Page 416 - Electrical Properties of Materials

398 Superconductivity

Amorphous Paramagnetic
semiconductor Strongly correlated metal metal

Temperature

Fig. 14.22 Antiferromagnetic
Generic phase diagram of the cuprate insulator
superconductors. The doping level is Superconductor
measured relative to the insulating
parent compound. Doping level

doubt that pairing is involved, and the effective charge is 2e. It is also known
that the pairs are made up of electrons with opposite momenta, just as was
shown in Fig. 14.3(a). Interaction of the electrons with the lattice might play a
role, but it is certainly not the full story. Another possible mechanism is pairing
by spin waves, as already referred to earlier in this chapter. An important ex-
periment is to measure the magnetic flux in superconducting rings containing
Josephson junctions. With conventional superconductors, the enclosed flux is
always an integer multiple of the flux quantum. With cuprate superconductors,
the enclosed flux turns out to be an odd multiple of the half-flux quantum. This
is no proof for spin-wave pairing, but if pairing is by spin waves, then this is
one of the conditions that must be satisfied.
In what other directions could one look for a theory? One might possibly
rely on the analogy between the quantum Hall effect and superconductivity
in cuprates. Two-dimensional effects and sudden loss of resistance are char-
acteristic to both. An attempt along such lines has been made at explaining
the quantum Hall effect with a theory which treats electrons as some kind of
composite bosons.
Most theoreticians believe that an energy gap always exists, and for cuprate
superconductors the relationship between gap energy and critical temperature
is 2 (0) ≈ 6 kT c in contrast with 3.5 kT c , which we have come across for low
temperature superconductors. No one entertains great hopes that a theory able
to predict the critical temperatures of various compounds will be forthcoming
in the near future. The theoretical interest will be sustained, however, very
likely for decades. Pairing mechanisms have become popular. Neutron stars
are supposed to have pairing condensations, and it is also believed that quark
condensations began just one second after the Big Bang, although experimental
evidence is lacking for the moment.
How are these superconductors produced? Being ceramics, they were first
produced by mixing, grinding, and baking of powdered reagents. Single crystal
samples, as described above, greatly helped in advancing the understanding of
their properties, but they are not suitable for mass applications. For devices one
needs them as thin films. For high-field magnets they have to be in the form
of wires. Thin films are mostly made by sputtering and pulsed laser deposi-
tion (a pulsed excimer laser evaporates the material which is already available