The Path To Relaxation - Understanding thermal conduction in twisted stacks with non-equilibrium phonon dynamics
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Examensarbete för masterexamen
Program
Modellbyggare
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Sammanfattning
In this work, the details of thermal transport in graphite is studied. The aim is to un derstand a striking anisotropy observed in the thermal conductivity of twisted stacks
of van-der-Waals thin sheets, as well as to explore the limitations of the relaxation
time approximation of the Boltzmann transport equation. To this end, classical
molecular dynamics simulations were performed, and analyzed using projection onto
phonon modes. Atomic forces were modeled using neuroevolution potentials previ ously trained on results from density functional theory, and gpumd was used to run
simulations on graphical processing units. Phonon modes were defined from Second order force constants extracted from the neuroevolution potential using hiphive.
The simulations were done both in equilibrium and in variations of non-equilibrium.
The equilibrium phonons were modeled using Langevin dynamics to extract their
lifetimes and finite-temperature renormalized frequencies. Non-equilibrium states
were studied by exciting individual out-of-plane modes to extreme amplitudes be fore running the simulations, and then following the energy as it was distributed
from the pumped modes to the rest of the system. Contrary to the exponential
decay predicted by the relaxation time approximation, energy was observed to os cillate back and forth between modes when initially excited to a sufficient degree.
Furthermore, the system was observed to reach a long-lived quasi-equilibrium state,
where a set of coupled modes collectively retained an above-equilibrium energy for
an extended period of time. The quasi-equilibrium lifetime was found to depend
negatively on the size of the simulated system, but the limit at large sizes remains
unknown.
Beskrivning
Ämne/nyckelord
phonons, thermal transport, non-equilibrium, molecular dynamics, graphite, two-dimensional materials, moiré structures.