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High-temperature superconductivity might start with tiny spontaneous loops of current

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High-temperature superconductivity might start with tiny spontaneous loops of current. 11.11.2010


Neutron science provides the experimental evidence to understand the precursor to a superconducting state.

Scientists using neutron spectroscopy at the Institut Laue-Langevin (ILL) have unlocked some of the secrets behind the strange magnetic properties of high temperature superconductors. This is a scientific breakthrough for a class of copper and oxygen based materials that have potential commercial application within a range of industries.

An international team of researchers including those from the LLB (France) and the University of Minnesota (USA) used the IN20 polarized and IN8 unpolarized neutron spectrometers, to prove experimentally for the first time the existence of unusual magnetic excitations in the ‘pseudo-gap phase’ that precedes the transition to superconductivity at high temperatures.

“The pseudo-gap phase is an interesting phenomenon in its own right, but understanding the mechanism by which it arises is key to understanding the properties of high temperature superconductivity,” says Dr Paul Steffens, scientist at the ILL. “This understanding could help us to raise the critical temperature for superconductivity further, which would be of great benefit as it would increase the potential applications.”
Several theoretical models have been proposed to describe the pseudo-gap phase. One of them, put forward by Professor CM Varma (Riverside) in 1997 claims that the superconducting state emerges from a spontaneous formation of microscopic electrical current loops, thereby creating microscopic magnetic moments. The team at the ILL have now observed magnetic behaviour in a copper-oxide high temperature superconductor consistent with this spontaneous formation of tiny loops of current.
High temperatures superconductivity was discovered more than 20 years ago, but the secrets behind its existence remained a mystery.  The results from ILL, published in Nature, will generate a greater understanding of what creates this phenomenon. This opens up the possibility of understanding why the critical temperature for superconductivity is higher in some copper-oxides than in others.

Notes to editors

1.    Pseudo-gap phase – a transitional state exhibited by copper oxide materials before they become superconducting. During this phase the materials demonstrate unusual properties that deviate considerably from the behaviour of normal metals. Several theoretical models have been proposed to explain the mechanism. One of them, put forward by CM Varma, professor in Riverside (California), and supported by this experimental evidence, postulates the existence of a hidden order out of which the superconducting state emerges. A spontaneous formation of microscopic current loops creates microscopic magnetic moments. The pseudo-gap phase results from the appearance of these current loops.

2.    Superconductivity is a macroscopic quantum mechanical phenomenon in which the absence of electrical resistance allows the transport of electrical energy without loss, and superconductors are therefore extremely interesting for industry. However historically, the ‘critical’ temperature required to induce superconductivity is so low, a few ten degrees from absolute zero (-273oC), that its use had been restricted to very specific technologies.
Hopes for a greater commercial application of the property were raised in 1986 with the discovery of high temperature copper oxide superconductors at up to 135k (-138°C). Even higher temperatures appear possible, but there is currently no clear understanding of the mechanism.

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


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