I. Modena

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.

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.

Use of a superconducting instrumentation for biomagnetic measurements performed in a hospital

A superconducting instrumentation has been set up to perform biomagnetic measurements for screening analysis in one of Rome hospitals. The magnetic detector is a 2nd derivative gradiometer, which can be balanced against both spatially uniform fields and field gradients. The overall noise level is \sim 4 \times 10^{-14} T.Hz-½ in the frequency range 0.5 ÷ 3000Hz. Main efforts are devoted to the recording of magnetocardio-graphic maps of normal and abnormal subjects while a detailed analysis of some segments of the heart cycle is carried on in subjects previously investigated by means of other techniques. Magnetomyograms of normal and abnormal subjects are detected and studied in the frequency domain. Magnetoencephalographic power spectra of various subjects have been mapped as well. Most remarkable results so far collected are presented. Some of the measurements have been performed in the hospital and some in the laboratory, always without the aid of any shielding.

A SQUID device for ac current measurements down to 10−14 A

A new method for measurements of ultralow currents in the audio‐frequency range (from dc up to over 104 Hz) has been developed. The combined use of a ferromagnetic current transformer and of a superconducting flux transformer together with a rf‐biased SQUID allows measurements of currents as low as 10−14 A. A detailed circuital analysis and considerations on the input impedance of the device are presented. The agreement between predictions and experiments is quite good. The measured noise level beyond the low‐frequency region keeps at values typical of commercial SQUID’s.

Millimeter wave generation by fluxon oscillations in a Josephson junction

A small‐area Josephson tunnel junction is pumped by the radiation emitted by a long Josephson junction dc biased on a zero‐field singularity. Josephson constant voltage steps are observed in the current‐voltage characteristic of the small junction at voltages of ±150 μV corresponding to the fundamental frequency of emission of the oscillator. The radiation also gives rise to quasiparticle steps around the gap‐sum voltage of the small junction. From the measured current amplitude of the steps we estimate that the maximum power coupled to the small junction is about 90 nW.

Coherence of Josephson soliton oscillators in the millimeter-wave range

Abstract We report on the observation of phase-locking and radiation enhancement from two dc series-biased Josephson fluxon oscillators ac-coupled by a thin film-integrated capacitor. The radiation is detected by a small area Josephson junction displaying, under the effect of the radiation, critical current suppression and Shapiro steps at a voltage of 220μV. The phenomena that we observe are the higher frequency counterpart of the superradiant effects measured by means of X-band room temperature receivers detecting the radiation emitted by series arrays of long Josephson junctions.

Experiments on tunable Josephson millimeter-wave oscillators

The coupling of millimeter-wave radiation from a long Josephson junction to a small-area junction is discussed. The coupling is obtained by means of thin-film technology and the design parameters allowing radiation coupling are discussed. Since the frequency of the oscillator radiation is smaller than the plasma frequency of the detector junction, the latter shows evidence of chaotic dynamics in the current-voltage characteristic. However, very stable zero-crossing bias steps appear for high values of the applied radiation power. The tunability of the oscillator power allows measuring the dependence of the detector critical current on the external radiation to be measured; from this measured dependence and from the observed amplitude of the photon-assisted tunneling steps it is estimated that maximum coupled power is the range of tens of nanowatts.

Radiation detection from Fiske steps in Josephson junctions above 200 GHz

We have measured the Josephson radiation emitted at millimeter‐wave frequencies by a long Josephson junction dc‐current biased on Fiske steps of the current‐voltage characteristic. Our measurements demonstrate that differences between dynamic excitation generating Fiske steps and fluxon oscillations giving rise to zero‐field steps in long junctions may exist. The radiation, whose maximum measured frequency is 240 GHz, is generated by an in‐line junction and detected on a chip by a small planar tunnel junction. From measurements of Shapiro steps amplitude and photon‐assisted tunneling steps we find that the coupled radiation has a power tunability of 350 nW and the maximum coupled power is 20 nW in a 30‐Ω normal‐state resistance tunnel junction.We have measured the Josephson radiation emitted at millimeter‐wave frequencies by a long Josephson junction dc‐current biased on Fiske steps of the current‐voltage characteristic. Our measurements demonstrate that differences between dynamic excitation generating Fiske steps and fluxon oscillations giving rise to zero‐field steps in long junctions may exist. The radiation, whose maximum measured frequency is 240 GHz, is generated by an in‐line junction and detected on a chip by a small planar tunnel junction. From measurements of Shapiro steps amplitude and photon‐assisted tunneling steps we find that the coupled radiation has a power tunability of 350 nW and the maximum coupled power is 20 nW in a 30‐Ω normal‐state resistance tunnel junction.

A thin‐film second‐order gradiometer with integrated dc‐SQUID

We have fabricated and characterized a superconducting planar gradiometer sensitive to the second‐order spatial derivative of the magnetic field. Our device has been patterned on a 12.8×12.8 mm silicon chip on which the sensitive area of the gradiometer and the SQUID inductance are generated by the same four 0.5‐mm‐square holes. We measured the sensitivity both in flux‐locked‐loop (FLL) and in open‐loop mode. With the second method we used another dc‐SQUID as amplifier and in this case we obtained the best noise performance. The gradiometer sensitivity was 18 fT/cm2 √Hz. The magnetic isoflux line distribution generated by a dipolar source was measured by the gradiometer in FLL mode.

Detection of fluxon oscillator radiation by a planar Josephson junction

Abstract A planar Josephson junction is used as a detector for the radiation emitted by a long Josephson junction oscillator dc-biased on a zero-field singularity. We observe Josephson current suppression and 300 μV radiation induced singularities in the current-voltage characteristic of the detector junction.