Inst. Theoretische Physik

191 Items

Recent Submissions
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.

Thermodynamic interaction between two discrete systems in non-equilibrium

Muschik, Wolfgang ; Berezovski, Arkadi (2004)

A systematic description of the interaction between two systems is presented within the framework of thermodynamics of discrete systems. We distinguish between two situations: a contact between an equilibrium system and a non-equilibrium one, and a contact between two non-equilibrium systems. In the second case the state space includes replacement quantities, which reduce the situation to a con...

Mesoscopic theory of liquid crystals

Muschik, Wolfgang ; Papenfuss, Christina ; Ehrentraut, Harald (2004)

Liquid crystals of uniaxial and biaxial molecules are considered in the framework of the mesoscopic description which is a general tool of continuum theory. A mesoscopic theory introduces the fields beyond hydrodynamics as additional variables of a configuration space, called mesoscopic space, on which the fields appearing in balances are defined. Besides the mesoscopic space, a mesoscopic dist...

Flow Velocity and Effective Viscosity of a Fluid Containing Rigid Cylindrical Inclusions

Hess, Siegfried (2005)

The determination of the flow properties of a fluid containing a cylindrical inclusion with its long axis oriented parallel to the vorticity direction is a 2-dimensional problem which is treated as a special case in a calculation of the corresponding D-dimensional problem. The velocity and pressure are obtained from the solution of the equations of hydrodynamics where D-dimensional multipole po...

Close-to-Fourier heat conduction equation of solids of constant mass density

Muschik, Wolfgang ; Siebert, Jürgen ; Herrmann, Heiko ; Rückner, Gunnar (2005)

Heat conduction close-to-Fourier means that we look for a minimal extension of heat conduction theory using the usual Fourier expression of the heat flux density and modifying that of the internal energy as minimally as possible by choosing the minimal state space. Applying Liu's procedure results in the class of materials and a dierential equation both belonging to the close-to-Fourier case of...

Kann die Physik das Leben wissenschaftlich erfassen?

Borzeszkowski, Horst-Heino von (2006)

Oft wird die in der Überschrift gestellte Frage kurz und bündig mit dem Satz beantwortet: »Natürlich nicht,denn Physik ist nicht Biologie!« Im hiesigen Kontext wiegt allerdings schwerer, daß Hegel ebenfalls zu einer verneinenden Antwort auf diese Frage kommt. Um Hegels Argumente hinsichtlich ihrer Stichhaltigkeit zu analysieren, hat man nicht eine Gegenüberstellung dessen, was wir heute unter P...

Endoreversible Thermodynamics: A Tool for Simulating and Comparing Processes of Discrete Systems

Muschik, Wolfgang ; Hoffmann, Karl Heinz (2006)

Endoreversible thermodynamics is concerned with reversible sub-systems that are in irreversible interaction with each other. Consequently, endoreversible thermodynamics represents the analogue for discrete systems to the local equilibrium hypothesis in continuum thermodynamics. Here a real cyclic 2-reservoir process is simulated by different endoreversible model processes. Simulation means that...