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 $w$ in terms of $p$ and $q$. 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 $w$. We reveal a peculiar dependence of the electronic property of nanoribbons on the width $w$.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 $N$ and $N_f\ll N$ at $T\neq 0$ and $\mu\neq 0$ is triggered by the drastic change in $\theta$ behavior. The conjecture is motivated by the holographic model of QCD where confinement -deconfinement phase transition indeed happens precisely at $T=T_c$ where $\theta$ dependence experiences a sudden change in behavior. The conjecture is also supported by quantum field theory arguments when the instanton calculations (which trigger the $\theta$ dependence) are under complete theoretical control for $T>T_c$, suddenly break down immediately below $T<T_c$ with sharp changes in the $\theta$ 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 $N$ limit by analyzing the $\theta$ dependence. We estimate the critical values for $T_c$ and $\mu_c$ 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 mp/mem_p/m_e 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 $\alpha$ and the proton-to-electron mass ratio $m_p/m_e$ 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 $\alpha$ and $m_p/m_e$. 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 $\alpha$. 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 ($D = 3+\epsilon$) 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 $\approx 6.42$. It turns out that the $\epsilon$-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 $= -0.00000657 \pm .10003032$. The probability that $\neq 0$ is one in 7794, only 850 million years (using the standard cosmology) after the Big Bang.Comment: 17 pages, 2 figure

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$_{120}$ nanocapsule from two C$_{60}$ 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

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$_8$ 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

Electronic Structure of Disclinated Graphene in an Uniform Magnetic Field

The electronic structure in the vicinity of the 1-heptagonal and 1-pentagonal defects in the carbon graphene plane is investigated. Using a continuum gauge field-theory model the local density of states around the Fermi energy is calculated for both cases. In this model, the disclination is represented by an SO(2) gauge vortex and corresponding metric follows from the elasticity properties of the graphene membrane. To enhance the interval of energies, a self-consistent perturbation scheme is used. The Landau states are investigated and compared with the predicted values.Comment: keywords: graphene, heptagonal defect, elasticity, carbon nanohorns, 13 page

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 $\mu$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