24 research outputs found
Nonequilibrium phonon mean free paths in anharmonic chains
Harnessing the power of low-dimensional materials in thermal applications
calls for a solid understanding of the anomalous thermal properties of such
systems. We analyze thermal conduction in one-dimensional systems by
determining the frequency-dependent phonon mean free paths (MFPs) for an
anharmonic chain, delivering insight into the diverging thermal conductivity
observed in computer simulations. In our approach, the MFPs are extracted from
the length-dependence of the spectral heat current obtained from nonequilibrium
molecular dynamics simulations. At low frequencies, the results reveal a
power-law dependence of the MFPs on frequency, in agreement with the diverging
conductivity and the recently determined equilibrium MFPs. At higher
frequencies, however, the nonequilibrium MFPs consistently exceed the
equilibrium MFPs, highlighting the differences between the two quantities.
Exerting pressure on the chain is shown to suppress the mean free paths and to
generate a weaker divergence of MFPs at low frequencies. The results deliver
important insight into anomalous thermal conduction in low-dimensional systems
and also reveal differences between the MFPs obtained from equilibrium and
nonequilibrium simulations.Comment: 8 pages, 7 figures, minor changes to v
Role of anharmonic phonon scattering in the spectrally decomposed thermal conductance at planar interfaces
Detailed understanding of vibrational heat transfer mechanisms between solids
is essential for the efficient thermal engineering and control of
nanomaterials. We investigate the frequency dependence of anharmonic scattering
and interfacial thermal conduction between two acoustically mismatched solids
in planar contact by calculating the spectral decomposition of the heat current
flowing through an interface between two materials. The calculations are based
on analyzing the correlations of atomic vibrations using the data extracted
from non-equilibrium molecular dynamics simulations. Inelastic effects arising
from anharmonic interactions are shown to significantly facilitate heat
transfer between two mass-mismatched face-centered cubic lattices even at
frequencies exceeding the cut-off frequency of the heavier material due to (i)
enhanced dissipation of evanescent vibrational modes and (ii)
frequency-doubling and frequency-halving three-phonon energy transfer processes
at the interface. The results provide substantial insight into interfacial
energy transfer mechanisms especially at high temperatures, where inelastic
effects become important and other computational methods are ineffective.Comment: minor changes to v
Local Semiconducting Transition in Armchair Carbon Nanotubes: The Effect of Periodic Bi-site Perturbation on Electronic and Transport Properties of Carbon Nanotubes
In carbon nanotubes, the most abundant defects, caused for example by
irradiation or chemisorption treatments, are small perturbing clusters, i.e.
bi-site defects, extending over both A and B sites. The relative positions of
these perturbing clusters play a crucial role in determining the electronic
properties of carbon nanotubes. Using bandstructure and electronic transport
calculations, we find out that in the case of armchair metallic nanotubes a
band gap opens up when the clusters fulfill a certain periodicity condition.
This phenomenon might be used in future nanoelectronic devices in which certain
regions of single metallic nanotubes could be turned to semiconducting ones.
Although in this work we study specifically the effect of hydrogen adatom
clusters, the phenomenon is general for different types of defects. Moreover,
we study the influence of the length and randomness of the defected region on
the electron transport through it.Comment: 5 Pages, 5 Figure
Spectral mapping of heat transfer mechanisms at liquid-solid interfaces
Thermal transport through liquid-solid interfaces plays an important role in many chemical and biological processes, and better understanding of liquid-solid energy transfer is expected to enable improving the efficiency of thermally driven applications. We determine the spectral distribution of thermal current at liquid-solid interfaces from nonequilibrium molecular dynamics, delivering a detailed picture of the contributions of different vibrational modes to liquid-solid energy transfer. Our results show that surface modes located at the Brillouin zone edge and polarized along the liquid-solid surface normal play a crucial role in liquid-solid energy transfer. Strong liquid-solid adhesion allows also for the coupling of in-plane polarized modes in the solid with the liquid, enhancing the heat-transfer rate and enabling efficient energy transfer up to the cutoff frequency of the solid. Our results provide fundamental understanding of the energy-transfer mechanisms in liquid-solid systems and enable detailed investigations of energy transfer between, e.g., water and organic molecules.Peer reviewe
Vibrational mean free paths and thermal conductivity of amorphous silicon from non-equilibrium molecular dynamics simulations
International audienc
Vibrational mean free paths and thermal conductivity of amorphous silicon from non-equilibrium molecular dynamics simulations
The frequency-dependent mean free paths (MFPs) of vibrational heat carriers in amorphous silicon are predicted from the length dependence of the spectrally decomposed heat current (SDHC) obtained from non-equilibrium molecular dynamics simulations. The results suggest a (frequency)−2 scaling of the room-temperature MFPs below 5 THz. The MFPs exhibit a local maximum at a frequency of 8 THz and fall below 1 nm at frequencies greater than 10 THz, indicating localized vibrations. The MFPs extracted from sub-10 nm system-size simulations are used to predict the length-dependence of thermal conductivity up to system sizes of 100 nm and good agreement is found with independent molecular dynamics simulations. Weighting the SDHC by the frequency-dependent quantum occupation function provides a simple and convenient method to account for quantum statistics and provides reasonable agreement with the experimentally-measured trend and magnitude.Peer reviewe