21 research outputs found
Engineering Quantum Spin Hall Effect in Graphene Nanoribbons via Edge Functionalization
Kane and Mele predicted that in presence of spin-orbit interaction graphene
realizes the quantum spin Hall state. However, exceptionally weak intrinsic
spin-orbit splitting in graphene ( eV) inhibits experimental
observation of this topological insulating phase. To circumvent this problem,
we propose a novel approach towards controlling spin-orbit interactions in
graphene by means of covalent functionalization of graphene edges with
functional groups containing heavy elements. Proof-of-concept first-principles
calculations show that very strong spin-orbit coupling can be induced in
realistic models of narrow graphene nanoribbons with tellurium-terminated
edges. We demonstrate that electronic bands with strong Rashba splitting as
well as the quantum spin Hall state spanning broad energy ranges can be
realized in such systems. Our work thus opens up new horizons towards
engineering topological electronic phases in nanostructures based on graphene
and other materials by means of locally introduced spin-orbit interactions.Comment: 5 pages, 3 figure
Electronic properties of one-dimensional nanostructures of the BiSe topological insulator
We theoretically study the electronic structure and spin properties of
one-dimensional nanostructures of the prototypical bulk topological insulator
BiSe. Realistic models of experimentally observed BiSe
nanowires and nanoribbons are considered using the tight-binding method. At low
energies, the band structures are composed of a series of evenly spaced
degenerate sub-bands resulting from circumferential confinement of the
topological surface states. The direct band gaps due to the non-trivial
Berry phase show a clear dependence on the circumference. The spin-momentum
locking of the topological surface states results in a pronounced 2 spin
rotation around the circumference with the degree of spin polarization
dependent on the the momentum along the nanostructure. Overall, the band
structures and spin textures are more complicated for nanoribbons, which expose
two distinct facets. The effects of reduced dimensionality are rationalized
with the help of a simple model that considers circumferential quantization of
the topological surface states. Furthermore, the surface spin density induced
by electric current along the nanostructure shows a pronounced oscillatory
dependence on the charge-carrier energy, which can be exploited in spintronics
applications.Comment: 10 pages, 9 figure
Orbital contribution to the magnetic properties of iron as a function of dimensionality
The orbital contribution to the magnetic properties of Fe in systems of
decreasing dimensionality (bulk, surfaces, wire and free clusters) is
investigated using a tight-binding hamiltonian in an and atomic
orbital basis set including spin-orbit coupling and intra-atomic electronic
interactions in the full Hartree-Fock (HF) scheme, i.e., involving all the
matrix elements of the Coulomb interaction with their exact orbital dependence.
Spin and orbital magnetic moments and the magnetocrystalline anisotropy energy
(MAE) are calculated for several orientations of the magnetization. The results
are systematically compared with those of simplified hamiltonians which give
results close to those obtained from the local spin density approximation. The
full HF decoupling leads to much larger orbital moments and MAE which can reach
values as large as 1 and several tens of meV, respectively, in the
monatomic wire at the equilibrium distance. The reliability of the results
obtained by adding the so-called Orbital Polarization Ansatz (OPA) to the
simplified hamiltonians is also discussed. It is found that when the spin
magnetization is saturated the OPA results for the orbital moment are in
qualitative agreement with those of the full HF model. However there are large
discrepancies for the MAE, especially in clusters. Thus the full HF scheme must
be used to investigate the orbital magnetism and MAE of low dimensional
systems
Multiplet features and magnetic properties of Fe on Cu(111): From single atoms to small clusters
The observation of sharp atomiclike multiplet features is unexpected for individual 3d atoms adsorbed on transition-metal surfaces. However, we show by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism that individual Fe atoms on Cu(111) exhibit such features. They are reminiscent of a low degree of hybridization, similar to 3d atoms adsorbed on alkali-metal surfaces. We determine the spin, orbital, and total magnetic moments, as well as magnetic anisotropy energy for the individual Fe atoms and for small Fe clusters that we form by increasing the coverage. The multiplet features are smoothened and the orbital moment rapidly decreases with increasing cluster size. For Fe monomers, comparison with density functional theory and multiplet calculations reveals a d(7) electronic configuration, owing to the transfer of one electron from the 4s to the 3d states
Localized electronic states at grain boundaries on the surface of graphene and graphite
Recent advances in large-scale synthesis of graphene and other 2D materials
have underscored the importance of local defects such as dislocations and grain
boundaries (GBs), and especially their tendency to alter the electronic
properties of the material. Understanding how the polycrystalline morphology
affects the electronic properties is crucial for the development of
applications such as flexible electronics, energy harvesting devices or
sensors. We here report on atomic scale characterization of several GBs and on
the structural-dependence of the localized electronic states in their vicinity.
Using low temperature scanning tunneling microscopy (STM) and spectroscopy
(STS), together with tight binding and ab initio numerical simulations we
explore GBs on the surface of graphite and elucidate the interconnection
between the local density of states (LDOS) and their atomic structure. We show
that the electronic fingerprints of these GBs consist of pronounced resonances
which, depending on the relative orientation of the adjacent crystallites,
appear either on the electron side of the spectrum or as an electron-hole
symmetric doublet close to the charge neutrality point. These two types of
spectral features will impact very differently the transport properties
allowing, in the asymmetric case to introduce transport anisotropy which could
be utilized to design novel growth and fabrication strategies to control device
performance
A Novel Quasi-One-Dimensional Topological Insulator in Bismuth Iodide -BiI
Recent progress in the field of topological states of matter(1,2) has largely
been initiated by the discovery of bismuth and antimony chalcogenide bulk
topological insulators (TIs)(3-6), followed by closely related ternary
compounds(7-16) and predictions of several weak TIs(17-19). However, both the
conceptual richness of Z classification of TIs as well as their structural
and compositional diversity are far from being fully exploited. Here, a new
Z topological insulator is theoretically predicted and experimentally
confirmed in the -phase of quasi-one-dimensional bismuth iodide
BiI. The electronic structure of -BiI, characterized by
Z invariants (1;110), is in proximity of both the weak TI phase (0;001) and
the trivial insulator phase (0;000). Our angle-resolved photoemission
spectroscopy measurements on the (001) surface reveal a highly anisotropic
band-crossing feature located at the point of the surface Brillouin zone and
showing no dispersion with the photon energy, thus being fully consistent with
the theoretical prediction
Transport électronique polarisé en spin dans les contacts atomiques de fer
This thesis is dedicated to the theoretical study of spin-dependent transport in atomic contact. The main objective is to understand the giant anisotropic magnetoresistance experimentally measured in iron break junctions. On this purpose, we developed a method to calculate electron transport properties in magnetic nanostructures. The calculation is performed in two steps. First the electronic structure of the contact is determined in a basis of atomic orbitals spd using a tight-binding model extended to include magnetism. The magnetic properties are described at the atomic scale using an interelectronic interaction Hamiltonian. Two interaction models are compared : a simple Stoner-like model and a more complete Hartree-Fock model, developed to reproduce the orbital polarization effects which appear in one-dimensional nanostructure. To describe the magnetic anisotropy, noncollinear magnetism and spin-orbit coupling are taken in account. In the second step, the electron transport properties are derived in the Landauer formalism. In this approach, the transport of electron is supposed to be coherent and elastic. The conductance is directly proportional to the transmission probability of electrons through the contact. This transmission is calculated from the Green function of the system. This method is applied to the study of magnetoresistance in iron atomic contact. Several contact geometries, from the monatomic wire to realistic systems, are compared. The results reveal the importance of contact geometry and orbital polarization.Cette thèse est consacrée à l'étude théorique du transport électronique dans les contacts atomiques magnétiques. L'objectif principal est d'expliquer la magnétorésistance anisotrope géante mesurée expérimentalement dans les jonctions à cassure de fer. Dans ce but, on a développé une méthode de calcul de la conductance des nanostructures magnétiques.Le calcul est effectué en deux étapes. Dans un premier temps, la structure électronique du contact est déterminée de manière autocohérente dans une base d'orbitales atomiques spd à l'aide d'un modèle de liaisons fortes étendu au magnétisme. Les propriétés magnétiques sont décrites à l'échelle atomique par un modèle d'interaction inter-électronique. Deux modèles d'interactions sont comparés : un modèle simple de type Stoner et un modèle plus complet de type Hartree-Fock développé pour reproduire les effets de polarisation orbitale susceptibles d'apparaître au niveau du contact. En effet, dans les nanostructures unidimensionnelles, on observe une levée du blocage du moment orbital qui existe dans les cristaux cubiques en volume. Pour permettre la description de l'anisotropie magnétique du système, on prend aussi en compte le magnétisme non-colinéaire et le couplage spin-orbite.Dans un second temps, les propriétés de transport électroniques du système sont déterminées dans le formalisme de Landauer. Dans cette approche, on considère que le transport est cohérent et élastique. Cette approximation est valide quand étudie un conducteur de taille atomique à basse température sous de faibles tensions. La conductance est alors directement proportionnelle à la probabilité de transmission des électrons à travers le système. Cette transmission est calculée à partir de la fonction de Green du système. Cette méthode de calcul est appliquée à l'étude de la magnétorésistance anisotrope des contacts de fer. Plusieurs géométries de contact, allant du fil monoatomique parfait aux systèmes réalistes, sont comparées. Les résultats révèlent le rôle prépondérant joué par la géométrie et par la polarisation orbitale. Pour que l'anisotropie magnétique soit aussi élevée que dans les expériences, il est nécessaire que l'atome de contact soit dans une configuration de fil monoatomique. Les effets de polarisation orbitale permettent d'expliquer les deux plateaux de conductance mesurés expérimentalement. Ils sont liés à l'existence de deux états magnétiques métastables qui différent par la direction du moment orbital sur l'atome de contact