S. Barbanera

Selective localization of alpha brain activity with neuromagnetic measurements

A method of localizing brain activity by a new combination of magnetic and electrical recording, relative covariance, is described. The successful application of this method to alpha EEG is reported. Spontaneous human brain activity was recorded simultaneously with fixed scalp electrodes and a movable magnetometer. The analysis was restricted to the alpha rhythm, which was selected by a narrow bandpass filter centered at the observed alpha frequency. For each magnetometer location, the ratio of the covariance of the magnetic and electric signals to the electric variance was calculated, producing a map reflecting the magnetic field pattern. Clear maxima of opposite polarity over the left and right parietotemporal areas indicate bilateral current source areas near the midline, in the vicinity of the calcarine fissure, at a depth of 4-6 cm from the scalp. This relative covariance method may prove generally useful in localizing bioelectrical sources such as spontaneous brain rhythms.

Biomagnetic measurements of spontaneous brain activity in epileptic patients

In the last few years there has been an increasing interest in the magnetic activity due to bioelectrical currents flowing in the brain. In this paper preliminary results are reported concerning spontaneous magnetic brain activity in 36 patients affected by different kinds of brain disease; in most of these cases the symptoms were induced by localized pathology (atrophies, scars, tumors). Measurements were carried out with the simultaneous recording of the EEG. At present one of the most interesting features of magnetic detection seems to be its high localizing ability in cases of cortical foci, and sometimes its ability to show activities not evident in the EEG. These features seem to be very encouraging for the search for technical improvements, with the aim of making the magnetic technique a candidate for current diagnostic purposes.

Properties of in-situ laser-pulsed deposited YBCO thin films on MgO with SrTiO3 buffer layer

Abstract Epitaxial YBa 2 Cu 3 O 7− y films were grown by pulsed-laser ablation on SrTiO 3 buffered (100) MgO substrate without any pre- or postannealing of the substrate or STO buffer layer. The role of the SrTiO 3 layer is to provide a better lattice mach to the YBCO film and to prevent the interaction between YBCO and MgO. The as-obtained YBCO films exhibit excellent structural and superconducting properties with T c (R = 0) = 90 K , R(300) R(100) = 2.8, Δ T = 0.8 K , J c ≈ 10 6 A/cm 2 (at 77 K and zero magnetic field) and a full width half maximum of the (005) YBCO peak of about 0.18°. These characteristics are very close to those of the YBCO film deposited on SrTiO 3 single crystal.

SQUID instrumentation and the relative covariance method for magnetic 3D localization of pathological cerebral sources

SummaryMagnetic fields generated by bioelectric currents responsible for epileptic activity in the human brain have been measured by means of a movable magnetometer based on SQUID technology. Simultaneously detected magnetic and electric signals were studied by computing the covariance and dividing this quantity by the electric variance. The relative covariance so obtained was proved, to reliably reflect the distribution of magnetic field and, by means of an appropriate model, permitted 3D localization of the investigated epileptic source.

High-quality surface YBCO thin films prepared by off-axis pulsed laser deposition technique

Abstract Epitaxial YBCO superconducting thin films have been grown in-situ by XeCl excimer laser ablation using an off-axis deposition geometry. This PLD geometry allows to obtain films with an excellent surface morphology without droplets or other particulates. The as-obtained YBCO films exhibit good structural and superconducting properties with T c( R = 0) =89.5 K, R (300) / R (100) =2.8, ΔT = 1.3 K, J c = 2 × 10 6 A/cm 2 (at 77 K and zero magnetic field) and full width half maximum of the rocking curve of the (005) YBCO peak of about 0.16°. These structural and electrical properties are very close to those of the YBCO films deposited by the conventional PLD on-axis geometry.

Superconducting properties of Y-Ba-Cu-O thin films grown in situ by laser ablation

Abstract Thin films of YBa 2 Cu 3 O y have been grown in situ on (100) SrTiO 3 and (100)MgO, by pulsed excimer laser evaporation from a stoichiometric target. The substrate temperature was adjusted at 750°C by using a CO 2 laser as the heating source and the oxygen partial pressure during the deposition was about 0.3 mbar. The samples have been patterned in the shape of lines 0.2 mm long and of widths between 5μm and 20μm. DC transport critical currents have been investigated as a function of temperature and of applied magnetic field up to 6T.

In situ preparation of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ thin films deposited by ion beam sputtering: preliminary results

Thin films of the high-temperature superconductor Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ have been grown in situ by ion beam sputtering from metal binary alloys. The metal binary alloys used were YCu and BaCu. The correct composition of the deposited film was obtained by adjusting the position of the targets with respect to the ion beam. The films were deposited onto heated SrTiO/sub 3/ substrates in oxygen partial pressure. After deposition the samples were slowly cooled to room temperature with a short step at 450 degrees C. The films did not need any further annealing after deposition and showed superconducting properties.

X-rays operation of a thin film NbVN superconducting-strip particle detector

Using a current biased 2 μm wide, thin film (Nb0.8V0.2)N superconducting strip operated at 1.4 K, we were able to detect voltage pulses due to the superconducting to normal transition induced by a 55Fe 6 keV X-rays source. At high bias currents the estimated counting efficiency was close to 60%. The detector performance and its potential application for X-rays and minimum ionizing particles detection are discussed in the framework of the recent literature on the subject.

Manufacturing and Properties of YBCO Step-Edge Type Josephson Junctions

We report the manufacturing and the properties of two types of step-edge Josephson junctions: YBCO-YBCO (SEJ) and YBCO-Au-YBCO (SNS-SEJ), respectively. The junctions were prepared on step shaped SrTiO3 substrates using in-situ pulsed laser deposition technology for film deposition and ion-etching patterning. The YBCO films used for this purpose have zero resistance transition temperature Tc(R=0) in the range (89–91) K and a critical current density Jc(B=0)>106 A/cm2 at 77 K in zero applied magnetic field. For both types of junction the I-V characteristics can be well described in the limit of the resistively shunted junction (RSJ) model.

Anomalous voltage modulation observed in YBCO dc-SQUIDs based on step-edge junctions

Abstract We have observed anomalous voltage vs. magnetic flux modulations in YBCO dc-SQUIDs employing step-edge junctions. The anomaly consists inphase inversion and frequency doubling of the voltage modulations in a restricted range of bias currents. We ascribe this phenomenon to the non-negligible capacitance of the junctions in the SQUID, as confirmed by numerical simulations.