2,142 research outputs found
Multimode Phonon Cooling via Three Wave Parametric Interactions with Optical Fields
We discuss the possible cooling of different phonon modes via three wave
mixing interactions of vibrational and optical modes. Since phonon modes
exhibit a variety of dispersion relations or frequency spectra with diverse
spatial structures, depending on the shape and size of the sample, we formulate
our theory in terms of relevant spatial mode functions for the interacting
fields in any given geometry. We discuss the possibility of Dicke like
collective effects in phonon cooling and present explicit results for
simultaneous cooling of two phonon modes via the anti-Stokes up conversions. We
show that the bimodal cooling should be observable experimentally
Magnetic and Transport Properties of Fe-Ag granular multilayers
Results of magnetization, magnetotransport and Mossbauer spectroscopy
measurements of sequentially evaporated Fe-Ag granular composites are
presented. The strong magnetic scattering of the conduction electrons is
reflected in the sublinear temperature dependence of the resistance and in the
large negative magnetoresistance. The simultaneous analysis of the magnetic
properties and the transport behavior suggests a bimodal grain size
distribution. A detailed quantitative description of the unusual features
observed in the transport properties is given
Excitons and high-order optical transitions in individual carbon nanotubes
We examine the excitonic nature of high-lying optical transitions in
single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A
careful analysis of the principal transitions of individual semiconducting and
metallic nanotubes reveals that in both cases the lineshape is consistent with
an excitonic model, but not one of free-carriers. For semiconducting species,
side-bands are observed at ~200 meV above the third and fourth optical
transitions. These features are ascribed to exciton-phonon bound states. Such
side-bands are not apparent for metallic nanotubes,as expected from the reduced
strength of excitonic interactions in these systems
Bimodal grain-size scaling of thermal transport in polycrystalline graphene from large-scale molecular dynamics simulations
Grain boundaries in graphene are inherent in wafer-scale samples prepared by
chemical vapor deposition. They can strongly influence the mechanical
properties and electronic and heat transport in graphene. In this work, we
employ extensive molecular dynamics simulations to study thermal transport in
large suspended polycrystalline graphene samples. Samples of different
controlled grain sizes are prepared by a recently developed efficient
multiscale approach based on the phase field crystal model. In contrast to
previous works, our results show that the scaling of the thermal conductivity
with the grain size implies bimodal behaviour with two effective Kapitza
lengths. The scaling is dominated by the out-of-plane (flexural) phonons with a
Kapitza length that is an order of magnitude larger than that of the in-plane
phonons. We also show that in order to get quantitative agreement with the most
recent experiments, quantum corrections need to be applied to both the Kapitza
conductance of grain boundaries and the thermal conductivity of pristine
graphene and the corresponding Kapitza lengths must be renormalized
accordingly.Comment: Accepted to Nano Lett.; Numerical samples and computer codes
availabl
Bimodal grain-size scaling of thermal transport in polycrystalline graphene from large-scale molecular dynamics simulations
Grain boundaries in graphene are inherent in wafer-scale samples prepared by
chemical vapor deposition. They can strongly influence the mechanical
properties and electronic and heat transport in graphene. In this work, we
employ extensive molecular dynamics simulations to study thermal transport in
large suspended polycrystalline graphene samples. Samples of different
controlled grain sizes are prepared by a recently developed efficient
multiscale approach based on the phase field crystal model. In contrast to
previous works, our results show that the scaling of the thermal conductivity
with the grain size implies bimodal behaviour with two effective Kapitza
lengths. The scaling is dominated by the out-of-plane (flexural) phonons with a
Kapitza length that is an order of magnitude larger than that of the in-plane
phonons. We also show that in order to get quantitative agreement with the most
recent experiments, quantum corrections need to be applied to both the Kapitza
conductance of grain boundaries and the thermal conductivity of pristine
graphene and the corresponding Kapitza lengths must be renormalized
accordingly.Comment: Accepted to Nano Lett.; Numerical samples and computer codes
availabl
Polariton Bose-Einstein condensate at room temperature in a Al(Ga)N nanowire-dielectric microcavity with a spatial potential trap
A spatial potential trap is formed in a 6.0 {\mu}m Al(Ga)N nanowire by
varying the Al composition along its length during epitaxial growth. The
polariton emission characteristics of a dielectric microcavity with the single
nanowire embedded in-plane has been studied at room temperature. Excitation is
provided at the Al(Ga)N end of the nanowire and polariton emission is observed
from the lowest bandgap GaN region of the nanowire. Comparison of the results
with those measured in an identical microcavity with an uniform GaN nanowire
and having an identical exciton-photon detuning suggests evaporative cooling of
the polaritons as they are transported across the trap in the Al(Ga)N nanowire.
Measurement of the spectral characteristics of the polariton emission, their
momentum distribution, first-order spatial coherence and time-resolved
measurements of polariton cooling provide strong evidence of the formation of
an equilibrium Bose-Einstein condensate, a unique state of matter in solid
state systems, in the GaN region of the nanowire, at room temperature. An
equilibrium condensate is not formed in the GaN nanowire dielectric microcavity
without the spatial potential trap.Comment: 28 pages, 6 figures, Submitted to the Proceedings of the National
Academy of Sciences of the United States of Americ
Thermal Transport Across Graphene Step Junctions
Step junctions are often present in layered materials, i.e. where
single-layer regions meet multi-layer regions, yet their effect on thermal
transport is not understood to date. Here, we measure heat flow across graphene
junctions (GJs) from monolayer to bilayer graphene, as well as bilayer to
four-layer graphene for the first time, in both heat flow directions. The
thermal conductance of the monolayer-bilayer GJ device ranges from ~0.5 to
9.1x10^8 Wm-2K-1 between 50 K to 300 K. Atomistic simulations of such GJ device
reveal that graphene layers are relatively decoupled, and the low thermal
conductance of the device is determined by the resistance between the two
dis-tinct graphene layers. In these conditions the junction plays a negligible
effect. To prove that the decoupling between layers controls thermal transport
in the junction, the heat flow in both directions was measured, showing no
evidence of thermal asymmetry or rectification (within experimental error
bars). For large-area graphene applications, this signifies that small bilayer
(or multilayer) islands have little or no contribution to overall thermal
transport
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