Conductance scaling at the band center of wide wires with pure non--diagonal disorder

Kubo formula is used to get the scaling behavior of the static conductance distribution of wide wires showing pure non-diagonal disorder. Following recent works that point to unusual phenomena in some circumstances, scaling at the band center of wires of odd widths has been numerically investigated. While the conductance mean shows a decrease that is only proportional to the inverse square root of the wire length, the median of the distribution exponentially decreases as a function of the square root of the length. Actually, the whole distribution decays as the inverse square root of the length except close to G=0 where the distribution accumulates the weight lost at larger conductances. It accurately follows the theoretical prediction once the free parameter is correctly fitted. Moreover, when the number of channels equals the wire length but contacts are kept finite, the conductance distribution is still described by the previous model. It is shown that the common origin of this behavior is a simple Gaussian statistics followed by the logarithm of the E=0 wavefunction weight ratio of a system showing chiral symmetry. A finite value of the two-dimensional conductance mean is obtained in the infinite size limit. Both conductance and the wavefunction statistics distributions are given in this limit. This results are consistent with the 'critical' character of the E=0 wavefunction predicted in the literature.Comment: 10 pages, 9 figures, RevTeX macr

Strong covalent bonding between two graphene layers

We show that two graphene layers stacked directly on top of each other (AA stacking) form strong chemical bonds when the distance between planes is 0.156 nm. Simultaneously, C-C in-plane bonds are considerably weakened from partial double-bond (0.141 nm) to single bond (0.154 nm). This polymorphic form of graphene bilayer is meta-stable w.r.t. the one bound by van der Waals forces at a larger separation (0.335 nm) with an activation energy of 0.16 eV/cell. Similarly to the structure found in hexaprismane, C forms four single bonds in a geometry mixing 90^{0} and 120^{0} angles. Intermediate separations between layers can be stabilized under external anisotropic stresses showing a rich electronic structure changing from semimetal at van der Waals distance, to metal when compressed, to wide gap semiconductor at the meta-stable minimum.Comment: tar gzip latex 4 pages 4 figure

Conductance as a Function of the Temperature in the Double Exchange Model

We have used the Kubo formula to calculate the temperature dependence of the electrical conductance of the double exchange Hamiltonian. We average the conductance over an statistical ensemble of clusters, which are obtained by performing Monte Carlo simulations on the classical spin orientation of the double exchange Hamiltonian. We find that for electron concentrations bigger than 0.1, the system is metallic at all temperatures. In particular it is not observed any change in the temperature dependence of the resistivity near the magnetical critical temperature. The calculated resistivity near $T_c$ is around ten times smaller than the experimental value. We conclude that the double exchange model is not able to explain the metal to insulator transition which experimentally occurs at temperatures near the magnetic critical temperature.Comment: 6 pages, 5 figures included in the tex

Finite size effects and localization properties of disordered quantum wires with chiral symmetry

Finite size effects in the localization properties of disordered quantum wires are analyzed through conductance calculations. Disorder is induced by introducing vacancies at random positions in the wire and thus preserving the chiral symmetry. For quasi one-dimensional geometries and low concentration of vacancies, an exponential decay of the mean conductance with the wire length is obtained even at the center of the energy band. For wide wires, finite size effects cause the conductance to decay following a non-pure exponential law. We propose an analytical formula for the mean conductance that reproduces accurately the numerical data for both geometries. However, when the concentration of vacancies increases above a critical value, a transition towards the suppression of the conductance occurs. This is a signature of the presence of ultra-localized states trapped in finite regions of the sample.Comment: 5 figures, revtex

Vortex, skyrmion and elliptical domain wall textures in the two-dimensional Hubbard model

The spin and charge texture around doped holes in the two-dimensional Hubbard model is calculated within an unrestricted spin rotational invariant slave-boson approach. In the first part we examine in detail the spin structure around two holes doped in the half-filled system where we have studied cluster sizes up to 10 x 10. It turns out that the most stable configuration corresponds to a vortex-antivortex pair which has lower energy than the Neel-type bipolaron even when one takes the far field contribution into account. We also obtain skyrmions as local minima of the energy functional but with higher total energy than the vortex solutions. Additionally we have investigated the stability of elliptical domain walls for commensurate hole concentrations. We find that (i) these phases correspond to local minima of the energy functional only in case of partially filled walls, (ii) elliptical domain walls are only stable in the low doping regime.Comment: 7 pages, 6 figures, accepted for Phys. Rev.

Resistivity of a Metal between the Boltzmann Transport Regime and the Anderson Transition

We study the transport properties of a finite three dimensional disordered conductor, for both weak and strong scattering on impurities, employing the real-space Green function technique and related Landauer-type formula. The dirty metal is described by a nearest neighbor tight-binding Hamiltonian with a single s-orbital per site and random on-site potential (Anderson model). We compute exactly the zero-temperature conductance of a finite size sample placed between two semi-infinite disorder-free leads. The resistivity is found from the coefficient of linear scaling of the disorder averaged resistance with sample length. This ``quantum'' resistivity is compared to the semiclassical Boltzmann expression computed in both Born approximation and multiple scattering approximation.Comment: 5 pages, 3 embedded EPS figure

Scattering phases in quantum dots: an analysis based on lattice models

The properties of scattering phases in quantum dots are analyzed with the help of lattice models. We first derive the expressions relating the different scattering phases and the dot Green functions. We analyze in detail the Friedel sum rule and discuss the deviation of the phase of the transmission amplitude from the Friedel phase at the zeroes of the transmission. The occurrence of such zeroes is related to the parity of the isolated dot levels. A statistical analysis of the isolated dot wave-functions reveals the absence of significant correlations in the parity for large disorder and the appearance, for weak disorder, of certain dot states which are strongly coupled to the leads. It is shown that large differences in the coupling to the leads give rise to an anomalous charging of the dot levels. A mechanism for the phase lapse observed experimentally based on this property is discussed and illustrated with model calculations.Comment: 18 pages, 9 figures. to appear in Physical Review

Quantifying deformation processes in the SE Pyrenees using U-Pb dating of fracture-filling calcites

It is difficult to quantify the timing of the deformation processes in brittle fold-thrust belts because minerals suitable for dating and well-preserved growth strata sediments are scarce or absent. Here, we quantify the duration of thrust sheet emplacement and shortening rates in the SE Pyrenean thrust sequence using U-Pb dating of fracture-filling calcites. The obtained U-Pb dates reveal a minimum duration for the emplacement of each thrust unit (18.7 Ma for the Bóixols-Upper Pedraforca, 11.6 Ma for the Lower Pedraforca and 14.3 Ma for the Cadí thrust sheets) and show that piggy-back thrusting was accompanied by post-emplacement deformation of the upper thrust sheets above the lower sheets during their south-directed tectonic transport. We calculated shortening rates of 0.6, 3.1 and 1.1 mm a−1 from the older to younger emplaced thrust sheets. Our results also reveal the formation of local normal faults during the late Oligocene as a result of the late stages of compression and exhumation in the SE Pyrenees. We observed that temperatures >110 °C could be a limiting factor when applying the U-Pb dating method