Biomedical subjects

R S Gonnelli

Publications and source records attributed to R S Gonnelli.

5 recordsLinked to original sources

PubMed2006

Effect of magnetic impurities in a two-band superconductor: a point-contact study of mn-substituted single crystals.

We present the first results of directional point-contact measurements in Mg1-xMnxB2 single crystals, with x up to 0.015 and bulk Tc down to 13.3 K. The order parameters Deltasigma and Deltapi were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk model. BothDeltapi and Deltasigma decrease with the critical temperature of the junctions TAC, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principles band-structure calculations.

Journal Article↗

PubMed2002

Direct evidence for two-band superconductivity in MgB2 single crystals from directional point-contact spectroscopy in magnetic fields.

We present the results of the first directional point-contact spectroscopy experiments in high-quality MgB2 single crystals. Because of the directionality of the current injection into the samples, the application of a magnetic field allowed us to separate the contributions of the sigma and pi bands to the total conductance of our point contacts. By using this technique, we were able to obtain the temperature dependency of each gap independent of the other. The consequent, strong reduction of the error on the value of the gap amplitude as a function of temperature allows a stricter test of the predictions of the two-band model for MgB2.

Journal Article↗

PubMed2001

Josephson effect in MgB(2) break junctions.

We present the first observation of the dc and ac Josephson effect in MgB(2) break junctions. The junctions, obtained at 4.2 K in high-quality, high-density polycrystalline metallic MgB(2) samples, show a nonhysteretic dc Josephson effect. By irradiating the junctions with microwaves we observe clear Shapiro steps spaced by the ideal Delta V value. The temperature dependence of the dc Josephson current and the dependence of the height of the steps on the microwave power are obtained. These results directly prove the existence of pairs with charge 2e in MgB(2) and give evidence of the superconductor-normal metal-superconductor weak link character of these junctions.

Journal Article↗

PubMed1991

Automatic model selection for the 'best localization' of cardiac sources.

In the present paper a simulation study on the influence of noise and source model on the accuracy of localization of the sources of biomagnetic fields is presented. Applying a statistical analysis (F test) to the localization results obtained by various models on the simulated maps calculated using different theoretical sources and different noise levels we were able to define a 'best localization' (BL) method. It allows an automatic determination of the particular source model able to represent in the best statistical way a specific field distribution obtaining the best source localization for that distribution. We applied this method to the localization of cardiac sources in the experimental maps of the magnetic field produced by isolated rabbit hearts completely immersed in a conductive medium. The results clearly indicate that the proposed method is very effective in determining the 'best localization' for every particular field distribution while the use of the same source model for every field map often produces a source localization completely in contrast with the position of cardiac structures of the isolated hearts.

Animals↗

PubMed1987

Inverse problem solution in cardiomagnetism using a current multipole expansion of the primary sources.

The equivalent current dipole (ECD) model is only the first-order approximation in modelling the primary sources of the magnetic field of the heart. From the experimental point of view this fact is particularly evident during the onset of ventricular depolarisation. In this paper we have tried to explain the departures of the experimental maps from the dipolar pattern in terms of the second-order component of the current multipole expansion for the primary source density. The antisymmetric part of this second-order component produces the external magnetic field and is equivalent to a magnetic dipole. To a first approximation it could represent circular currents flowing within the heart. We have derived the analytical expression for the magnetic field normal to the frontal plane produced by an equivalent current dipole and by an equivalent magnetic dipole (EMD) lying in a homogeneous conducting half-space. Using this expression in a least-squares fit computer program we have obtained the appropriate set of ECD and EMD parameters producing the best matching between theoretical and experimental field distributions in normal subjects. The results are in good agreement with the anatomical features of the heart and with the electrophysiology of ventricular activation. With previous theoretical considerations they strongly suggest the presence of divergence-free circular current sources within the heart.

Electric Conductivity↗