6,289 research outputs found
Carbon Nanotube Thermal Transport: Ballistic to Diffusive
We propose to use l_0/(l_0+L) for the energy transmission covering both
ballistic and diffusive regimes, where l_0 is mean free path and L is system
length. This formula is applied to heat conduction in carbon nanotubes (CNTs).
Calculations of thermal conduction show: (1) Thermal conductance at room
temperature is proportional to the diameter of CNTs for single-walled CNTs
(SWCNTs) and to the square of diameter for multi-walled CNTs (MWCNTs). (2)
Interfaces play an important role in thermal conduction in CNTs due to the
symmetry of CNTs vibrational modes. (3) When the phonon mean free path is
comparable with the length L of CNTs in ballistic-diffusive regime, thermal
conductivity \kappa goes as L^{\alpha} . The effective exponent \alpha is
numerically found to decrease with increasing temperature and is insensitive to
the diameter of SWCNTs for Umklapp scattering process. For short SWCNTs (<0.1
\mu m) we find \alpha \approx 0.8 at room temperature. These results are
consistent with recent experimental findings.Comment: 4 pages, two figure
Dimensional crossover of thermal conductance in nanowires
Dimensional dependence of thermal conductance at low temperatures in
nanowires is studied using the nonequilibrium Green's function (NEGF) method.
Our calculation shows a smooth dimensional crossover of thermal conductance in
nanowire from one-dimensional to three-dimensional behavior with the increase
of diameters. The results are consistent with the experimental findings that
the temperature dependence of thermal conductance at low temperature for
diameters from tens to hundreds nanometers will be close to Debye law. The
calculation also suggests that universal thermal conductance is only observable
in nanowires with small diameters. We also find that the interfacial thermal
conductance across Si and Ge nanowire is much lower than the corresponding
value in bulk materials.Comment: 4 figure
A Worm Algorithm for Two-Dimensional Spin Glasses
A worm algorithm is proposed for the two-dimensional spin glasses. The method
is based on a low-temperature expansion of the partition function. The
low-temperature configurations of the spin glass on square lattice can be
viewed as strings connecting pairs of frustrated plaquettes. The worm algorithm
directly manipulates these strings. It is shown that the worm algorithm is as
efficient as any other types of cluster or replica-exchange algorithms. The
worm algorithm is even more efficient if free boundary conditions are used. We
obtain accurate low-temperature specific heat data consistent with a form c =
T^{-2} exp(-2J/(k_BT)), where T is temperature and J is coupling constant, for
the +/-J two-dimensional spin glass.Comment: 4 pages, 3 figure
A fast algorithm for random sequential adsorption of discs
An efficient algorithm for random sequential adsorption of hard discs in two
dimensions is implemented. A precise value for the coverage is obtained:
theta(infty) = 0.547069. The asymptotic law theta(t) = theta(infty) - ct^{-1/2}
is verified to a high accuracy. Pair correlation function is analyzed.Comment: 7 pages + 4 figures, Plain TeX 3.14
Transition Matrix Monte Carlo Method
We analyze a new Monte Carlo method which uses transition matrix in the space
of energy. This method gives an efficient reweighting technique. The associated
artificial dynamics is a constrained random walk in energy, producing the
result that correlation time is proportional to the specific heat.Comment: LaTeX, 8 pages, 1 figur
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