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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.

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