Kontakt: Kurt Schönhammer

Kontakt: Prof. Dr. M. Müller

Introduction of new doctoral students and staff; info about the renewal process of the RTS 1493.

Kontakt: K.-H. Rehren

Systems with even number of f electrons per site can be a non-Kramers doublet ground state. When these f electrons couple with conduction electrons, a two-channel Kondo system can be realized. The two-channel Kondo impurity has residual entropy in the ground state. Then the two-channel Kondo lattice (2ch KL) should undergo some phase transitions. We report on a novel symmetry breaking between channels in the 2ch KL at half filling, by combining the dynamical mean-field theory and continuous-time quantum Monte Carlo method. The channel moment is zero in this phase even though the channel symmetry is broken. Instead, the ordered state is characterized by a correlation function representing the Kondo effect. It is shown that the present order can be interpreted also as an odd-frequency order.

Kontakt: Pruschke

Kontakt: Marcus Müller

Kontakt: K.-H. Rehren

Physical, biological, and engineered materials ranging from foams and emulsions to biopolymer and bar-joint networks can be modeled as random networks of springs. We study the oscillatory rheology of random networks immersed in a viscous background fluid, and show how their response is intimately tied to the presence or absence of floppy modes in the zero frequency limit. The rheology displays dynamic critical scaling with three different regimes: viscous fluid, elastic solid, and shear thinning power law fluid. We give scaling arguments to explain all of the critical exponents and confirm our predictions with numerics.

Kontakt: Heussinger

Kontakt: Claus Heussinger

Kontakt: K.-H. Rehren

Kontakt: K.-H. Rehren

Kontakt: Rehren

Kontakt: Marcus Müller

Quantum field theories are mathematical models that are supposed to describe physics at high energies and small scales (such as in a particle accelerator). Mathematically, a quantum field theory is given by certain sets of operators on a Hilbert space - for convenience, say, C* algebras or von Neumann algebras - which fulfil a number of properties imposed by physics, such as covariance under a representation of the Poincare´ group. While these axioms are easy to write down, it is very hard to "construct models", that is, to give nontrivial examples that fulfil the assumptions.

One class of tractable examples in this context are so-called integrable models in 1+1 dimensions. Here the mathematical construction has been completed by Lechner (for the case of massive particles) and more recently by Bostelmann, Lechner and Morsella (for the case of massless particles). This involves methods from operator theory and Tomita-Takesaki modular theory. In the talk, we review this construction, and point out interesting differences between the massive and the massless case.

Kontakt: K.-H. Rehren

Longo and Witten have studied endomorphisms of the chiral component of the free massless field algebra to construct QFT models with boundary. We further find such endomorphisms and construct two-dimensional models without boundary.

Kontakt: K.-H. Rehren

There is growing interest in correlated materials driven far from equilibrium conditions. On one side, probing the time evolution of these materials away from equilibrium may give access to dynamical properties otherwise unaccessible by other techniques. On a more practical side, since correlated materials are often on the verge of a Mott metal-to-insulator transition, it could be feasible to switch on/off their conducting properties much faster than e.g. changing temperature or pressure, a great opportunity for possible applications.

Kontakt: Prof. Dr. T. Pruschke

Kontakt: Claus Heussinger

Kontakt: K.-H. Rehren

Kontakt: K.-H. Rehren

Kontakt: Claus Heussinger

Kontakt: C. Heussinger

Kontakt: K.-H. Rehren

I will review the papers [1] and [2] and apply them to the problem of classification of four point conformal invariant function with values in arbitrary irreducible representations of the Lorentz group.

References:

[1] Petko A. Nikolov and Nikola P. Petrov, Dimensional Reduction of Invariant Fields and Differential Operators. I. Reduction of Invariant Fields, Ann. Henri Poincare, DOI 10.1007/s00023-011-0133-0

[2] Petko A. Nikolov and Tihomir Valchev, Description of all conformally invariant differential operators, acting on scalar functions, Journ. Math. Phys. 48, 123516 (2007)

Kontakt: K.-H. Rehren

Kontakt: Annette Zippelius

The nature of fluctuations in the flow of glasses and a variety of amorphous materials has been a topic of much current interest. Soft glasses made of dense colloidal suspensions have been used as model systems to understand the behavior of hard sphere glasses. We use the technique of confocal microscopy to follow the trajectories of individual particles in a sheared colloidal glass. This enables us to measure the local strain and diffusion in the system. Remarkably, the local strain and the diffusion is heterogeneous, and most of the plastic deformation is concentrated in localized regions that are termed shear transformation zones (STZ). The formation of a shear transformation zone induces a long-ranged quadrupolar strain field around it. This long-range interaction between the STZs give rise to a scale-free deformation of glasses. Further, I will discuss the role of the long-range strain correlations in the transition from homogeneous flow to inhomogeneous (shear banded) flow.

Kontakt: Claus Heussinger

The well-defined nanostructures with long-range order are technologically and industrially important. Because of the self-assembly into periodic arrays block copolymers (BCP) form films with different patterns. For many applications where regular periodic arrays are required, it is necessary to generate specific alignment of the nanostructures in block copolymer films. Conventional lithography techniques are not suitable and disadvantageous because of multistep process, which makes them very slow and high cost for large-area applications at high densities. The unconventional approaches based on self-assembly of BCPs have been suggested using mesoscopic simulation methods such as dissipative particle dynamics and dynamic density functional theory. We study the self-assembly of the cylinder-forming and lamellar-forming copolymer films near patterned surfaces (hexagonal, sparse rectangular and spare triangular patterns), having periodically distributed regions of different wettability. The parallel alignment of microdomains between preferentially attractive homogeneous surfaces is shown to transform into the stable perpendicular hexagonal phase in the case of the substrate patterns, which commensurate with the hexagonal morphology in the bulk. The new strategy based on double phase separation of the binary mixture of copolymers is suggested. The role of a solvent in the self-assembly of block copolymer in films have been shown.

Kontakt: Richard Vink

Kontakt: K.-H. Rehren

Kontakt: K.-H. Rehren

Kontakt: Claus Heussinger

Kontakt: Marcus Müller

Kontakt: K.-H. Rehren

Kontakt: M. Müller

We present a new determination of the local dark matter phase-space density. This result is obtained implementing, in the limit of isotropic velocity distribution and spherical symmetry, Eddington's inversion formula, which links univocally the dark matter distribution function to the density profile. The derived dark matter phase-space density differs significantly - most dramatically in the high velocity tail - from the model usually taken as a reference in dark matter detection studies, a Maxwell-Boltzmann distribution with velocity dispersion fixed in terms of the local circular velocity and with a sharp truncation at a given value of the escape velocity. We discuss the impact of astrophysical uncertainties on dark matter scattering rates and direct detection exclusion limits, considering a few sample cases and showing that the most sensitive ones are those for light dark matter particles and for particles scattering inelastically. As a general trend, when adopting a self-consistent phase-space density, we find that rates are larger, and hence exclusion limits stronger, than with the standard Maxwell-Boltzmann approximation.

Kontakt: K.-H. Rehren

Many-body effects are of crucial importance both for the thermodynamics and for effective single-band descriptions of interacting bosons in optical lattices. I will discuss two recent results:

1) We theoretically investigate finite-temperature thermodynamics and demagnetization cooling of two-component Bose-Bose mixtures in a cubic optical lattice, by using bosonic dynamical mean field theory (BDMFT). We calculate the finite-temperature phase diagram, and remarkably find that the system can be heated from the superfluid into the Mott insulator at low temperature, analogous to the Pomeranchuk effect in 3He. This provides a promising many-body cooling technique. We examine the entropy distribution in the trapped system and discuss its dependence on temperature and an applied magnetic field gradient.

2) We construct the effective lowest-band Bose-Hubbard model incorporating interaction-induced on-site correlations. The model is based on generalized ladder operators for local correlated states, allowing for a systematic construction of the optimal single-band low-energy description in the form of the extended Bose-Hubbard model. This approach not only yields the previously found effective multi-body interactions, but also contains multi-body-induced single-particle tunneling, pair tunneling and nearest-neighbor interaction processes of higher orders. These multi-particle effects can be enhanced using Feshbach resonances, leading to corrections which are well within experimental reach and of significance to the phase diagram of ultracold bosonic atoms in an optical lattice. We analyze the energy reduction mechanism of interacting atoms on a local lattice site and show that it cannot be explained on a single-particle level by a spatial broadening of Wannier orbitals.

Kontakt: St. Kehrein

Kontakt: K.-H. Rehren

Kontakt: Claus Heussinger

We propose an easily implemented approach to study time-dependent correlation functions of one dimensional systems at finite temperature T using the density matrix renormalization group. The entanglement growth inherent to any time-dependent calculation is significantly reduced if the auxiliary degrees of freedom which purify the statistical operator are time evolved with the physical Hamiltonian but reversed time. We exploit this to investigate the long time behavior of current correlation functions of the XXZ spin-1/2 Heisenberg chain. This allows a direct extraction of the Drude weight D at intermediate to large T. We find that D is nonzero -- and thus transport is dissipationless -- everywhere in the gapless phase. At low temperatures we establish an upper bound to D by comparing with bosonization.

Kontakt: Prof. K. Schönhammer

Kontakt: K.-H. Rehren

In the talks, the relation between pure Micro States and the corresponding thermal Macro States will be analysed. This is relevant in the context of the discussion of the Nature of Thermality of e.g. Black Holes and the Problem of Unitarity.

Kontakt: K.-H. Rehren

Kontakt: Claus Heussinger

Kontakt: K.-H. Rehren

Kontakt: K.-H. Rehren

Kontakt: Prof. T. Pruschke

Kontakt: Prof. Laura Covi

Kontakt: K.-H. Rehren

Kontakt: K.-H. Rehren

Kontakt: Prof. T. Pruschke

Kontakt: Claus Heussinger

- all day -

Kontakt: K.-H. Rehren

Kontakt: Prof. T. Pruschke

Kontakt: Claus Heussinger

We study kinks in the electronic dispersion of a generic strongly correlated system by dynamic mean-field theory (DMFT) using three different algorithms to clearly distinguish physical features from numerical artefacts. The focus is on doped systems where no particle-hole symmetry holds and valence fluctuations matter potentially.

Our findings extend the picture that the kinks reflect the coupling of the fermionic quasiparticles to emergent collective modes, namely the spin fluctuations, beyond half-filling to finite doping. The energies of the kinks and their doping dependence fit well to the kinks in the cuprates, which is surprising in view of the spatial correlations neglected by DMFT.

Kontakt: Prof. Stefan Kehrein

A generalization of Connes' noncommutative geometry is required in order to deal with Lorentzian signature space-times. We will discuss the difficulties arising from such generalization and present some approaches to deal with. In particular, we will show how the Riemannian distance formula can be adapted to Lorentzian manifolds as well as how to introduce a notion of global time through the construction of Lorentzian spectral triples.

Kontakt: K.-H. Rehren

Kontakt: K.-H. Rehren

I present how QCD has been introduced and will be completed in the future by solving the strong CP problem. If the axion solves the strong CP problem, it can constitute the dark matter of our universe. I introduce two existing claims for its existence. I also compare it with another dark matter candidate, the LSP of supersymmetric models, and point out its feasibility in the LHC era

Kontakt: Prof. Laura Covi

We propose a mesoscopic computational model for studying the mechanics of fibrils or, in general, bundles of optionally cross-linked polymers. The model offers a set of parameters whose values can be tuned to match the properties or ambient conditions of the fibril under consideration, such as the extent of cross-linking, the temperature, and the dimensions of the fibril. Although it is fairly general in its applicability, the model is discussed in the context of collagen fibrils. Monte Carlo simulation of this model is made possible by means of a recently introduced move known as TRACTRIX, and we outline an adaptation of this technique which is specially suited for bundles of many polymers. In particular, this adaptation enables the simultaneous deformation, in one simulation step, of several polymers spanning the entire cross-section of the fibril. By subjecting our model fibril to external forces that cause it to bend, estimates of the axial Young's Modulus of collagen fibrils can be made after comparing the bent profile with a Timoshenko beam in a tip-loaded cantilever setup. The results are in good agreement with those obtained by laboratory experiment.

Kontakt: R. Vink

The construction of an infinite tensor product of the C*-algebra C_0(R) is not obvious, because it has no unit, and it has no nonzero projection. Based on a choice of an approximate identity, we construct here an infinite tensor product of C_0(R), denoted L_V, and use it to find (partial) group algebras for the full continuous representation theory of R^(N). We obtain an interpretation of the Bochner–Minlos theorem in R^(N) as the pure state space decomposition of the partial group algebras which generate L_V. We analyze the representation theory of L_V, and show that there is a bijection between a natural set of representations of L_V and Rep(R^(N),H) , but that there is an extra part which essentially consists of the representation theory of a multiplicative semigroup Q which depends on the initial choice of approximate identity.

Kontakt: K.-H. Rehren