The 3d-to-4s-by-2p highway to superconductivity in cuprates

High-temperature superconductors are nowadays found in great variety and hold technological promise. It is still an unsolved mystery that the critical temperature T_c of the basic cuprates is so high. The answer might well be hidden in a conventional corner of theoretical physics, overlooked in the recent hunt for exotic explanations of new effects in these materials. A forgotten intra-atomic s-d two-electron exchange in the Cu atom is found to provide a strong (~eV) electron pairing interaction. A Bardeen-Cooper-Schrieffer approach can explain the main experimental observations and predict the correct d_{x^2-y^2} symmetry of the gap.Comment: 4 pages, 3 figures, LaTeX2

Specific Heat Discontinuity in Impure Two-Band Superconductors

The Ginzburg-Landau coefficients, and the jump of the specific heat are calculated for a disordered two-band superconductor. We start with the analysis of a more general case arbitrary anisotropy. While the specific heat discontinuity at the critical temperature T_c decreases with increasing disorder, its ratio to the normal state specific heat at T_c increases and slowly converges to the isotropic value. For a strong disorder the deviation from the isotropic value is proportional to the elastic electron scattering time. In the case of a two-band superconductor we apply a simplified model of the interaction independent on momentum within a band. In the framework of this model all thermodynamic values can be found explicitly at any value of the scattering rate. This solution explains the sample dependence of the specific heat discontinuity in MgB_2 and the influence of the disorder on the critical temperature.Comment: New results relate to two-band superconductors, 9 pages, 2 figure

Effect of strain on surface diffusion in semiconductor heteroepitaxy

We present a first-principles analysis of the strain renormalization of the cation diffusivity on the GaAs(001) surface. For the example of In/GaAs(001)-c(4x4) it is shown that the binding of In is increased when the substrate lattice is expanded. The diffusion barrier \Delta E(e) has a non-monotonic strain dependence with a maximum at compressive strain values (e 0) studied. We discuss the consequences of spatial variations of both the binding energy and the diffusion barrier of an adatom caused by the strain field around a heteroepitaxial island. For a simplified geometry, we evaluate the speed of growth of two coherently strained islands on the GaAs(001) surface and identify a growth regime where island sizes tend to equalize during growth due to the strain dependence of surface diffusion.Comment: 10 pages, 8 figures, LaTeX2e, to appear in Phys. Rev. B (2001). Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

Mechanisms and theoretical simulations of the catalytic growth of nanocarbons

Nanocarbons have been catalytically grown since 1993. However, even today, the formation mechanisms of carbon nanotubes (CNTs) and graphene are not sufficiently understood. This sustained challenge has been an engine for the development in theory concepts and computational methods, tackling the problem of well-controlled production of these nanomaterials. This article discusses how experimental discoveries and theoretical approaches evolved hand-in-hand for the successful understanding of challenging issues, highlighting parallels and distinctions between graphene and CNTs. Key aspects include the mechanisms of nucleation and CNT-liftoff, chiral symmetry selection and control, rates of growth and island shapes, mechanisms defining single chirality of the nanotubes, and ways to suppress grain boundaries in the quest for ever larger and faster growing single-crystal graphene, or longest defect-free CNTs. The theme of catalyst chemistry and structure, either as a nanoparticle or a planar substrate, is traced through the stages of nanocarbon formation, with focus on theoretically generalizable findings

Armchair or Zigzag? A tool for characterizing graphene edge

Electronic, magnetic, and structural properties of graphene flakes depend sensitively upon the type of edge atoms. We present a simple software tool for determining the type of edge atoms in a honeycomb lattice. The algorithm is based on nearest neighbor counting. Whether an edge atom is of armchair or zigzag type is decided by the unique pattern of its nearest neighbors. Particular attention is paid to the practical aspects of using the tool, as additional features such as extracting out the edges from the lattice could help in analyzing images from transmission microscopy or other experimental probes. Ultimately, the tool in combination with density-functional theory or tight-binding method can also be helpful in correlating the properties of graphene flakes with the different armchair-to-zigzag ratios. Program summary Program title: edgecount Catalogue identifier: AEIA_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEIA_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 66685 No. of bytes in distributed program, including test data, etc.: 485 381 Distribution format: tar.gz Programming language: FORTRAN 90/95 Computer: Most UNIX-based platforms Operating system: Linux, Mac OS Classification: 16.1, 7.8 Nature of problem: Detection and classification of edge atoms in a finite patch of honeycomb lattice. Solution method: Build nearest neighbor (NN) list; assign types to edge atoms on the basis of their NN pattern. Running time: Typically similar to second(s) for all examples. (C) 2010 Elsevier B.V. All rights reserved

Can Two-Dimensional Boron Superconduct?

Two-dimensional boron is expected to exhibit various structural polymorphs, all being metallic. Additionally, its small atomic mass suggests strong electron–phonon coupling, which in turn can enable superconducting behavior. Here we perform first-principles analysis of electronic structure, phonon spectra, and electron–phonon coupling of selected 2D boron polymorphs and show that the most stable structures predicted to feasibly form on a metal substrate should also exhibit intrinsic phonon-mediated superconductivity, with estimated critical temperature in the range of <i>T</i>_c ≈ 10–20 K