arXiv subjects

J. Ehrens

Publications and source records attributed to J. Ehrens.

4 recordsLinked to original sources

arXiv2024

Comparison of various schemes to determine the Young's modulus of disordered carbon nanomembranes compared to crystalline graphene

The determination of mechanical properties such as the Young's modulus provides an important means to compare classical molecular dynamics simulations with materials. In this respect, ultra-thin materials hold several challenges: their volume is ambiguous, and different methods to determine a stress-strain relation deliver different result in particular for disordered systems. Using the example of carbon nanomembranes we discuss three common approaches to the problem and show that stress-strain simulations following experimental setups deliver correct results if adjusted carefully. We provide step-by-step instructions how to perform trustworthy simulations.

cond-mat.mtrl-sci↗

arXiv2021

Theoretical formation of carbon nanomembranes under realistic conditions using classical molecular dynamics

Carbon nanomembranes made from aromatic precursor molecules are free standing nanometer thin materials of macroscopic lateral dimensions. Although produced in various versions for about two decades not much is known about their internal structure. Here we present a first systematic theoretical attempt to model the formation, structure, and mechanical properties of carbon nanomembranes using classical molecular dynamics simulations. We find theoretical production scenarios under which stable membranes form. They possess pores as experimentally observed. Their Young's modulus, however, is systematically larger than experimentally determined.

cond-mat.mtrl-sci↗

arXiv2018

Rotational magnetocaloric effect of anisotropic giant spin systems

The magnetocaloric effect, that consists of adiabatic temperature changes in a varying external magnetic field, appears not only when the amplitude is changed, but in cases of anisotropic magnetic materials also when the direction is varied. In this article we investigate the magnetocaloric effect theoretically for the archetypical single molecule magnets Fe8 and Mn12 that are rotated with respect to a magnetic field. We complement our calculations for equilibrium situations with investigations of the influence of non-equilibrium thermodynamic cycles.

cond-mat.str-el↗

arXiv2017

Young's moduli of carbon materials investigated by various classical molecular dynamics schemes

Classical carbon potentials together with classical molecular dynamics are employed to calculate structures and physical properties of such carbon-based materials where quantum mechanical methods fail either due to the excessive size, irregular structure or long-time dynamics. Examples are given by recently synthesized free-standing carbon nanomembranes (CNM) with molecular thickness and macroscopic lateral size as well as by amorphous carbon. Although such potentials, as for instance implemented in LAMMPS, yield reasonably accurate bond lengths and angles for several carbon materials such as graphene, it is not clear how accurate they are in terms of mechanical properties such as Young's moduli. We performed large-scale classical molecular dynamics investigations of three carbon-based materials using the various potentials implemented in LAMMPS as well as the highly sophisticated EDIP potential of Nigel Marks. We demonstrate how the Young's moduli vary with classical potentials and compare to experimental results. Since classical descriptions of carbon are bound to be approximations it is not astonishing that different realizations yield differing results. One should therefore carefully check for which observables a certain potential is suited. We hope to contribute to such a clarification.

cond-mat.mtrl-sci↗