


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 5 Dec 2014 11:00  Professor Neelima Gupte  Dynamics of impurities in a threedimensional volumepreserving map 

Tea room F1. Abstract: We study the dynamics of inertial particles in threedimensional incompressible maps, as representations of volumepreserving flows. The impurity dynamics has been modeled, in the Lagrangian framework, by a sixdimensional dissipative bailout embedding map. The fluidparcel dynamics of the base map is embedded in the particle dynamics governed by the map. The base map considered for the present study is the ArnoldBeltramiChildress (ABC) map. We consider the behavior of the system both in the aerosol regime, where the density of the particle is larger than that of the base flow, as well as the bubble regime, where the particle density is less than that of the base flow. The phase spaces in both the regimes show rich and complex dynamics with three types of dynamical behaviors—chaotic structures, regular orbits, and hyperchaotic regions. In the oneaction case, the aerosol regime is found to have periodic attractors for certain values of the dissipation and inertia parameters. For the aerosol regime of the twoaction ABC map, an attractor merging and widening crisis is identified using the bifurcation diagram and the spectrum of Lyapunov exponents. After the crisis an attractor with two parts is seen, and trajectories hop between these parts with period 2. The bubble regime of the embedded map shows strong hyperchaotic regions as well as crisis induced intermittency with characteristic times between bursts that scale as a power law behavior as a function of the dissipation parameter. Furthermore, we observe a riddled basin of attraction and unstable dimension variability in the phase space in the bubble regime. The bubble regime in the oneaction case shows similar behavior. This study of a simple model of impurity dynamics may shed light upon the transport properties of passive scalars in threedimensional flows. We also compare our results with those seen earlier in twodimensional flows. 
4 Nov 2014 15:00  Mikhail Kiselev  Nanoelectromechanics with spin: effects of resonance scattering and strong electron correlations 

Seminar room (106). Abstract:
We consider the electromechanical properties of a singleelectronic device
consisting of a movable quantum dot attached to a vibrating cantilever,
forming a tunnel contact with a nonmovable source electrode. We show that
the resonance Kondo tunneling of electrons amplifies exponentially the
strength of nanoelectromechanical (NEM) coupling in such a device and
makes the latter insensitive to mesoscopic fluctuations of electronic
levels in a nanodot. It is also shown that the study of a KondoNEM
phenomenon provides additional (as compared with standard conductance
measurements in a nonmechanical device) information on retardation effects
in the formation of a manyparticle cloud accompanying the Kondo
tunneling. A possibility for superhigh tunability of mechanical
dissipation as well as supersensitive detection of mechanical displacement
is demonstrated. 
10 Jun 2014 14:15  Darko Tanasković  Bad metal behavior reveals quantum criticality in the doped Hubbard model 

Seminar room (106). Abstract:
Scaling of physical quantities at finite temperature can reveal the existence of an otherwise experimentally inaccessible zerotemperature phase transition, or quantum critical point (QCP). However, it is not clear whether this generic QCPdominated hightemperature behavior survives when the QCP is accompanied by a finitetemperature first order transition, or even completely masked by the emergence of an additional ordered phase in its vicinity. Here we show, using the dynamical mean field theory on the frustrated Hubbard model, that such phenomenology is possible in the case of the Mott metalinsulator transition. We find that the quantum critical scaling of resistivity is always present at temperatures sufficiently above the critical temperature Tc of the first order transition. Furthermore, in the limit of strong onsite interaction, Tc is significantly reduced and the scaling becomes valid even at very low temperature, revealing an almost fully blown quantum critical region. At high temperature, the quantum critical region extends to high doping and perfectly matches the region of the typical bad metal behavior with a linear growth of resistivity with temperature, which we interpret as a signature of connection between these two ubiquitous phenomena.
References:
1) J. Vucicevic, D. Tanaskovic, M.J. Rozenberg and V. Dobrosavljevic,
Bad metal behavior reveals quantum criticality in the doped Hubbard model,
preprint
2) J. Vucicevic, H. Terletska, D. Tanaskovic and V. Dobrosavljevic,
Finitetemperature Crossover and the Quantum Widom Line Near the Mott Transition,
Phys. Rev. B 88 (2013) 075143
3) H. Terletska, J. Vucicevic, D. Tanaskovic and V. Dobrosavljevic,
Quantum Critical Transport Near the Mott Transition,
Phys. Rev. Lett. 107 (2011) 026401

20 May 2014 12:00  BingSu Lu  Statistical physics of isotropicgenesis nematic elastomers 

Tea room F1. Abstract:
I will present an overview of nematic elastomers, which are nematic polymers that have been randomly, permanently crosslinked to form an equilibrium random solid network. I explain how such systems involve both annealed (thermal) as well as quenched (architectural) disorder. The distribution of thermal configurations of nematic polymers can be memorized by the elastomer at the instant of crosslinking, and I will show how a replica field theory formalism (inspired by Deam and Edwards) manages to capture this piece of physics. The random solid network restricts the motion of nematic polymer segments, and we address how this restriction modifies the liquid crystalline structure of nematic elastomers at high temperatures. In particular, we show that shortranged oscillatory spatial correlations in the nematic alignment can emerge for a class of nematic elastomers at a sufficiently high density of crosslinks. 
15 May 2014 15:15  Lev Vidmar  Ultracold bosons on optical lattices: Ideal testbed to study nonequilibrium dynamics 

Seminar room (106). Abstract:
When initially a closed quantum system in prepared in a state which is not an eigenstate of the system, it undergoes a unitary timeevolution and relaxes to some steady state. Investigation of the steady state properties of fundamental models represents a very active research topic of manybody quantum physics. In generic (nonintegrable) systems, the longtime values of (local) observables eventually approach expectation values of a thermal ensemble where only a few quantities, like the energy density, characterize the state. The framework to study these properties is the socalled eigenstate thermalization hypothesis, which, however, does not represent the necessary condition to observe thermal properties. In contrast, thermal ensembles usually fail to describe integrable systems, where the dynamics is constrained by the set of integrals of motion. Beside these scenarios, the system may first evolve into some transient metastable states, which feature nontrivial properties such as quasicondensation of bosons in a comoving frame. In my talk, I will show that all these features mentioned above can be studied experimentally with ultracold bosons on optical lattices using the same initial state, i.e., a product of local Fock states with one boson per site. While most of the results will be discussed using exact numerical methods on onedimensional lattice systems, these results open a new way towards studying the interplay of dimensionality, interactions and different particle species in a wellcontrolled nonequilibrium environment. 
25 Apr 2014 11:00  Jacek Herbrych  Effective S=1/2 description of S=1 chain with strong anisotropy 

Tea room F1. Abstract:
I will present a study of the one–dimensional S=1 antiferromagnetic spin chain with large easy plane anisotropy, with special emphasis on field–induced quantum phase transitions. Temperature and magnetic field dependence of magnetization, specific heat, and thermal conductivity is presented using a combination of numerical methods. In addition, the original S=1 model is mapped into the low–energy effective S=1/2 XXZ Heisenberg chain, a model which is exactly solvable using the Bethe ansatz technique. The effectiveness of the mapping is explored, and we show that all considered quantities are in qualitative, and in some cases quantitative, agreement. The thermal conductivity of the considered S=1 model is found to be strongly influenced by the underlying effective description. 
22 Apr 2014 14:15  Yigal Meir  Emerging localized states and alternating Kondo effects in quantum point contacts 

Seminar room (106). Abstract:
Quantum point contacts (QPCs), are the basic building blocks of any mesoscopic structure, and display quantized conductance, reflecting the quantization of the number of
transparent channels. An additional feature, coined the 0.7 anomaly, has been observed in almost all QPCs, and has been a subject of intensive debate in the last couple of decades. In the past we have attributed this feature to the emergence of a quasi
localized state at the QPC, which explains all the phenomenology of the effect. In this
talk I will describe two new experiments, and relevant theories, one which measured the
thermoelectric power through the QPC, and another which measured the conductance
through lengthtunable QPC. The experimental findings support the picture of the localized state(s). Interestingly, with increasing QPC length, it was found that both the
0.7 anomaly and the zero bias peak in the differential conductance oscillate and periodically split with channel length, supporting the idea that the number of the localized
state increases with length, leading to an alternating Kondo effect. 
15 Apr 2014 14:00  Konrad Hoppe  Mutual selection in timevarying networks 

Tea room F1. Abstract:
Timevarying networks play an important role in the investigation of the stochastic processes that occur on complex networks. The ability to formulate the development of the network topology on the same time scale as the evolution of the random process is important for a variety of applications including the spreading of diseases. The networks under investigation are constructed by microscopic rules based on a static vertex intrinsic fitness variable. These models are characterised by a probability distribution of fitness and a linking function that describes the probability of the existence of an edge, which depends mutually on the attractiveness of the nodes on both ends of that edge. This class of attachment mechanisms has been considered before in the fitness based complex networks literature, but not on timevarying networks. The network of international trade relationships builds an excellent example of a static fitness driven network and I will present some highlights of it during this talk.
Furthermore, I will give insights into the structure of the time varying network, using arbitrary attachment rules and illustrate results about opinion formation and epidemic outbreaks. Interestingly, the voter model exhibits an unanticipated behaviour as the network never reaches consensus in the case of mutual selection, but stays forever in its initial macroscopic configuration, which is a further piece of evidence that timevarying networks are very different from their static counterpart with respect to random processes that take place on them. Furthermore, epidemic outbreaks are found to be accelerated by uncorrelated mutual selection compared to previously considered random attachment. 
8 Apr 2014 14:00  Jure Kokalj  Electronic properties of systems close to or in the Mott insulating phase 

Seminar room (106). Abstract:
In this talk I will present my work on several topics of strongly
correlated
electron systems close to or in the Mott insulating phase. I will start
with
short comments on the question on the validity of Luttingers
theorem, then move on to spin chains by introducing new method and some
interesting finitetemperature dynamical properties of frustrated and
randomJ spin
chains. This will be followed by the presentation of phenomenological
model for
overdoped cuprates and several experimental results it can describe, after
which
I will finally move on to the interesting physics of organic
superconductors with the latest
numerical results. During the talk I will point out also future challenges
and
future interests or directions regarding the presented topics. 
1 Apr 2014 14:00  Adriano Amaricci  Strongly correlated electrons systems in and outofequilibrium: tales from dynamical meanfield theory 

Tea room F1. Abstract:
Strongly correlated materials are systems whose macroscopic state is the result of the competition of the large interactions among constituents. The rather localized nature of the d or forbital electrons gives rise to countless unusual effects like Mott transition, unconventional superconductivity or quantum criticality. In this talk I will briefly review some aspects of the physics of strongly correlated systems, both in and outofequilibrium, which more attracted my attention during the last few years, such as the metalinsulator transition in Mott and Chargetransfer systems or 2dimensional electrons gas, the emergence of incoherent metallic states in doped Mott insulators, the effects of optical trapping in superconducting ultracold gases or the formation of nonequilibrium states in driven correlated systems.
Finally I will outline my future research plans in the field of nonequilibrium dynamics of quantum materials, e.g. transitionmetal oxides or topological insulators, motivated by the recent progresses in the development of timeresolved spectroscopy. In particular, I shall discuss some fundamental, yet open, challenges to the theoretical understanding of the mechanisms governing the offequilibrium transformations in such systems. Facing these issues represents the necessary step to take to build an ultrafast control theory of quantum materials properties. 
27 Mar 2014 11:00  Ali Naji  Fluctuationinduced forces in disordered soft matter 

Tea room F1. Abstract:
I will present a short overview of fluctuationinduced forces in the con text of confined Coulomb fluids and liquid crystals as two wellestablished examples in soft matter physics where such forces have been thoroughly explored. I will then discuss how these forces are influenced by the pres ence of quenched disordered source fields on the bounding surfaces. I will consider the specific cases of monopolar surface charge disorder in the case of Coulomb fluids and easy direction (or preferred anchoring axis) disorder in the case of liquid crystalline films and discuss also their similarities and differences. It is shown that the presence of disorder can lead to significant contributions in the total interaction force between two planar bounding surfaces in a way that they can even completely mask the Casimir force at intermediate to large intersurface separations. By considering the special (and simpler) example of two chargedisordered dielectric surfaces in vac uum, it is further shown that monopolar charge randomness can generate very longranged normal and lateral interaction forces (and torques) even if the surfaces are chargeneutral on the average. 
18 Mar 2014 14:00  Luca Celardo  Robustness of collective properties to disorder: the case of Superradiance 

Seminar room F1 (C106). Abstract:
Open quantum systems are at the center of many research fields in condensed matter physics.
We will introduce the non Hermitian Hamiltonian approach to open quantum systems,
showing that this approach, in its simplest form, can be viewed as an extension of the Fermi Golden Rule.
This approach allows to take into account the effect of the opening beyond the perturbation limit,
where novel collective effects can arise.
As an example of quantum collective property we consider here
single excitation Superradiance.
Among the many fascinating aspects of these properties, one important open question regards their robustness to the effects induced by the presence of an environment. This robustness might enable to exploit coherent quantum effects to
build quantum devices for information technologies and basic energy science.
Specifically, we analyze the interplay of Superradiance, induced by the coupling
to a continuum of states, and disorder, induced by the coupling with another environment, in one dimensional nanostructures. We consider static disorder (position dependent), which leads to Anderson localization.
Superradiance is shown to be robust with respect to disorder.
Moreover the interplay of different environments is shown to lead to novel cooperative regimes, such as the subradiant hybrid regime. 
11 Mar 2014 15:00  Jernej Mravlje  Discriminating Fermi liquids from nonFermi liquids in optical spectrocopy 

Seminar room (106). Abstract:
The optical spectroscopy is a sensitive probe of correlated electrons, which
can access broad range of frequency scales. The deviations of the structure
of the low frequency peak from the simple Drude form are often
misinterpreted in experimental literature with the signatures of the non
Fermi liquid behavior. We recently derived the optical response of the
local Fermi liquid and found that it is described in terms of the universal
scaling form that deviates in a prononced way from Drude behavior.
I will discuss new experimental results that fully validate our theory and
demonstrate that the optical spectroscopy is a powerful tool to identify the
nonFermi liquid behavior, but only provided correct placebo reference
is taken. A combination of the density functional theory with the dynamical
meanfield theory reproduces the experimental results fully and points
that the electrical conduction in the 10005000cm1 range is dominated by
the resillient quasiparticle excitations.
In addition to presenting these new, yet unpublished experimental and
theoretical results I will be giving a brief overview of my past and ongoing
work and stress my future interests. 
3 Mar 2014 11:00  Ryan Requist  Kondo conductance through molecular radicals on Au(111) 

Tea room F1. Abstact:
Molecular radicals have unpaired spins which are screened at low temperatures on metal surfaces by the Kondo effect, resulting in zerobias conductance anomalies in scanning tunneling spectroscopy. Using a scheme bridging density functional theory and numerical renormalization group, we have investigated the Kondo conductance anomalies of two molecular radicals  nitric oxide (NO) and alpha,gammabisdiphenylenebetaphenylallyl (BDPA)  on the Au(111) surface. NO is a spin 1/2 radical with two molecular orbitals near the Fermi energy. Our calculations predicted a Kondo resonance in the more weakly hybridized orbital with an observable anomaly subsequently verified in experiment [1]. The second radical, BDPA, selfassembles into dimers, trimers and chains on the Au(111) surface, all displaying Kondo conductance anomalies [2]. I will present DFT calculations aimed at understanding the geometric and electronic structure of the isolated adsorbed molecule and molecular chains.
[1] R. Requist, S. Modesti, P. P. Baruselli, A. Smogunov, M. Fabrizio and E. Tosatti, PNAS, vol. 11, pp. 6974 (2014).
[2] S. Muellegger, M. Rashidi, M. Fattinger and R. Koch, J. Phys. Chem. C, vol. 117, pp. 57185721 (2013).

6 Jan 2014 14:00  Andrej Košmrlj  Elasticity, Geometry and Buckling 

Tea room F1. Abstract:
In this talk I present how geometrical shape affects the mechanical properties of thin solid membranes and how buckling instabilities change the geometry of periodic microstructures in materials. Using methods rooted in statistical physics, we find that the bending rigidity of planar thin solid membranes characterized by a frozen random height profile diverges, while the Young?s and shear moduli tend to zero at long wavelengths. We assess the relative importance of geometrical warping and thermal fluctuations. Finally, we show how methods from solid state physics, such as theory of dislocations, Ising spins and geometrical frustration, can help us deduce the change in the geometry of periodic microstructures due to buckling instabilities. Buckling instabilities can change the microstructure symmetries, including a spontaneous chiral symmetry breaking, which drastically modifies the material properties.





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