Electronic Structure and Magnetic Properties of β\beta-Ti6_{6}Sn5_{5}

The electronic structure of $\beta$-Ti$_{6}$Sn$_{5}$ has been studied based on the density functional theory within the local-density approximation. The calculation indicates that $\beta$-Ti$_{6}$Sn$_{5}$ is very close to ferromagnetic instability and shows ferromagnetic ordering after rare earth element doping. Large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level

Surface state scattering by adatoms on noble metals

When surface state electrons scatter at perturbations, such as magnetic or nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized at the adatom site, can develop below the bottom of the surface state band for both spin channels. In the case of adatoms, these states have been found very recently in scanning tunneling spectroscopy experiments\cite{limot,olsson} for the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried out a systematic theoretical investigation of the electronic structure of these surface states in the presence of magnetic and non-magnetic atoms on Cu(111). We found that Ca and all 3$d$ adatoms lead to a split-off state at the bottom of the surface band which is, however, not seen for the $sp$ elements Ga and Ge. The situation is completely reversed if the impurities are embedded in the surface: Ga and Ge are able to produce a split-off state whereas the 3$d$ impurities do not. The resonance arises from the s-state of the impurities and is explained in terms of strength and interaction nature (attraction or repulsion) of the perturbing potential.Comment: 6 pages, 5 figure

Hysteretic resistance spikes in quantum Hall ferromagnets without domains

We use spin-density-functional theory to study recently reported hysteretic magnetoresistance \rho_{xx} spikes in Mn-based 2D electron gases [Jaroszy\'{n}ski et al. Phys. Rev. Lett. (2002)]. We find hysteresis loops in our calculated Landau fan diagrams and total energies signaling quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation effects are crucial to stabilize the relevant magnetic phases arising from distinct symmetry-broken excited- and ground-state solutions of the Kohn-Sham equations. Besides hysteretic spikes in \rho_{xx}, we predict hysteretic dips in the Hall resistance \rho_{xy}. Our theory, without domain walls, satisfactorily explains the recent data.Comment: 4 pages, 4 figures, published version (some changes to the text; same figures as in v1

Quantum interference in deformed carbon nanotube waveguides

Quantum interference (QI) in two types of deformed carbon nanotubes (CNTs), i.e., axially stretched and AFM tip-deformed CNTs, has been investigated by the pi-electron only and four-orbital tight-binding (TB) method. It is found that the rapid conductance oscillation (RCO) period is very sensitive to the applied strains, and decreases in an inverse proportion to the deformation degree, which could be used as a powerful experimental tool to detect precisely the deformation degree of the deformed CNTs. Also, the sigma-pi coupling effect is found to be negligible under axially stretched strain, while it works on the transport properties of the tip-deformed CNTs.Comment: 14 pages and 5 figure

Intrinsic spin Hall effect in platinum metal

Spin Hall effect in metallic Pt is studied with first-principles relativistic band calculations. It is found that intrinsic spin Hall conductivity (SHC) is as large as $\sim 2000 (\hbar/e)(\Omega {\rm cm})^{-1}$ at low temperature, and decreases down to $\sim 200 (\hbar/e)(\Omega {\rm cm})^{-1}$ at room temperature. It is due to the resonant contribution from the spin-orbit splitting of the doubly degenerated $d$-bands at high-symmetry $L$ and $X$ points near the Fermi level. By modeling these near degeneracies by effective Hamiltonian, we show that SHC has a peak near the Fermi energy and that the vertex correction due to impurity scattering vanishes. We therefore argue that the large spin Hall effect observed experimentally in platinum is of intrinsic nature.Comment: Accepted for publication in Phys. Rev. Let

A New Approach to Regulating Temporary Agency Work in Ontario or Back to the Future?

In 2009, the province of Ontario, Canada adopted the Employment Standards Amendment Act (Temporary Help Agencies) partly in response to public concern over temporary agency workers’ limited access to labour protection. This article examines its “new” approach in historical and international context, illustrating that the resulting section of the Employment Standards Act (ESA) reflects continuity through change in its continued omissions and exclusions.The article begins by defining temporary agency work and describing its significance, explaining how it exemplifies precarious employment, partly by virtue of the triangular employment relationship at its heart. Next it traces three eras of regulation, from the early 20th to the early 21st centuries: in the first era, against the backdrop of the federal government’s forays into regulation through the Immigration Act, Ontario responded to abusive practices of private employment agencies, with strict regulations, directed especially at those placing recent immigrants in employment. In the second era, restrictions on private employment agencies were gradually loosened, resulting in modest regulation; in this era, there was growing space for the emergence of “new” types of agencies providing “employment services,” including temporary help agencies, which carved out a niche for themselves by targeting marginalized social groups, such as women. The third era was characterized by the legitimization of private employment agencies and, in particular, temporary help agencies, both in a passive sense by government inaction in response to growing complexities surrounding their operation, and in an active sense by the repeal of Ontario’s Employment Agencies Act in 2000.Despite a consultative process aimed, in the words of Ontario’s then Minister of Labour, at “enhanc [ing] protections for employees working for temporary help agencies,” the new section of the ESA adopted in 2009 reproduces outdated approaches to regulation through its omissions and exclusions; specifically, it focuses narrowly on temporary help agencies rather than including an overlapping group of private employment agencies with which they comprise the employment services industry and its denial of access to protection to workers from a particular occupational group (i.e., workers placed by a subset of homecare agencies otherwise falling within the definition of “assignment employees”). Highlighting the importance of looking back in devising new regulations, the article concludes by advancing a more promising approach for the future that would address more squarely the triangular employment relationship as the basis for extending greater protection to workers.En 2009, l’Ontario a adopté la Loi modifiant la Loi sur les normes d’emploi en ce qui concerne les agences temporaires de placement et certaines autres questions afin de répondre, du moins en partie, à la préoccupation publique à l’égard de l’accès limité en matière de protection des conditions de travail des travailleurs de ces agences. Le présent article examine cette « nouvelle » approche dans une perspective historique et internationale, ce qui permet d’observer que cette nouvelle section de la Loi sur les normes d’emploi (LNE) s’inscrit, à travers le changement, dans une continuité en ce qui concerne les exclusions et les omissions de la LNE.L’article débute par une définition de l’expression « agence temporaire de placement » (ATP), tout en faisant ressortir sa signification en termes de précarité d’emploi grâce à la relation d’emploi triangulaire qui est au coeur de celle-ci. Puis il retrace trois périodes de régulation. Première période du début du 20e siècle au début du 21e siècle, avec en toile de fond les incursions du gouvernement fédéral en matière de réglementation via la Loi sur l’immigration, la province d’Ontario a répondu aux pratiques abusives des agences de placement privées par des règles strictes s’adressant directement aux agences faisant le placement des nouveaux immigrants. Durant la seconde période, les restrictions envers les agences de placement temporaires ont été graduellement relâchées, donnant lieu à une régulation plus modeste; c’est aussi la période d’émergence de « nouveaux » types d’agences procurant des « services d’emploi », incluant des agences temporaires d’aide qui se découvraient une niche en ciblant des groupes sociaux marginalisés, comme les femmes. La troisième période est caractérisée par la légitimation des agences privées de placement, en particulier des agences temporaires d’aide, de façon passive via l’inaction du gouvernement en réponse à la complexité croissante de leurs opérations, et de façon active par l’abolition la Loi sur les agences de placement de l’Ontario en 2000.En dépit de la tenue d’un processus de consultation dont le but, selon les dires du Ministre du travail, était d’ « élargir les protections pour les employés travaillant pour des agences temporaires d’aide », cette nouvelle section de la LNE adoptée en 2009 ne fait que reproduire les approches dépassées de régulations à travers ses exclusions et ses omissions. Plus spécifiquement, on vise de façon étroite les agences temporaires d’aide plutôt que le groupe plus large des agences privées d’emploi qui englobent l’industrie des services d’emploi et son dénie d’accès à la protection des travailleurs d’un groupe professionnel particulier (soit les travailleurs placés en emploi par un sous-ensemble d’agences de soins à domicile qui seraient autrement considérés légalement comme travailleurs de l’agence, i.e. selon une relation « employeur-employé »). Mettant en lumière l’importance de jeter un regard sur le passé pour développer de nouvelles régulations, l’article conclut en proposant une approche plus prometteuse pour l’avenir et qui s’adresserait plus carrément au problème de la relation d’emploi triangulaire comme support à une protection plus étendue des travailleurs.En 2009, la provincia de Ontario en Canadá adoptó la revisión de la ley de normas de empleo (Agencias de ayuda temporaria) parcialmente en respuesta a la preocupación pública sobre el acceso limitado a la protección laboral de los trabajadores temporales de agencias. Este artículo analiza su “nuevo” enfoque en un contexto histórico e internacional, ilustrando que dicha sección de la Ley de normas de empleo refleja una continuidad a través del cambio en omisiones y exclusiones persistentes.Este artículo comienza definiendo el trabajo temporario de agencias y describe su significado, explica cómo esto ilustra el empleo precario, parcialmente en virtud de la relación triangular de empleo que constituye su esencia. Luego, se esbozan tres eras de regulación, desde el comienzo del siglo XX hasta el comienzo del siglo XXI: en la primera era, frente a las incursiones del gobierno federal en la regulación mediante la Ley de inmigración, Ontario respondía a las prácticas abusivas de las empresas privadas de empleo con estrictas regulaciones, dirigidas en particular contra aquellas que ofrecían servicios de empleo a inmigrantes recientes.En la segunda era, las restricciones respecto a las agencias de empleo privado fueron relajadas gradualmente, resultando en una regulación modesta; en esta era, hubo un espacio creciente para la emergencia de “nuevos” tipos de agencias proveedoras de servicios de empleo, incluyendo las agencias de ayuda temporal que se abrieron un espacio propio focalizando los grupos sociales marginalizados, como las mujeres. La tercera era fue caracterizada por la legitimación de las agencias privadas de empleo y, en particular, de las agencias de ayuda temporal, legitimación en sentido pasivo mediante la inacción del gobierno en respuesta a las crecientes complejidades que rodean el funcionamiento de dichas agencias, y en un legitimación en sentido activo mediante la revocación de la Ley de las agencias de empleo de Ontario en el año 2000.A pesar de un proceso consultativo orientado, según las palabras del entonces Ministro de trabajo de Ontario, a “ampliar las protecciones para los empleados que trabajan para las agencias de ayuda temporal”, la nueva sección de la Ley de normas de empleo adoptada en 2009, con sus omisiones y exclusiones, reproduce los enfoques obsoletos de regulación; en particular, la ley se centra casi exclusivamente en las agencias de ayuda temporal en vez de incluir un amplio grupo de agencias privadas de empleo que comprenden la industria de servicios de empleo y la negativa de acceso a la protección de los trabajadores de un grupo ocupacional particular (es decir, los trabajadores colocados por un subconjunto de agencias de servicios a domicilio que de otra manera caen en la definición de “empleados por asignación”).Destacando la importancia de ver retrospectivamente el diseño de nuevas regulaciones, este artículo concluye con la propuesta de un enfoque más prometedor para el futuro que se dirigiría más directamente a la relación de empleo triangular como base para lograr una más amplia protección para los trabajadores

Extracting convergent surface energies from slab calculations

The formation energy of a solid surface can be extracted from slab calculations if the bulk energy per atom is known. It has been pointed out previously that the resulting surface energy will diverge with slab thickness if the bulk energy is in error, in the context of calculations which used different methods to study the bulk and slab systems. We show here that this result is equally relevant for state-of-the-art computational methods which carefully treat bulk and slab systems in the same way. Here we compare different approaches, and present a solution to the problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies.Comment: 3 revtex pages, 1 figure, in print on J. Phys. Cond. Mat

A simple, efficient, and general treatment of the singularities in Hartree-Fock and exact-exchange Kohn-Sham methods for solids

We present a general scheme for treating the integrable singular terms within exact exchange (EXX) Kohn-Sham or Hartree-Fock (HF) methods for periodic solids. We show that the singularity corrections for treating these divergencies depend only on the total number and the positions of k-points and on the lattice vectors, in particular the unit cell volume, but not on the particular positions of atoms within the unit cell. The method proposed here to treat the singularities constitutes a stable, simple to implement, and general scheme that can be applied to systems with arbitrary lattice parameters within either the EXX Kohn-Sham or the HF formalism. We apply the singularity correction to a typical symmetric structure, diamond, and to a more general structure, trans-polyacetylene. We consider the effect of the singularity corrections on volume optimisations and k-point convergence. While the singularity corrections clearly depends on the total number of k-points, it exhibits a remarkably small dependence upon the choice of the specific arrangement of the k-points.Comment: 24 pages, 5 Figures, re-submitted to Phys. Rev. B after revision

Magnetism of 3d transition metal atoms on W(001): submonolayer films

We have investigated random submonolayer films of 3d transition metals on W(001). The tight-binding linear muffin-tin orbital method combined with the coherent potential approximation was employed to calculate the electronic structure of the films. We have estimated local magnetic moments and the stability of different magnetic structures, namely the ferromagnetic order, the disordered local moments and the non-magnetic state, by comparing the total energies of the corresponding systems. It has been found that the magnetic moments of V and Cr decrease and eventually disappear with decreasing coverage. On the other hand, Fe retains approximately the same magnetic moment throughout the whole concentration range from a single impurity to the monolayer coverage. Mn is an intermediate case between Cr and Fe since it is non-magnetic at very low coverages and ferromagnetic otherwise.Comment: 5 pages, 3 figures in 6 files; presented at ICN&T 2006, Basel, Switzerlan