Institut für Theoretische Physik

148 Items

Recent Submissions
Quantum light driven many level systems in the weak and strong coupling limits

Chatraee Azizabadi, Shahabedin (2017)

We propose a possibility to study the quantum light excitation of atomic systems. To confine our study, we limit the configuration to two and four level emitters as target systems. Source and target are coupled via unidirectional waveguide. We numerically implement the cascaded formalism and use the source-target approach. Herein, we focus on continuous excitation scenarios where the source s...

Multidimensional high harmonic spectroscopy

Serbinenko, Valeriya (2017)

Multidimensional high harmonic spectroscopy is a cutting-edge technique for studying the effect of the intense low-frequency laser fields on atoms and molecules. The high energy photons, emitted by a particle as a result of the interaction with the strong laser field, carry the information about the processes in atoms and molecules, which lead to the emission. Multidimensional character of the te...

Quantum control of light and matter fields in the nonlinear regime

Naumann, Nicolas Ludwig (2017)

In this thesis three different approaches to control the quantum statistics in nanoscopic devices are investigated. In the first part of the thesis, mechanical oscillations in solid state devices are manipulated by an external laser to show bistabilities, coherent amplification, and enhanced damping. The dynamics is controlled by the frequency of the external laser. This study is performed in a...

Identical emitters, collective effects and dissipation in quantum optics

Gegg, Michael (2017)

In this thesis a formalism for indistinguishable multi-level quantum emitters in quantum master equations is developed. The complexity of the approach scales only polynomially in the number of quantum emitters. Complexity here means the number of coupled equations or rather the dimension of the Liouville space. This approach opens new possibilities for calculating open quantum systems. The meth...

Brief remark on material motion dependence

Muschik, Wolfgang (2013)

Acceleration-sensitive materials have properties which depend on the motion of the material. This motion refers to an arbitrary chosen standard frame of reference and does not depend on observers, which can be changed without changing the material properties. Consequently, one has properly to distinguish between changing the observer and changing the motion of the material. This is achieved by ...

Wie philosophisch dürfen bzw. müssen die Naturwissenschaften sein?

Borzeszkowski, Horst-Heino von (2015)

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Manipulating shear-induced non-equilibrium transitions in colloidal films by feedback control

Vezirov, Tarlan A. ; Gerloff, Sascha ; Klapp, Sabine H. L. (2015)

Using Brownian Dynamics (BD) simulations we investigate non-equilibrium transitions of sheared colloidal films under controlled shear stress sigma(xz). In our approach the shear rate (gamma) over dot is a dynamical variable, which relaxes on a time scale tau(c) such that the instantaneous, configuration-dependent stress sigma(xz)(t) approaches a pre-imposed value. Investigating the dynamics und...

Tunable structures of mixtures of magnetic particles in liquid-crystalline matrices

Peroukidis, Stavros D. ; Lichtner, Ken ; Klapp, Sabine H. L. (2015)

We investigate the self-organization of a binary mixture of similar sized rods and dipolar soft spheres by means of Monte-Carlo simulations. We model interparticle interactions by employing anisotropic Gay-Berne, dipolar and soft-sphere interactions. In the limit of vanishing magnetic moments we obtain a variety of fully miscible liquid crystalline phases including nematic, smectic and lamellar...

Generic model for tunable colloidal aggregation in multidirectional fields

Kogler, Florian ; Velev, Orlin D. ; Hall, Carol K. ; Klapp, Sabine H. L. (2015)

Based on Brownian Dynamics computer simulations in two dimensions we investigate aggregation scenarios of colloidal particles with directional interactions induced by multiple external fields. To this end we propose a model which allows continuous change in the particle interactions from point-dipole-like to patchy-like (with four patches). We show that, as a result of this change, the non-equi...

Effects of magnetic field gradients on the aggregation dynamics of colloidal magnetic nanoparticles

Heinrich, Dirk ; Goñi, Alejandro R. ; Osán, Tristán ; Cerioni, L. M. C. ; Smessaert, Anton ; Klapp, Sabine H. L. ; Faraudo, Jordi ; Pusiol, Daniel Jose ; Thomsen, Christian (2015)

We have used low-field H-1 nuclear-magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) to investigate the aggregation dynamics of magnetic particles in ionic ferrofluids (IFFs) in the presence of magnetic field gradients. At the beginning of the experiments, the measured NMR spectra were broad and asymmetric, exhibiting two features attributed to different dynamical environments o...

Self-assembly of three-dimensional ensembles of magnetic particles with laterally shifted dipoles

Yener, Arzu B. ; Klapp, Sabine H. L. (2016)

We consider a model of colloidal spherical particles carrying a permanent dipole moment which is laterally shifted out of the particles' geometrical centres, i.e. the dipole vector is oriented perpendicular to the radius of the particles. Varying the shift delta from the centre, we analyse ground state structures for two, three and four hard spheres, using a simulated annealing procedure. We al...

The effect of charge separation on the phase behavior of dipolar colloidal rods

Rutkowski, David M. ; Velev, Orlin D. ; Klapp, Sabine H. L. ; Hall, Carol K. (2016)

Colloids with anisotropic shape and charge distribution can assemble into a variety of structures that could find use as novel materials for optical, photonic, electronic and structural applications. Because experimental characterization of the many possible types of multi-shape and multipolar colloidal particles that could form useful structures is difficult, the search for novel colloidal mat...

Dynamics of a bacterial flagellum under reverse rotation

Adhyapak, Tapan Chandra ; Stark, Holger (2016)

To initiate tumbling of an E. coli, one of the helical flagella reverses its sense of rotation. It then transforms from its normal form first to the transient semicoiled state and subsequently to the curly-I state. The dynamics of polymorphism is effectively modeled by describing flagellar elasticity through an extended Kirchhoff free energy. However, the complete landscape of the free energy r...

Orientational order and translational dynamics of magnetic particle assemblies in liquid crystals

Peroukidis, Stavros D. ; Klapp, Sabine H. L. (2016)

Implementing extensive molecular dynamics simulations we explore the organization of magnetic particle assemblies (clusters) in a uniaxial liquid crystalline matrix comprised of rodlike particles. The magnetic particles are modelled as soft dipolar spheres with diameter significantly smaller than the width of the rods. Depending on the dipolar strength coupling the magnetic particles arrange in...

Multidirectional colloidal assembly in concurrent electric and magnetic fields

Bharti, Bhuvnesh ; Kogler, Florian ; Hall, Carol K. ; Klapp, Sabine H. L. ; Velev, Orlin D. (2016)

Dipolar interactions between nano- and micron sized colloids lead to their assembly into domains with well-defined local order. The particles with a single dipole induced by an external field assemble into linear chains and clusters. However, to achieve the formation of multidirectionally organized nano-or microassemblies with tunable physical characteristics, more sophisticated interaction too...

Phase separation and coexistence of hydrodynamically interacting microswimmers

Blaschke, Johannes ; Maurer, Maurice ; Menon, Karthik ; Zöttl, Andreas ; Stark, Holger (2016)

A striking feature of the collective behavior of spherical microswimmers is that for sufficiently strong self-propulsion they phase-separate into a dense cluster coexisting with a low-density disordered surrounding. Extending our previous work, we use the squirmer as a model swimmer and the particle-based simulation method of multi-particle collision dynamics to explore the influence of hydrody...

Taylor line swimming in microchannels and cubic lattices of obstacles

Münch, Jan L. ; Alizadehrad, Davod ; Babu, Sujin B. ; Stark, Holger (2016)

Microorganisms naturally move in microstructured fluids. Using the simulation method of multi-particle collision dynamics, we study in two dimensions an undulatory Taylor line swimming in a microchannel and in a cubic lattice of obstacles, which represent simple forms of a microstructured environment. In the microchannel the Taylor line swims at an acute angle along a channel wall with a clearl...

Active Brownian motion of emulsion droplets: coarsening dynamics at the interface and rotational diffusion

Schmitt, Max ; Stark, Holger (2016)

A micron-sized droplet of bromine water immersed in a surfactant-laden oil phase can swim (S. Thutupalli, R. Seemann, S. Herminghaus, New J. Phys. 13073021 (2011). The bromine reacts with the surfactant at the droplet interface and generates a surfactant mixture. It can spontaneously phase-separate due to solutocapillary Marangoni flow, which propels the droplet. We model the system by a diffus...

Shear banding in nematogenic fluids with oscillating orientational dynamics

Lugo-Frias, Rodrigo ; Reinken, Henning ; Klapp, Sabine H. L. (2016)

We investigate the occurrence of shear banding in nematogenic fluids under planar Couette flow, based on mesoscopic dynamical equations for the orientational order parameter and the shear stress. We focus on parameter values where the sheared homogeneous system exhibits regular oscillatory orientational dynamics, whereas the equilibrium system is either isotropic (albeit close to the isotropic-...

Active Brownian particles moving in a random Lorentz gas

Zeitz, Maria ; Wolff, Katrin ; Stark, Holger (2017)

Biological microswimmers often inhabit a porous or crowded environment such as soil. In order to understand how such a complex environment influences their spreading, we numerically study non-interacting active Brownian particles (ABPs) in a two-dimensional random Lorentz gas. Close to the percolation transition in the Lorentz gas, they perform the same subdiffusive motion as ballistic and diff...