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New superconductors 401

14.10 New superconductors
The phenomenon of superconductivity never ceases to surprise us. There are
lots of recently discovered superconductors which are most reluctant to fit into
the general framework. The situation reminds me of Pope’s well known epitaph
intended for Newton,
Nature, and Nature’s laws lay hid in the night
God said, Let Newton be! and all was light

and of Squire’s addition to it a couple of centuries later,
It did not last: the Devil howling “Ho!
Let Einstein be! restored the status quo.
Well, this is what happened to superconductivity. After the formulation of
the BCS theory in 1957 all was light for a long time. But then, in 1986 our
confidence in understanding the physics was shattered by the arrival of high
T c oxide superconductors. So, we could say at the time, there are conventional
superconductors and oxide superconductors and one day we shall understand
how those in the latter family work. But nowadays nothing can be taken for
granted. The old type of intermetallic compounds reappear with much higher
critical temperature, organic materials join the club, and it turns out that an
applied magnetic field is not necessarily a bad thing. We do not really know
any more what the limits are, what is achievable, and what is not. The status
quo of ignorance has been restored.
Let us start with magnesium boride, a simple intermetallic compound with a
crystalline structure shown in Fig. 14.25. The boron atoms arrange themselves
in two-dimensional hexagonal sheets, like graphite, within a cubic structure
of magnesium. What is extraordinary about it is its critical temperature well
above that of other intermetallic compounds. It does obey though BCS theory
in one respect: it has an isotope effect. The critical temperature is 40.2 K for
atomic weight 10 and 39.2 K for atomic weight 11. It differs, however, from

B

Mg Fig. 14.25
Crystal structure of MgB .
2