Forthcoming Seminars at F-1
22 Jun 2018
|Yicheng Zhang||Information measures for a local quantum phase transition:|
Lattice fermions in a one-dimensional harmonic trap
|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~. 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.
 Zhang, Vidmar and Rigol, Phys. Rev. A 97, 023605 (2018)
F1 tea room.
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|5 Dec 2014|
|Professor Neelima Gupte||Dynamics of impurities in a three-dimensional volume-preserving map|
|Tea room F1. Abstract: We study the dynamics of inertial particles in three-dimensional incompressible maps, as representations of volume-preserving flows. The impurity dynamics has been modeled, in the Lagrangian framework, by a six-dimensional dissipative bailout embedding map. The fluid-parcel 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 Arnold-Beltrami-Childress (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 one-action 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 two-action 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 one-action case shows similar behavior. This study of a simple model of impurity dynamics may shed light upon the transport properties of passive scalars in three-dimensional flows. We also compare our results with those seen earlier in two-dimensional flows.|
|4 Nov 2014|
|Mikhail Kiselev||Nanoelectromechanics with spin: effects of resonance scattering and strong electron correlations|
|Seminar room (106). Abstract: We consider the electromechanical properties of a single-electronic device
consisting of a movable quantum dot attached to a vibrating cantilever,
forming a tunnel contact with a non-movable 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 Kondo-NEM
phenomenon provides additional (as compared with standard conductance
measurements in a nonmechanical device) information on retardation effects
in the formation of a many-particle cloud accompanying the Kondo
tunneling. A possibility for superhigh tunability of mechanical
dissipation as well as supersensitive detection of mechanical displacement
|10 Jun 2014|
|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 zero-temperature phase transition, or quantum critical point (QCP). However, it is not clear whether this generic QCP-dominated high-temperature behavior survives when the QCP is accompanied by a finite-temperature 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 metal-insulator 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 on-site 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.
1) J. Vucicevic, D. Tanaskovic, M.J. Rozenberg and V. Dobrosavljevic,
Bad metal behavior reveals quantum criticality in the doped Hubbard model,
2) J. Vucicevic, H. Terletska, D. Tanaskovic and V. Dobrosavljevic,
Finite-temperature 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|
|Bing-Su Lu||Statistical physics of isotropic-genesis nematic elastomers|
|Tea room F1. Abstract:
I will present an overview of nematic elastomers, which are nematic polymers that have been randomly, permanently cross-linked 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 cross-linking, 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 short-ranged oscillatory spatial correlations in the nematic alignment can emerge for a class of nematic elastomers at a sufficiently high density of cross-links. |
|15 May 2014|
|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 time-evolution and relaxes to some steady state. Investigation of the steady state properties of fundamental models represents a very active research topic of many-body quantum physics. In generic (non-integrable) systems, the long-time 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 so-called 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 quasi-condensation of bosons in a co-moving 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 one-dimensional lattice systems, these results open a new way towards studying the interplay of dimensionality, interactions and different particle species in a well-controlled nonequilibrium environment.|
|25 Apr 2014|
|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|
|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 length-tunable 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|
|Konrad Hoppe||Mutual selection in time-varying networks|
|Tea room F1. Abstract: Time-varying 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 time-varying 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 time-varying 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|
|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
electron systems close to or in the Mott insulating phase. I will start
short comments on the question on the validity of Luttingers
theorem, then move on to spin chains by introducing new method and some
interesting finite-temperature dynamical properties of frustrated and
chains. This will be followed by the presentation of phenomenological
overdoped cuprates and several experimental results it can describe, after
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
future interests or directions regarding the presented topics.|
|1 Apr 2014|
|Adriano Amaricci||Strongly correlated electrons systems in- and out-of-equilibrium: tales from dynamical mean-field 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 f-orbital 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 out-of-equilibrium, which more attracted my attention during the last few years, such as the metal-insulator transition in Mott and Charge-transfer systems or 2-dimensional electrons gas, the emergence of incoherent metallic states in doped Mott insulators, the effects of optical trapping in superconducting ultra-cold gases or the formation of non-equilibrium states in driven correlated systems.
Finally I will outline my future research plans in the field of non-equilibrium dynamics of quantum materials, e.g. transition-metal oxides or topological insulators, motivated by the recent progresses in the development of time-resolved spectroscopy. In particular, I shall discuss some fundamental, yet open, challenges to the theoretical understanding of the mechanisms governing the off-equilibrium transformations in such systems. Facing these issues represents the necessary step to take to build an ultra-fast control theory of quantum materials properties. |
|27 Mar 2014|
|Ali Naji||Fluctuation-induced forces in disordered soft matter|
|Tea room F1. Abstract:
I will present a short overview of fluctuation-induced forces in the con- text of confined Coulomb fluids and liquid crystals as two well-established 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 inter-surface separations. By considering the special (and simpler) example of two charge-disordered dielectric surfaces in vac- uum, it is further shown that monopolar charge randomness can generate very long-ranged normal and lateral interaction forces (and torques) even if the surfaces are charge-neutral on the average.|
|18 Mar 2014|
|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|
|Jernej Mravlje||Discriminating Fermi liquids from non-Fermi 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
non-Fermi liquid behavior, but only provided correct placebo reference
is taken. A combination of the density functional theory with the dynamical
mean-field theory reproduces the experimental results fully and points
that the electrical conduction in the 1000-5000cm-1 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|
|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 zero-bias 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,gamma-bisdiphenylene-beta-phenylallyl (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 . The second radical, BDPA, self-assembles into dimers, trimers and chains on the Au(111) surface, all displaying Kondo conductance anomalies . I will present DFT calculations aimed at understanding the geometric and electronic structure of the isolated adsorbed molecule and molecular chains.
 R. Requist, S. Modesti, P. P. Baruselli, A. Smogunov, M. Fabrizio and E. Tosatti, PNAS, vol. 11, pp. 69-74 (2014).
 S. Muellegger, M. Rashidi, M. Fattinger and R. Koch, J. Phys. Chem. C, vol. 117, pp. 5718-5721 (2013).
|6 Jan 2014|
|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.