Page 412 - Electrical Properties of Materials

394 Superconductivity

values of T c and H c . The two superconductors used in practical devices are the
ductile Nb–Ti alloy and the brittle intermetallic compound Nb 3 Sn, the latter
being used at the highest magnetic fields.

14.8.2 Switches and memory elements
The use of superconductors as switches follows from their property of becom-
ing normal in the presence of a magnetic field. We can make a superconducting
wire resistive by using the magnetic field produced by a current flowing in an-
other superconducting wire. Memory elements based on such switches have
indeed been built, but they were never a commercial success.
Superconducting memory elements based on the properties of Josephson
junctions have a much better chance. As we have mentioned before, and may
be seen in Fig. 14.19, the junction has two stable states, with zero voltage
and the other with a finite voltage. It may be switched from one state into the
other one by increasing or decreasing the magnetic field threading the junc-
tion. The advantage of this Josephson junction memory is that there is no
normal-to-superconducting phase transition necessary, only the type of tun-
nelling is changed, which is a much faster process. Switching times as short as
10 ps have been measured.
Will it ever be worthwhile to go to the trouble and expense of cooling
memory stores to liquid helium temperatures? So far computer manufactur-
ers have been rather reluctant (understandably, it is a high risk business) to
introduce superconducting memories. It is difficult to predict, but the latest
members of the family, Rapid Single Flux Quantum (RSFQ) devices, might
have the chance to be introduced in practice some time in the future, when
high speed becomes the principal requirement. The basic architecture is a ring
containing a Josephson junction. A large number of such rings coupled mag-
netically make up the device that can serve both as a memory element (it stores
a single flux quantum) and a logic device. The latter property is due to the fact
that voltage pulses can travel from element to element extremely rapidly along
such a line. The highest speed of operation observed so far for these devices is
770 GHz. Apart from speed a further advantage is the quantized nature of the
storage mechanism, providing protection against both noise and cross-talk.

14.8.3 Magnetometers

A further important application of Josephson junctions is in a magnetometer
called SQUID (Superconducting Quantum Interference Device). Its operation
is based on the previously mentioned property that the maximum supercurrent
through two junctions in parallel is dependent on the magnetic flux enclosed
by the loop. It follows from eqn (14.74) that there is a complete period in I max ,
while varies from 0 to 0 . Thus, if we can tell to an accuracy of 1% the
2
magnitude of the supercurrent, and we take a loop area of 1 cm , the smallest
magnetic field that can be measured is 10 –12 T. Commercially available devices
(working on roughly the same principle) can offer comparable sensitivities.
Although the Josephson junction does many things superlatively well, like
other topics in superconductivity, its applications (so far) are few. However, it