Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100)

The lifetimes of spin excitations of Mn adsorbates on CuN/Cu(100) are computed from first-principles. The theory is based on a strong-coupling T-matrix approach that evaluates the decay of a spin excitation due to electron-hole pair creation. Using a previously developed theory [Phys. Rev. Lett. {\bf 103}, 176601 (2009) and Phys. Rev. B {\bf 81}, 165423 (2010)], we compute the excitation rates by a tunneling current for all the Mn spin states. A rate equation approach permits us to simulate the experimental results by Loth and co-workers [Nat. Phys. {\bf 6}, 340 (2010)] for large tunnelling currents, taking into account the finite population of excited states. Our simulations give us insight into the spin dynamics, in particular in the way polarized electrons can reveal the existence of an excited state population. In addition, it reveals that the excitation process occurs in a way very different from the deexcitation one. Indeed, while excitation by tunnelling electrons proceeds via the s and p electrons of the adsorbate, deexcitation mainly involves the d electrons

Creating pseudo Kondo-resonances by field-induced diffusion of atomic hydrogen

In low temperature scanning tunneling microscopy (STM) experiments a cerium adatom on Ag(100) possesses two discrete states with significantly different apparent heights. These atomic switches also exhibit a Kondo-like feature in spectroscopy experiments. By extensive theoretical simulations we find that this behavior is due to diffusion of hydrogen from the surface onto the Ce adatom in the presence of the STM tip field. The cerium adatom possesses vibrational modes of very low energy (3-4meV) and very high efficiency (> 20%), which are due to the large changes of Ce-states in the presence of hydrogen. The atomic vibrations lead to a Kondo-like feature at very low bias voltages. We predict that the same low-frequency/high-efficiency modes can also be observed at lanthanum adatoms.Comment: five pages and four figure

Magnetic transitions induced by tunnelling electrons in individual adsorbed M-Phthalocyanine molecules (M ≡\equiv Fe, Co)

We report on a theoretical study of magnetic transitions induced by tunnelling electrons in individual adsorbed M-Phthalocyanine (M-Pc) molecules where M is a metal atom: Fe-Pc on a Cu(110)(2$\times$1)-O surface and Co-Pc layers on Pb(111) islands. The magnetic transitions correspond to the change of orientation of the spin angular momentum of the metal ion with respect to the surroundings and possibly an applied magnetic field. The adsorbed Fe-Pc system is studied with a Density Functional Theory (DFT) transport approach showing that i) the magnetic structure of the Fe atom in the adsorbed Fe-Pc is quite different from that of the free Fe atom or of other adsorbed Fe systems and ii) that injection of electrons (holes) into the Fe atom in the adsorbed Fe-Pc molecule dominantly involves the Fe $3d_{z^2}$ orbital. These results fully specify the magnetic structure of the system and the process responsible for magnetic transitions. The dynamics of the magnetic transitions induced by tunnelling electrons is treated in a strong-coupling approach. The Fe-Pc treatment is extended to the Co-Pc case. The present calculations accurately reproduce the strength of the magnetic transitions as observed by magnetic IETS (Inelastic Electron Tunnelling Spectroscopy) experiments; in particular, the dominance of the inelastic current in the conduction of the adsorbed M-Pc molecule is accounted for

The double torus as a 2D cosmos: groups, geometry and closed geodesics

The double torus provides a relativistic model for a closed 2D cosmos with topology of genus 2 and constant negative curvature. Its unfolding into an octagon extends to an octagonal tessellation of its universal covering, the hyperbolic space H^2. The tessellation is analysed with tools from hyperbolic crystallography. Actions on H^2 of groups/subgroups are identified for SU(1, 1), for a hyperbolic Coxeter group acting also on SU(1, 1), and for the homotopy group \Phi_2 whose extension is normal in the Coxeter group. Closed geodesics arise from links on H^2 between octagon centres. The direction and length of the shortest closed geodesics is computed.Comment: Latex, 27 pages, 5 figures (late submission to arxiv.org

Osteogénesis Imperfecta en la infancia: Consideraciones clínicas y aspectos del tratamiento quirúrgico

Se ha llevado a cabo un estudio retrospective de los pacientes afectados de Osteogénesis Imperfecta que fueron diagnosticados durante los últimos 20 años en nuestro Hospital Infantil. Comprende un total de 26 casos que se han clasificado siguiendo los criterios de Sillence. El estudio se centró especialmente en aquellos enfermos incluidos en el tipo III, que precisaron tratamiento quirúrgico para tratar de conseguir la deambulación. Describimos las técnicas quirúrgicas empleadas, en función del diámetro de la cavidad medular, ya que la utilización de los clavos telescópicos, que es la preferible, sólo es posible en los niños mayores. Fueron precisas varias intervenciones en cada paciente para corregir y mantener la alineación de los huesos largos durante el crecimiento y permitir la marcha. Se analizan las complicaciones más frecuentes asociadas al tratamiento quirúrgico así como las anomalías del desarrollo derivadas de la propia afección.A retrospective review was made of 26 patients with Osteogenesis Imperfecta treated in our Hospital during the past 20 years. All of them were classified according to the Sillence Criteria. The study was focused on the type III patients, who underwent surgical treatment in order to achieve ambulation. We also describe the surgical technique, which as chosen according to the width of the medullary channel, being extensible nails the best choice; they could only be use in older children. Several surgical procedures were needed on each patient to correct or maintain the normal alignment of the long bones during the growth period. Surgical complications were analyzed as well as those produced by the natural course of the disease

Surface embedding, topology and dualization for spin networks

Spin networks are graphs derived from 3nj symbols of angular momentum. The surface embedding, the topology and dualization of these networks are considered. Embeddings into compact surfaces include the orientable sphere S^2 and the torus T, and the not orientable projective space P^2 and Klein's bottle K. Two families of 3nj graphs admit embeddings of minimal genus into S^2 and P^2. Their dual 2-skeletons are shown to be triangulations of these surfaces.Comment: LaTeX 17 pages, 6 eps figures (late submission to arxiv.org

Energetics and stability of dangling-bond silicon wires on H passivated Si(100)

We evaluate the electronic, geometric and energetic properties of quasi 1-D wires formed by dangling bonds on Si(100)-H (2 x 1). The calculations are performed with density functional theory (DFT). Infinite wires are found to be insulating and Peierls distorted, however finite wires develop localized electronic states that can be of great use for molecular-based devices. The ground state solution of finite wires does not correspond to a geometrical distortion but rather to an antiferromagnetic ordering. For the stability of wires, the presence of abundant H atoms in nearby Si atoms can be a problem. We have evaluated the energy barriers for intradimer and intrarow diffusion finding all of them about 1 eV or larger, even in the case where a H impurity is already sitting on the wire. These results are encouraging for using dangling-bond wires in future devices.Comment: 8 pages, 6 figure

Análisis experimental de los déficits motivacionales y cognitivos del "Learned Helplessness" en una muestra de adolescentes no depresivos

En este trabajo se analizan los deficits motivacionales y cognitivos del learned helplessness en función de las expectativas de autoeficacia, expectativas de contingencia de respuesta-resultado, locus de control y sexo de los sujetos en una muestra de 274 adolescentes no depresivos. Los resultados indican que los déficits motivacionales solo se presentan en el grupo de baja expectativa de autoeficacia y alta expectativa de contingencia de respuesta-resultado. No se detectaron déficits cognitivos en ninguno de los grupos.Within the present papel the authors analyse the motivational and cognitive deficits of the model of learned heiplessness from the expectations of self-eficacy, expectations of response-outcome contingency, locus of control, and sex of the subjets; using a sample of 274 non depressed teenagers. The results suggest that the motivational deficits are only present within the group that shows low expectations of self-eficacy and high expectations of response-outcome contingency. The cognitive deficits were not found in either of the group

Inelastic current-voltage characteristics of atomic and molecular junctions

We report first-principles calculations of the inelastic current-voltage (I-V) characteristics of a gold point contact and a molecular junction in the nonresonant regime. Discontinuities in the I-V curves appear in correspondence to the normal modes of the structures. Due to the quasi-one-dimensional nature of these systems, specific modes with large longitudinal component dominate the inelastic I-V curves. In the case of the gold point contact, our results are in good agreement with recent experimental data. For the molecular junction, we find that the inelastic I-V curves are quite sensitive to the structure of the contact between the molecule and the electrodes thus providing a powerful tool to extract the bonding geometry in molecular wires.Comment: 4 pages, 3 figure