950 research outputs found
The formation and structure of circumstellar and interstellar dust
The intriguing abundance of long linear carbon chain molecules in some dark clouds and in circumstellar shells is still not well understood. Recent laboratory studies which have probed this problem indicate that when carbon vapor nucleates to form particles, linear chains and hollow cage molecules (fullerenes) also form at more-or-less the same time. The results have consequences for the formation, structures and spectroscopic properties of the molecular and dust components ejected from cool carbon-rich stars. A most interesting result of the experimental observations relates to the probability that a third character in addition to the chains and grains, the C(sub 60) molecule probably in the form of the ion C(sub 60)(sup +) in the less shielded regions, is present and perhaps responsible for some of the ubiquitously observed interstellar spectroscopic features such as the Diffuse Interstellar Features, the 2170A UV Absorption or perhaps some of the Unidentified Infrared Bands. Further study of small carbon particles which form in the gas phase has resulted in the discovery that they have quasi-icosahedral spiral shell structures. The role that such species may play in the interstellar medium as well as that played by C(sub 60) (or C sub 60 sup +) should soon be accessible to verification by a combination of laboratory experiment and astronomical spectroscopy
Peculiar Width Dependence of the Electronic Property of Carbon Nanoribbons
Nanoribbons (nanographite ribbons) are carbon systems analogous to carbon
nanotubes. We characterize a wide class of nanoribbons by a set of two integers
, and then define the width in terms of and . Electronic
properties are explored for this class of nanoribbons. Zigzag (armchair)
nanoribbons have similar electronic properties to armchair (zigzag) nanotubes.
The band gap structure of nanoribbons exhibits a valley structure with
stream-like sequences of metallic or almost metallic nanoribbons. These
sequences correspond to equi-width curves indexed by . We reveal a peculiar
dependence of the electronic property of nanoribbons on the width .Comment: 8 pages, 13 figure
Deconfinement Phase Transition in Hot and Dense QCD at Large N
We conjecture that the confinement- deconfinement phase transition in QCD at
large number of colors and at and is
triggered by the drastic change in behavior. The conjecture is
motivated by the holographic model of QCD where confinement -deconfinement
phase transition indeed happens precisely at where dependence
experiences a sudden change in behavior. The conjecture is also supported by
quantum field theory arguments when the instanton calculations (which trigger
the dependence) are under complete theoretical control for ,
suddenly break down immediately below with sharp changes in the
dependence. Finally, the conjecture is supported by a number of
numerical lattice results. We employ this conjecture to study confinement
-deconfinement phase transition of hot and dense QCD in large limit by
analyzing the dependence. We estimate the critical values for
and where the phase transition happens by approaching the critical
values from the hot and/or dense regions where the instanton calculations are
under complete theoretical control. We also describe some defects of various
codimensions within a holographic model of QCD by focusing on their role around
the phase transition point.Comment: Talk at the Workshop honoring 60th anniversary of Misha Shifma
Enhanced Sensitivity to the Time Variation of the Fine-Structure Constant and in Diatomic Molecules: A Closer Examination of Silicon Monobromide
Recently it was pointed out that transition frequencies in certain diatomic
molecules have an enhanced sensitivity to variations in the fine-structure
constant and the proton-to-electron mass ratio due to a near
cancellation between the fine-structure and vibrational interval in a ground
electronic multiplet [V.~V.~Flambaum and M.~G.~Kozlov, Phys. Rev. Lett.~{\bf
99}, 150801 (2007)]. One such molecule possessing this favorable quality is
silicon monobromide. Here we take a closer examination of SiBr as a candidate
for detecting variations in and . We analyze the rovibronic
spectrum by employing the most accurate experimental data available in the
literature and perform \emph{ab initio} calculations to determine the precise
dependence of the spectrum on variations in . Furthermore, we calculate
the natural linewidths of the rovibronic levels, which place a fundamental
limit on the accuracy to which variations may be determined.Comment: 8 pages, 2 figure
Searching for Extra Dimensions in the Early Universe
We investigate extra spatial dimensions () in the early
universe using very high resolution molecular rotational spectroscopic data
derived from a large molecular cloud containing moderately cold carbon monoxide
gas at Z . It turns out that the -dependent quantum
mechanical wavelength transitions are solvable for a linear molecule and we
present the solution here. The CO microwave data allows a very precise
determination of . The probability
that is one in 7794, only 850 million years (using the
standard cosmology) after the Big Bang.Comment: 17 pages, 2 figure
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
Dynamics of fullerene coalescence
Fullerene coalescence experimentally found in fullerene-embedded single-wall
nanotubes under electron-beam irradiation or heat treatment is simulated by
minimizing the classical action for many atom systems. The dynamical trajectory
for forming a (5,5) C nanocapsule from two C fullerene molecules
consists of thermal motions around potential basins and ten successive
Stone-Wales-type bond rotations after the initial cage-opening process for
which energy cost is about 8 eV. Dynamical paths for forming large-diameter
nanocapsules with (10,0), (6,6), and (12,0) chiral indexes have more bond
rotations than 25 with the transition barriers in a range of 10--12 eV.Comment: 4 pages, 2 figures, 1 supplementary movie at
http://dielc.kaist.ac.kr/yonghyun/coal.mpeg. To be published in Physical
Review Letter
Raman imaging and electronic properties of graphene
Graphite is a well-studied material with known electronic and optical
properties. Graphene, on the other hand, which is just one layer of carbon
atoms arranged in a hexagonal lattice, has been studied theoretically for quite
some time but has only recently become accessible for experiments. Here we
demonstrate how single- and multi-layer graphene can be unambiguously
identified using Raman scattering. Furthermore, we use a scanning Raman set-up
to image few-layer graphene flakes of various heights. In transport experiments
we measure weak localization and conductance fluctuations in a graphene flake
of about 7 monolayer thickness. We obtain a phase-coherence length of about 2
m at a temperature of 2 K. Furthermore we investigate the conductivity
through single-layer graphene flakes and the tuning of electron and hole
densities via a back gate
Formation of a "Cluster Molecule" (C20)2 and its thermal stability
The possible formation of a "cluster molecule" (C20)2 from two single C20
fullerenes is studied by the tight-binding method. Several (C20)2 isomers in
which C20 fullerenes are bound by strong covalent forces and retain their
identity are found; actually, these C20 fullerenes play the role of "atoms" in
the "cluster molecule". The so-called open-[2+2] isomer has a minimum energy.
Its formation path and thermal stability at T = 2000 - 4000 K are analyzed in
detail. This isomer loses its molecular structure due to either the decay of
one of C20 fullerenes or the coalescence of two C20 fullerenes into a C40
cluster. The energy barriers for the metastable open-[2+2] configuration are
calculated to be U = 2 - 5 eV.Comment: 21 pages, 8 figure
Magnetic Properties of Undoped
The Heisenberg antiferromagnet, which arises from the large Hubbard
model, is investigated on the molecule and other fullerenes. The
connectivity of leads to an exotic classical ground state with
nontrivial topology. We argue that there is no phase transition in the Hubbard
model as a function of , and thus the large solution is relevant for
the physical case of intermediate coupling. The system undergoes a first order
metamagnetic phase transition. We also consider the S=1/2 case using
perturbation theory. Experimental tests are suggested.Comment: 12 pages, 3 figures (included
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