Model for Vortex Pinning in a Two-Dimensional Inhomogeneous d-wave Superconductor

We study a model for the pinning of vortices in a two-dimensional, inhomogeneous, Type-II superconductor in its mixed state. The model is based on a Ginzburg-Landau (GL) free energy functional whose coefficients are determined by the mean-field transition temperature T_{c0} and the zero-temperature penetration depth \lambda(0). We find that if (i) T_{c0} and \lambda(0) are functions of position, and (ii) \lambda^2(0) is proportional to T_{c0}^y, with y greater than 0, then the vortices tend to be pinned where T_{c0}, and hence the magnitude of the superconducting order parameter \Delta, are large. This behavior is in contrast to the usual picture of pinning in Type-II superconductors, where pinning occurs in the small-gap regions. We also compute the local density of states of a model BCS Hamiltonian with d-wave symmetry, in which the pairing field is obtained from Monte Carlo simulations of a GL free energy. Several features observed in scanning tunneling spectroscopy measurements on YBa_2Cu_3O_{6+x} and Bi_2Sr_2CaCu_2O_{8+x} are well reproduced by our model: far from the cores, the local density of states spectrum has a small gap and sharp coherence peaks, while near the cores it has a larger gap with low, broad peaks. Additionally, also in agreement with experiment, the spectrum near the core does not exhibit a zero-energy peak which is, however, observed in other theoretical studies.Comment: 25 pages, 11 figures. Accepted for publication in Phys. Rev.

Effects of inhomogeneities and thermal fluctuations on the spectral function of a model d-wave superconductor

We compute the spectral function $A({\bf k}, \omega)$ of a model two-dimensional high-temperature superconductor, at both zero and finite temperatures $T$. We assume that an areal fraction $c_{\beta}$ of the superconductor has a large gap $\Delta$ ($\beta$ regions), while the rest has a smaller $\Delta$ ($\alpha$ regions), both of which are randomly distributed in space. We find that $A({\bf k}, \omega)$ is most strongly affected by inhomogeneity near the point $\mathbf k = (\pi, 0)$ (and the symmetry-related points). For $c_\beta\simeq 0.5$, $A({\bf k}, \omega)$ exhibits two double peaks (at positive and negative energy) near this k-point if the difference between $\Delta_\alpha$ and $\Delta_\beta$ is sufficiently large in comparison to the hopping integral. The strength of the inhomogeneity required to produce a split spectral function peak suggests that inhomogeneity is unlikely to be the cause of a second branch in the dispersion relation. Thermal fluctuations also affect $A({\bf k}, \omega)$ most strongly near $\mathbf k = (\pi,0)$. Typically, peaks that are sharp at $T = 0$ become reduced in height, broadened, and shifted toward lower energies with increasing $T$; the spectral weight near $\mathbf k = (\pi, 0)$ becomes substantial at zero energy for $T$ greater than the phase-ordering temperature.Comment: Accepted for publication in Phys. Rev. B. Scheduled Issue: 01 Jan 2008. 26 Pages and 10 figure

Finite-Size-Scaling at the Jamming Transition: Corrections to Scaling and the Correlation Length Critical Exponent

We carry out a finite size scaling analysis of the jamming transition in frictionless bi-disperse soft core disks in two dimensions. We consider two different jamming protocols: (i) quench from random initial positions, and (ii) quasistatic shearing. By considering the fraction of jammed states as a function of packing fraction for systems with different numbers of particles, we determine the spatial correlation length critical exponent $\nu\approx 1$, and show that corrections to scaling are crucial for analyzing the data. We show that earlier numerical results yielding $\nu<1$ are due to the improper neglect of these corrections.Comment: 5 pages, 4 figures -- slightly revised version as accepted for Phys. Rev. E Rapid Communication

Smoothed particle hydrodynamics on GPU computing

Smoothed Particle Hydrodynamics (SPH) is a powerful technique used to simulate complex free-surface flows. However one of the main drawbacks of this method is the expensive computational runtime and the large number of particles needed when 3D simulations are performed. High Performance Computing (HPC) therefore becomes essential to accelerate these codes and perform simulations. In this study, parallelization using Graphics Processing Units (GPU) is applied to the SPHysics code (www.sphysics.org) dedicated to free-surface flows with SPH. Simulations involving several million particles on a single GPU exhibit speedups of up to two orders of magnitude over the same calculations using CPU codes, while parallelization using MPI for multi-GPU leads to further acceleration. This cheap technology allows studying real-life engineering problems at reasonable computational runtimes

Descripción del cráneo de Bramocharax caballeroi Contreras & Rivera 1985 (Pisces, Characidae), pez endémico del Lago de Catemaco, Veracruz, México.

The Bramocharax genus is distributed from Central America to Mexico. Until now the species known are Bramocharax bransfordi collected in Nicaragua and Costa Rica, B. dorioni and B. baileyi from Guatemala and B. caballeroi from Catemaco Lake, Veracruz, Mexico. The aim of this work is to realize a detailed description of the skull for Bramocharax caballeroi from the analysis of 37 different topotypes. The skull of this species has 58 osteological elements, 49 of them coupled. Shape is oval in lateral view, because the bones of the snout (ethmoid, vomer, lateral ethmoid, dentary, maxillae, premaxillae, ecto and mesopterygoids) and the frontal and circunorbitals are elongated. The dorsal margin is slightly concave and the anterior part is elongated. In specimens less than 80 mm, all the bones are smoother, with more regular edges. With the age increase, they become more robust with more irregular edges. A comparison of the skull between B. caballeroi and Astyanax mexicanus resulted in 25 bones with differences, while with Astyanax fasciatus, main contrasts were the supraoccipital length, the shape of the opercles and the infraorbital 3. In addition there are important differences in the dentition, gill rackers, as well as suspensory pharingeals and pharyngeal plates with respect to Astyanax.El género Bramocharax se distribuye del centro-norte de Centro América a México. Hasta este momento se conoce a Bramocharax bransfordi de Nicaragua y Costa Rica, a B. dorioni y B. baileyi de Guatemala y a B. caballeroi del lago de Catemaco, Veracruz, México. El trabajo tuvo por objetivo estudiar detalladamente el cráneo de Bramocharax caballeroi a partir del análisis de 37 topotipos de diferentes tallas y compararlo con otras especies relacionadas. B. caballeroi presenta 58 huesos en su cráneo, de los cuales 49 son elementos pareados. En vista lateral su perfil es ovalado, debido a los huesos que conforman el hocico (etmoides, vómer, etmoides lateral, mandíbula, maxila, premaxila, ecto y mesopterigoides), así como el frontal y los circunorbitales, son alargados. El margen dorsal es ligeramente cóncavo y la parte anterior alargada. En los ejemplares menores a 80 mm todos los huesos son más lisos y conforme los organismos tienen una talla mayor, se hacen más robustos y los bordes de cada hueso se hacen más irregulares. Al comparar el cráneo de B. caballeroi con el de Astyanax mexicanus se encontraron diferencias en 25 huesos, mientras que al hacerlo con A. fasciatus, los principales contrastes fueron la longitud del supraoccipital, la forma del opérculo y del infraorbital 3. Adicionalmente, se encontraron diferencias importantes en la dentición y en las branquiespinas así, como en las placas y suspensores faríngeos con respecto a los Astyanax

Towards accelerating smoothed particle hydrodynamics simulations for free-surface flows on multi-GPU clusters

Starting from the single graphics processing unit (GPU) version of the Smoothed Particle Hydrodynamics (SPH) code DualSPHysics, a multi-GPU SPH program is developed for free-surface flows. The approach is based on a spatial decomposition technique, whereby different portions (sub-domains) of the physical system under study are assigned to different GPUs. Communication between devices is achieved with the use of Message Passing Interface (MPI) application programming interface (API) routines. The use of the sorting algorithm radix sort for inter-GPU particle migration and sub-domain “halo” building (which enables interaction between SPH particles of different sub-domains) is described in detail. With the resulting scheme it is possible, on the one hand, to carry out simulations that could also be performed on a single GPU, but they can now be performed even faster than on one of these devices alone. On the other hand, accelerated simulations can be performed with up to 32 million particles on the current architecture, which is beyond the limitations of a single GPU due to memory constraints. A study of weak and strong scaling behaviour, speedups and efficiency of the resulting program is presented including an investigation to elucidate the computational bottlenecks. Last, possibilities for reduction of the effects of overhead on computational efficiency in future versions of our scheme are discussed.Xunta de GaliciaEngineering and Physical Sciences Research Council (EPSRC)Research Councils UK (RCUK

Anti-cancer effect of Cissus quadrangularis on human glioblastoma cells

Objectives Glioblastoma multiforme (GBM) is a common and fatal brain tumour in the central nervous system with a poor survival rate and a median survival time of 15 months only. The standard treatment is aggressive surgical resection followed by radiotherapy and chemotherapy. However, effective drugs available in chemotherapy are limited. This study was designed to evaluate, for the first time, the potential therapeutic effect of Cissus quadrangularis (CQ) in human glioblastoma cells and to investigate its possible mechanisms of action. Methods In this study, we examined the anticancer activity of CQ in human glioblastoma U87 MG cells by cell viability assay, cell migration assay, immunofluorescence staining and Western blot. Results Our results demonstrated that CQ treatment induced U87 cytotoxicity, cell cycle arrest and cell death. The cytotoxicity of CQ mediates ER stress, autophagy and mitochondrial apoptosis by suppressing pro-survival signalling pathways (extracellular signal-regulated kinase and signal transducer and activator of transcription 3 pathways). Conclusions The findings of this study imply that CQ is a promising anti-cancer candidate for the treatment of GBM. Highlights The anticancer effect of Cissus quadrangularis (CQ) was studied in human glioblastoma U87 MG cells. It was demonstrated that CQ treatment induced cytotoxicity, cell cycle arrest and cell death in U87 MG cells. CQ may become a potential chemotherapy component for the treatment of glioblastoma multiforme

Single-Particle Density of States of a Superconductor with a Spatially Varying Gap and Phase Fluctuations

Recent experiments have shown that the superconducting energy gap in some cuprates is spatially inhomogeneous. Motivated by these experiments, and using exact diagonalization of a model d-wave Hamiltonian, combined with Monte Carlo simulations of a Ginzburg-Landau free energy functional, we have calculated the single-particle density of states LDOS$(\omega,r)$ of a model high-T$_c$ superconductor as a function of temperature. Our calculations include both quenched disorder in the pairing potential and thermal fluctuations in both phase and amplitude of the superconducting gap. Most of our calculations assume two types of superconducting regions: $\alpha$, with a small gap and large superfluid density, and $\beta$, with the opposite. If the $\beta$ regions are randomly embedded in an $\alpha$ host, the LDOS on the $\alpha$ sites still has a sharp coherence peak at $T = 0$, but the $\beta$ component does not, in agreement with experiment. An ordered arrangement of $\beta$ regions leads to oscillations in the LDOS as a function of energy. The model leads to a superconducting transition temperature $T_c$ well below the pseudogap temperature $T_{c0}$, and has a spatially varying gap at very low $T$, both consistent with experiments in underdoped Bi2212. Our calculated LDOS$(\omega,r)$ shows coherence peaks for $T T_c$, in agreement with previous work considering phase but not amplitude fluctuations in a homogeneous superconductor. Well above $T_c$, the gap in the LDOS disappears.Comment: 37 pages, 12 figures. Accepted by Phys. Rev. B. Scheduled Issue: 01 Nov 200