54,943 research outputs found
Conducting a thermal conductivity survey
A physically transparent approximate theory of phonon decay rates is presented starting from a pair potential model of the interatomic forces in an insulator or semiconductor. The theory applies in the classical regime and relates the 3-phonon decay rate to the third derivative of the pair potential. Phonon dispersion relations do not need to be calculated, as sum rules relate all the needed quantities directly to the pair potential. The Brillouin zone averaged phonon lifetime turns out to involve a dimensionless measure of the anharmonicity multiplied by an effective density of states for 3-phonon decay. Results are given for rare gas and alkali halide crystals. For rare gases, the results are in good agreement with more elaborate perturbation calculations. Comparison to experimental data on phonon linewidths and thermal conductivity are made
Cosmic string loops and large-scale structure
We investigate the contribution made by small loops from a cosmic string
network as seeds for large-scale structure formation. We show that cosmic
string loops are highly correlated with the long-string network on large scales
and therefore contribute significantly to the power spectrum of density
perturbations if the average loop lifetime is comparable to or above one Hubble
time. This effect further improves the large-scale bias problem previously
identified in earlier studies of cosmic string models.Comment: 5 pages, 5 figure
Simultaneous calculation of the helical pitch and the twist elastic constant in chiral liquid crystals from intermolecular torques
We present a molecular simulation method that yields simultaneously the equilibrium pitch wave number q and the twist elastic constant K2 of a chiral nematic liquid crystal by sampling the torque density. A simulation of an untwisted system in periodic boundary conditions gives the product K2q; a further simulation with a uniform twist applied provides enough information to separately determine the two factors. We test our new method for a model potential, comparing the results with K2q from a thermodynamic integration route, and with K2 from an order fluctuation analysis. We also present a thermodynamic perturbation theory analysis valid in the limit of weak chirality
Non-adiabatic Kohn-anomaly in a doped graphene monolayer
We compute, from first-principles, the frequency of the E2g, Gamma phonon
(Raman G-band) of graphene, as a function of the charge doping. Calculations
are done using i) the adiabatic Born-Oppenheimer approximation and ii)
time-dependent perturbation theory to explore dynamic effects beyond this
approximation. The two approaches provide very different results. While, the
adiabatic phonon frequency weakly depends on the doping, the dynamic one
rapidly varies because of a Kohn anomaly. The adiabatic approximation is
considered valid in most materials. Here, we show that doped graphene is a
spectacular example where this approximation miserably fails.Comment: 5 pages, 3 figures, Accepted by Phys. Rev. Let
Numerical study of resistivity of model disordered three-dimensional metals
We calculate the zero-temperature resistivity of model 3-dimensional
disordered metals described by tight-binding Hamiltonians. Two different
mechanisms of disorder are considered: diagonal and off-diagonal. The
non-equilibrium Green function formalism provides a Landauer-type formula for
the conductance of arbitrary mesoscopic systems. We use this formula to
calculate the resistance of finite-size disordered samples of different
lengths. The resistance averaged over disorder configurations is linear in
sample length and resistivity is found from the coefficient of proportionality.
Two structures are considered: (1) a simple cubic lattice with one s-orbital
per site, (2) a simple cubic lattice with two d-orbitals. For small values of
the disorder strength, our results agree with those obtained from the Boltzmann
equation. Large off-diagonal disorder causes the resistivity to saturate,
whereas increasing diagonal disorder causes the resistivity to increase faster
than the Boltzmann result. The crossover toward localization starts when the
Boltzmann mean free path relative to the lattice constant has a value between
0.5 and 2.0 and is strongly model dependent.Comment: 4 pages, 5 figure
Self-referential Monte Carlo method for calculating the free energy of crystalline solids
A self-referential Monte Carlo method is described for calculating the free energy of crystalline solids. All Monte Carlo methods for the free energy of classical crystalline solids calculate the free-energy difference between a state whose free energy can be calculated relatively easily and the state of interest. Previously published methods employ either a simple model crystal, such as the Einstein crystal, or a fluid as the reference state. The self-referential method employs a radically different reference state; it is the crystalline solid of interest but with a different number of unit cells. So it calculates the free-energy difference between two crystals, differing only in their size. The aim of this work is to demonstrate this approach by application to some simple systems, namely, the face centered cubic hard sphere and Lennard-Jones crystals. However, it can potentially be applied to arbitrary crystals in both bulk and confined environments, and ultimately it could also be very efficient
Small-q electron-phonon scattering and linear dc resistivity in high-T_c oxides
We examine the effect on the DC resistivity of small-q electron-phonon
scattering, in a system with the electronic topology of the high-T_c oxides.
Despite the fact that the scattering is dominantly forward, its contribution to
the transport can be significant due to ``ondulations'' of the bands in the
flat region and to the umpklapp process. When the extended van-Hove
singularities are sufficiently close to the acoustic branch of the
phonons contribute significantly to the transport. In that case one can obtain
linear dependent resistivity down to temperatures as low as 10 K, even if
electrons are scattered also by optical phonons of about 500 K as reported by
Raman measurements.Comment: LATEX file and 4 Postscript figure
Chi-square test on candidate events from CW signal coherent searches
In a blind search for continuous gravitational wave signals scanning a wide
frequency band one looks for candidate events with significantly large values
of the detection statistic. Unfortunately, a noise line in the data may also
produce a moderately large detection statistic.
In this paper, we describe how we can distinguish between noise line events
and actual continuous wave (CW) signals, based on the shape of the detection
statistic as a function of the signal's frequency. We will analyze the case of
a particular detection statistic, the F statistic, proposed by Jaranowski,
Krolak, and Schutz.
We will show that for a broad-band 10 hour search, with a false dismissal
rate smaller than 1e-6, our method rejects about 70 % of the large candidate
events found in a typical data set from the second science run of the Hanford
LIGO interferometer.Comment: proceedings of GWDAW8, 2003 conference, 12pages, 6 figure
The relationship between induced fluid structure and boundary slip in nanoscale polymer films
The molecular mechanism of slip at the interface between polymer melts and
weakly attractive smooth surfaces is investigated using molecular dynamics
simulations. In agreement with our previous studies on slip flow of
shear-thinning fluids, it is shown that the slip length passes through a local
minimum at low shear rates and then increases rapidly at higher shear rates. We
found that at sufficiently high shear rates, the slip flow over atomically flat
crystalline surfaces is anisotropic. It is demonstrated numerically that the
friction coefficient at the liquid-solid interface (the ratio of viscosity and
slip length) undergoes a transition from a constant value to the power-law
decay as a function of the slip velocity. The characteristic velocity of the
transition correlates well with the diffusion velocity of fluid monomers in the
first fluid layer near the solid wall at equilibrium. We also show that in the
linear regime, the friction coefficient is well described by a function of a
single variable, which is a product of the magnitude of surface-induced peak in
the structure factor and the contact density of the adjacent fluid layer. The
universal relationship between the friction coefficient and induced fluid
structure holds for a number of material parameters of the interface: fluid
density, chain length, wall-fluid interaction energy, wall density, lattice
type and orientation, thermal or solid walls.Comment: 33 pages, 14 figure
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