81 research outputs found
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
Unsupervised landmark analysis for jump detection in molecular dynamics simulations
Molecular dynamics is a versatile and powerful method to study diffusion in
solid-state ionic conductors, requiring minimal prior knowledge of equilibrium
or transition states of the system's free energy surface. However, the analysis
of trajectories for relevant but rare events, such as a jump of the diffusing
mobile ion, is still rather cumbersome, requiring prior knowledge of the
diffusive process in order to get meaningful results. In this work, we present
a novel approach to detect the relevant events in a diffusive system without
assuming prior information regarding the underlying process. We start from a
projection of the atomic coordinates into a landmark basis to identify the
dominant features in a mobile ion's environment. Subsequent clustering in
landmark space enables a discretization of any trajectory into a sequence of
distinct states. As a final step, the use of the smooth overlap of atomic
positions descriptor allows distinguishing between different environments in a
straightforward way. We apply this algorithm to ten Li-ionic systems and
conduct in-depth analyses of cubic LiLaZrO, tetragonal
LiGePS, and the -eucryptite LiAlSiO. We
compare our results to existing methods, underscoring strong points,
weaknesses, and insights into the diffusive behavior of the ionic conduction in
the materials investigated
AiiDA: Automated Interactive Infrastructure and Database for Computational Science
Computational science has seen in the last decades a spectacular rise in the
scope, breadth, and depth of its efforts. Notwithstanding this prevalence and
impact, it is often still performed using the renaissance model of individual
artisans gathered in a workshop, under the guidance of an established
practitioner. Great benefits could follow instead from adopting concepts and
tools coming from computer science to manage, preserve, and share these
computational efforts. We illustrate here our paradigm sustaining such vision,
based around the four pillars of Automation, Data, Environment, and Sharing. We
then discuss its implementation in the open-source AiiDA platform
(http://www.aiida.net), that has been tuned first to the demands of
computational materials science. AiiDA's design is based on directed acyclic
graphs to track the provenance of data and calculations, and ensure
preservation and searchability. Remote computational resources are managed
transparently, and automation is coupled with data storage to ensure
reproducibility. Last, complex sequences of calculations can be encoded into
scientific workflows. We believe that AiiDA's design and its sharing
capabilities will encourage the creation of social ecosystems to disseminate
codes, data, and scientific workflows.Comment: 30 pages, 7 figure
BoltzWann: A code for the evaluation of thermoelectric and electronic transport properties with a maximally-localized Wannier functions basis
We present a new code to evaluate thermoelectric and electronic transport
properties of extended systems with a maximally-localized Wannier function
basis set. The semiclassical Boltzmann transport equations for the homogeneous
infinite system are solved in the constant relaxation-time approximation and
band energies and band derivatives are obtained via Wannier interpolations.
Thanks to the exponential localization of the Wannier functions obtained, very
high accuracy in the Brillouin zone integrals can be achieved with very
moderate computational costs. Moreover, the analytical expression for the band
derivatives in the Wannier basis resolves any issues that may occur when
evaluating derivatives near band crossings. The code is tested on binary and
ternary skutterudites CoSb_3 and CoGe_{3/2}S_{3/2}.Comment: 19 pages, 7 figure
Dielectric response and interactions in low-dimensional carbon materials from first principles calculations
Includes bibliographical references (p. 155-162).Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.by Boris Kozinsky.Ph.D
Anomalous thermoelectric transport phenomena from interband electron-phonon scattering
The Seebeck coefficient and electrical conductivity are two critical
quantities to optimize simultaneously in designing thermoelectric materials,
and they are determined by the dynamics of carrier scattering. We uncover a new
regime where the co-existence at the Fermi level of multiple bands with
different effective masses leads to strongly energy-dependent carrier lifetimes
due to intrinsic electron-phonon scattering. In this anomalous regime,
electrical conductivity decreases with carrier concentration, Seebeck
coefficient reverses sign even at high doping, and power factor exhibits an
unusual second peak. We discuss the origin and magnitude of this effect using
first-principles Boltzmann transport calculations and simplified models. We
also identify general design rules for using this paradigm to engineer enhanced
performance in thermoelectric materials.Comment: 11 pages, 7 figure
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