Type of Paper:
Topological defects in Topological Insulators and bound states at Topological Superconductor vorticesAuthors:
, G. Campagnano2
, A. Tagliacozzo1,2
and F. Guinea3
Università degli studi di Napoli “Federico II”, Napoli, Italy; 2
CNR-SPIN, Napoli, Italy; 3
Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, SpainAbstract:
Scattering of Dirac electrons by topological defects and bound states have been proposed as the most relevant source of resistance in graphene  and at the boundary surfaces of three dimensional Topological Insulators (3D-TI). Striking enough, when a 3D-TI is proximized by a superconductor, a vortex can bind a Majorana fermionic quasiparticle at the interface .
We will discuss the electron cross section for intravalley scattering at an edge dislocation in graphene and its contribution to the resistivity r within the Boltzmann theory of transport, including corrections coming from the local stress. The result will be compared to the resistivity induced by a localized gaussian bump , curving the surface. The long wavelength continuous limit will be used throughout. The unifying scheme of the Dirac equation on a curved space[4,5] allows us to consider topological point-like defects and bumps for an unrelaxed lattice, on the same foot. While r is proportional to the carrier concentration n when electrons scatter off a bump, an edge dislocation provides the expected inverse dependence rµn-1 . Smooth potentials are likely not to change the isospin and the valley of the scattering electrons, nor their energy spectrum drastically, unless the strain produces gaps or zero energy states[7,8].
Next we present the solution for a bound state at a screw disclination in a two band 3D-TI like Bi2Se3. When the TI is proximized by a superconductor and a vortex pierces the TI, a bound state forms at the interfaces in the vortex core, which acts in analogy with the singularity of a screw disclination. If the parity of the order parameter of the superconductor is even, the bound state excitation is a Majorana bound state. This is not the case if the parity is odd .
 M. I. Katsnelson, F. Guinea and A. K. Geim, Phys.Rev. B 2009, 79, 195426.
 L. Fu and C.L. Kane, Phys. Rev. Lett. 2008, 100, 096407.
 V. Parente, P. Vitale, A. Tagliacozzo and F. Guinea, unpublished.
 F. De Juan, A. Cortijo and M.A. Vozmediano, Phys. Rev. B 2007, 76, 165409.
 V. Parente, P. Lucignano, P. Vitale, A. Tagliacozzo and F. Guinea, Phys. Rev. B 2011, 83, 075424.
 F.Guinea, J. Low. Temp. Phys.2008, 153, 359.
 V.M. Pereira, A.H. Castro Neto, and N.M.R. Peres, Phys. Rev. B 2009, 80, 045401.
 F. Guinea, M. I. Katsnelson and A. K. Geim, Nat.Phys. 2010, 6, 30.
 A. Tagliacozzo, P. Lucignano and F. Tafuri, Phys. Rev. B 2012, 86, 045435.
Keywords: Dirac electrons; topological effects; two band topological insulators; Majorana bound state
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Type of Paper: Article
Title: Atomic Ordering Effects in Highly Spin-Polarized Co-based Full Heusler Alloys
Authors: P.J. Hasnip, C. Loach, J.H. Smith, M.I.J. Probert and V.K. Lazarov
Affiliation: Department of Physics, University of York, York YO10 5DD, UK
Abstract: Co-based full Heusler alloys are predicted to be half-metallic in bulk, making them excellent candidates for spintronics devices such as current-perpendicular-to-plane spin valves. This half-metallicity has yet to be realised in device or thin film heterostructures, a discrepancy often attributed to the level of chemical and structural disorder in the thin films. Such disorder ultimately reduces the spin polarization of the Heusler alloys at the Fermi level, destroying their half-metallicity.
We present a first principles study of the effect of disorder on the spin polarization of the bulk Co-based Heusler alloys Co2MnSi, Co2FeSi and Co2MnxFe1-xSi. The half-metallicity is shown to be lost in the presence of most types of disorder, though in several cases the spin polarization at the Fermi level remains high. Whilst the spin polarization is lost or even reversed when intermixing occurs between the Co sublattice and the Fe/Mn or Si sublattices, the polarization is relatively robust when the Co sublattice is kept intact. Calculations of the disordering energies show that it is particularly favourable energetically to keep the Co sublattice intact, suggesting that high spin polarization may be recovered by suitable annealing. A comparison of the disordering energies across the various Co-based Heuslers provides insight into the ordering trend amongst these materials and the qualitative benefits of annealing at various temperatures. This latter insight is important in guiding the future growth and post-growth treatment of such Heusler-based heterostructures.
Type of Paper: Review
Title: Spin and Charge Excitations in Low Dimensional Electron Liquids
Author: Florent Perez
Affiliation: Institut des NanoSciences de Paris, Université Pierre et Marie Curie, Paris, France
Abstract: We review 15 years of optical exploration of low energy spin and charge excitations, like plasmons, spin-plasmons, spin-waves and individual excitations, of electron gases confined in semi-conductor quantum wells and wires. The electronic Raman scattering probe has, during these years, proven its efficiency in the accurate determination of the dispersion of these excitations. These explorations aimed at improving the understanding of the Coulomb interaction between confined electrons. Recently, the introduction of test-bed spin-polarized electron gases gave an experimental access to spin-resolved Coulomb interactions. Very recently, intriguing interplay between spin-orbit and Coulomb interactions has been discovered.
Keywords: spin-plasmon, plasmon, spin-wave, electron gas, quantum wells, Coulomb interaction, spin-orbit interaction, Raman scattering