1,336 research outputs found
Lattice thermal conductivity of freestanding gallium nitride nanowires
aip Publishers Publications Topics | Librarians Authors Your access is provided by: Eastern Illinois University Register to create your user account, or sign in if you have an existing account Additional sign in Sign in via Shibboleth/Athens My cart Export citations Add to my favorites Recommend to library Subscribe to email alerts Submit an article Reprints & Permissions Subscribe to RSS Access Key Free Content Open Access Content Subscribed Content Free Trial Content Home \u3e Publishers \u3e AIP Publishing \u3e Journal of Applied Physics \u3e Volume 108, Issue 3 \u3e Article banner image F Lattice thermal conductivity of freestanding gallium nitride nanowires Download PDF Jie Zou1,a) + View Affiliations a) Electronic mail: [email protected]. J. Appl. Phys. 108, 034324 (2010); http://dx.doi.org/10.1063/1.3463358 Previous Article Table of Contents Next Article Back to Search Results facebook twitter Share this page separator email print this page Abstract Full Text References (28) Cited By (8) Data & Media Metrics Related We report detailed calculations of the lattice thermal conductivity of freestanding gallium nitride(GaN)nanowires with diameters ranging from 20 to 140 nm. Results are compared with experimental data on GaNnanowires grown by thermal chemical vapor deposition(CVD). Calculations are based on the Boltzmann transport equation and take into account the change in the nonequilibrium phonon distribution in the case of diffuse scattering at the surfaces. Phonon dispersion relation is obtained in the elastic continuum approximation for each given nanowire. For valid comparisons with the experimental data, simulations are performed with a dopant concentration and impurity profile characteristic of thermal CVDGaNnanowires. Our results show that the room-temperature thermal conductivity of the nanowires has very low values, ranging from 6.74 W/m K at 20 nm to 16.4 W/m K at 140 nm. The obtained results are in excellent agreement with the experimental data. We have also demonstrated that in addition to impurity scattering, boundary scattering, and phonon confinement, the change in the nonequilibrium phonon distribution leads to a further reduction in the thermal conductivity of the nanowires and has to be taken into account in the calculations. Our conclusion is different from that of an earlier study which attributed the very low thermal conductivity to the unusually large mass-difference scattering in the nanowires
Alpine plants are on the move: Quantifying distribution shifts of Australian alpine plants through time
Aim
Alpine plant species’ distributions are thought to have been shifting to higher elevations in response to climate change. By moving upslope, species can occupy cooler and more suitable environments as climate change warms their current ranges. Despite evidence of upslope migration in the northern hemisphere, there is limited evidence for elevational shifts in southern hemisphere plants. Our study aimed to determine if alpine plants in Australia have migrated upslope in the last 2 to 6 decades.
Location
Kosciuszko National Park, NSW, Australia.
Methods
We collated historic occurrence data for 36 Australian alpine plant species from herbarium specimens and historic field observations and combined these historic data with modern occurrence data collected in the field.
Results
Eleven of the thirty-six species had shifted upslope in mean elevation and four species showed downslope elevational shifts. The rate of change for upslope shifts varied between 4 and 10 m per year and the rate of change for most downslope shifts was between 4 and 8 m per year, with one species shifting downslope at a high rate of 18 m per year. Additionally, some species showed shifts upward in their upper range edge and/or upward or downward shifts in their lower range edge. Five species also showed range contractions in the difference between their lower and upper range edges over time, while two showed range expansions. We found no significant differences in elevational shifts through time among herbaceous dicotyledons, herbaceous monocotyledons and shrubs.
Main Conclusions
Plant elevational shifts are occurring rapidly in the Australian alpine zone. This may allow species to persist under climate change. However, if current warming trends continue, several species within the Australian alpine zone will likely run out of suitable habitat within a century
Quantum chaos in nanoelectromechanical systems
We present a theoretical study of the electron-phonon coupling in suspended
nanoelectromechanical systems (NEMS) and investigate the resulting quantum
chaotic behavior. The phonons are associated with the vibrational modes of a
suspended rectangular dielectric plate, with free or clamped boundary
conditions, whereas the electrons are confined to a large quantum dot (QD) on
the plate's surface. The deformation potential and piezoelectric interactions
are considered. By performing standard energy-level statistics we demonstrate
that the spectral fluctuations exhibit the same distributions as those of the
Gaussian Orthogonal Ensemble (GOE) or the Gaussian Unitary Ensemble (GUE),
therefore evidencing the emergence of quantum chaos. That is verified for a
large range of material and geometry parameters. In particular, the GUE
statistics occurs only in the case of a circular QD. It represents an anomalous
phenomenon, previously reported for just a small number of systems, since the
problem is time-reversal invariant. The obtained results are explained through
a detailed analysis of the Hamiltonian matrix structure.Comment: 14 pages, two column
Anomalous quantum chaotic behavior in nanoelectromechanical structures
It is predicted that for sufficiently strong electron-phonon coupling an
anomalous quantum chaotic behavior develops in certain types of suspended
electro-mechanical nanostructures, here comprised by a thin cylindrical quantum
dot (billiard) on a suspended rectangular dielectric plate. The deformation
potential and piezoelectric interactions are considered. As a result of the
electron-phonon coupling between the two systems the spectral statistics of the
electro-mechanic eigenenergies exhibit an anomalous behavior. If the center of
the quantum dot is located at one of the symmetry axes of the rectangular
plate, the energy level distributions correspond to the Gaussian Orthogonal
Ensemble (GOE), otherwise they belong to the Gaussian Unitary Ensemble (GUE),
even though the system is time-reversal invariant.Comment: 4 pages, pdf forma
Thermo-mechanical behavior of surface acoustic waves in ordered arrays of nanodisks studied by near infrared pump-probe diffraction experiments
The ultrafast thermal and mechanical dynamics of a two-dimensional lattice of
metallic nano-disks has been studied by near infrared pump-probe diffraction
measurements, over a temporal range spanning from 100 fs to several
nanoseconds. The experiments demonstrate that, in these systems, a
two-dimensional surface acoustic wave (2DSAW), with a wavevector given by the
reciprocal periodicity of the array, can be excited by ~120 fs Ti:sapphire
laser pulses. In order to clarify the interaction between the nanodisks and the
substrate, numerical calculations of the elastic eigenmodes and simulations of
the thermodynamics of the system are developed through finite-element analysis.
At this light, we unambiguously show that the observed 2DSAW velocity shift
originates from the mechanical interaction between the 2DSAWs and the
nano-disks, while the correlated 2DSAW damping is due to the energy radiation
into the substrate.Comment: 13 pages, 10 figure
Excitation Theory for Space-Dispersive Active Media Waveguides
A unified electrodynamic approach to the guided-wave excitation theory is
generalized to the waveguiding structures containing a hypothetical
space-dispersive medium with drifting charge carriers possessing simultaneously
elastic, piezoelectric and magnetic properties. Substantial features of our
electrodynamic approach are: (i) the allowance for medium losses and (ii) the
separation of potential fields peculiar to the slow quasi-static waves. It is
shown that the orthogonal complementary fields appearing inside the external
source region are just associated with a contribution of the potential fields
inherent in exciting sources. Taking account of medium losses converts the
usual orthogonality relation into a novel form called the quasi-orthogonality
relation. It is found that the separation of potential fields reveals the fine
structure of interaction between the exciting sources and mode eigenfields: in
addition to the exciting currents interacting with the curl fields, the
exciting charges and the double charge (surface dipole) layers appear to
interact with the quasi-static potentials and the displacement currents,
respectively.Comment: LaTeX 2.09, 28 pages with mathematical appendi
Quantum statistical effects in nano-oscillator arrays
We have theoretically predicted the density of states(DOS), the low
temperature specific heat, and Brillouin scattering spectra of a large, free
standing array of coupled nano-oscillators. We have found significant gaps in
the DOS of 2D elastic systems, and predict the average DOS to be nearly
independent of frequency over a broad band f < 50GHz. At low temperatures, the
measurements probe the quantum statistics obeyed by rigid body modes of the
array and, thus, could be used to verify the quantization of the associated
energy levels. These states, in turn, involve center-of mass motion of large
numbers of atoms, N > 1.e14, and therefore such observations would extend the
domain in which quantum mechanics has been experimentally tested. We have found
the required measurement capability to carry out this investigation to be
within reach of current technology.Comment: 1 tex file, 3 figures, 1 bbl fil
The Arecibo Galaxy Environment Survey: II. A HI view of the Abell cluster 1367 and its outskirts
We present 21 cm HI line observations of 5x1 square degrees centered on the
local Abell cluster 1367 obtained as part of the Arecibo Galaxy Environment
Survey. One hundred sources are detected (79 new HI measurements and 50 new
redshifts), more than half belonging to the cluster core and its infalling
region. Combining the HI data with SDSS optical imaging we show that our HI
selected sample follows scaling relations similar to the ones usually observed
in optically selected samples. Interestingly all galaxies in our sample appear
to have nearly the same baryon fraction independently of their size, surface
brightness and luminosity. The most striking difference between HI and
optically selected samples resides in their large scale distribution: whereas
optical and X-ray observations trace the cluster core very well, in HI there is
almost no evidence of the presence of the cluster. Some implications on the
determination of the cluster luminosity function and HI distribution for
samples selected at different wavelength are also discussed.Comment: 22 pages, 15 figures, 4 tables. Accepted for publication on MNRAS
Main Journal. High resolution version of this paper can be downloaded at
http://www.astro.cf.ac.uk/pub/Luca.Cortese/papers/ages_a1367.pdf . Datacubes
and catalogs can be downloaded at http://www.naic.edu/~ages/public_data.htm
Role of confined phonons in thin film superconductivity
We calculate the critical temperature and the superconducting energy
gaps of a thin film superconductor system, where is the
superconducting energy gap of the -th subband. Since the quantization of
both the electron energy and phonon spectrum arises due to dimensional
confinement in one direction, the effective electron-electron interaction
mediated by the quantized confined phonons is different from that mediated by
the bulk phonon, leading to the modification of in the thin film system.
We investigate the dependence of and on the film thickness
with this modified interaction.Comment: 4 pages, 2 figure
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