Fluctuations of the correlation dimension at metal-insulator transitions

We investigate numerically the inverse participation ratio, $P_2$, of the 3D Anderson model and of the power-law random banded matrix (PRBM) model at criticality. We found that the variance of $\ln P_2$ scales with system size $L$ as $\sigma^2(L)=\sigma^2(\infty)-A L^{-D_2/2d}$, being $D_2$ the correlation dimension and $d$ the system dimension. Therefore the concept of a correlation dimension is well defined in the two models considered. The 3D Anderson transition and the PRBM transition for $b=0.3$ (see the text for the definition of $b$) are fairly similar with respect to all critical magnitudes studied.Comment: RevTex, 5 pages, 4 eps figures, to be published in Phys. Rev. Let

Spin quantum Hall effect and plateau transitions in multilayer network models

We study the spin quantum Hall effect and transitions between Hall plateaus in quasi two-dimensional network models consisting of several coupled layers. Systems exhibiting the spin quantum Hall effect belong to class C in the symmetry classification for Anderson localisation, and for network models in this class there is an established mapping between the quantum problem and a classical one involving random walks. This mapping permits numerical studies of plateau transitions in much larger samples than for other symmetry classes, and we use it to examine localisation in systems consisting of $n$ weakly coupled layers. Standard scaling ideas lead one to expect $n$ distinct plateau transitions, but in the case of the unitary symmetry class this conclusion has been questioned. Focussing on a two-layer model, we demonstrate that there are two separate plateau transitions, with the same critical properties as in a single-layer model, even for very weak interlayer coupling.Comment: 5 pages, 6 figure

Localized to extended states transition for two interacting particles in a two-dimensional random potential

We show by a numerical procedure that a short-range interaction $u$ induces extended two-particle states in a two-dimensional random potential. Our procedure treats the interaction as a perturbation and solve Dyson's equation exactly in the subspace of doubly occupied sites. We consider long bars of several widths and extract the macroscopic localization and correlation lengths by an scaling analysis of the renormalized decay length of the bars. For $u=1$, the critical disorder found is $W_{\rm c}=9.3\pm 0.2$, and the critical exponent $\nu=2.4\pm 0.5$. For two non-interacting particles we do not find any transition and the localization length is roughly half the one-particle value, as expected.Comment: 4 two-column pages, 4 eps figures, Revtex, to be published in Europhys. Let

Plastia sustitutiva del LCA con tendones de la pata de ganso cuadruples

Se han estudiado retrospectivamente 44 casos de pacientes a los que se les realizó por vía artrocóspica una plastia sustitutiva intraticular de reconstrucción del LCA, con tendones de Semitendinoso y Recto Interno antólogos, en 4 fascículos. La edad media de los pacientes fue de 25 años. Los resultados a corto plazo (2 años) obtenidos con KT-1000 a 30 Ib han sido excelente-bueno en 40 casos y aceptable en 4 casos. Cuatro casos referían sensación de inestabilidad. Los test de Lachman y Pívot-Shift fueron positivos en 3 casos. Con el test de Lysholm se obtuvo resultado excelente-bueno en 36 casos, regular en 6 y malo en 2 casos. Este tipo de plastia presenta una baja morbilidad de la zona dadora del injerto, comparativamente con la plastia HTH patelar antóloga, y sin alteración del aparato extensor.A retrospective study of 44 patients, who underwent an arthroscopically assisted ACL (anterior cruciata ligament) reconstruction with quadrupled semitendinosus/gracilis tendon autografts was performed. The mean age was 25 years. Short term outcomes of knee stability (2 years follow-up) with KT-1000 arthrometer at 30 pounds were excellent or good in 40 cases and acceptable in 4. Pain or joint swelling were not found. Instability complaints were found in 4 cases. Lachman and Pivot-shift were found positive in 3 cases. Lysholm questionnaire scores were excellent or good in 36 cases, fair in 6 cases and per in two cases, the donor site morbidity associated with hamstring harvest seems to be minimal compared with bonepatellar tendon-bone autograft without impairment of the extensor mechanism

New Results for Diffusion in Lorentz Lattice Gas Cellular Automata

New calculations to over ten million time steps have revealed a more complex diffusive behavior than previously reported, of a point particle on a square and triangular lattice randomly occupied by mirror or rotator scatterers. For the square lattice fully occupied by mirrors where extended closed particle orbits occur, anomalous diffusion was still found. However, for a not fully occupied lattice the super diffusion, first noticed by Owczarek and Prellberg for a particular concentration, obtains for all concentrations. For the square lattice occupied by rotators and the triangular lattice occupied by mirrors or rotators, an absence of diffusion (trapping) was found for all concentrations, except on critical lines, where anomalous diffusion (extended closed orbits) occurs and hyperscaling holds for all closed orbits with {\em universal} exponents ${\displaystyle{d_f = \frac{7}{4}}}$ and ${\displaystyle{\tau = \frac{15}{7}}}$. Only one point on these critical lines can be related to a corresponding percolation problem. The questions arise therefore whether the other critical points can be mapped onto a new percolation-like problem, and of the dynamical significance of hyperscaling.Comment: 52 pages, including 18 figures on the last 22 pages, email: [email protected]

Magnetic Field Effect for Two Electrons in a Two Dimensional Random Potential

We study the problem of two particles with Coulomb repulsion in a two-dimensional disordered potential in the presence of a magnetic field. For the regime, when without interaction all states are well localized, it is shown that above a critical excitation energy electron pairs become delocalized by interaction. The transition between the localized and delocalized regimes goes in the same way as the metal-insulator transition at the mobility edge in the three dimensional Anderson model with broken time reversal symmetry.Comment: revtex, 7 pages, 6 figure

New embedded digital front-end for high resolution PET scanner

This work describes a new digital front-end for a high-resolution low-cost animal PET scanner which is currently under development. The advances in flexibility and size of modern FPGAs together with the release of new tools enable the integration of most of the front-end electronics in a single FPGA. The implemented system includes a small 32-bit RISC processor, several peripherals attached to the internal buses and a special DSP unit closely attached to the processor which is dedicated to the detection of the gamma events. On top of these, a small footprint real time operating system abstracts the underlying hardware, providing the mechanisms to combine on-chip slow control and data streamingThis work was supported in part by the FPU Research Grant from the Spanish Education and Science Ministry, by the Spanish Thematic Network IM3 (PI052204) and project TEC2004-07052-C02-02Publicad

Anomalously large critical regions in power-law random matrix ensembles

We investigate numerically the power-law random matrix ensembles. Wavefunctions are fractal up to a characteristic length whose logarithm diverges asymmetrically with different exponents, 1 in the localized phase and 0.5 in the extended phase. The characteristic length is so anomalously large that for macroscopic samples there exists a finite critical region, in which this length is larger than the system size. The Green's functions decrease with distance as a power law with an exponent related to the correlation dimension.Comment: RevTex, 4 pages, 4 eps figures. Final version to be published in Phys. Rev. Let

Episodic displacement on a sackung scarp in Benasque Valley (Central Spanish Pyrenees). Paleoseismic record?

A sackung scarp has been investigated by trenching in the El Ubago glacial valley (central Spanish Pyrenees). The scarp is located 18 km to the SW of the North Maladeta Fault, which was the source of the Mw 5.3 Vielha earthquake of 1923. Three displacement events have been inferred based on colluvial wedge stratigraphy and fault truncation. Event X at ca. 16.9 ka created the sackung. Events Y and Z have been constrained at 15-8.3 and 8.3-5.3 ka. The timing of event Y partially overlaps with that of a faulting event recognised in a previously investigated sackung at about 2 km. A minimum vertical slip rate of 0.18 mm/yr has been calculated for the sackung. The spatial association of the sackung features in this sector of the Pyrenees with the North Maladeta Fault and the millennial recurrence of the inferred displacement events (5.6 kyr) suggests that the kinematics of the studied sackung has been controlled by seismic activity. Demonstrating in future investigations that the sackung features in the area constitute archives of large paleoearthquake would be of great interest for seismic hazard assessments. They might help to improve the catalogue of paleoearthquakes and might provide information on earthquake recurrence intervals and the age of the MRE

Multipurpose Monte Carlo simulator for photon transport in turbid media

Proceeding of: 2009 IEEE Nuclear Science Symposium Conference Record (NSS/MIC), Orlando, Florida, 25-31 October 2009Monte Carlo methods provide a flexible and rigorous solution to the problem of light transport in turbid media, which enable approaching complex geometries for a closed analytical solution is not feasible. The simulator implements local rules of propagation in the form of probability density functions that depend on the local optical properties of the tissue. This work presents a flexible simulator that can be applied in multiple applications related to optical tomography. In particular, unlike previous codes, the simulator explicitly supports fluorescent-tissues and variance reduction and code parallelization techniques are implemented in order to speed up the execution with fluorochrome-labelled agents. The simulator is validated with simple geometries for which an analytical solution exists, as well as with an experimental polyester resin based optical phantom.This work was supported in part by the Ministry of Science and Innovation under projects TEC2008-06715 and TEC2007-64731/TCM and by the EU’s 7th Frame Programme under contract HEALTH-F5-2008-20179