Biomedical subjects

Marvin L Cohen

Publications and source records attributed to Marvin L Cohen.

17 recordsLinked to original sources

PubMed2006

Energy gaps in graphene nanoribbons.

Based on a first-principles approach, we present scaling rules for the band gaps of graphene nanoribbons (GNRs) as a function of their widths. The GNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen passivation. Both varieties of ribbons are shown to have band gaps. This differs from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them. Our ab initio calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges. For GNRs with zigzag shaped edges, gaps appear because of a staggered sublattice potential on the hexagonal lattice due to edge magnetization. The rich gap structure for ribbons with armchair shaped edges is further obtained analytically including edge effects. These results reproduce our ab initio calculation results very well.

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PubMed2006

Half-metallic graphene nanoribbons.

Electrical current can be completely spin polarized in a class of materials known as half-metals, as a result of the coexistence of metallic nature for electrons with one spin orientation and insulating nature for electrons with the other. Such asymmetric electronic states for the different spins have been predicted for some ferromagnetic metals--for example, the Heusler compounds--and were first observed in a manganese perovskite. In view of the potential for use of this property in realizing spin-based electronics, substantial efforts have been made to search for half-metallic materials. However, organic materials have hardly been investigated in this context even though carbon-based nanostructures hold significant promise for future electronic devices. Here we predict half-metallicity in nanometre-scale graphene ribbons by using first-principles calculations. We show that this phenomenon is realizable if in-plane homogeneous electric fields are applied across the zigzag-shaped edges of the graphene nanoribbons, and that their magnetic properties can be controlled by the external electric fields. The results are not only of scientific interest in the interplay between electric fields and electronic spin degree of freedom in solids but may also open a new path to explore spintronics at the nanometre scale, based on graphene.

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PubMed2006

Giant wave-drag enhancement of friction in sliding carbon nanotubes.

Molecular dynamics simulations of coaxial carbon nanotubes in relative sliding motion reveal a striking enhancement of friction when phonons whose group velocity is close to the sliding velocity of the nanotubes are strongly excited. The effect is analogous to the dramatic increase in air drag experienced by aircraft flying close to the speed of sound but differs in that it can occur in multiple velocity ranges with varying magnitude, depending on the atomic level structures of the nanotubes. The phenomenon is a general one that may occur in other nanoscale mechanical systems.

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PubMed2006

Symmetry breaking in boron nitride nanotubes.

We have imaged boron nitride nanotubes with atomic scale resolution using scanning tunneling microscopy. While some nanotubes show the expected triangular lattice pattern, the majority of the nanotubes show unusual stripe patterns which break the underlying symmetry of the boron nitride lattice. We identify the origin of the symmetry breaking and demonstrate that conventional STM imaging analysis is inadequate for boron nitride nanotubes.

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PubMed2006

Temperature dependence of the optical transition energies of carbon nanotubes: the role of electron-phonon coupling and thermal expansion.

Tunable Raman spectroscopy is used to measure the optical transition energies Eii of individual single wall carbon nanotubes. Eii is observed to shift down in energy by as much as 50 meV, from -160 to 300 degrees C, in contrast with previous measurements performed on nanotubes in alternate environments, which show upshifts and downshifts in Eii with temperature. We determine that electron-phonon coupling explains our experimental observations of nanotubes suspended in air, neglecting thermal expansion. In contrast, for nanotubes in surfactant or in bundles, thermal expansion of the nanotubes' environment exerts a nonisotropic pressure on the nanotube that dominates over the effect of electron-phonon coupling.

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PubMed2005

Reentrant semiconducting behavior of zigzag carbon nanotubes at substitutional doping by oxygen dimers.

The electronic structures of carbon nanotubes doped with oxygen dimers are studied using the ab initio pseudopotential density functional method. The fundamental energy gap of zigzag semiconducting nanotubes exhibits a strong dependence on both the concentration and configuration of oxygen-dimer defects that substitute for carbon atoms in the tubes and on the tube chiral index. For a certain type of zigzag nanotube when doped with oxygen dimers, the energy gap is closed and the tube becomes semimetallic. At higher oxygen-dimer concentrations the gap reopens, and the tube exhibits semiconducting behavior again. The change of the band gap of the zigzag tube is understood in terms of their response to the strains caused by the dimer substitutional doping.

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PubMed2005

Electrical switching in metallic carbon nanotubes.

We present first-principles calculations of quantum transport which show that the resistance of metallic carbon nanotubes can be changed dramatically with homogeneous transverse electric fields if the nanotubes have impurities or defects. The change of the resistance is predicted to range over more than 2 orders of magnitude with experimentally attainable electric fields. This novel property has its origin that backscattering of conduction electrons by impurities or defects in the nanotubes is strongly dependent on the strength and/or direction of the applied electric fields. We expect this property to open a path to new device applications of metallic carbon nanotubes.

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PubMed2005

Nonlocal screening, electron-phonon coupling, and phonon renormalization in metals.

A method for calculating the phonon self-energy in metals arising from the coupling between phonons and electrons near the Fermi surface is developed. The essence of this scheme is the separation of the inter- and intraband parts of the electron polarizability. The intraband contribution provides extra screenings and is closely related to the electron-phonon coupling and phonon softening in metals. Applications of this scheme to phonons in MgB2 give excellent results when compared with experiments and previous theoretical work. In addition, both electron and hole dopings are found to reduce the renormalization effect of the E(2g) phonon mode, which indicates a weakened electron-phonon coupling in the doped systems.

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PubMed2005

Observation of the giant stark effect in boron-nitride nanotubes.

Bias dependent scanning tunneling microscopy and scanning tunneling spectroscopy have been used to characterize the influence of transverse electric fields on the electronic properties of boron-nitride nanotubes (BNNTs). We find experimental evidence for the theoretically predicted giant Stark effect. The observed giant Stark effect significantly reduces the band gap of BNNTs and thus greatly enhances the utility of BNNTs for nanoscale electronic, electromechanical, and optoelectronic applications.

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PubMed2005

Temperature dependence of the band gap of semiconducting carbon nanotubes.

The temperature dependence of the band gap of semiconducting single-wall carbon nanotubes (SWNTs) is calculated by direct evaluation of electron-phonon couplings within a "frozen-phonon" scheme. An interesting diameter and chirality dependence of E(g)(T) is obtained, including nonmonotonic behavior for certain tubes and distinct "family" behavior. These results are traced to a strong and complex coupling between band-edge states and the lowest-energy optical phonon modes in SWNTs. The E(g)(T) curves are modeled by an analytic function with diameter- and chirality-dependent parameters; these provide a valuable guide for systematic estimates of E(g)(T) for any given SWNT. The magnitudes of the temperature shifts at 300 K are smaller than 12 meV and should not affect (n,m) assignments based on optical measurements.

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PubMed2005

Structural transformations of carbon nanotubes under hydrostatic pressure.

We used simulations with a classical force field to study the transformation under hydrostatic pressure of isolated single-walled nanotubes (SWNT) from a circular to a collapsed cross section. Small-diameter SWNTs deform continuously under pressure, whereas larger-diameter SWNTs display hysteresis and undergo a first-order-like transformation. The different behavior is due to the changing proportions in the total energy of the wall-curvature energy and the van der Waals attraction between opposite walls of the tube.

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PubMed2004

Fermi surface of NaxCoO2.

Doping evolution of the Fermi surface topology of Na(x)CoO(2) is studied systematically. Both local density approximation (LDA) and local spin density approximation (LSDA) predict a large Fermi surface as well as small hole pockets for doping levels x approximately 0.5. In contrast, the hole pockets are completely absent for all doping levels within LSDA+U. More importantly, we find no violation of Luttinger's rule in this system. The measured Fermi surface of Na(0.7)CoO(2) can be explained by its half-metallic behavior and agrees with our LSDA+U calculations.

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PubMed2004

Identifying defects in nanoscale materials.

We have developed a novel iterative experimental-theoretical technique which can identify the atomic structure of defects in many-atom nanoscale materials from scanning tunneling microscopy and spectroscopy data. A given model for a defect structure is iteratively improved until calculated microscopy and spectroscopy data based on the model converge on the experimental results. We use the technique to identify a defect responsible for the electronic properties of a carbon nanotube intramolecular junction. Our technique can be extended for analysis of defect structures in nanoscale materials in general.

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PubMed2004

Dynamic sliding friction between concentric carbon nanotubes.

Molecular dynamics simulations are used to study mechanical energy dissipation in carbon nanotube oscillators of lengths of tens of nanometers. The principal source of friction is found to be the ends of the tubes and hence dynamical friction is virtually independent of the overlap area between tubes. As a result of this, tube commensuration does not lead to significantly increased frictional forces. The friction force is found to depend strongly and nonlinearly on the relative velocity of the tubes. It is suggested that a strong velocity dependence and strong contributions from surface edges may be quite general features of friction at the nanoscale.

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PubMed2003

Phonon instabilities and the ideal strength of aluminum.

We have calculated the phonon spectra of aluminum as a function of strain using density functional perturbation theory for <110>, <100>, and <111> uniaxial tension, as well as relaxed <112>[111] shear. In all four cases, phonon instabilities occur at points away from the center of the Brillouin zone and intrude before the material becomes unstable according to elastic stability criteria. This is the first time the ideal strength of a metal has been shown to be dictated by instabilities in the acoustic phonon spectra. We go on to describe the crystallography of the unstable modes, all of which are shear in character. This work further suggests that shear failure is an inherent property of aluminum even in an initially dislocation-free perfect crystal.

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PubMed2003

Band structure and Fermi surface of electron-doped C60 monolayers.

C60 fullerides are challenging systems because both the electron-phonon and electron-electron interactions are large on the energy scale of the expected narrow band width. We report angle-resolved photoemission data on the band dispersion for an alkali-doped C60 monolayer and a detailed comparison with theory. Compared to the maximum bare theoretical band width of 170 meV, the observed 100-meV dispersion is within the range of renormalization by electron-phonon coupling. This dispersion is only a fraction of the integrated peak width, revealing the importance of many-body effects. Additionally, measurements on the Fermi surface indicate the robustness of the Luttinger theorem even for materials with strong interactions.

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PubMed2002

The origin of the anomalous superconducting properties of MgB(2).

Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB(2) is different. Some early experimental studies have indicated that MgB(2) has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB(2) and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.

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