Institut für Theoretische Physik
Start / Aktuell / Institut
Dezember  2014
Mo
01.12.2014
Sitzungszimmer Mathematik
Theoretische Physik
14:15
Born-Hilbert-Seminar

Jan Philip Solovej
University of Copenhagen

TBA



Kontakt: K.-H. Rehren
Mo
01.12.2014
Seminarraum A3.101
Theoretische Physik
14:15
Aktuelle Probleme der theoretischen Festkörperphysik (Forschungsseminar M.Phy.411)

Manuel Kreye
Universität Göttingen

'The Unruh Effect in Condensed Matter'

We want to understand (quantum) phase transitions in non-equilibrium where recent progress has been made in the field of quantum quench dynamics. One open question is, under what circumstances a quenched system reaches a stationary state and whether this can be described by a thermal density matrix. There is an interesting formal analogy between quantum quenches and the transformation to uniformly accelerating coordinate frames. In the latter the Unruh effect is observed that predicts a thermal distribution in the accelerating frame while in the rest frame there is none. A short overview of the theoretical foundations of the Unruh effect will be given and some analogies to quantum quenches will be explored.

Kontakt: S. Kehrein
Di
02.12.2014
SR 3, Institut für Theoretische Physik, A03.101
SFB 937
17:15
Seminar

Yael Roichman
University of Tel Aviv

Dynamics in Steady State in-Vitro Acto-Myosin Networks

We show that complex fluids such as actin networks respond to deformations differently at intermediate length scales than they do as a bulk material, at large distances. When characterizing passive entangled F-actin networks we found that this intermediate response regime, characterized by an decay with distance, persists to surprisingly large distances of 2-6 µm, which is comparable the size of a cell, and over ten times larger than the mesh size of the actin network. A similar cross-over was observed previously in microtubule networks at r_c = 15 µm, if not as clearly, and without explanation. We use this newfound understanding to extract structural information of active in-vitro reconstituted cytoskeleton networks, in which such analysis can be done in a controlled fashion.

Kontakt: Glormann
Mi
03.12.2014
Seminarraum A 03.101
Theoretische Physik
12:00
Statistische Mechanik komplexer Systeme (Forschungsseminar M.Phy.410)

Qiyun Tang
Göttingen

Physical Aging in Ultrathin Polymer Films: A Molecular Simulation Study

Physical aging in glassy polymer films has attracted great attention in the past two decades due to their strong correlation with the lifetime of polymer-based nano-devices. Recently we developed a simulation routine by introducing vacancy diffusion and annihilation processes into the Monte Carlo simulation to study these phenomena. We found that the responses of film aging rates to temperatures are consistent with that for glassy polymer films in experiments. Especially, we demonstrated that the accelerated physical aging, commonly observed in experiments, slows down at extremely small film thicknesses. This phenomenon can be attributed to an inversed vacancy diffusion process caused by the sliding motion of chain molecules. Our findings provide direct evidence of the relationship between the sliding motion of short chain fragments and the physical aging of ultrathin polymer films, and also verify the existence of a new confinement effect at the nanoscale.

Kontakt: Marcus Müller
Mo
08.12.2014
Seminarraum A.04.101
Theoretische Physik
14:15
Quantenfeldtheorie (Forschungsseminar M.Phy.412)

Federico Dradi
Göttingen

TBA



Kontakt: K.-H. Rehren
Mo
08.12.2014
Seminarraum A3.101
Theoretische Physik
14:15
Aktuelle Probleme der theoretischen Festkörperphysik (Forschungsseminar M.Phy.411)

Debanjan Basu
Institute for Theoretical Physics Technical University Clausthal

Complex Bandstructure for calculating Thermal Conductivity across Nanostrucutred Interfaces

Thermal conductivity is an important factor affecting the efficiency of thermoelectric devices. Our goal is to explore the thermal transmission due to phonons in multilayered structures on a mode-by-mode basis using the Beam Matching Technique. For this purpose, we determine the “complex bandstructure”, which describes propagating as well as evanescent phonon modes of the individual materials of this multilayer.We describe how to extract the matching conditions from the classical equations of motion for the atoms.

Kontakt: S. Kehrein
Mo
15.12.2014
SR4 oder Sitzungszimmer Mathematik
Theoretische Physik
14:15
Born-Hilbert-Seminar

Rainer Verch
Leipzig

TBA



Kontakt: K.-H. Rehren
Mo
15.12.2014
Seminarraum A3.101
Theoretische Physik
14:15
Aktuelle Probleme der theoretischen Festkörperphysik (Forschungsseminar M.Phy.411)

Benjamin Lenz
Universität Göttingen

TBA

TBA

Kontakt: S. Kehrein
Mo
15.12.2014
HS 4
SFB 937
14:15
Seminar

Peter J.M. van Haastert
University of Groningen, The Netherlands

How cells move and navigate in shallow gradients of chemoattractant

Central to chemotaxis is the molecular mechanism by which a shallow spatial gradient of chemoattractant induces protrusions in the direction of the gradient. We use Dictyostelium to investigate the minimal requirements for chemotaxis, and identified a basal signaling module providing activation of Ras and F-actin at the leading edge. I will present a conceptual framework of gradient sensing and chemotaxis that is based on basal cell movement with on top of that a directional bias by the cAMP gradient. In the absence of external cues, cells extend protrusions in an ordered stochastic process with a mixture of alternating right-left splitting of an existing protrusion, by which cells move persistently in one direction, and extension of new protrusions in random directions. The place where these protrusions are formed is highly associated with patches of activated Ras. A cAMP gradient induces the local activation of Ras at the side of the cell facing the highest concentration of cAMP. In shallow gradients, this cAMP-induced activated Ras combines with endogenous Ras activation to increase the probability to extend protrusions in the direction of the cAMP gradient. Other signaling pathway such as PI3K, TORC2 and guanylyl cyclace/myosin contribute to chemotaxis as memory of direction and amplifiers of spatial information.

Kontakt: Glormann
Di
16.12.2014
Seminarraum A3.101
Theoretische Physik
14:15
Theoretisch-physikalisches Seminar

Markus Heyl
Universität Innsbruck

Many-body localization and quantum ergodicity in disordered long-range Ising model

Ergodicity in quantum many-body systems is —despite its fundamental importance — still an open problem. Many-body localization provides a general framework for quantum ergodicity, and may therefore offer important insights. In this talk, it will be shown using both numerical and analytical methods that long-range interacting Ising models with transverse-field disorder enter a many-body localized phase at infinite temperature, irrespective of the disorder strength. As a consequence, these systems are nonergodic. To characterize and quantify quantum ergodicity, a measure for distances in Hilbert space will be introduced. It will be shown that in spin-1/2 systems it is equivalent to a simple local observable in real space, which can be measured in current experiments of superconducting qubits, polar molecules, Rydberg atoms, and trapped ions.

Kontakt: S. Kehrein
Do
18.12.2014
Sitzungssaal Mathematik
Theoretische Physik
15:15

We study harmonic maps between hyperbolic spaces. In particular, we focus on a famous conjecture due to Rick Schoen. According to this conjecture every quasi-symmetric map: S1 to S1 can be uniquely extended to a quasi-conformal harmonic diffeomorphism: D2 to D2, where here D2 stands for the unit open disk of R2 equipped with the hyperbolic metric. The uniqueness has been settled by Li and Tam. However, the existence part remains still open. We shall confirm affirmatively the conjecture if we impose some additional conditions on the initial data.

Kontakt: K.-H. Rehren

Januar  2015
Mo
05.01.2015
Seminarraum A.04.101
Theoretische Physik
14:15
Quantenfeldtheorie (Forschungsseminar M.Phy.412)

Robert Czechowski
Göttingen

TBA



Kontakt: K.-H. Rehren
Do
08.01.2015
Sitzungssaal Mathematik
Theoretische Physik
15:15

Defects in two-dimensional quantum field theories are lines across which fields of the theory may have discontinuities. Such defects are called "topological" if their precise location does not matter when evaluating correlators. Topological defects contain a lot of information about the field theory in question. In this talk I will concentrate on two-dimensional topological and conformal field theories, and I would like to explain how two superficially quite different constructions turn out to be closely related when formulated via defects. These are 1) the orbifolding procedure, and 2) building field theories on surfaces with spin structure.

Kontakt: K.-H. Rehren
Mo
12.01.2015
Seminarraum A3.101
Theoretische Physik
14:15
Aktuelle Probleme der theoretischen Festkörperphysik (Forschungsseminar M.Phy.411)

David Storch
Universität Göttingen

TBA

TBA

Kontakt: S. Kehrein
Mo
19.01.2015
Seminarraum A3.101
Theoretische Physik
14:15
Aktuelle Probleme der theoretischen Festkörperphysik (Forschungsseminar M.Phy.411)

Ingo Homrighausen
Universität Göttingen

TBA

TBA

Kontakt: S. Kehrein
Di
20.01.2015
Seminarraum A3.101
Theoretische Physik
14:15
Theoretisch-physikalisches Seminar

Robin Steinigeweg
Technical University of Braunschweig

Real-time relaxation of currents in spin-1/2 chains: Progress by quantum typicality

We use the concept of typicality to study the real-time dynamics of spin and energy currents in spin-1/2 models in one dimension and at nonzero temperatures [1,2]. These chains are the integrable XXZ chain and a nonintegrable modification due to the presence of a staggered magnetic field oriented in z direction. In the framework of linear response theory, we numerically calculate autocorrelation functions by propagating a single pure state, drawn at random as a typical representative of the full statistical ensemble. By comparing to small-system data from exact diagonalization (ED) and existing short-time data from time-dependent density matrix renormalization group (tDMRG), we show that typicality is satisfied in finite systems over a wide range of temperature and is fulfilled in both, integrable and nonintegrable systems. For the integrable case [1], we calculate the long-time dynamics of the spin current and extract the spin Drude weight for large systems outside the range of ED. We particularly provide strong evidence that the high-temperature Drude weight vanishes at the isotropic point. For the nonintegrable case [2], we obtain the full relaxation curve of the energy current and determine the heat conductivity as a function of magnetic field, exchange anisotropy, and temperature.

Do
22.01.2015
Sitzungssaal Mathematik
Theoretische Physik
14:15

Finite renormalization freedom in locally covariant quantum field theories on curved spacetime is known to be tightly constrained, under certain standard hypotheses, to the same terms as in flat spacetime up to finitely many curvature dependent terms. These hypotheses include locality, covariance, scaling, and continuous and analytic dependence on the metric and coupling parameters. The analytic dependence hypothesis is somewhat unnatural, because it requires that locally covariant observables (which are simultaneously defined on all spacetimes) depend continuously on an arbitrary metric, with the dependence strengthened to analytic on analytic metrics. Moreover the fact that analytic metrics are globally rigid makes the implementation of this requirement at the level of local algebras of observables rather technically cumbersome. We show that the conditions of locality, covariance and scaling, in conjunction with the microlocal spectral condition, are actually sufficient to constrain the allowed finite renormalizations equally strongly, making both the continuity and the analyticity hypotheses unnecessary. The key step in the proof uses the non-linear Peetre theorem on the characterization of differential operators. Based on [arXiv:1411.1302].

Kontakt: K.-H. Rehren
Mo
26.01.2015
Seminarraum A3.101
Theoretische Physik
14:15
Aktuelle Probleme der theoretischen Festkörperphysik (Forschungsseminar M.Phy.411)

Davide Fioretto
Universität Göttingen

TBA

TBA

Kontakt: S. Kehrein
Di
27.01.2015
SR 3, Institut für Theoretische Physik, A03.101
SFB 937
17:15
Seminar

Markus Maniak
University of Kassel

Biogenesis of Lipid Droplets from the Endoplasmic Reticulum

If grown axenically, Dictyostelium amoebae fail to build one cellular organelle, the lipid droplet. As soon as free fatty acids are added to the growth medium, or when amoebae are fed with bacteria, fat synthesis begins and lipid droplets are formed.

In the first step of fat synthesis, one coenzyme A-activated fatty acid is linked to glycerol-3-phosphate, mainly through the activity of GPAT3, because the knockout mutant has a strikingly “lean” phenotype. Two candidate enzymes for conducting the addition of the second fatty acid, AGPAT3 and PLSC1, failed to affect fat synthesis if knocked out singly or in combination. The third fatty acid is added by enzymes called DGATs. Whereas DGAT2 provides the main activity in mammals, DGAT1 clearly is the dominant enzyme activity in Dictyostelium. Although no defects were observed in a DGAT2 knockout, this protein can restore the defects seen in a DGAT1 KO, if overexpressed.

Most of the above enzymes reside in the Endoplasmic Reticulum when fatty acids are scarce, but move to the surface of lipid droplets when fatty acids are provided in excess. We saw this dual localization also for many other lipid droplets constituents, among these a number of proteins with as yet unknown functions. This step of protein trafficking is remarkable, because the ER is delimited by a normal membrane bilayer, whereas the hydrophobic core of fat in the lipid droplet is surrounded by just a single membrane leaflet. We are currently investigating the basis of the protein redistribution in vivo and in vitro, and the progress will be reported.

Kontakt: Glormann
Do
29.01.2015
Sitzungssaal Mathematik
Theoretische Physik
15:15

The cobordism hypothesis classifies fully local topological field theories. It is an open problem to realize quantum Chern-Simons theory as a fully local field theory in this sense. I show how classical Chern-Simons theory and its "pre-quantization" (in the sense of geometric quantization) has a fully local incarnation, and how it implies the need to cancel the quantum anomaly by means of "p1-structure" (Atiyah 2-framings). I close with an outlook on how this could be used to get the fully local quantum theory.

Kontakt: K.-H. Rehren

Februar  2015
Mo
23.02.2015
SR4 oder Sitzungszimmer Mathematik
Theoretische Physik
14:15
Born-Hilbert-Seminar

Jakob Yngvason
Wien

TBA



Kontakt: K.-H. Rehren

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