Institut Laue-Langevin

High-Temperature Superconductors reveal part of their mystery!

In collaboration with the ILL, scientists from the LLB (France) and the University of Minnesota (USA) have published in the November issue of the journal Nature the results of experiments performed on the polarized neutron spectrometer IN20, currently the most powerful of its kind worldwide. They have succeeded in experimentally verifying the theory that there exists in these materials an ordered state of matter with very unusual magnetic properties that precedes the superconducting phase. This discovery is an important step towards understanding high-temperature superconductivity.


Superconductivity, discovered nearly one hundred years ago, is a very particular state of matter which is characterized by the complete absence of electrical resistance and the repulsion of magnetic fields. These properties make superconductors extremely interesting for a wide range of technical applications, in particular in magnetism, for the transport of electrical energy without loss, and many others. However, in most materials superconductivity appears only at extremely low temperatures – a few Kelvin, i.e. a few degrees above absolute zero (-273°C).
In 1987 and the following years, it was discovered that certain materials based on copper oxide already become superconducting at temperatures of up to 138K (-135°C). Although this is still cold and far away from the “dream” of a room-temperature superconductor, it is technically much easier to reach these temperatures and means that these materials already play an important role in various technical applications of today.

Up to now, despite many years of intense research, there has been no conclusive explanation for the high transition temperature of these superconductors. It is however known that before becoming superconducting, these materials pass through a state - known as the “pseudo-gap phase”- in which they demonstrate unusual properties which deviate considerably from the behaviour of normal metals. Several theoretical models have been proposed to describe this pseudo-gap phase. One of them, put forward by C. M. Varma, professor in Riverside (California), postulates the existence of a hidden order out of which the superconducting state emerges. Prof. Varma claims there is a spontaneous formation of microscopic current loops, thereby creating microscopic magnetic moments. According to this theory, the pseudo-gap phase results from the appearance of these current loops.

The team of scientists from France and the USA has been working for several years on the experimental verification of this theory. This is an extremely difficult task requiring the polarized neutron scattering technique and the highest performance spectrometers available. In 2006, they found the first indication of the postulated magnetic order created by the microscopic current loops. On the IN20 spectrometer at the ILL, they have now succeeded in observing for the first time an excitation of this ordered state of magnetic moments in the copper-oxide superconductor HgBa₂CuO₄₊δ. Based on this result, complimentary measurements have then been performed on the IN8 spectrometer at the ILL, and at the Frm2 reactor in Munich. By studying different samples and observing how the signal varies as a function of temperature, the scientists have been able to prove that this excitation is directly related to the pseudo-gap phase, and that its properties are consistent with the predictions of the theory.

It is possible that the understanding of the pseudo-gap phase is the key to understanding the superconducting phase. Secondly, as superconductivity is often thought to have a magnetic origin, the discovery of new magnetic excitations is of course highly intriguing.
If it can be shown that these results are also valid for other high-temperature superconductors, they would in all probability bring us a huge step forward in the major scientific challenge of understanding high-temperature superconductivity, which may ultimately allow superconductors to play an increasingly important role in the technology of the future.

Yuan Li (right), from Stanford University, with ILL local contact Paul Steffens (left)

Re.: Nature, Volume:468,Pages:283–285.