Inst. Theoretische Physik

197 Items

Recent Submissions
Out-of-equilibrium dynamics of open quantum many-body systems

Droenner, Leon Janek (2019)

Many-body localization, which prevents a many-body quantum systems from reaching thermal equilibrium has opened the possibility to study out-of-equilibrium dynamics of isolated quantum systems, leading to new phases of matter such as the discrete time crystal. However, little is known about these effects when the quantum many-body system is coupled to an external environment. An intuitive assum...

Is Spreading Depolarization Characterized by an Abrupt, Massive Release of Gibbs Free Energy from the Human Brain Cortex?

Dreier, Jens P. ; Isele, Thomas ; Reiffurth, Clemens ; Offenhauser, Nikolas ; Kirov, Sergei A. ; Dahlem, Markus A. ; Herreras, Oscar (2013)

In the evolution of the cerebral cortex, the sophisticated organization in a steady state far away from thermodynamic equilibrium has produced the side effect of two fundamental pathological network events: ictal epileptic activity and spreading depolarization. Ictal epileptic activity describes the partial disruption, and spreading depolarization describes the near-complete disruption of the p...

The Divergence of the Viscosity of a Fluid of Hard Spheres as an Indicator for the Fluid-Solid Phase Transition

Koo, Hyearn-Maw ; Hess, Siegfried (1987)

Solution of the Kirkwood-Smoluchowski equation for a hard sphere fluid yields an expression for the viscosity which shows a dramatic pretransitional increase and a divergence at a number density close to that one observed in computer simulations and in colloidal dispersions. The value for the transition density stems from a boundary condition at the surface of the hard sphere in the configurati...

The Viscosity Coefficients of Oriented Nematic and Nematic Discotic Liquid Crystals; Affine Transformation Model

Baalss, D. ; Hess, Siegfried (1988)

General expressions are derived for the anisotropy of the viscosity and for the Leslie coefficients governing the flow alignment subject to the assumption that the equipotential surfaces for the interaction of oriented nonspherical molecules in the nematic or nematic discotic phase of a liquid crystal are related to a spherical interaction potential by an affine transformation. In particular, f...

Exciton-phonon coupling in monolayers of transition metal dichalcogenides

Selig, Malte (2018)

Monolayers of transition metal dichalcogenides (TMDs) attracted much attention in recent research due to their promising optical and electronic properties for future technological applications. In this thesis, an excitonic description within the Heisenberg equation of motion formalism for the optical response of these semiconducting, ultra-thin materials is developed. Hereby the main focus lies...

Derivation and Application of an Algorithm for the Numerical Calculation of the Local Orientation of Nematic Liquid Crystals

Kilian, A. ; Hess, Siegfried (1989)

Starting from a relaxation equation for the alignment tensor, an algorithm is derived which allows the numerical calculation of the dynamic and static behavior of the director field n with the correct nematic symmetry property, where n and - n are equivalent. As a first application, a two-dimensional problem is treated where the typical nematic defects with half-integer winding numbers only occ...

Dissipative systems with nonlocal delayed feedback control

Grawitter, Josua ; van Buel, Reinier ; Schaaf, Christian ; Stark, Holger (2018-11-07)

We present a linear model, which mimics the response of a spatially extended dissipative medium to a distant perturbation, and investigate its dynamics under delayed feedback control. The time a perturbation needs to propagate to a measurement point is captured by an inherent delay time (or latency). A detailed linear stability analysis demonstrates that a nonzero system delay acts to destabili...

The Viscosity Coefficients of Biaxial-Nematic Liquid Crystals. Phenomenology and Affine Transformation Model

Baalss, Dieter (1990)

The viscosity tensor of biaxial-nematic liquid crystals contains 16 independent elements. A complete set of these viscosity coefficients is introduced and related to experimentally accessible quantities. Furthermore, the flow alignment and its stability against an arbitrary infinitesimal disturbance are discussed. By the affine transformation model, formerly established for the uniaxial symmetr...

Static and Dynamic Properties of Ferroelectric Liquid Crystals in the Vicinity of a First-Order SmA-SmC* Phase Transition

Netz, Roland R. ; Hess, Siegfried (1992)

A unified theoretical description of static and dynamic properties of ferroelectric liquid crystals in terms of a Landau free energy is presented. The shear deformation tensor is used as principal order parameter. Results for the stationary case for tilt angle variation and dielectric properties are compared with experiments on a compound with a pronounced first-order SmA-SmC* phase transition ...

Tunable Real Space Transfer Oscillator by Delayed Feedback Control of Chaos

Cooper, D. P. ; Schöll, E. (1995)

It is demonstrated numerically that by using Pyragas' method of chaos self-control a stable semiconductor oscillator can be designed based on driven real-space transfer oscillations in a modulation-doped heterostructure. By application of a small time-continuous delayed feedback voltage control signal, different unstable periodic orbits embedded in the chaotic attractor can be stabilized. Thus ...

A Simplified Thermodynamic Theory for Biaxial Nematics

Papenfuß, Christina ; Verhás, Jozsef ; Muschik, Wolfgang (1995)

A continuum theory of a biaxial nematic phase in liquid crystals is presented. The liquid crystalline ordering is described by a field of three unit vectors. Because these are macroscopic directors, the present approach is a generalization of the Ericksen-Leslie theory to biaxial nematics. The applied tools from nonequilibrium thermodynamics are classical Linear Irreversible Thermodynamics and ...

Monte Carlo Simulation of the Director Field of a Nematic Liquid Crystal with Three Elastic Coefficients

Gruhn, Thomas ; Hess, Siegfried (1996)

In this article, a Monte Carlo simulation is presented, which generates the equilibrium director field of a nematic liquid crystal under the influence of an external field and fixed boundary conditions. The liquid crystal is characterized by a set of directors on a spatially fixed lattice. The simulation is based on an expression for the Frank free energy with three elastic coefficients. The ch...

On the Shock Front Thickness in Water and other Molecular Liquids

Hess, Siegfried (1997)

Theoretical explanations are presented for the deviation of the shock front thickness from linear hydrodynamics, as observed by W. Eisenmenger (1964) in water and several molecular liquids for large driving pressure differences. Two mechanisms are proposed, which are based on generalizations of the Maxwell relaxation equation for the friction pressure tensor. One is due to the spatial inhomogen...

Pressure and Isotropic-Nematic Transition Temperature of Model Liquid Crystals

Hess, Siegfried ; Su, Bin (1999)

The pressure in the gaseous, the isotropic liquid and nematic liquid crystalline states, as well as the isotropic-nematic transition temperature are calculated for a model system composed of non-spherical particles. The potential is a generalization of the Lennard-Jones interaction where the attractive part depends on the relative orientations of the particles and the vector joining their cente...

Irreversibility and Second Law

Muschik, Wolfgang (1998)

According to M. Eigen there are two reasons for irreversibility: weak and strict temporality. Measures for irreversibility are defined by different formulations of the 2nd law which can be divided into two classes: the more general in time integrated formulations as Clausius' inequality and the more special in time local formulations as the non-negativity of the entropy production density. The ...

Second Law Induced Existence Conditions for Isothermal 2-Phase Region Cyclic Processes in Binary Mixtures

Bauer, W. D. ; Muschik, Wolfgang (1998)

Conditions describing the behaviour of the kink of isobars at the dew line in v-x-phase diagrams of binary mixtures are proved using the second law. These conditions are interesting in connection to the compatibility of Serogodsky's and van Platen's cycles with the second law of thermodynamics.

Weakly Nonlocal And Nonlinear Heat Transport In Rigid Solids

Lebon, Georgy ; Jou, David ; Casas-Vázquez, José ; Muschik, Wolfgang (1998)

A weakly nonlocal and nonlinear theory of heat conduction in rigid bodies is proposed. The constitutive equations generalize these of Fourier, Maxwell-Cattaneo and Guyer-Krumhansl. The proposed model uses the fundaments and the technique of extended irreversible thermodynamics. The main conclusion is that the presence of nonlocal terms in the transport equation for the heat flux implies a modif...

Does Clausius' Inequality Analogue Exist for Open Discrete Systems?

Muschik, Wolfgang ; Gümbel, Sebastian (1999)

We derive two versions of Clausius inequality for open discrete systems, one formulated for the non-equilibrium system itself using contact quantities, the other one using equilibrium quantities belongs to the system's environment. Consequently, Bhalekar's conjecture that there is no Clausius inequality for open systems is disproved.

Modeling and simulation of electrically driven quantum dot based single-photon sources

Kantner, Markus (2018)

Semiconductor quantum optics is on the leap from the lab to real world applications. The currently unfolding "second quantum revolution" aims at the development of novel quantum technologies that exploit inherent quantum mechanical phenomena like entanglement and quantum superposition for communication and information processing tasks. Many applications, such as eavesdropping-secure encryption ...

Remarks on thermodynamical terminology

Muschik, Wolfgang (2004)

Often difficulties in understanding thermodynamics arise from using different terminology or semantics. Therefore here some few remarks are made for tending towards a unified thermodynamical terminology.