1,428 research outputs found
Thermal rectification in carbon nanotube intramolecular junctions: Molecular dynamics calculations
We study heat conduction in (n, 0)/(2n, 0) intramolecular junctions by using
molecular dynamics method. It is found that the heat conduction is asymmetric,
namely, heat transports preferably in one direction. This phenomenon is also
called thermal rectification. The rectification is weakly dependent on the
detailed structure of connection part, but is strongly dependent on the
temperature gradient. We also study the effect of the tube radius and
intramolecular junction length on the rectification. Our study shows that the
tensile stress can increase rectification. The physical mechanism of the
rectification is explained
Electrical conductivity measured in atomic carbon chains
The first electrical conductivity measurements of monoatomic carbon chains
are reported in this study. The chains were obtained by unraveling carbon atoms
from graphene ribbons while an electrical current flowed through the ribbon
and, successively, through the chain. The formation of the chains was
accompanied by a characteristic drop in the electrical conductivity. The
conductivity of carbon chains was much lower than previously predicted for
ideal chains. First-principles calculations using both density functional and
many-body perturbation theory show that strain in the chains determines the
conductivity in a decisive way. Indeed, carbon chains are always under varying
non-zero strain that transforms its atomic structure from cumulene to polyyne
configuration, thus inducing a tunable band gap. The modified electronic
structure and the characteristics of the contact to the graphitic periphery
explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure
Strain Modulated Superlattices in Graphene
Strain engineering of graphene takes advantage of one of the most dramatic
responses of Dirac electrons enabling their manipulation via strain-induced
pseudo-magnetic fields. Numerous theoretically proposed devices, such as
resonant cavities and valley filters, as well as novel phenomena, such as snake
states, could potentially be enabled via this effect. These proposals, however,
require strong, spatially oscillating magnetic fields while to date only the
generation and effects of pseudo-gauge fields which vary at a length scale much
larger than the magnetic length have been reported. Here we create a periodic
pseudo-gauge field profile using periodic strain that varies at the length
scale comparable to the magnetic length and study its effects on Dirac
electrons. A periodic strain profile is achieved by pulling on graphene with
extreme (>10%) strain and forming nanoscale ripples, akin to a plastic wrap
pulled taut at its edges. Combining scanning tunneling microscopy and atomistic
calculations, we find that spatially oscillating strain results in a new
quantization different from the familiar Landau quantization observed in
previous studies. We also find that graphene ripples are characterized by large
variations in carbon-carbon bond length, directly impacting the electronic
coupling between atoms, which within a single ripple can be as different as in
two different materials. The result is a single graphene sheet that effectively
acts as an electronic superlattice. Our results thus also establish a novel
approach to synthesize an effective 2D lateral heterostructure - by periodic
modulation of lattice strain.Comment: 18 pages, 5 figures and supplementary informatio
Issues for the Next Generation of Galaxy Surveys
I argue that the weight of the available evidence favours the conclusions
that galaxies are unbiased tracers of mass, the mean mass density (excluding a
cosmological constant or its equivalent) is less than the critical Einstein-de
Sitter value, and an isocurvature model for structure formation offers a viable
and arguably attractive model for the early assembly of galaxies. If valid
these conclusions complicate our work of adding structure formation to the
standard model for cosmology, but it seems sensible to pay attention to
evidence.Comment: 14 pages, 3 postscript figures, uses rspublic.st
Test of a Jastrow-type wavefunction for a trapped few-body system in one dimension
For a system with interacting quantum mechanical particles in a
one-dimensional harmonic oscillator, a trial wavefunction with simple structure
based on the solution of the corresponding two-particle system is suggested and
tested numerically. With the inclusion of a scaling parameter for the distance
between particles, at least for the very small systems tested here the ansatz
gives a very good estimate of the ground state energy, with the error being of
the order of ~1% of the gap to the first excited state
Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes
The gap oscillations caused by a magnetic flux penetrating a carbon nanotube
represent one of the most spectacular observation of the Aharonov-Bohm effect
at the nano--scale. Our understanding of this effect is, however, based on the
assumption that the electrons are strictly confined on the tube surface, on
trajectories that are not modified by curvature effects. Using an ab-initio
approach based on Density Functional Theory we show that this assumption fails
at the nano-scale inducing important corrections to the physics of the
Aharonov-Bohm effect. Curvature effects and electronic density spilled out of
the nanotube surface are shown to break the periodicity of the gap
oscillations. We predict the key phenomenological features of this anomalous
Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of
a large metallic phase in the low flux regime of Multi-walled nanotubes, also
suggesting possible experiments to validate our results.Comment: 7 figure
Ten-year change in sedentary behaviour, moderate-to-vigorous physical activity, cardiorespiratory fitness and cardiometabolic risk: independent associations and mediation analysis.
BACKGROUND: We aimed to study the independent associations of 10-year change in sedentary behaviour (SB), moderate-to-vigorous physical activity (MVPA) and objectively measured cardiorespiratory fitness (CRF), with concurrent change in clustered cardiometabolic risk and its individual components (waist circumference, fasting glucose, high-density lipoprotein (HDL) cholesterol, triglycerides and blood pressure). We also determined whether associations were mediated by change in CRF (for SB and MVPA), waist circumference (for SB, MVPA and CRF) and dietary intake (for SB). METHODS: A population-based sample of 425 adults (age (mean±SD) 55.83±9.40; 65% men) was followed prospectively for 9.62±0.52 years. Participants self-reported SB and MVPA and performed a maximal cycle ergometer test to estimate peak oxygen uptake at baseline (2002-2004) and follow-up (2012-2014). Multiple linear regression and the product of coefficients method were used to examine independent associations and mediation effects, respectively. RESULTS: Greater increase in SB was associated with more detrimental change in clustered cardiometabolic risk, waist circumference, HDL cholesterol and triglycerides, independently of change in MVPA. Greater decrease in MVPA was associated with greater decrease in HDL cholesterol and increase in clustered cardiometabolic risk, waist circumference and fasting glucose, independent of change in SB. Greater decrease in CRF was associated with more detrimental change in clustered cardiometabolic risk and all individual components. Change in CRF mediated the associations of change in SB and MVPA with change in clustered cardiometabolic risk, waist circumference and, only for MVPA, HDL cholesterol. Change in waist circumference mediated the associations between change in CRF and change in clustered cardiometabolic risk, fasting glucose, HDL cholesterol and triglycerides. CONCLUSIONS: A combination of decreasing SB and increasing MVPA, resulting in positive change in CRF, is likely to be most beneficial towards cardiometabolic health.This work was supported by a British Heart Foundation Intermediate Basic Science Research Fellowship to KW (grant number FS/12/58/29709), the UK Medical Research Council (grant number MC_UU_12015/3) to KW, the Research Foundation Flanders (grant number G.0194.11N) and the Flemish Policy Research Centre Sport
Silicon and III-V compound nanotubes: structural and electronic properties
Unusual physical properties of single-wall carbon nanotubes have started a
search for similar tubular structures of other elements. In this paper, we
present a theoretical analysis of single-wall nanotubes of silicon and group
III-V compounds. Starting from precursor graphene-like structures we
investigated the stability, energetics and electronic structure of zigzag and
armchair tubes using first-principles pseudopotential plane wave method and
finite temperature ab-initio molecular dynamics calculations. We showed that
(n,0) zigzag and (n,n) armchair nanotubes of silicon having n > 6 are stable
but those with n < 6 can be stabilized by internal or external adsorption of
transition metal elements. Some of these tubes have magnetic ground state
leading to spintronic properties. We also examined the stability of nanotubes
under radial and axial deformation. Owing to the weakness of radial restoring
force, stable Si nanotubes are radially soft. Undeformed zigzag nanotubes are
found to be metallic for 6 < n < 11 due to curvature effect; but a gap starts
to open for n > 12. Furthermore, we identified stable tubular structures formed
by stacking of Si polygons. We found AlP, GaAs, and GaN (8,0) single-wall
nanotubes stable and semiconducting. Our results are compared with those of
single-wall carbon nanotubes.Comment: 11 pages, 10 figure
Identification of Electron Donor States in N-doped Carbon Nanotubes
Nitrogen doped carbon nanotubes have been synthesized using pyrolysis and
characterized by Scanning Tunneling Spectroscopy and transmission electron
microscopy. The doped nanotubes are all metallic and exhibit strong electron
donor states near the Fermi level. Using tight-binding and ab initio
calculations, we observe that pyridine-like N structures are responsible for
the metallic behavior and the prominent features near the Fermi level. These
electron rich structures are the first example of n-type nanotubes, which could
pave the way to real molecular hetero-junction devices.Comment: 5 pages, 4 figures, revtex, submitted to PR
- …