2,051 research outputs found
First principle study of the thermal conductance in graphene nanoribbon with vacancy and substitutional silicon defect
The thermal conductance in graphene nanoribbon with a vacancy or silicon
point defect (substitution of C by Si atom) is investigated by non-equilibrium
Green's function (NEGF) formalism combined with first-principle calculations
density-functional theory with local density approximation. An efficient
correction to the force constant matrix is presented to solve the conflict
between the long-range character of the {\it ab initio} approach and the
first-nearest-neighboring character of the NEGF scheme. In nanoribbon with a
vacancy defect, the thermal conductance is very sensitive to the position of
the vacancy defect. A vacancy defect situated at the center of the nanoribbon
generates a saddle-like surface, which greatly reduces the thermal conductance
by strong scattering to all phonon modes; while an edge vacancy defect only
results in a further reconstruction of the edge and slightly reduces the
thermal conductance. For the Si defect, the position of the defect plays no
role for the value of the thermal conductance, since the defective region is
limited within a narrow area around the defect center.Comment: accepted by AP
Phonon Modes in Single-Walled Molybdenum Disulphide (MoS2) Nanotubes: Lattice Dynamics Calculation and Molecular Dynamics Simulation
We study the phonon modes in single-walled MoS nanotubes via the
lattice dynamics calculation and molecular dynamics simulation. The phonon
spectra for tubes of arbitrary chiralities are calculated from the dynamical
matrix constructed by the combination of an empirical potential with the
conserved helical quantum numbers . In particular, we show that
the frequency () of the radial breathing mode is inversely proportional
to the tube diameter () as {cm}. The eigen vectors
of the first twenty lowest-frequency phonon modes are illustrated. Based on
these eigen vectors, we demonstrate that the radial breathing oscillation is
disturbed by phonon modes of three-fold symmetry initially, and the tube is
squashed by the modes of two-fold symmetry eventually. Our study provides
fundamental knowledge for further investigations of the thermal and mechanical
properties of the MoS nanotubes.Comment: Nanotechnology, publishe
Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects
We perform molecular dynamics simulations to investigate the reduction of the
thermal conductivity by kinks in silicon nanowires. The reduction percentage
can be as high as 70% at room temperature. The temperature dependence of the
reduction is also calculated. By calculating phonon polarization vectors, two
mechanisms are found to be responsible for the reduced thermal conductivity:
(1) the interchanging effect between the longitudinal and transverse phonon
modes and (2) the pinching effect, i.e a new type of localization, for the
twisting and transverse phonon modes in the kinked silicon nanowires. Our work
demonstrates that the phonon interchanging and pinching effects, induced by
kinking, are brand new and effective ways in modulating heat transfer in
nanowires, which enables the kinked silicon nanowires to be a promising
candidate for thermoelectric materials.Comment: Nano. Lett. accepted (2013
Correlation effects for semiconducting single wall carbon nanotube: a density matrix renormalization group study
In this paper, we report the applicability of the density matrix
renormalization group(DMRG) approach to the cylindrical single wall carbon
nanotube (SWCN) for purpose of its correlation effect. By applying the DMRG
approach to the ++ model, with and being the hopping and
Coulomb energies between the nearest neighboring sites, respectively, and
the onsite Coulomb energy, we calculate the phase diagram for the SWCN with
chiral numbers (), which reflects the competition between the
correlation energy and . Within reasonable parameter ranges, we
investigate possible correlated groundstates, the lowest excitations and the
corresponding correlation functions in which the connection with the excitonic
insulator is particularly addressed.Comment: 1 source files, 5 figure
Perturbative corrections to form factors in QCD
We compute perturbative QCD corrections to form factors at leading
power in , at large hadronic recoil, from the light-cone sum rules
(LCSR) with -meson distribution amplitudes in HQET. QCD factorization for
the vacuum-to--meson correlation function with an interpolating current for
the -meson is demonstrated explicitly at one loop with the power counting
scheme . The jet
functions encoding information of the hard-collinear dynamics in the
above-mentioned correlation function are complicated by the appearance of an
additional hard-collinear scale , compared to the counterparts entering
the factorization formula of the vacuum-to--meson correction function for
the construction of from factors. Inspecting the
next-to-leading-logarithmic sum rules for the form factors of indicates that perturbative corrections to the hard-collinear functions
are more profound than that for the hard functions, with the default theory
inputs, in the physical kinematic region. We further compute the subleading
power correction induced by the three-particle quark-gluon distribution
amplitudes of the -meson at tree level employing the background gluon field
approach. The LCSR predictions for the semileptonic form
factors are then extrapolated to the entire kinematic region with the
-series parametrization. Phenomenological implications of our determinations
for the form factors are explored by investigating the
(differential) branching fractions and the ratio of
and by determining the CKM matrix element from the total decay rate
of .Comment: 49 pages, 8 figures, version accepted for publication in JHE
QCD calculations of form factors with higher-twist corrections
We update QCD calculations of form factors at large hadronic
recoil by including the subleading-power corrections from the higher-twist
-meson light-cone distribution amplitudes (LCDAs) up to the twist-six
accuracy and the strange-quark mass effects at leading-power in
from the twist-two -meson LCDA . The higher-twist
corrections from both the two-particle and three-particle -meson LCDAs are
computed from the light-cone QCD sum rules (LCSR) at tree level. In particular,
we construct the local duality model for the twist-five and -six -meson
LCDAs, in agreement with the corresponding asymptotic behaviours at small quark
and gluon momenta, employing the QCD sum rules in heavy quark effective theory
at leading order in . The strange quark mass effects in semileptonic
form factors yield the leading-power contribution in the heavy quark
expansion, consistent with the power-counting analysis in soft-collinear
effective theory, and they are also computed from the LCSR approach due to the
appearance of the rapidity singularities. We further explore the
phenomenological aspects of the semileptonic decays and
the rare exclusive processes , including the determination of
the CKM matrix element , the normalized differential
distributions and precision observables defined by the ratios of branching
fractions for the above-mentioned two channels in the same intervals of .Comment: 36 pages, 9 figure
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