INSTRUMENTATION
Advisory Panel Jonathan W. Amy Glenn L. Booman Robert L. Bowman
Jack W . Frazer Howard V. Malmstadt William F. Ulrich
Novel Superconducting Laboratory Devices Part II. Operating characteristics of a magnetometer, voltmeter, and radiation detector based on the Josephson Effect By Juri Matisoo
L A S T M O N T H t h e general effects of
superconductivity, t h e Josephson Effect, a n d tunneling were dis cussed as a prelude t o t h e under standing of t h e operation of certain laboratory devices. T h e operating principles a n d characteristics of a magnetometer, a voltmeter, a n d a radiation detector will now be dis cussed.
First, there existe a critical current value, IC) below which current can flow through t h e junction with n o voltage drop a n d above which a voltage appears. T h e value of Ic depends strongly on applied mag netic fields, b u t in a rather unex pected w a y ; rather t h a n decreasing monotonically with increasing field, it is periodic in field a n d (ideally) is given b y
Devices A:
TTAHH
MAGNETOMETER. T h e m a g n e
tometer a n d voltmeter, operate on the same principle ; t h a t is, t h e volt meter is also basically a magnetic field detector. T h e magnetometer is largely t h e development of Zim merman a n d Silver a n d their col leagues a t Ford (1) while t h e volt meter was developed b y Clarke a t Cambridge (2). Let us review briefly t h e proper ties of a Josephson tunnel junction.
SUPERCONDUCTOR
BARRIER
°OXIDE
SUPERCONDUCTOR
Figure 1. This shows (schematically) a tunnel junction. Two superconductors are separated by a barrier of oxide of thickness, dOKUU:. The barrier is of area A and the area of field penetration, A H , is indicated by the dotted lines (A H = L χ (2λ + dmii,)
Ij(H)
sin ——— = 7,(0) j-±-
(5)
3>o
where Η is the applied field ; Φ0 is a constant (2 χ Ι Ο " 7 G-cm 2 or 2 χ 1 0 - 1 5 W b ) and AH is t h e effective area of t h e field penetration, L(2\ + dox) (Figure 1). This dependence is t h e result of summing t h e current density over the area of t h e junction. Because of t h e magnetic field (as discussed) there is a spatial dependence t o Φ. If t h e field is just equal to ηΦ0/ΑΗ (η = 1, 2, . . . ) , one half of t h e current in t h e junction flows in one direction a n d half in the other, and the externally measured I0 is zero. If the field has some other value, a nonzero Ic is measured. If one could m a k e a junction of sufficient length so t h a t Au = 1 cm 2 , then t h e period of Ic in field is 2 X 1 0 _ 7 G . Since one can cer tainly detect a change of a fraction of a period, magnetic fields on t h e order of 1 0 _ 9 G could be detected. Unfortunately, i t is difficult in practice to m a k e such a long junc tion (L
