Rashba spin-orbit interaction enhanced by graphene in-plane deformations

Graphene consists in a single-layer carbon crystal where 2$p_z$ electrons display a linear dispersion relation in the vicinity of the Fermi level, conveniently described by a massless Dirac equation in $2+1$ spacetime. Spin-orbit effects open a gap in the band structure and offer perspectives for the manipulation of the conducting electrons spin. Ways to manipulate spin-orbit couplings in graphene have been generally assessed by proximity effects to metals that do not compromise the mobility of the unperturbed system and are likely to induce strain in the graphene layer. In this work we explore the $\rm{U(1)}\times SU(2)$ gauge fields that result from the uniform stretching of a graphene sheet under a perpendicular electric field. Considering such deformations is particularly relevant due to the counter-intuitive enhancement of the Rashba coupling between 30-50% for small bond deformations well known from tight-binding and DFT calculations. We report the accessible changes that can be operated in the band structure in the vicinity of the K points as a function of the deformation strength and direction.Comment: 10 pages, 7 figure

Intrinsic switching field distribution of arrays of Ni80Fe20 nanowires probed by in situ\mathit{in\, situ} magnetic force microscopy

The progress of magnetization reversal of weakly packed ferromagnetic Ni80Fe20 nanowire arrays of different diameters (40, 50, 70 and 100 nm) electrodeposited in polycarbonate membranes was studied by magnetic force microscopy (MFM). For such a low packing density of nanomagnets, the dipolar interactions between neighbouring wires can be neglected. The intrinsic switching field distribution has been extracted from in situ MFM images and its width was found to be considerably smaller than for densely packed nanowire arrays.Comment: 4 pages, 5 figures. To appear in Journal of Superconductivity and Novel Magnetis

Efeito da irrigação no crescimento de plantas de seringueira em viveiro.

Durante tres anos (1981-1983) foi conduzido, em viveiro, na Estacao Experimental do CNPSD, em Manaus-AM, um estudo objetivando avaliar os efeitos da irrigacao e da interacao irrigacao x fertilizantes sobre a altura e diametro de caule da seringueira (Hevea spp.). O delineamento experimental utilizado foi o de blocos ao acaso em esquema fatorial, contendo os seguintes tratamentos: irrigado/adubado (IA), irrigado/nao adubado (INA), nao irrigado/adubado (NIA) e nao irrigado/nao adubado (NINA). Nos tratamentos irrigados, o solo foi abastecido ate a capacidade de campo, a profundidade de 25 cm, toda vez que o potencial matricial por metro atingia -0,5 bar, ao passo que, nos adubados, uma mistura de NPKMg foi parcelada em cinco aplicacoes a cada 30 dias, sendo a primeira efetuada aos 60 dias apos o plantio. Nos dois primeiros anos analisados, verificou-se que os tratamentos irrigados foram significativamente superiores aos nao irrigados, particularmente quando a irrigacao esteve associada a adubacao. Em geral, o comportamenteo das diferentes praticas testadas apresentaram a seguinte ordem decrescente: IA>INA>NIA>NINA. Nao foi observada interacao dos efeitos principais (irrigacao x adubacao)

Magneto-Conductance Anisotropy and Interference Effects in Variable Range Hopping

We investigate the magneto-conductance (MC) anisotropy in the variable range hopping regime, caused by quantum interference effects in three dimensions. When no spin-orbit scattering is included, there is an increase in the localization length (as in two dimensions), producing a large positive MC. By contrast, with spin-orbit scattering present, there is no change in the localization length, and only a small increase in the overall tunneling amplitude. The numerical data for small magnetic fields $B$, and hopping lengths $t$, can be collapsed by using scaling variables $B_\perp t^{3/2}$, and $B_\parallel t$ in the perpendicular and parallel field orientations respectively. This is in agreement with the flux through a `cigar'--shaped region with a diffusive transverse dimension proportional to $\sqrt{t}$. If a single hop dominates the conductivity of the sample, this leads to a characteristic orientational `finger print' for the MC anisotropy. However, we estimate that many hops contribute to conductivity of typical samples, and thus averaging over critical hop orientations renders the bulk sample isotropic, as seen experimentally. Anisotropy appears for thin films, when the length of the hop is comparable to the thickness. The hops are then restricted to align with the sample plane, leading to different MC behaviors parallel and perpendicular to it, even after averaging over many hops. We predict the variations of such anisotropy with both the hop size and the magnetic field strength. An orientational bias produced by strong electric fields will also lead to MC anisotropy.Comment: 24 pages, RevTex, 9 postscript figures uuencoded Submitted to PR

Magnetic force microscopy investigation of arrays of nickel nanowires and nanotubes

The magnetic properties of arrays of nanowires (NWs) and nanotubes (NTs), 150 nm in diameter, electrodeposited inside nanoporous polycarbonate membranes are investigated. The comparison of the nanoscopic magnetic force microscopy (MFM) imaging and the macroscopic behavior as measured by alternating gradient force magnetometry (AGFM) is made. It is shown that MFM is a complementary technique that provides an understanding of the magnetization reversal characteristics at the microscopic scale of individual nanostructures. The local hysteresis loops have been extracted by MFM measurements. The influence of the shape of such elongated nanostructures on the dipolar coupling and consequently on the squareness of the hysteresis curves is demonstrated. It is shown that the nanowires exhibit stronger magnetic interactions than nanotubes. The non-uniformity of the magnetization states is also revealed by combining the MFM and AGFM measurements.Comment: 7 pages, 5 figure

Comparação entre a erodibilidade medida diretamente e a estimada pelo método nomográfico em latossolo amarelo textura muito argilosa.

bitstream/item/32758/1/CPATU-BP164.pd

Directed paths on hierarchical lattices with random sign weights

We study sums of directed paths on a hierarchical lattice where each bond has either a positive or negative sign with a probability $p$. Such path sums $J$ have been used to model interference effects by hopping electrons in the strongly localized regime. The advantage of hierarchical lattices is that they include path crossings, ignored by mean field approaches, while still permitting analytical treatment. Here, we perform a scaling analysis of the controversial ``sign transition'' using Monte Carlo sampling, and conclude that the transition exists and is second order. Furthermore, we make use of exact moment recursion relations to find that the moments always determine, uniquely, the probability distribution $P(J)$. We also derive, exactly, the moment behavior as a function of $p$ in the thermodynamic limit. Extrapolations ($n\to 0$) to obtain for odd and even moments yield a new signal for the transition that coincides with Monte Carlo simulations. Analysis of high moments yield interesting ``solitonic'' structures that propagate as a function of $p$. Finally, we derive the exact probability distribution for path sums $J$ up to length L=64 for all sign probabilities.Comment: 20 pages, 12 figure

Spin superfluidity and spin-orbit gauge symmetry fixing

The Hamiltonian describing 2D electron gas, in a spin-orbit active medium, can be cast into a consistent non-Abelian gauge field theory leading to a proper definition of the spin current. The generally advocated gauge symmetric version of the theory results in current densities that are gauge covariant, a fact that poses severe concerns on their physical nature. We show that in fact the problem demands gauge fixing, leaving no room to ambiguity in the definition of physical spin currents. Gauge fixing also allows for polarized edge excitations not present in the gauge symmetric case. The scenario here is analogous to that of superconductivity gauge theory. We develop a variational formulation that accounts for the constraints between U(1) physical fields and SU(2) gauge fields and show that gauge fixing renders a physical matter and radiation currents and derive the particular consequences for the Rashba SO interaction.Comment: to appear in EP

Efectividad de la miotomía de aductores y tenotomía de psoasiliaco en la prevención de la luxación de cadera en la parálisis cerebral espástica

Se evaluaron 208 pacientes con parálisis cerebral infantil espástica, sin retardo mental, con potencia de marcha; la edad osciló entre los 16 meses y los 13 años de edad con un promedio de 5 años. El seguimiento fue de la 10 años, con promedio de 6,2 años. A los que se realizó miotomía de aductores y tenotomía del psoasiliaco. Los resultados fueron valorados con el índice de migración lateral. Los mejores resultados se logran en niños de 2 años con 8 5% de caderas reducidas.We rewiewd 208 spastic cerebral palsied patients, without mental ratardation and with gait potential. They were 16 months to 13 years old (average 5 years). Follow-up was 1 to 10 years (average: 6,2 years). Myotomy were evaluated by lateral migration index. Best results were obteined in children of 2 years with 85% of reducted hips