arXiv subjects

Asegun Henry

Publications and source records attributed to Asegun Henry.

24 records · Page 2Linked to original sources

arXiv2016

Understanding Divergent Thermal Conductivity in Single Polythiophene Chains using Modal Analysis and Sonification

We used molecular dynamics simulations, the Green-Kubo Modal Analysis (GKMA) method and sonification to study the modal contributions to thermal conductivity in individual polythiophene chains. The simulations suggest that it is possible to achieve divergent thermal conductivity and the GKMA method allowed for exact pinpointing of the modes responsible for the anomalous behavior. The analysis showed that transverse vibrations in the plane of the aromatic rings at low frequencies ~ 0.05 THz are primarily responsible. Further investigation showed that the divergence arises from persistent correlation between the three lowest frequency modes on chains. Sonification of the mode heat fluxes revealed regions where the heat flux amplitude yields a somewhat sinusoidal envelope with a long period similar to the relaxation time. This characteristic in the divergent mode heat fluxes gives rise to the overall thermal conductivity divergence, which strongly supports earlier hypotheses that attribute the divergence to correlated phonon-phonon scattering/interactions.

cond-mat.soft↗

arXiv2016

Phonon Optimized Potentials

Molecular dynamics (MD) simulations have been extensively used to study phonons and gain insight, but direct comparisons to experimental data are often difficult, due to a lack of empirical interatomic potentials (EIPs) for different systems. As a result, this issue has become a major barrier to realizing the promise associated with advanced atomistic level modeling techniques. Here, we present a general method for specifically optimizing EIPs from ab initio inputs for the study of phonon transport properties, thereby resulting in phonon optimized potentials (POPs). The method uses a genetic algorithm (GA) to directly fit to the key properties that determine whether or not the atomic level dynamics and most notably the phonon transport are described properly.

cond-mat.mtrl-sci↗

arXiv2015

Non-negligible Contributions to Thermal Conductivity From Localized Modes in Amorphous Silicon Dioxide

Thermal conductivity is an important property for almost all applications involving heat transfer, ranging from energy and microelectronics to food processing and textiles. The theory and modeling of crystalline materials is in some sense a solved problem, where one can now calculate the thermal conductivity of any crystalline line compound from first principles [1,2] using expressions based on the phonon gas model (PGM)[3,4]. However, modeling of amorphous materials still has many open questions, because the PGM itself becomes questionable when one cannot rigorously define the phonon velocities. New theories and methods are therefore needed to understand phonon transport in amorphous materials. In this letter, we used our recently developed Green-Kubo modal analysis (GKMA) method to study amorphous silica (a-SiO2). The predicted thermal conductivities exhibit excellent agreement with experiments at all temperatures and anharmonic effects are included in the thermal conductivity calculation for all types of modes in a-SiO2 for the first time. Previously, localized modes (locons) have been thought to have a negligible contribution to thermal conductivity, due to their highly localized nature, which conceptually should prevent them from moving energy to another location. However, in a-SiO2 our results indicate that locons contribute more than 10% to the total thermal conductivity from 400K to 800K and they are largely responsible for the increase in thermal conductivity of a-SiO2 above room temperature. This is an effect that cannot be explained by previous methods and therefore offers new insight into the nature of phonon transport in amorphous/glassy materials.

cond-mat.mtrl-sci↗

arXiv2015

Direct Calculation of Modal Contributions to Thermal Conductivity via Green-Kubo Modal Analysis: Crystalline and Amorphous Silicon

In this letter we derive a new method for direct calculation of the modal contributions to thermal conductivity, which is termed Green-Kubo modal analysis (GKMA). The GKMA method combines the lattice dynamics formalism with the Green-Kubo formula for thermal conductivity, such that the thermal conductivity becomes a direct summation of modal contributions, where one need not define the phonon velocity. As a result the GKMA method can be applied to any material/group of atoms where the atoms vibrate around stable equilibrium positions, which includes not only crystalline line compounds, but also random alloys, amorphous materials and even molecules. The GKMA method provides new insight into the nature of phonon transport, as it casts the problem in terms of mode-mode correlation instead of scattering, and provides a general unified formalism that can be used to understand phonon-phonon interactions in essentially any class of materials or structures where the atoms vibrate around stable equilibrium sites.

cond-mat.mtrl-sci↗

arXiv2014

A New Formalism for Calculating Modal Contributions to Thermal Interface Conductance from Molecular Dynamics Simulations

A new formalism for extracting the modal contributions to thermal interface conductance with full inclusion of anharmonicity is presented. The results indicate that when two materials are joined a new set of vibrational modes are required to correctly describe the transport across the interface. Among these new modes, certain classifications emerge, as most modes extend at least partially into the other material. Localized interfacial modes are also present and exhibit the highest conductance contributions on a per mode basis. The results also show that anharmonicity enables inelastic scattering at temperatures as low as 10K and inelastic processes contribute 20% of the conductance for the system studied.

cond-mat.mtrl-sci↗

arXiv2014

Thermal Interface Conductance between Aluminum and Silicon by Molecular Dynamics Simulations

The thermal interface conductance between Al and Si was simulated by a non-equilibrium molecular dynamics method. In the simulations, the coupling between electrons and phonons in Al are considered by using a stochastic force. The results show the size dependence of the interface thermal conductance and the effect of electron-phonon coupling on the interface thermal conductance. To understand the mechanism of interface resistance, the vibration power spectra are calculated. We find that the atomic level disorder near the interface is an important aspect of interfacial phonon transport, which leads to a modification of the phonon states near the interface. There, the vibrational spectrum near the interface greatly differs from the bulk. This change in the vibrational spectrum affects the results predicted by AMM and DMM theories and indicates new physics is involved with phonon transport across interfaces. Keywords:

cond-mat.mtrl-sci↗