2,199 research outputs found
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 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)(21)-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 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
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
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
Quantum kernels to learn the phases of quantum matter
Classical machine learning has succeeded in the prediction of both classical and quantum phases of matter. Notably, kernel methods stand out for their ability to provide interpretable results, relating the learning process with the physical order parameter explicitly. Here we exploit quantum kernels instead. They are naturally related to the fidelity, and thus it is possible to interpret the learning process with the help of quantum information tools. In particular, we use a support vector machine (with a quantum kernel) to predict and characterize second-order quantum phase transitions. We explain and understand the process of learning when the fidelity per site (rather than the fidelity) is used. The general theory is tested in the Ising chain in transverse field. We show that for small-sized systems, the algorithm gives accurate results, even when trained away from criticality. Besides, for larger sizes we confirm the success of the technique by extracting the correct critical exponent ¿. Finally, we present two algorithms, one based on fidelity and one based on the fidelity per site, to classify the phases of matter in a quantum processor
Osteogénesis Imperfecta en la infancia: Consideraciones clÃnicas 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 asà como las anomalÃas 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
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