Download Mr. SQUID User`s Guide

Mr. SQUID User’s Guide
1 microamp. Since the junction on the left has to carry 1 microamp of screening current, it can
now carry only 4 microamps of bias current before it becomes resistive. It doesn’t distinguish
between bias currents and screening currents; it just detects the flow of the electron pairs. When
it carries a total of 5 microamps, it becomes resistive.
When the junction on the bottom goes normal, all the current goes through the junction on the
top, which makes it go normal. That means both paths are now resistive, so a voltmeter will
register a voltage across the SQUID.
As we increase the applied magnetic flux, the screening current increases. But when the applied
magnetic flux reaches half a flux quantum, something interesting happens. The junctions
momentarily go normal. The continuity of the superconducting loop is destroyed long enough
for one quantum of magnetic flux to pop inside the loop. Then superconductivity around the
loop is restored. (This is illustrated in Figure 5-7.) Thus, the junctions serve as gates that allow
magnetic flux to enter (or leave) the loop. The voltage read with an oscilloscope or x-y recorder
is the average voltage across the SQUID. Although the experimenter observes a non-zero dc
voltage (hence the SQUID appears “resistive” just above Ic as shown in Figure 5-4 and the left
side of Figure 5-8) the instantaneous voltage across the SQUID and the circulating current are
actually oscillating at high frequencies in the microwave range in response to an applied
magnetic field.3
The phenomenon is not so surprising, if you notice that it makes things easier for the SQUID.
Consider what happens to the screening current. Rather than generating enough screening
current to keep 0.51 flux quanta out, now all the SQUID has to do is generate enough screening
current to keep 0.49 flux quanta in, which is, of course, a little easier (that is, lower in energy.)
Of course, the screening current has to change direction, as shown in Figure 5-6 below.
V
Ibia s /2
Ibia s
Is
Ibia s /2
Figure 5-6 The screening current Is has reversed its direction.
If we consider the behavior of the screening current as more and more magnetic flux is applied,
we would obtain the plot shown in Figure 5-7. As you see, the screening current changes sign
(really, it changes direction), when the applied flux reaches half of a flux quantum. Then, as the
applied flux goes from half a flux quantum toward one flux quantum, the screening current
decreases. When the applied flux reaches exactly one flux quantum, the screening current goes
3An
article which describes this is Ryhänen et al, "SQUID Magnetometers for Low-Frequency Applications,"
Journal of Low Temperature Physics 76, 287 (1989).
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