum control. One of the most important problems that arise in quantum control is decoherence. These devices are affected by a Broad Band Colored and Structured (BBCS) noise, which is qualitatively different to what encountered in atomic physics, since it is chatacterized by a strong non-Markovian low-frequency component with a characteristic power spectrum $S(omega)propto frac{1}{omega}$.
The STImulated Raman Adiabatic Passage (STIRAP) protocol allows to transfer population between two levels using coupling with a third level that remains empty during the whole dynamics. This is an effect due to quantum coherence, that uses a series of phenomena (Autler-Townes splitting, electromagnetic induced transparency, adiabatic dynamics) features of multilevel atomic and molecular systems. The observation of this effect in superconducting nanodevices would pave the way for various application, for example to microwave photon manipulation or to quantum computation with dressed states. However symmetries which on one hand protect the system against decoherence, yield selection rules which may cancel coupling to the pump external drive. A careful study demonstrated the possibility to realize a Lambda configuration allowing for STIRAP implementation in a superconducting device such as the Cooper Pair Box (CPB) biased off-symmetry, in the presence of BBCS noise.

22 Feb 2013
Pieralberto MarchettiNon-BCS superconductivity in cuprates from attraction of quantum spin vortices
Tea room F1. Abstract:

We propose a mechanism for pairing and superconductivity in hole-doped cuprates based on a gauge approach to the t-t-J model taken as hamiltonian for the CuO planes. The pairing force is a long-range attraction between quantum spin vortices centered on the empty sites, described by spinless holons, of two opposite Neel sublattices, leading to pairing of charge carriers. We show that three crossover temperatures found in the experimental data associated to different features of pseudogap phenomenology can be interpreted in terms of this pairing mechanism, which crucially depends on the concept of the hole as a composite excitation.
The spin fluctuations in fact are described by bosonic spinons with a gap generated by the spin vortices. Due to the no-double occupation constraint, there is a gauge attraction between holon and spinon binding them into a physical hole. Lowering the temperature, through gauge interaction the spin vortex attraction on holons induces the formation of spin-singlet (RVB) spinon pairs, thus leading to the formation of incoherent composite hole-pairs.
The true superconducting transition occurs at an even lower temperature via a planar XY-type transition and it involves a kinetic energy gain due to lowering of the spinon gap. Since the short-range antiferromagnetic (AF) order and the holon pairing originate from the same term of the t- t-J model, this approach incorporates a strong interplay between AF and SC, giving rise to a universal relation between the energy of the resonance mode (bound state of spinons) and Tc, as observed in neutron scattering experiments.

P.A. Marchetti, F. Ye, Z.B. Su and L. Yu, Phys.Rev. B 84 (2011) 214525
P.A. Marchetti and M. Gambaccini, J. Phys. Cond. Matter 24 (2012) 475601

18 Feb 2013
Kenji TsutsuiTheoretical Study of Electronic States and Resonant Inelastic X-ray Scattering Spectra in Ni-substituted Cuprates
Physics seminar room (106). Abstract:

Impurity substitutions for copper in high-Tc cuprate superconductors have attracted much attentions. For Ni substitution, it has been argued from several experiments that hole carriers are strongly localized at around Ni sites. We have demonstrated theoretically that a hole can be bound in NiO4 plaquettes forming the Zhang-Rice doublet [1,2]. In this study, we examine the electronic state in case that one more hole is introduced in the Zhang-Rice doublet state. We carry out the numerically exact diagonalization on small clusters of the d-p model with a Ni impurity site. The phase diagram of the ground states is obtained as a function of parameters such as the charge transfer energy at Ni site. In the realistic parameter values, we find that the hole is located in neighboring Cu sites, meaning that the Zhang-Rice doublet behaves as an attractive impurity. In the resonant inelastic x-ray scattering for Ni K-edge, we propose the characteristic dependence of spectral weight on hole concentration. This work has been done in collaboration with T. Tohyama and S. Maekawa.

[1] K. Tsutsui, A. Toyama, T. Tohyama, S. Maekawa, Phys. Rev. B 80, 224519 (2009).
[2] K. Tsutsui, T. Tohyama, S. Maekawa, J. Phys. Chem. Sol. 72, 354 (2011).

14 Feb 2013
David Sherrington, FRSUnderstanding glassy materials: Pseudo-spin glasses and random field systems
Joint Seminar in Condensed Matter Physics, Room 106.

This talk will consider effects of quenched disorder (alloying) in structurally deformable materials with a particular orientation towards glassy behaviour, using minimalist modelling and mappings to analogues of spin glasses and random field magnets. As well as spin glass experience bearing on the behaviour of the materials systems, the study suggests new issues for spin glass/random field magnets and new “laboratories” to study spin models.
  The main emphasis will be on martensitic alloys but thoughts and questions concerning relaxors will also be exposed, hoping for criticism and feedback in the light of long-time experience and expertise at Jozef Stefan Institute.      more...

14 Feb 2013
Marcin MierzejewskiReduced dynamics of non-equilibrium interacting system
Tea room F1. more...

24 Jan 2013
Robin SteinigewegEigenstate thermalization within isolated spin-chain systems
Tea room F1. Abstract:

The thermalization phenomenon and many-body quantum statistical properties are studied on the example of several observables in isolated spin-chain systems, both integrable and generic non-integrable ones. While diagonal matrix elements for non-integrable models comply with the eigenstate thermalization hypothesis, the integrable systems show evident deviations and similarity to properties of noninteracting many-fermion models. The finite-size scaling reveals that the crossover between two regimes is given by a scale closely related to the scattering length. Low-frequency off-diagonal matrix elements related to d.c. transport quantities also follow in a generic system a behavior analogous to the eigenstate thermalization hypothesis, however unrelated to the one of diagonal matrix elements. more...

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