301 research outputs found
Boltzmann Transport in Nanostructures as a Friction Effect
Surface scattering is the key limiting factor to thermal transport in
dielectric crystals as the length scales are reduced or when temperature is
lowered. To explain this phenomenon, it is commonly assumed that the mean free
paths of heat carriers are bound by the crystal size and that thermal
conductivity is reduced in a manner proportional to such mean free paths. We
show here that these conclusions rely on simplifying assumptions and
approximated transport models. Instead, starting from the linearized Boltzmann
transport equation in the relaxon basis, we show how the problem can be reduced
to a set of decoupled linear differential equations. Then, the heat flow can be
interpreted as a hydrodynamic phenomenon, with the relaxon gas being slowed
down in proximity of a surface by friction effects, similar to the flux of a
viscous fluid in a pipe. As an example, we study a ribbon and a trench of
monolayer molybdenum disulphide, describing the procedure to reconstruct the
temperature and thermal conductivity profile in the sample interior and showing
how to estimate the effect of nanostructuring. The approach is general and
could be extended to other transport carriers, such as electrons, or extended
to materials of higher dimensionality and to different geometries, such as thin
films
Static dielectric properties of carbon nanotubes from first principles
We characterize the response of isolated single- (SWNT) and multi-wall (MWNT)
carbon nanotubes and bundles to static electric fields using first-principles
calculations and density-functional theory. The longitudinal polarizability of
SWNTs scales as the inverse square of the band gap, while in MWNTs and bundles
it is given by the sum of the polarizabilities of the constituent tubes. The
transverse polarizability of SWNTs is insensitive to band gaps and chiralities
and is proportional to the square of the effective radius; in MWNTs the outer
layers dominate the response. The transverse response is intermediate between
metallic and insulating, and a simple electrostatic model based on a
scale-invariance relation captures accurately the first-principles results.
Dielectric response of non-chiral SWNTs in both directions remains linear up to
very high values of applied field.Comment: Submitted to Phys. Rev. Lett. on 09/28/200
Berry Phase and Pseudospin Winding Number in Bilayer Graphene
Ever since the novel quantum Hall effect in bilayer graphene was discovered,
and explained by a Berry phase of 2pi [K. S. Novoselov et al., "Unconventional
quantum Hall effect and Berry's phase of 2pi in bilayer graphene", Nature Phys.
2, 177 (2006)], it has been widely accepted that the low-energy electronic
wavefunction in this system is described by a non-trivial Berry phase of 2pi,
different from the zero phase of a conventional two-dimensional electron gas.
Here, we show that (i) the relevant Berry phase for bilayer graphene is not
different from that for a conventional two-dimensional electron gas (as
expected, given that Berry phase is only meaningful modulo 2pi) and that (ii)
what is actually observed in the quantum Hall measurements is not the absolute
value of the Berry phase but the pseudospin winding number.Comment: 6 pages, 3 figures, published versio
Phonon anharmonicities in graphite and graphene
We determine from first-principles the finite-temperature
properties--linewidths, line shifts, and lifetimes--of the key vibrational
modes that dominate inelastic losses in graphitic materials. In graphite, the
phonon linewidth of the Raman-active E2g mode is found to decrease with
temperature; such anomalous behavior is driven entirely by electron-phonon
interactions, and does not appear in the nearly-degenerate infrared-active E1u
mode. In graphene, the phonon anharmonic lifetimes and decay channels of the
A'1 mode at K dominate over E2g at G and couple strongly with acoustic phonons,
highlighting how ballistic transport in carbon-based interconnects requires
careful engineering of phonon decays and thermalization.Comment: 5 pages, 4 figures; typos corrected and reference adde
Dynamics and Thermodynamics of a Novel Phase of NaAlH4
We characterize a novel orthorhombic phase (gamma) of NaAlH4, discovered
using first-principles molecular dynamics, and discuss its relevance to the
dehydrogenation mechanism. This phase is close in energy to the known
low-temperature structure and becomes the stabler phase above 320 K, thanks to
a larger vibrational entropy associated with AlH4 rotational modes. The
structural similarity of gamma-NaAlH4 to alpha-Na3AlH6 suggests it acts as a
key intermediate during hydrogen release. Findings are consistent with recent
experiments recording an unknown phase during dehydrogenation.Comment: 10 pages, 4 figures, 1 table + supplementary info; In press (Physical
Review Letters
Bulk Aluminum at High Pressure: A First-Principles Study
The behavior of metals at high pressure is of great importance to the fields
of shock physics, geophysics, astrophysics, and nuclear materials. In order to
further understand the properties of metals at high pressures we studied the
equation of state of aluminum using first-principles techniques up to 2500 GPa,
pressures within reach of the planned L.L.N.L. National Ignition Facility. Our
simulations use density-functional theory and density-functional perturbation
theory in the generalized gradient approximation at 0K. We found core overlaps
to become relevant beyond pressures of 1200 GPa. The equations of state for
three phases (fcc, bcc, and hcp) were calculated predicting the fcc-hcp,
fcc-bcc, and hcp-bcc transitions to occur at 215 GPa, 307 GPa, and 435 GPa
respectively. From the phonon dispersions at increasing pressure, we predict a
softening of the lowest transverse acoustic vibrational mode along the [110]
direction, which corresponds to a Born instability of the fcc phase at 725 GPa.Comment: 4 pages, 5 figures, accepted to Phys. Rev. B as a Brief Report. This
version has update many figures. Moreover we provided updated and more
accurate numbers based on further in-depth analyses of potential
computational error
Ballistic Transport in Nanostructures, and its Application to Functionalized Nanotubes
We developed and implemented a first-principles based theory of the Landauer ballistic conductance, to determine the transport properties of nanostructures and molecular-electronics devices. Our approach starts from a quantum-mechanical description of the electronic structure of the system under consideration, performed at the density-functional theory level and using finite-temperature molecular dynamics simulations to obtain an ensemble of the most likely microscopic configurations. The extended Bloch states are then converted into maximally-localized Wannier functions to allow us to construct the Green's function of the conductor, from which we obtain the density of states (confirming the reliability of our microscopic calculations) and the Landauer conductance. A first application is presented to the case of pristine and functionalized carbon nanotubes.Singapore-MIT Alliance (SMA
First-principles investigation of organic photovoltaic materials C, C, [C]PCBM, and bis-[C]PCBM using a many-body -Lanczos approach
We present a first-principles investigation of the excited-state properties
of electron acceptors in organic photovoltaics including C, C,
[6,6]-phenyl-C-butyric-acid-methyl-ester ([C]PCBM), and
bis-[C]PCBM using many-body perturbation theory within the Hedin's
approximation and an efficient Lanczos approach. Calculated vertical
ionization potentials (VIP) and vertical electron affinities (VEA) of C
and C agree very well with experimental values measured in gas phase.
The density of states of all three molecules is also compared to photoemission
and inverse photoemission spectra measured on thin-films, exhibiting a close
agreement - a rigid energy-gap renormalization owing to intermolecular
interactions in the thin-films. In addition, it is shown that the low-lying
unoccupied states of [C]PCBM are all derived from the highest-occupied
molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene
C. The functional side group in [C]PCBM introduces a slight
electron transfer to the fullerene cage, resulting in small decreases of both
VIP and VEA. This small change of VEA provides a solid justification for the
increase of open-circuit voltage when replacing fullerene C with
[C]PCBM as the electron acceptor in bulk heterojunction polymer solar
cells.Comment: 9 pages, 4 figures, and 7 table
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