Elliptic Annular Josephson Tunnel Junctions in an external magnetic field: The statics

We have investigated the static properties of one-dimensional planar Josephson tunnel junctions in the most general case of elliptic annuli. We have analyzed the dependence of the critical current in the presence of an external magnetic field applied either in the junction plane or in the perpendicular direction. We report a detailed study of both short and long elliptic annular junctions having different eccentricities. For junctions having a normalized perimeter less than one the threshold curves are derived and computed even in the case with one trapped Josephson vortex. For longer junctions a numerical analysis is carried out after the derivation of the appropriate Perturbed sine-Gordon Equation. For a given applied field we find that a number of different phase profiles exist which differ according to the number of fluxon-antifluxon pairs. We demonstrate that in samples made by specularly symmetric electrodes a transverse magnetic field is equivalent to an in-plane field applied in the direction of the current flow. Varying the ellipse eccentricity we reproduce all known results for linear and ring-shaped Josephson tunnel junctions. Experimental data on high-quality Nb/Al-AlOx/Nb elliptic annular junctions support the theoretical analysis provided self-field effects are taken into account.Comment: 30 pages, 13 figure

SQUID based multichannel system for brain functional imaging

A multichannel system for brain imaging containing 163 SQUID magnetometers arranged in a helmet shaped multisensorial array has been developed. To this aim, a previous investigation of a several SQUID configurations has been performed in order to choose a SQUID sensor having best performance for brain imaging on the basis of system working conditions. In particular, magnetometer and planar gradiometer have been designed, fabricated and characterized. Furthermore, a small magnetometer has been also taken into account. Since it has been decided to work in a magnetically shielding room a SQUID magnetometer has been chosen in order to guarantees high magnetic field sensitivity. These SQUID magnetometers are based on an integrated Ketchen design including a superconducting flux transformer consisting of a pickup coil and a multiturn input coil inductively coupled to the SQUID loop in a washer shape. The circuits for Flux Locked loop (FLL) operations and for Additional Positive Feedback including a thin film resistor network for gain adjusting, are integrated on the same chip containing the SQUID magnetometer. A magnetic field spectral noise as low as 1.8 fT/Hz measured in FLL operation, ensures the sensor capability to detect the tiny magnetic filed arising from brain activity. More than 200 SQUID magnetometers have been fabricated and characterized selecting the sensors with best performance. 154 measurement sensors are arranged in a SQUID multisensorial array properly designed and customized realized. Further 9 channels are located far from the scalp on three bakelite towers in order to realize via software a gradiometer to background noise rejection. The system properties have been investigated including the estimation of background noise, the shielding factor of the magnetically shielding room and the Dewar performance. Finally, preliminary measurements have been successfully performed. In particular, some brain activities such as the alpha rhythm, a spontaneous activity of a human having closed eyes and the evoked activity concerning the tapping of left and right forefinger have been analyzed. The same activities have been co-registered using the 32-channels EEG to measure bioelectric activity. The good agreement between EEG and MEG data indicates that the system operates properly

Fine-Tuning and Optimization of Superconducting Quantum Magnetic Sensors by Thermal Annealing

In the present article, we present the experimental results concerning the fine-tuning and optimization of superconducting quantum interference device (SQUID) parameters by thermal annealing. This treatment allows for the modification of the parameters in order to meet a specific application or to adjust the device parameters to prevent the increase of magnetic field noise and work instability conditions due to a different critical current with respect to the design value. In particular, we report the sensor critical current, the voltage–flux (V–Φ) characteristics and the spectral density of the magnetic field of SQUID magnetometers for different annealing temperatures. The measurements demonstrate that it is possible to achieve a fine control of the most important device parameters. In particular, we show that thermal annealing allows for the reduction of SQUID noise by more than a factor of 5 and makes the device working operations very stable. These results are very useful in view of quantum technology applications related to superconducting quantum computing where the correct functioning of the quantum bit depends on the fine control of the superconducting quantum device parameters and selectable annealing is possible by using a suitable laser as a thermal source

Picoammeters Based on Gradiometric Superconducting Quantum Interference Devices

High-sensitivity ac current sensors based on a superconducting quantum interference device have been designed, fabricated and characterized. In particular, double-washer schemes in either parallel or series configurations have been considered. The advantages and the drawbacks of both configurations have been examined by measuring the main features and parameters, such as the flux-to-voltage characteristic, the magnetic field spectral noise and flux-to-current transfer factor. The devices are designed to have similar flux-to-current transfer factors and are fabricated on the same chip to avoid differences in parameters due to the fabrication process. Both devices exhibited a current sensitivity as low as 1–2 pA per bandwidth unit, allowing for their use in ultrahigh-sensitivity applications

Highly Sensitive Tunable Magnetometer Based on Superconducting Quantum Interference Device

In the present article, experimental results regarding fully integrated superconducting quantum interference devices (SQUID), including a circuit to tune and optimize the main sensor device characteristics, are reported. We show the possibility of modifying the critical current of a SQUID magnetometer in liquid helium by means of a suitable heating circuit. This allows us to improve the characteristics of the SQUID sensor and in particular to optimize the voltage–magnetic flux characteristic and the relative transfer factor (responsivity) and consequently to also improve the flux and magnetic field noise. It is also possible to reset the SQUID sensor in case of entrapment of magnetic flux, avoiding taking it out of the helium bath. These results are very useful in view of most SQUID applications such as those requiring large multichannel systems in which it is desirable to optimize and eventually reset the magnetic sensors in a simple and effective way

Multichannel System Based on a High Sensitivity Superconductive Sensor for Magnetoencephalography

We developed a multichannel system based on superconducting quantum interference devices (SQUIDs) for magnetoencephalography measurements. Our system consists of 163 fully-integrated SQUID magnetometers, 154 channels and 9 references, and all of the operations are performed inside a magnetically-shielded room. The system exhibits a magnetic field noise spectral density of approximatively 5 fT/Hz1=2. The presented magnetoencephalography is the first system working in a clinical environment in Italy