518 research outputs found
Thermal conductivity and diffusion-mediated localization in Fe_{1-x}Cr_{x} Alloys
We apply a new Kubo-Greenwood type formula combined with a generalized
Feynman diagram- matic technique to report a first principles calculation of
the thermal transport properties of disordered Fe_{1-x}Cr_{x} alloys. The
diagrammatic approach simplifies the inclusion of disorder-induced scattering
effects on the two particle correlation functions and hence renormalizes the
heat current operator to calculate configuration averaged lattice thermal
conductivity and diffusivity. The thermal conductivity K(T) in the present case
shows an approximate quadratic T-dependence in the low temperature regime (T <
20 K), which subsequently rises smoothly to a T-independent saturated value at
high T . A numerical estimate of mobility edge from the thermal diffusivity
data yields the fraction of localized states. It is concluded that the complex
disorder scattering processes, in force-constant dominated disorder alloys such
as Fe-Cr, tend to localize the vibrational modes quite significantly.Comment: 5 pages, 5 figure
Tracer diffusion inside fibrinogen layers
We investigate the obstructed motion of tracer (test) particles in crowded
environments by carrying simulations of two-dimensional Gaussian random walk in
model fibrinogen monolayers of different orientational ordering. The fibrinogen
molecules are significantly anisotropic and therefore they can form structures
where orientational ordering, similar to the one observed in nematic liquid
crystals, appears. The work focuses on the dependence between level of the
orientational order (degree of environmental crowding) of fibrinogen molecules
inside a layer and non-Fickian character of the diffusion process of spherical
tracer particles moving within the domain. It is shown that in general
particles motion is subdiffusive and strongly anisotropic, and its
characteristic features significantly change with the orientational order
parameter, concentration of fibrinogens and radius of a diffusing probe.Comment: 8 pages, 12 figure
Generation and detection of very high frequency acoustic waves in solids Final report
Techniques for generation and detection of very high frequency acoustic waves in solid
Thermoelectric properties of Co, Ir, and Os-Doped FeSi Alloys: Evidence for Strong Electron-Phonon Coupling
The effects of various transition metal dopants on the electrical and thermal
transport properties of Fe1-xMxSi alloys (M= Co, Ir, Os) are reported. The
maximum thermoelectric figure of merit ZTmax is improved from 0.007 at 60 K for
pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal
conductivity data among Os, Ir and Co doped alloys indicates strong
electron-phonon coupling in this compound. Because of this interaction, the
common approximation of dividing the total thermal conductivity into
independent electronic and lattice components ({\kappa}Total =
{\kappa}electronic + {\kappa}lattice) fails for these alloys. The effects of
grain size on thermoelectric properties of Fe0.96Ir0.04Si alloys are also
reported. The thermal conductivity can be lowered by about 50% with little or
no effect on the electrical resistivity or Seebeck coefficient. This results in
ZTmax = 0.125 at 100 K, still about a factor of five too low for solid-state
refrigeration applications
Effects of nano-void density, size, and spatial population on thermal conductivity: a case study of GaN crystal
The thermal conductivity of a crystal is sensitive to the presence of
surfaces and nanoscale defects. While this opens tremendous opportunities to
tailor thermal conductivity, a true "phonon engineering" of nanocrystals for a
specific electronic or thermoelectric application can only be achieved when the
dependence of thermal conductivity on the defect density, size, and spatial
population is understood and quantified. Unfortunately, experimental studies of
effects of nanoscale defects are quite challenging. While molecular dynamics
simulations are effective in calculating thermal conductivity, the defect
density range that can be explored with feasible computing resources is
unrealistically high. As a result, previous work has not generated a fully
detailed understanding of the dependence of thermal conductivity on nanoscale
defects. Using GaN as an example, we have combined physically-motivated
analytical model and highly-converged large scale molecular dynamics
simulations to study effects of defects on thermal conductivity. An analytical
expression for thermal conductivity as a function of void density, size, and
population has been derived and corroborated with the model, simulations, and
experiments
Towards the grain boundary phonon scattering problem: an evidence for a low-temperature crossover
The problem of phonon scattering by grain boundaries is studied within the
wedge disclination dipole (WDD) model. It is shown that a specific q-dependence
of the phonon mean free path for biaxial WDD results in a low-temperature
crossover of the thermal conductivity, . The obtained results allow to
explain the experimentally observed deviation of from a
dependence below in and .Comment: 4 pages, 2 figures, submitted to J.Phys.:Condens.Matte
Heat transport in silicon from first principles calculations
Using harmonic and anharmonic force constants extracted from
density-functional calculations within a supercell, we have developed a
relatively simple but general method to compute thermodynamic and thermal
properties of any crystal. First, from the harmonic, cubic, and quartic force
constants we construct a force field for molecular dynamics (MD). It is exact
in the limit of small atomic displacements and thus does not suffer from
inaccuracies inherent in semi-empirical potentials such as Stillinger-Weber's.
By using the Green-Kubo (GK) formula and molecular dynamics simulations, we
extract the bulk thermal conductivity. This method is accurate at high
temperatures where three-phonon processes need to be included to higher orders,
but may suffer from size scaling issues. Next, we use perturbation theory
(Fermi Golden rule) to extract the phonon lifetimes and compute the thermal
conductivity from the relaxation time approximation. This method is
valid at most temperatures, but will overestimate at very high
temperatures, where higher order processes neglected in our calculations, also
contribute. As a test, these methods are applied to bulk crystalline silicon,
and the results are compared and differences discussed in more detail. The
presented methodology paves the way for a systematic approach to model heat
transport in solids using multiscale modeling, in which the relaxation time due
to anharmonic 3-phonon processes is calculated quantitatively, in addition to
the usual harmonic properties such as phonon frequencies and group velocities.
It also allows the construction of accurate bulk interatomic potentials
database.Comment: appear in PRB (2011
Enhancement of the Thermal Conductivity in gapped Quantum Spin Chains
We study mechanism of magnetic energy transport, motivated by recent
measurements of the thermal conductivity in low dimensional quantum magnets. We
point out a possible mechanism of enhancement of the thermal conductivity in
gapped magnetic system, where the magnetic energy transport plays a crucial
role. This mechanism gives an interpretation for the recent experiment of
CuGeO_3, where the thermal conductivity depends on the crystal direction.Comment: 4 pages, 2 figure
First-principles analysis of lattice thermal conductivity in monolayer and bilayer graphene
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