The s- and d- of it

IN 1987, a group of researchers had discovered the intriguing new phenomenon of high temperature superconductivity. The conventional superconductors had a critical temperature (temperature below which the material is superconducting) which was extremely low - typically, about -250'c. However,
the new materials, which were mostly
very complex crystals like yttriumbarium-copper-oxide (YBCO), were
superconducting at much higher
temperatures.

Since then, a host of other materials
with varying properties have been discovered and studied by scientists all over the world. The situation on the
theoretical front has, however, not been
too encouraging; the conventional
explanation of superconductivity fails
to enumerate the phenomenon of
high temperature superconductivity.
Despite many attempts for a suitable
explanation, a satisfactory theory of
this tantalising phenomenon still eludes
scientists.

Recently, a team of researchers at
the IBm Thomas J Watson research laboratories at Yorktown Heights, us, has
reported that the behaviour of electrons
within the superconductor may be
described by a function called the
d-wave. In traditional superconductors,
electrons continue in twos to form
Cooper pairs, which can then move
through the material without much
resistance, giving rise to the almost zero
resistance to electric current which is
the remarkable hallmark of superconductivity. The pairs are formed because
of the wave of lattice vibration called a
phonon. This produces a spherically
symmetrical wave function (the mathematical function which carries all the
information about the electron) called
an s-wave (Science, Vol 271, No 5247).

A rival theory which tries to explain
high temperature superconductivity
does away with this mechanism of pairing of the electrons. According to this
theory, the electrons get paired because
of a magnetic interaction (spin fluctuation) and the resulting wave function is
called d-wave which is very different
from the s-wave. Experiments carried
out over the last few years have generated conflicting evidence as to which of
these mechanisms is at work in these
materials.

But now, Chang-Chyi Tsuei and
John Kirtley at IBM have reported an
experiment which is the strongest evidence available so far of the d-wave
behaviour. Using a ring made up of
thallium-barium-copper-oxide, which
has a much simpler crystal structure
than YBCO, they have studied the behaviour of electrons moving in the ring.
They conclude that there is convincing
evidence for the electrons behaving
according to the d-wave theories.

Though there have been many
experimental results which favour the
s-wave explanation, more and more
people in the field are now convinced
that the simplest phonon theories
are not sufficient to explain the
phenomenon. Whether the correct
explanation lies in the d-wave theory or
some -other unknown enumeration, the
field is sure to witness much excitement
in the future.