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Forthcoming Seminars at F-1
Friday
22 Jun 2018
13:00 | Yicheng Zhang | Information measures for a local quantum phase transition:
Lattice fermions in a one-dimensional harmonic trap |
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We use quantum information measures to study the local quantum phase transition that occurs for trapped
spinless fermions in one-dimensional lattices. We focus on the case of a harmonic confinement~[1]. The transition
occurs upon increasing the characteristic density and results in the formation of a band-insulating domain in the
center of the trap. We show that the ground-state bipartite entanglement entropy can be used as an order parameter
to characterize this local quantum phase transition.We also study excited eigenstates by calculating the average von
Neumann and second Renyi eigenstate entanglement entropies, and compare the results with the thermodynamic
entropy and the mutual information of thermal states at the same energy density. While at low temperatures we
observe a linear increase of the thermodynamic entropy with temperature at all characteristic densities, the average
eigenstate entanglement entropies exhibit a strikingly different behavior as functions of temperature below and
above the transition. They are linear in temperature below the transition but exhibit activated behavior above it.
Hence, at nonvanishing energy densities above the ground state, the average eigenstate entanglement entropies
carry fingerprints of the local quantum phase transition.
[1] Zhang, Vidmar and Rigol, Phys. Rev. A 97, 023605 (2018)
F1 tea room. |
Seminars Archive
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
19 Dec 2011
14:30 | Vera M. O. Batista | Shape shifters: Crystallization and packing of deformable colloids |
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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
15:15 | Artem Badasyan | Physics of helix formation in biopolymers. What we know and what we dont. |
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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
15:15 | Natasa Przulj | Network mining uncovers new biology |
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Physics seminar room (106).
Abstract:
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.
more... |
25 Oct 2011
15:15 | Soham Biswas | Novel features and new universality class of a recently proposed opinion dynamics model |
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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].
References
[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
14:00 | Bogdan R. Bulka | Spin Stark Effect in a Triangular Molecule |
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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
15:15 | Veljko Zlatic | Thermal transport in strongly correlated multilayers |
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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
11:00 | Dr. Antun Balaz | Faraday Waves in Binary Non-Miscible Bose-Einstein Condensates |
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| 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
15:00 | prof. Takami Tohyama | Spin and Charge Excitations in the Antiferromagnetic Metallic Phase of Iron Arsenides: Inelastic Neutron Scattering and Resonant Inelastic X-Ray Scattering |
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| Tea Room F1. |
22 Jun 2011
11:00 | Andreas Weichselbaum | Quantum impurity models and tensor networks |
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| 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
14:00 | Jure Kokalj | Model self energy for the metallic phase of overdoped cuprate superconductors |
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| 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.
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10 Jun 2011
11:00 | Giniyat Khaliullin | Spin excitations in cuprates and iridates probed by resonant x-ray scattering |
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| 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
15:00 | Marcin Mierzejewski | Nonlinear current response of interacting fermions in metallic and insulating regimes |
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| 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
15:00 | Indu Satija | Phase Transitions, Entanglement and Quantum Noise Interferometry in Ultracold Atoms |
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| 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
15:00 | Vincent Demery | Diffusion in fluctuating fields: perturbative methods |
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| 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
15:00 | Dr. Rok Zitko | Edge states of topological insulators |
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| 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.
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24 Feb 2011
16:15 | Dr. Mike Fearn | Negative bend resistance ballistic sensors for high spatial resolution magnetic field detection |
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| 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
11:00 | Prof. Takami Tohyama | Spin and orbital dynamics in AF metallic phase of iron arsenides |
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| Tea room F1. |
21 Jan 2011
14:00 | Prof Dr Stefan Thurner | Deriving Human Behavioral Laws from a Large-Scale Computer Game Society |
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| 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
15:15 | Robin Steinigeweg | Projection Operator Approach to Spin Diffusion in the Anisotropic Heisenberg Chain at High Temperatures |
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| 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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