3,141 research outputs found
Why edge effects are important on the intrinsic loss mechanisms of graphene nanoresonators?
Molecular dynamics simulations are performed to investigate edge effects on
the quality factor of graphene nanoresonators with different edge
configurations and of various sizes. If the periodic boundary condition is
applied, very high quality factors () are obtained for all kinds
of graphene nanoresonators. However, if the free boundary condition is applied,
quality factors will be greatly reduced by two effects resulting from free
edges: the imaginary edge vibration effect and the artificial effect. Imaginary
edge vibrations will flip between a pair of doubly degenerate warping states
during the mechanical oscillation of nanoresonators. The flipping process
breaks the coherence of the mechanical oscillation of the nanoresonator, which
is the dominant mechanism for extremely low quality factors. There is an
artificial effect if the mechanical oscillation of the graphene nanoresonator
is actuated according to an artificial vibration (non-natural vibration of the
system), which slightly reduce the quality factor. The artificial effect can be
eliminated by actuating the mechanical oscillation according to a natural
vibration of the nanoresonator. Our simulations provide an explanation for the
recent experiment, where the measured quality factor is low and varies between
identical samples with free edges.Comment: accepted by J. Appl. Phy
A theoretical study of thermal conductivity in single-walled boron nitride nanotubes
We perform a theoretical investigation on the thermal conductivity of
single-walled boron nitride nanotubes (SWBNT) using the kinetic theory. By
fitting to the phonon spectrum of boron nitride sheet, we develop an efficient
and stable Tersoff-derived interatomic potential which is suitable for the
study of heat transport in sp2 structures. We work out the selection rules for
the three-phonon process with the help of the helical quantum numbers attributed to the symmetry group (line group) of the SWBNT. Our calculation
shows that the thermal conductivity diverges with length as
with exponentially decaying , which results from the competition between boundary scattering
and three-phonon scattering for flexure modes. We find that the two flexure
modes of the SWBNT make dominant contribution to the thermal conductivity,
because their zero frequency locates at where is
the rotational angle of the screw symmetry in SWBNT.Comment: accepted by PR
Elastic and non-linear stiffness of graphene: a simple approach
The recent experiment [Science \textbf{321}, 385 (2008)] on the Young's
modulus and third-order elastic stiffness of graphene are well explained in a
very simple approach, where the graphene is described by a simplified system
and the force constant for the non-linear interaction is estimated from the
Tersoff-Brenner potential.Comment: 4 pages, 4 figure
Energy-Delay Tradeoffs of Virtual Base Stations With a Computational-Resource-Aware Energy Consumption Model
The next generation (5G) cellular network faces the challenges of efficiency,
flexibility, and sustainability to support data traffic in the mobile Internet
era. To tackle these challenges, cloud-based cellular architectures have been
proposed where virtual base stations (VBSs) play a key role. VBSs bring further
energy savings but also demands a new energy consumption model as well as the
optimization of computational resources. This paper studies the energy-delay
tradeoffs of VBSs with delay tolerant traffic. We propose a
computational-resource-aware energy consumption model to capture the total
energy consumption of a VBS and reflect the dynamic allocation of computational
resources including the number of CPU cores and the CPU speed. Based on the
model, we analyze the energy-delay tradeoffs of a VBS considering BS sleeping
and state switching cost to minimize the weighted sum of power consumption and
average delay. We derive the explicit form of the optimal data transmission
rate and find the condition under which the energy optimal rate exists and is
unique. Opportunities to reduce the average delay and achieve energy savings
simultaneously are observed. We further propose an efficient algorithm to
jointly optimize the data rate and the number of CPU cores. Numerical results
validate our theoretical analyses and under a typical simulation setting we
find more than 60% energy savings can be achieved by VBSs compared with
conventional base stations under the EARTH model, which demonstrates the great
potential of VBSs in 5G cellular systems.Comment: 5 pages, 3 figures, accepted by ICCS'1
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
Edge states induce boundary temperature jump in molecular dynamics simulation of heat conduction
We point out that the origin of the commonly occurred boundary temperature
jump in the application of No\'se-Hoover heat bath in molecular dynamics is
related to the edge modes, which are exponentially localized at the edge of the
system. If heat baths are applied to these edge regions, the injected thermal
energy will be localized thus leading to a boundary temperature jump. The jump
can be eliminated by shifting the location of heat baths away from edge
regions. Following this suggestion, a very good temperature profile is obtained
without increasing any simulation time, and the accuracy of thermal
conductivity calculated can be largely improved.Comment: accepted by PRB, brief report, references added, typo correcte
Isotopic effects on the thermal conductivity of graphene nanoribbons: localization mechanism
Thermal conductivity of graphene nanoribbons (GNR) with length 106~{\AA} and
width 4.92~{\AA} after isotopic doping is investigated by molecular dynamics
with quantum correction. Two interesting phenomena are found: (1) isotopic
doping reduces thermal conductivity effectively in low doping region, and the
reduction slows down in high doping region; (2) thermal conductivity increases
with increasing temperature in both pure and doped GNR; but the increasing
behavior is much more slowly in the doped GNR than that in pure ones. Further
studies reveal that the physics of these two phenomena is related to the
localized phonon modes, whose number increases quickly (slowly) with increasing
isotopic doping in low (high) isotopic doping region.Comment: 6 fig
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