Hydrogenated Graphene Nanoribbons for Spintronics

We show how hydrogenation of graphene nanoribbons at small concentrations can open new venues towards carbon-based spintronics applications regardless of any especific edge termination or passivation of the nanoribbons. Density functional theory calculations show that an adsorbed H atom induces a spin density on the surrounding $\pi$ orbitals whose symmetry and degree of localization depends on the distance to the edges of the nanoribbon. As expected for graphene-based systems, these induced magnetic moments interact ferromagnetically or antiferromagnetically depending on the relative adsorption graphene sublattice, but the magnitude of the interactions are found to strongly vary with the position of the H atoms relative to the edges. We also calculate, with the help of the Hubbard model, the transport properties of hydrogenated armchair semiconducting graphene nanoribbons in the diluted regime and show how the exchange coupling between H atoms can be exploited in the design of novel magnetoresistive devices

Coherent transport in graphene nanoconstrictions

We study the effect of a structural nanoconstriction on the coherent transport properties of otherwise ideal zig-zag-edged infinitely long graphene ribbons. The electronic structure is calculated with the standard one-orbital tight-binding model and the linear conductance is obtained using the Landauer formula. We find that, since the zero-bias current is carried in the bulk of the ribbon, this is very robust with respect to a variety of constriction geometries and edge defects. In contrast, the curve of zero-bias conductance versus gate voltage departs from the $(2n+1) e^2/h$ staircase of the ideal case as soon as a single atom is removed from the sample. We also find that wedge-shaped constrictions can present non-conducting states fully localized in the constriction close to the Fermi energy. The interest of these localized states in regards the formation of quantum dots in graphene is discussed.Comment: 9 pages, 9 figure

Spin splitting in a polarized quasi-two-dimensional exciton gas

We have observed a large spin splitting between "spin" $+1$ and $-1$ heavy-hole excitons, having unbalanced populations, in undoped GaAs/AlAs quantum wells in the absence of any external magnetic field. Time-resolved photoluminescence spectroscopy, under excitation with circularly polarized light, reveals that, for high excitonic density and short times after the pulsed excitation, the emission from majority excitons lies above that of minority ones. The amount of the splitting, which can be as large as 50% of the binding energy, increases with excitonic density and presents a time evolution closely connected with the degree of polarization of the luminescence. Our results are interpreted on the light of a recently developed model, which shows that, while intra-excitonic exchange interaction is responsible for the spin relaxation processes, exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors.Comment: Revtex, four pages; four figures, postscript file Accepted for publication in Physical Review B (Rapid Commun.

Granuloma reparativo de células gigantes en el escafoides tarsiano: a propósito de 1 caso

Se presenta el caso de una mujer de 22 años con dolor en la región medialarsiana del pie izquierdo de 1 año de evolución sin antecedente traumático. La paciente presentaba una tumoración dolorosa en la cara medial de escafoides. La movilidad del tobillo era completa. La radiología mostraba una lesión radiolucente insuflante en el escafoides. En la TAC se apreciaba una ruptura de la cortical y la ausencia de afectación de partes blandas. La captación con 99Tc mostraba una alta concentración de radioisótopo en la lesión. El tratamiento quirúrgico incluyó curetaje e injerto óseo. La microscopía de la muestra quirúrgica ofreció el diagnóstico de granuloma reparativo de células gigantes. Se discute el diagnóstico diferencial de esta lesión.A 22-year-old women with pain in her left midle foot since one year before the first visit to the hospital, without any previous trauma is presented. At physical examination there was a tender point and tumor at the medial aspect of the foot, over the navicular bone. Complete range of motion was present. Plain X-rays showed a radiolucent lesion in the navicular. Computed tomography showed cortical bone disruption and no soft tissue involvement. The tecnetium-99 bone scan disclosed a high concentration of radioisotope at the lesion. Surgical treatment included curetagge and bone grafting. Microscopic assesment of the resected tissue was compatible with giant cell reparative granuloma. The differential diagnosis is discussed