113 research outputs found
Recursion and Path-Integral Approaches to the Analytic Study of the Electronic Properties of
The recursion and path-integral methods are applied to analytically study the
electronic structure of a neutral molecule. We employ a tight-binding
Hamiltonian which considers both the and valence electrons of carbon.
From the recursion method, we obtain closed-form {\it analytic} expressions for
the and eigenvalues and eigenfunctions, including the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) states, and the Green's functions. We also present the local densities
of states around several ring clusters, which can be probed experimentally by
using, for instance, a scanning tunneling microscope. {}From a path-integral
method, identical results for the energy spectrum are also derived. In
addition, the local density of states on one carbon atom is obtained; from this
we can derive the degree of degeneracy of the energy levels.Comment: 19 pages, RevTex, 6 figures upon reques
Computational study of the thermal conductivity in defective carbon nanostructures
We use non-equilibrium molecular dynamics simulations to study the adverse
role of defects including isotopic impurities on the thermal conductivity of
carbon nanotubes, graphene and graphene nanoribbons. We find that even in
structurally perfect nanotubes and graphene, isotopic impurities reduce thermal
conductivity by up to one half by decreasing the phonon mean free path. An even
larger thermal conductivity reduction, with the same physical origin, occurs in
presence of structural defects including vacancies and edges in narrow graphene
nanoribbons. Our calculations reconcile results of former studies, which
differed by up to an order of magnitude, by identifying limitations of various
computational approaches
Effect of electron and hole doping on the structure of C, Si, and S nanowires
We use ab initio density functional calculations to study the effect of
electron and hole doping on the equilibrium geometry and electronic structure
of C, Si, and S monatomic wires. Independent of doping, all these nanowires are
found to be metallic. In absence of doping, C wires are straight, whereas Si
and S wires display a zigzag structure. Besides two preferred bond angles of 60
deg and 120 deg in Si wires, we find an additional metastable bond angle of 90
deg in S wires. The equilibrium geometry and electronic structure of these
nanowires is shown to change drastically upon electron and hole doping.Comment: 5 pages including 5 figure
Shot Noise and Full Counting Statistics from Non-equilibrium Plasmons in Luttinger-Liquid Junctions
We consider a quantum wire double junction system with each wire segment
described by a spinless Luttinger model, and study theoretically shot noise in
this system in the sequential tunneling regime. We find that the
non-equilibrium plasmonic excitations in the central wire segment give rise to
qualitatively different behavior compared to the case with equilibrium
plasmons. In particular, shot noise is greatly enhanced by them, and exceeds
the Poisson limit. We show that the enhancement can be explained by the
emergence of several current-carrying processes, and that the effect disappears
if the channels effectively collapse to one due to, {\em e.g.}, fast plasmon
relaxation processes.Comment: 9 pages; IOP Journal style; several changes in the tex
Molecular geometry optimization with a genetic algorithm
We present a method for reliably determining the lowest energy structure of
an atomic cluster in an arbitrary model potential. The method is based on a
genetic algorithm, which operates on a population of candidate structures to
produce new candidates with lower energies. Our method dramatically outperforms
simulated annealing, which we demonstrate by applying the genetic algorithm to
a tight-binding model potential for carbon. With this potential, the algorithm
efficiently finds fullerene cluster structures up to starting
from random atomic coordinates.Comment: 4 pages REVTeX 3.0 plus 3 postscript figures; to appear in Physical
Review Letters. Additional information available under "genetic algorithms"
at http://www.public.iastate.edu/~deaven
Prismane C_8: A New Form of Carbon?
Our numerical calculations on small carbon clusters point to the existence of
a metastable three-dimensional eight-atom cluster C which has a shape of a
six-atom triangular prism with two excess atoms above and below its bases. We
gave this cluster the name "prismane". The binding energy of the prismane
equals to 5.1 eV/atom, i.e., is 0.45 eV/atom lower than the binding energy of
the stable one-dimensional eight-atom cluster and 2.3 eV/atom lower than the
binding energy of the bulk graphite or diamond. Molecular dynamics simulations
give evidence for a rather high stability of the prismane, the activation
energy for a prismane decay being about 0.8 eV. The prismane lifetime increases
rapidly as the temperature decreases indicating a possibility of experimental
observation of this cluster.Comment: 5 pages (revtex), 3 figures (eps
Theory of Spontaneous Polarization of Endohedral Fullerenes
A pseudo-Jahn-Teller model describing central atom distortions is proposed
for endohedral fullerenes of the form A@C where A is either a rare gas
or a metal atom. A critical (dimensionless) coupling is found, below
which the symmetric configuration is stable and above which inversion symmetry
is broken. Vibronic parameters are given for selected endohedral fullerenes.Comment: 4 pages, REVTEX, 1 Postscript figure. [Phys. Rev. Lett. (in press)
Unusually High Thermal Conductivity of Carbon Nanotubes
Combining equilibrium and non-equilibrium molecular dynamics simulations with
accurate carbon potentials, we determine the thermal conductivity of
carbon nanotubes and its dependence on temperature. Our results suggest an
unusually high value ~W/mK for an isolated
(10,10) nanotube at room temperature, comparable to the thermal conductivity of
a hypothetical isolated graphene monolayer or diamond. Our results suggest that
these high values of are associated with the large phonon mean free
paths in these systems; substantially lower values are predicted and observed
for the basal plane of bulk graphite.Comment: 4 pages 3 figures (5 postscript files), submitted for publicatio
Orientational Melting in Carbon Nanotube Ropes
Using Monte Carlo simulations, we investigate the possibility of an
orientational melting transition within a "rope" of (10,10) carbon nanotubes.
When twisting nanotubes bundle up during the synthesis, orientational
dislocations or twistons arise from the competition between the anisotropic
inter-tube interactions, which tend to align neighboring tubes, and the torsion
rigidity that tends to keep individual tubes straight. We map the energetics of
a rope containing twistons onto a lattice gas model and find that the onset of
a free "diffusion" of twistons, corresponding to orientational melting, occurs
at T_OM > 160 K.Comment: 4 page LaTeX file with 3 figures (10 PostScript files
Thermal effects on atomic friction
We model friction acting on the tip of an atomic force microscope as it is
dragged across a surface at non-zero temperatures. We find that stick-slip
motion occurs and that the average frictional force follows ,
where is the tip velocity. This compares well to recent experimental work
(Gnecco et al, PRL 84, 1172), permitting the quantitative extraction of all
microscopic parameters. We calculate the scaled form of the average frictional
force's dependence on both temperature and tip speed as well as the form of the
friction-force distribution function.Comment: Accepted for publication, Physical Review Letter
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