Authors:	Dittrich, Florian
Advisor:	Virnau, Peter
Title:	Simulations of phase behavior in non-equilibrium systems
Online publication date:	4-Jan-2024
Year of first publication:	2024
Language:	english
Abstract:	Non-equilibrium systems cover a tremendously wide range of different systems, in experiments and in simulations as well as in the real world. The fact that these systems are not in thermodynamic equilibrium is in many cases responsible for unique effects and behavior. A fundamental understanding of non-equilibrium systems is crucial to gain insights into such behavior and exploit it in a manifold of use cases. A recent growth in attention to non-equilibrium systems is a consequence. Especially deep insights into the nature of certain non-equilibrium systems can be gained through the study of phase behavior in these very systems. To do so, this thesis utilizes computer simulations of different systems: Discrete and continuous active matter systems on the one hand and skyrmion lattices on the other hand. The active matter systems being discussed in this work consist of active particles. These particles are not only subject to Brownian motion but they are in addition “actively” performing directed motion, which drives the systems out of equilibrium. Active lattice gas models are studied as a discrete realization of such particles with comparably low computational effort. At sufficiently high activity they undergo a motility induced phase separation (MIPS) that closely resembles the gas-liquid transition known from equilibrium. However, a determination of critical points and exponents for different model realizations performed in this work showed some model dependent deviations from 2D Ising universality class. This raises the question, whether the concept of universality holds for active matter and non-equilibrium systems at all. The critical behavior of active Brownian particles (ABPs) around MIPS has already been studied before and showed even stronger deviations. In this work additional focus is put on quenches of ABPs from homogeneous phase right into the phase separated regime and to the critical point. Following the quench, the systems far-from-steady-state dynamics, structure growth and aging can be studied. Results obtained in this work appear to be similar to those observed during phase separation in the 2D Ising model. However, for the active lattice models, there are deviations in the case of quenches inside the coexistence regions. Skyrmion lattices consist of densely packed skyrmions. These topologically stabilized whirls of magnetization can be described as quasiparticles. By modelling them as soft disks similar to ABPs, the underlying interaction potential of experimental skyrmions was determined with the help of computer simulations in this work. Furthermore, different experimental skyrmion lattices were characterized according to their phase state and degree of hexagonal order with the help of a newly developed parameter. Thereby the onset of a hexatic phase was found.
DDC:	530 Physik 530 Physics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-9878
URN:	urn:nbn:de:hebis:77-openscience-c8a04109-cf7a-4c55-8152-97d52824288f1
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	http://rightsstatements.org/vocab/InC/1.0/
Extent:	101 Seiten ; Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen