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Graphene has been identified as a promising material with numerous applications, particularly in spintronics. In this paper we investigate the peculiar features of spin excitations of magnetic units deposited on graphene nanoribbons and how they can couple through a dynamical interaction mediated by spin currents. We examine in detail the spin lifetimes and identify a pattern caused by vanishing density of states sites in pristine ribbons with armchair borders. Impurities located on these sites become practically invisible to the interaction, but can be made accessible by a gate voltage or doping. We also demonstrate that the coupling between impurities can be turned on or off using this characteristic, which may be used to control the transfer of information in transistor-like devices., Comment: 10 pages, 10 figures

We demonstrate that ferromagnetic and antiferromagnetic excitations can be triggered by the dynamical spin accumulations induced by the bulk and surface contributions of the spin Hall effect. Due to the spin-orbit interaction, a time-dependent spin density is generated by an oscillatory electric field applied parallel to the atomic planes of Fe/W(110) multilayers. For symmetric trilayers of Fe/W/Fe in which the Fe layers are ferromagnetically coupled, we demonstrate that only the collective out-of-phase precession mode is excited, while the uniform (in-phase) mode remains silent. When they are antiferromagnetically coupled, the oscillatory electric field sets the Fe magnetizations into elliptical precession motions with opposite angular velocities. The manipulation of different collective spin-wave dynamical modes through the engineering of the multilayers and their thicknesses may be used to develop ultrafast spintronics devices. Our work provides a general framework that probes the realistic responses of materials in the time or frequency domain., Comment: 5 pages, 4 figures

We review the basic definitions of the direct microscopic formalism (DMF) and the corresponding model code TRANSNU, to describe pre-equilibrium nuclear systems, as elements of the grand canonical ensemble. We analyze its inconsistencies, especially the impossibility of exact solution of the nuclear many-body problem, and propose solutions. We use the strong dependence of pre-equilibrium emissions on the ratio of transition rates, in code ${\bf \mbox{TNG}}$, to propose a redefinition of the parameters in the master equation, and obtain smoother excitation functions in the energy regions where different exciton classes contribute. We compare the transition rates of TRANSNU with the phenomenological ones for the estimation the excitation function of a few p-induced reactions on $^{56}$Fe, and obtain reasonable descriptions for activation energy, local maxima and average magnitude. Despite the inconsistencies and a few remaining numerical problems, related with non meaningful noises in the strong oscillations of the TST, especially with the excitation energy and for low exciton number, we find the results of the present work very promising, indicating that the DFM can be used as a more precise and physically meaningful approach for the study of nuclear systems in pre-equilibrium than the traditional statistical models., Comment: 50 pages, 11 figures

We present an analysis of the use of the Darwin-Fowler approximation in connection with the statistical IPM, by comparing the results of our recent studies with the occupation number approach (OCN) and some traditional statistical independent particle model (IPM) approaches. The analysis of level density works based on the statistical IPM reveals that the use of the the Darwin-Fowler approximation, in some of them, is theoretically inconsistent and some of their results should rather be considered as theoretical coincidences with other consistent approaches, than proofs of their validity. We conclude that, in general, the use of the Darwin-Fowler approximation with the statistical IPM should be used criteriously or, if possible, avoided altogether and suggest that the combinatorial IPM approaches have important advantages over the other models and formalisms analyzed in this paper, especially regarding the consistency of the microscopic description of the nuclear structure and dynamics of non highly excited systems.

In ultrathin ferromagnets deposited on metallic substrates, excitation of precessional motion of the spins produces a spin current in the substrate that transports angular momentum out of the film. This phenomenon is referred to as spin pumping, and is a source of damping of the spin motion. Spin pumping enters importantly in the description of spin dynamics in other nanoscale and subnanoscale systems as well. In this paper, we present an approach based on the Kubo formalism that allows the explicit calculation of this spin current and its spatial variation. We use the formalism to explore features of the spin current generated by spin motions in a simple model system., Comment: 10 pages, 7 figures

The growing interest in carbon-based spintronics has stimulated a number of recent theoretical studies on the RKKY interaction in graphene, based on which the energetically favourable alignment between magnetic moments embedded in this material can be calculated. The general consensus is that the strength of the RKKY interaction in graphene decays as 1/D3 or faster, where D is the separation between magnetic moments. Such an unusually fast decay for a 2-dimensional system suggests that the RKKY interaction may be too short ranged to be experimentally observed in graphene. Here we show in a mathematically transparent form that a far more long ranged interaction arises when the magnetic moments are taken out of their equilibrium positions and set in motion. We not only show that this dynamic version of the RKKY interaction in graphene decays far more slowly but also propose how it can be observed with currently available experimental methods., Comment: 7 pages, 2 figures, submitted

We present a brief numerical study of the Darwin-Fowler method applied to the analysis of the energy partition of essembles of bosons and fermions. We analyze the assertion of the existence of a "strong maximum" made in the original paper of Darwin and Fowler and other studies and show that although the presumed saddle point along the real axis of the grand canonical parameters may exist it cannot, in general, be characterized as "strong", in the sense of having much larger magnitude than the other points along the path of integration. We show that in some cases the saddle point is not even present and the various approximations of the method can be interpreted as a tricky reformulation of usual thermodynamic relations. The close connection of the method with the formalism of the Laplace transform may produce wrong results if the internal energy of the components of the ensemble is not large enough. Therefore, although useful in many applications the Darwin-Fowler method may not be suitable, in general, for a detailed microscopic analysis of the nuclear structure in connection with the Shell Model approach, as it is usually done in studies of the pre-equilibrium stage of nuclear reactions.

We present a microscopic formalism that extends the traditional formulation of Williams, Ericson and Bloch and permits to obtain the transition strengths (TS) of pre-equilibrium nuclear reactions directly from their quantum microscopic description. We calculate the TS without resorting to the Laplace transform approach and the use of the saddle point approximation. We also analyze some problems that may appear in connection with these mathematical tools and the Darwin-Fowler approach in this case. We show that, analogously to the nuclear densities, the strengths for transitions that change the exciton number by two or leave it unchanged can be estimated microscopically as convolutions of the functions of simpler states. When using the HO basis for the Model Space we obtained important departure from the results of the exciton model (EXM), which can partially invalidate our previous analysis on the attainment of equilibrium during the PE stage. On the other hand, by using constant grid of energies for the sp-basis we were able to reproduce the results of EXM quite well in a large range of excitation energies. A new model code, TRANSNU, was developed that can be ported to traditional semi-classical codes like TNG for nuclear data evaluation.

In spintronics, the ability to transport magnetic information often depends on the existence of a spin current traveling between two different magnetic objects acting as source and probe. A large fraction of this information never reaches the probe and is lost because the spin current tends to travel omni-directionally. We propose that a curved boundary between a gated and a non-gated region within graphene acts as an ideal lens for spin currents despite being entirely of non-magnetic nature. We show as a proof of concept that such lenses can be utilized to redirect the spin current that travels away from a source onto a focus region where a magnetic probe is located, saving a considerable fraction of the magnetic information that would be otherwise lost., Comment: 9 pages, 3 figures

We demonstrate with a fully quantum-mechanical approach that graphene can function as gate-controllable transistors for pumped spin currents, i.e., a stream of angular momentum induced by the precession of adjacent magnetizations, which exists in the absence of net charge currents. Furthermore, we propose as a proof of concept how these spin currents can be modulated by an electrostatic gate. Because our proposal involves nano-sized systems that function with very high speeds and in the absence of any applied bias, it is potentially useful for the development of transistors capable of combining large processing speeds, enhanced integration and extremely low power consumption.

We demonstrate with a quantum-mechanical approach that carbon nanotubes are excellent spin-current waveguides and are able to carry information stored in a precessing magnetic moment for long distances with very little dispersion and with tunable degrees of attenuation. Pulsed magnetic excitations are predicted to travel with the nanotube Fermi velocity and are able to induce similar excitations in remote locations. Such an efficient way of transporting magnetic information suggests that nanotubes are promising candidates for memory devices with fast magnetization switchings.

Using the Dirac-Hartree-Fock-Bogoliubov approximation to study nuclear pairing, we have found the short-range correlations of the Dirac $^1$S$_0$ pairing fields to be essentially identical to those of the two-nucleon virtual state at all values of the baryon density. We make use of this fact to develop a relativistic separable potential that correctly describes the pairing fields., Comment: 17 pages, 4 eps-figures

We use the Gorkov formulation of the Dirac-Hartree-Fock-Bogoliubov approximation to nuclear pairing to study the ^1S_0 nucleon-nucleon correlations in nuclear matter. We find the short-range correlations of the ^1S_0 pairing fields to be almost identical to those of the two-nucleon virtual state. We obtain mutually consistent results for the pairing fields, using several different sets of effective interaction parameters, when we demand that each of these sets places the virtual-state pole at its physical location., Comment: 11 pages, 9 PostScript figures, RevTex, submitted to Phys. Rev. C