Showing papers by "José María Gómez published in 2004"

Theoretical derivation of 1/f noise in quantum chaos.

Abstract: It was recently conjectured that 1/ƒ noise is a fundamental characteristic of spectral fluctuations in chaotic quantum systems. This conjecture is based on the power spectrum behavior of the excitation energy fluctuations, which is different for chaotic and integrable systems. Using random matrix theory, we derive theoretical expressions that explain without free parameters the universal behavior of the excitation energy fluctuations power spectrum. The theory gives excellent agreement with numerical calculations and reproduces to a good approximation the 1/ƒ (1/ƒ^(2)) power law characteristic of chaotic (integrable) systems. Moreover, the theoretical results are valid for semiclassical systems as well.

Early cyclosporine a withdrawal in kidney-transplant recipients receiving sirolimus prevents progression of chronic pathologic allograft lesions.

Abstract: Background. Nephrotoxicity of calcineurin inhibitors (CNIs) is partially responsible for the development of chronic allograft nephropathy (CAN). Sirolimus has demonstrated its potential to substitute for CNIs because it lacks significant nephrotoxicity and shows a short-term immunosuppressive capacity comparable with that of cyclosporine. This results in the maintenance of better renal function when cyclosporine is eliminated, but it has not been demonstrated whether this benefit is associated with an improvement in the pathologic substrate and a reduction in CAN. Methods. We analyzed pretransplant and 1-year renal-allograft biopsies from 64 patients enrolled in a multicenter trial. Patients received cyclosporine and sirolimus during the first 3 months after transplant and were then randomly assigned to continue with cyclosporine or have it withdrawn. Histologic chronic allograft lesions were compared between groups. Results. The percentage of patients in whom chronic pathologic lesions progressed was lower in the group of cyclosporine elimination. Significant differences were observed in chronic interstitial and tubular lesions (70% vs. 40.9% [P<0.05] and 70% vs. 47.8% [P<0.05], respectively), whereas no differences were observed in acute lesions (subclinical rejection). Prevalence of CAN at 1 year was lower in this group, as was the severity and incidence of new cases (P<0.05). Conclusions. Early cyclosporine withdrawal associated with sirolimus administration is followed by an improvement in renal function, a reduction in the progression of chronic pathologic allograft lesions, and a lower incidence of new cases and severity of CAN during the first year after transplantation. This benefit may result in better long-term graft outcome.

Stringent numerical test of the Poisson distribution for finite quantum integrable Hamiltonians.

Abstract: Using a class of exactly solvable models based on the pairing interaction, we show that it is possible to construct integrable Hamiltonians with a Wigner distribution of nearest-neighbor level spacings. However, these Hamiltonians involve many-body interactions and the addition of a small integrable perturbation very quickly leads the system to a Poisson distribution. Besides this exceptional case, we show that the accumulated distribution of an ensemble of random integrable two-body pairing Hamiltonians is in perfect agreement with the Poisson limit. These numerical results for quantum integrable Hamiltonians provide a further empirical confirmation of the work of Berry and Tabor in the semiclassical limit.

O Methylation of Phenol in Liquid Phase over Basic Zeolites

Abstract: Liquid-phase O methylation of phenol was carried out with dimethyl carbonate (DMC) over as-synthesized KNaX zeolite with impregnated alkaline metal hydroxides. The presence of alkaline hydroxides is associated with hydroxysodalite impurity in the X zeolite. This zeolite exhibited better properties as basic catalyst than those prepared by postsynthesis treatments (ionic exchange). Phenol conversion and selectivity toward anisole were 100% and 85%, respectively, at 438 K with a DMC/phenol molar ratio of 2. The crystallinity and composition of the KNaX zeolite were retained after reaction, making it possible to recover the catalyst without loss of the impregnated alkaline hydroxides during the reaction. The reactivation of the KNaX zeolite was carried out at low temperature (373 K), achieving the same conversion and selectivity to anisole in the phenol methylation as in the first reaction.

Number of principal components and localization length in E 2 and M 1 transition strengths in V 46

Abstract: Predictions of the embedded Gaussian orthogonal random matrix ensemble of one plus two-body interactions, in the Gaussian domain, for the number of principal components and localization length, measuring complexity and chaos in transition strengths originating from an eigenstate with energy $E$, are tested successfully for the first time for $E2$ and $M1$ transition strengths. Shell-model results for the $2p1f$-shell nucleus $^{46}\mathrm{V}$ are used as the example.

PRINCIPAL COMPONENTS ANALYSIS OF EXTENSIVE AIR SHOWERS APPLIED TO THE IDENTIFICATION OF COSMIC TeV GAMMA-RAYS

Abstract: We apply a principal components analysis (PCA) to the secondary particle density distributions at ground level produced by cosmic gamma-rays and protons. For this purpose, high-energy interactions of cosmic rays with Earth's atmosphere and the resulting extensive air showers have been simulated by means of the CORSIKA Monte Carlo code. We show that a PCA of the two-dimensional particle density fluctuations provides a decreasing sequence of covariance matrix eigenvalues that have typical features of a polynomial law, which are different for different primary cosmic rays. This property is applied to the separation of electromagnetic showers from proton simulated extensive air showers, and it is proposed as a new discrimination method that can be used experimentally for gamma-proton separation. A cutting parameter related to the polynomial behavior of the decreasing sequence of covariance matrix eigenvalues is calculated, and the efficiency of the cutting procedure for gamma-proton separation is evaluated.

Impregnation of X Zeolite with Alkaline Hydroxide During the Synthesis

Abstract: Enhancement of the basic properties of the X zeolite with FAU framework has been carried out during synthesis, without further treatment such as ion-exchange or impregnation. Control of the washing stage enables retention of the optimum amount of alkaline hydroxides, which increases the basic catalytic activity of X zeolite in the alkylation of toluene. This alkaline hydroxides neither affect the FAU framework nor the silicon/aluminium molar ratio of the X zeolite. The optimum (Na + K)/Al molar ratio of the improved catalyst was 1.06 corresponding to a washing volume of 200 mL. This X zeolite presented better catalytic activity than a cesium-zeolite prepared by ion-exchange. The presence of hydroxysodalite impurity in the X zeolite increased the amount of impregnated alkaline hydroxides but not the catalytic activity. The present study shows that it is possible to increase the basic properties of the X zeolite directly in the synthesis process.