Page 420 - Electrical Properties of Materials

402 Superconductivity

other metallic superconductors by not having a high charge carrier density.
There is an energy gap but it is of a different kind. Two superimposed energy
gaps have to be assumed to explain its properties.
Another recently discovered superconductor is PuCoGa 5 which has a high
critical temperature of 18 K and in which induced magnetic fluctuations of the
electrons are supposed to be responsible for the superconducting transition.
Clearly, this is not an oxide superconductor but could the superconducting
mechanism be close to that of oxides? Will there be similar compounds found
with higher critical temperatures? The answers are not known at the moment.
A further interesting feature of the PuCoGa 5 superconductor is its extremely
high upper critical field estimated at 35 T. The tentative explanation is the ra-
dioactivity of plutonium 239, which is responsible for pinning the flux lines by
creating line defects.
Let us now come to the effect of a magnetic field. We have been happy to ac-
cept so far that the critical temperature is reduced by applying a magnetic field,
and a high enough magnetic field will completely destroy superconductivity.
This is not surprising at all. Cooper pairs are made up of electrons with oppos-
ite momenta and spins. Therefore a magnetic field, whether applied or internal
due to the ferromagnetic line-up of dipoles, may be expected to be harmful
because it affects differently the spin up and the spin down state. So the clear
conclusion is that superconductivity might coexist with antiferromagnetism
but never with ferromagnetism! Well, the discovery of superconductivity in
UGe 2 proved otherwise. If the material is kept all the time above the Curie
temperature so that its magnetic state is paramagnetic, then, however low the
temperature, no superconducting state exists. On the other hand, below the
Curie temperature, in the ferromagnetic state, there is a range of pressures
for which superconductivity is present below a critical temperature. This is so
much against the grain that a new theory is needed. The tentative answer is
that some other type of Cooper pair must exist in which electrons of opposite
momenta but identical spins pair up, and then an applied magnetic field might
actually be helpful. The likely reason why these materials (there are a num-
ber of them) have only recently been discovered is their anisotropic nature. If
anisotropic, then the state will crucially depend on the electron momenta in
various directions that can be seriously altered by impurity scattering. Hence,
superconductivity exists only when the material is made pure enough—and up
to now the technology has just not been available.
Next we wish to mention organic superconductors. All kinds of organic ma-
terials are in fashion nowadays, including superconductors. What is certainly
known about them is that the molecules are long, that they are close to each
other, so that electrons and holes can hop from one to the next; and that they
are stacked in two dimensions. They have some unusual properties; the most
outrageous among them being that the superconducting state can be brought
∗ The resemblance is probably the main on by applying a magnetic field. We know (see Fig. 11.34) that on the applica-
reason why they have been so diligently tion of a magnetic field the electronic bands split into a spin-up and spin-down
investigated in the last couple of years. band which have somewhat different momenta. When two electrons of differ-
Since the microscopic mechanism of
the cuprate superconductors is still un- ent spin pair up, the resulting momentum will be non-zero. Could that cause
known, clues from the behaviour of a the various anomalies observed? It remains to be shown.
similar superconductor might offer the The latest superconducting family is that of pnictides. These are layered
key to understanding both. iron arsenide materials. They bear a certain resemblance to cuprate ∗