Superconducting magnetometers

2014 Under the proper circumstances a current carrying superconductor can generate voltage. This voltage is produced by supercurrent oscillations at frequency 03C9 = 2e/0127 V 2014 a phenomena similar to the Josephson effect in superconducting tunnel junctions. Various structures have been fabricated in which this quantum relation dominates the electrical behavior. These thin film structures have been developed into reliable sensor elements which have been incorporated into various electronic instruments 2014 particularly magnetometers. These instruments include a differential magnetometer with a one second response time and sensitivity greater than 10-10 gauss and a digital magnetometer counting in increments of 1/4h/2e at a rate up to 104 per second. The fundamental principles of these instruments will be discussed as well as their present operational limitations. REVUE DE PHYSIQUE APPLIQUÉE TOME 5, FÉVRIER 1970, PAGE Zero electrical resistance has been the principal hallmark of the phenomena of superconductivity. It was, in fact, this effect which alerted Kamerlingh Onnes that he has uncovered a new physical phenomenon. This electrical aspect of superconductivity, the ability to carry finite current at zero voltage has received major attention from experimentalists for some time. This is primarily because there is at least an adequate phenomenological description of this aspect of superconductivity and also, in a practical sense, because of the vast economic implications of low cost power transmission and very high field superconducting magnets. However, there is another facet of superconductivity which in the past seems to have received somewhat less attention. Despite the apparent contradiction in terms, this kind of superconductivity can probably best be characterized as the resistive superconducting state that is to say, superconductivity at finite voltages. In contrast to zero voltage superconductivity there seems to be little theory to guide the experimenter into this intrinsically time dependent problem. The purpose of this paper is to indicate our use of superconductivity at finite voltages for instrumentation purposes and, in the absence of a more complete theory, to attempt to describe our results in physical terms. In order to sustain a voltage supporting superconducting state it is usually necessary that the super-electron density be inhomogeneous. Kim [1] and Gieaver [2] have studied this state in situations where the inhomogeneities are vortex lines produced by an external field. I want to concentrate here on the "field free" case where the inhomogeneity is determined by the material itself. The extreme of this inhomogeneity is the Josephson [3] tunnel junction, with an insulating barrier. However this structure has already had extensive treatment and since it is relatively delicate is not uniquely suitable for instrumentation. In what is to follow I would like to specifically exclude this particular inhomogeneity and consider only electron density variations resulting from current through the Dayem [4] bridge or a nonhomogeneous metal [5]. Voltage-supporting superconductivity. In figure 1, curve A shows a typical current-voltage characteristic across such a superconducting structure driven by a current source. For currents less than some temperature-dependent critical current, I,,,, current can flow FIG. 1. Voltage developed across a Dayem bridge driven from a current source. Curve A shows a critical current, I.1, of about .2 milliamperes. Curve B is the same structure irradiated with microwaves at about 35 GHz. without developing any voltage; while above this critical current, voltage ( ) exists. Voltage induced above, ICI’ indicates that power is being delivered to the superconductor from the current source. However, as I shall indicate, because of the peculiar process of voltage generation in a superconductor only part ofthis power is transformed into heat while the remainder can escape as high frequency radiation, principally at frequencies v = N(2e/h) V = Ncpo-1 V. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:019700050101300