On the interband pairing in doped graphane

An estimation shows that the interband pairing channel between the valence band components of doped graphane can support a superconducting transition temperature (or a contribution into this expected event) of the order of 100 K at the coupling strength near 1 eV.     

Ab initio studies of isolated hydrogen vacancies in graphane

We present a density functional study of various hydrogen vacancies located on a single hexagonal ring of graphane (fully hydrogenated graphene) considering the effects of charge states and the position of the Fermi level. We find that uncharged vacancies that lead to a carbon sublattice balance are energetically favorable and are wide band gap systems just like pristine graphane. Vacancies that do create a sublattice imbalance introduce spin polarized states into the band gap, and exhibit a half-metallic behavior with a magnetic moment of 1.00 μ_B per vacancy. The results show the possibility of using vacancies in graphane for novel spin-based applications. When charging such vacancy configurations, the deep donor (+1/0) and deep acceptor (0/−1) transition levels within the band gap are noted. We also note a half-metallic to metallic transition and a significant reduction of the induced magnetic moment due to both negative and positive charge doping.     

Thickness and in-plane elasticity of graphane

The thickness and in-plane mechanical properties (Young?s, shear modulus, Poisson?s ratios) of fully hydrogenated graphene (graphane) sheets are predicted using a molecular mechanics approach. The equilibrium lengths and bond angles distortions used for the graphane models are obtained from Density Functional Theory (DFT) simulations. Our models compare well with existing data on the uniaxial properties of graphane and graphene sheets from first principle and Molecular Dynamics (MD) simulations, highlight a special orthotropic mechanical behaviour for graphane, and identify thickness and shear stiffness values which are peculiar of hydrogenated graphene.     

Energy exchange between discrete breathers in graphane in thermal equilibrium

Graphane is a fully hydrogenated graphene which is practically interesting for application in electronics, hydrogen storage and transportation, in nanoscale devices. As it was previously shown, the energy of a discrete breather (nonlinear localized mode) in graphane close to the value of the energy barrier at which the dehydrogenation of graphene occurs. In the present work, molecular dynamics simulation is used to investigate the possibility of energy exchange between discrete breathers in graphane in thermal equilibrium at 400 K and 600 K. In thermally equilibrated graphane, hydrogen atoms are spontaneously excited and can be considered as discrete breathers. Comparison of the kinetic energy per atom as the function of time for the selected hydrogen atoms with their displacements along the z axis showed that there is an energy exchange between the discrete breathers at evaluated temperatures. Hydrogen atom, transmitting its energy to the neighboring atom no longer exists as discrete breather. At high temperatures (600 K) the energy exchange between closely located discrete breathers also take place but strong thermo-oscillations of atoms at high temperatures (above 400 K) considerably affect the process.     

Electron energy loss spectrum of graphane from first-principles calculations

In this study, the energy loss near edge structure (ELNES) of carbon atoms in chair and tricycle conformers of hydrogenated graphene, namely ‘graphane’, has been calculated in the density functional theory using FP-LAPW method, and then, it has been compared with that of graphite and graphene. Using ELNES from chair conformer, the carbon K-edge was found to have a few main features including electron transition from 1s orbital of carbon atom to π*, σ*, and a hybridization of these two states. The first feature in tricycle conformer, however, has contributions of both π* and σ* states. The comparison of ELNES and the unoccupied density of states in each structure also justifies this. The energy difference between π* and σ* features of graphane conformers was decreased relative to it in graphite and graphene. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in graphite and graphene, it may be essential as well for the ELNES spectra of graphane conformers.     

Structure,stability and electronic properties of tricycle type graphane

We propose a new allotrope of graphane (named as ‘tricycle') with equivalent 4up/2down UUUDUD hydrogenation in each six‐carbon ring, which can be considered as the combination of previously proposed 3up/3down chair graphane and stirrup graphane. We find that tricycle graphane is more stable than stirrup graphane and its negative Gibbs free energy (–91 meV/atom) is very close to that of the most stable chair one (–103 meV/atom). Investigations on its vibrational property confirm its dynamical stability. Such a new two‐dimensional hydrocarbon may be produced in the process of graphene hydrogenation with a relatively high probability due to its remarkable stability.

     

Electronic and dynamics properties of a molecular wire of graphane nanoclusters

In this work we study the electronics and dynamical properties of an array of graphane nanoclusters. The electronic properties are obtained from first principles calculations. The dynamical study is performed by solving the time-dependent Schrödinger equation, adopting the occupation number Hamiltonian and using parameters obtained with first principles calculations. The thermal behavior is simulated by a stochastic algorithm. Our results show that for a set of geometric parameters of the array of nanoclusters, the system exhibits bi-stability in the charge configuration, excitation energies that allow operation at room temperature, operation times below the picosecond and scalability.     

Spontaneous regeneration of an atomically sharp graphene/graphane interface under thermal disordering

The smearing of the graphene/graphane interface owing to the thermally activated migration of hydrogen atoms has been investigated by the molecular dynamics method. Contrary to expectations, it has been found that the fast spontaneous regeneration of this interface occurs even at a sufficiently high temperature T ≈ 1500 K; as a result, the average width of the disordered region does not exceed the length of a C-C bond; i.e., the interface remains almost atomically sharp. The cause of this effect appears to be the specific shape of the potential relief of the system, namely, the significant difference between the heights of the energy barriers for the direct and inverse migrations of hydrogen. A simple model that makes it possible to obtain the temperature dependence of the equilibrium distribution function of typical atomic configurations, to estimate the typical time of establishing the equilibrium state, and thereby to quantitatively describe the results of the computer experiment is presented.     

Theoretical investigation of manganese adsorption on graphene and graphane: A first-principles comparative study

Within the framework of spin-polarized generalized gradient approximation (σGGA) of the density functional theory (DFT) and pseudopotential method, the structural, magnetic, and electronic properties of graphene and graphane upon the adsorption of manganese atoms have been theoretically investigated. In contrast to the recent results (New J. Phys. 12, 063020 (2010)), Mn atom has been found to be adsorbed on a hollow-site configuration and no appreciable indication to substitute one of the C atoms of the graphene sheet. Unlike the recent results on Mn-doped graphane (Carbon 48, 3901 (2010)), the Mn adatom prefers to adsorb on the top of a carbon atom, forming a bridge with the uppermost hydrogen atoms. The magnetic moment of the Mn-doped graphene is found to be larger than that of the Mn-doped graphane. The structural parameters and electronic properties of both Mn-doped graphene and Mn-doped graphane are determined and compared with the available data.     

Elementary particles

Abstract

The main property criteria which determine whether a certain material shall be used for a given application are discussed in general terms. The increase in cost of any given material in progressing through the various stages of processing and manufacture is outlined for steel. The general growth trends of production of a range of common metals, cement and plastics in the U.S.A. up to 1960 shows the very high growth rate of aluminium and plastics in recent years compared to other materials. It is shown that by considering the cost per unit of property required that a very useful index is given of the competitiveness of any given material. By way of example, typical costs for one ton of tensile strength given by a range of materials in various forms are tabled.

Some general observations are made on specific areas of conflict between materials, e.g. steel and concrete, aluminium versus steel, and plastics versus metals.     

Elementary excitations in vibrational nuclei

We propose to describe the entire collective spectrum of vibrational nuclei in terms of few enteracting elementary excitation modes. We discuss in detail the case in which only two elementary modes are important (the quadrupole d and the octupole f-bosons). We give explicit expressions for the energy levels and transition matrix elements.     

苯基取代聚乙炔中的元激发

苯环取代聚乙炔是一类具有简并基态的发光聚合物.利用扩展的SuSchriefferHeeger模型,研究了这类聚合物链中的孤子、极化子等元激发特性.结果表明:由于苯环与聚乙炔主链间的强π电子耦合,苯环取代抑制了聚乙炔主链的二聚化,减小了导带与价带之间的能隙;因此取代聚乙炔中的元激发具有较小的激发能,同时具有更大的相干长度.尽管苯基取代对元激发有很大的影响,但是非取代聚乙炔中元激发的基本物理规律依然成立,表明这些元激发的基本特性不受取代基团的影响 关键词： 孤子 极化子 苯基取代聚乙炔     

Elementary excitations in quadrupolar systems

We consider $\text{S ?}\text{3}\text{2}$ isotropic quadrupolar ordered systems and derive elementary excitations at low temperature. A Holstein-Primakoff type transformation and a linear approximation are used. For $\text{S =}\text{3}\text{2}$, the spectrum is made of four degenerate acoustic branches. For S ? 2, only two degenerate branches satisfy the Goldstone theorem: they describe Δm = ± 1 excitations similar to librons in molecular crystals. The two degenerate branches describing Δm = ± 2 excitations have a gap at k = 0 although the hamiltonian is isotropic. For a special $\text{S =}\text{3}\text{2}$ cubic hamiltonian, a Goldstone mode is found in the spectrum and related to a continuous degeneracy of the ground state. A comparison between $\text{S =}\text{1}\text{2}$ dipolar and $\text{S =}\text{3}\text{2}$ quadrupolar systems is presented.     

Elementary particle symmetries

Problem-based learning (PBL) is an established pedagogy in many areas of education for the professions. Although there is an awareness of PBL in many departments of physics in the UK and many claim to include PBL-like instruction to some degree, it has made rather less impact in the physical sciences. This paper describes the aims of PBL and how these are implemented based on our experiences in Physics at the University of Leicester. It is not our purpose to discuss here the parochial details of this programme which are partly historical and adapted to local conditions. (The interested reader can find them on our web site.) Rather we look at general aspects of PBL in Physics in the light of our experience and that of others. In addition to numerous examples of PBL problems, our discussion includes the educational and philosophical underpinnings of PBL, the nature of the ‘problem’ in PBL, issues in facilitation and assessment as well as a brief review of the published evaluations of PBL. Space constraints mean we do not discuss the process of change management.     

Elementary particle symmetries

The number of known elementary particles is now very large, and to reduce their diverse properties to order it is useful to look for symmetries between them. Symmetry properties of various kinds, and the conservation laws associated with them, are discussed here. After giving a brief survey of the general types of particle encountered in nature, we discuss first the symmetries related to space and time, and associated with the conservation laws of momentum, energy, angular momentum, and parity. Later we discuss the so-called internal symmetries, in which different particles are regarded as different states of a single basic entity. Theories of this type–particularly the ‘eightfold way’–have had considerable success in systematizing the properties of the known particles.     

Elementary excitations in easy-plane Heisenberg ferromagnets

We study the influence of the non-diagnonal terms in an easy-plane Heisenberg ferromagnet on the cross-section for inelastic neutron scattering from single spin-wave states. We show that these terms modify the wavevector dependence of the cross-section for unpolarized neutrons, and the created polarization. An appropriate experiment on CsNiF₃, for example, should be a good test of these predictions, and would eventually clarify some uncertainties about the appropriate Boson Hamiltonian for an easy-plane ferromagnet.     

Elementary excitations in tunnel-coupled electron bilayers

A new class of single-particle excitations in tunnel-coupled electron bilayers is investigated by inelastic light scattering. The dispersion law and the dependence of the energies of these excitations on the degree of unbalance between the layers have been measured. A new spectroscopic method is proposed for determining the degree of unbalance between bilayers.