Figure 23: A SEM picture of a step edge Josephson junction [24]
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One of the most powerful systems currently available is the Neuromag Vectorview™. The detector array consists of 102 triple sensor elements.
Each element comprises of two orthogonal planar first-order gradiometers and a magnetometer. The two gradiometers are most sensitive to currents flowing directly below them, parallel to the sensor plane and in the direction of the sensor pick-up coils. The magnetometer on the other hand is most sensitive along the axis perpendicular to the plane of the sensor. Thus, the Vectorview™ has excellent spatial resolution.
C. T. F. systems also make a device with between 64 and 150 sensors. This device also includes a reference array of sensors positioned above the signal sensors. This measures the magnetic vector and first-order gradient tensor components of background noise. This information is then processed to give a virtual 3rd order response to any noise signals. Therefore, the system exhibits a magnetometer response to nearby signal sources and a 3rd order gradient response to distant noise sources. This enables the system in many cases to be operated in an unshielded environment.
SQUIDs are used in many other biomedical applications, the most popular of these being heart monitoring, [20] and [21]. However SQUIDs have also been used to observe much slower changing fields such as those due to currents in the legs believed to be responsible for organ development and healing.
Since the discovery of HTS superconductors, a large amount of research was put into applying them in many applications including that of SQUIDs. Bulk high temperature superconductors have very low critical current densities jc, so thin film technologies were developed giving a thousand-fold increase in jc. Standard barrier tunnel junctions are difficult to make due to short coherence length, the complex chemistry involved and the requirement to oxygenate the compounds at temperatures in excess of 600oC. So many different types of Josephson junctions have been investigated. The main applied types of Josephson junctions are based on weak links where the Josephson effect occurs across the link.
The first type of high temperature Josephson junction to be found and applied was based on natural grain boundary weak links [22]. However the junction properties were fairly unpredictable and unreproducible so were not useful for commercial applications. An evolution of this technology involves the use of artificial grain boundaries or bicrystal junctions. They work on the theory that two touching grains of superconducting material at angles larger than about 10o [23] can act as a weak link Josephson junction. These are quite common since it is possible to grow grains on a substrate that enables more control of crystal plane orientation.
The most commonly used type of junction is the step edge Josephson junction. A film of superconducting material is grown on a substrate and then etched producing a step with bridges, which act as a weak link. Advantages of this type are low noise while also they can be freely placed on the substrate. However, they suffer from low reproducibility compared to bicrystal junctions.
Figure 23: A SEM picture of a step edge Josephson junction
[24]
More recently, some LTS Josephson junctions have been translated into HTS equivalents by taking account of the issues involved for HTS. Superconducting-Normal-Superconducting (SNS) or tri-layer junctions in HTS are similar to the traditional junction showing how some of the initial problems have been overcome. Some of the issues involved include the barrier which needs to have the correct thermal and connection properties with the superconducting material [25]. Other modern junctions include ramp like structures, a hybrid of step and SNS type.
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This document was saved as HTML from a Word 97 document and then labouriously converted from its nasty output. This document was last updated on Wednesday 28th October 1998.