Scientists using neutron diffraction have made a significant step forward in the quest to understand high-temperature superconductivity.
The international team of researchers, which included members from the Insitut Laue-Langevin (ILL) and the Laboratoire Léon Brillouin (LLB) in France, and the University of Minnesota in the USA, has observed unusual magnetic excitations in the so-called pseudo-gap phase of copper oxide superconductors [Nature 468, 283 (2010)]. This is a precursor to the superconducting state, and a clear understanding of its properties is regarded by many as essential to the discovery of the physical principles behind the copper oxides’ behavior.
Several theoretical models have been proposed to describe the pseudo-gap phase. One of them, put forward in 1997, suggests that the superconducting state emerges from a spontaneous formation of microscopic electrical current loops, thereby creating microscopic magnetic moments. The new observations are consistent with this mechanism, and significantly narrow the range of directions likely to be fruitful in explaining high-temperature superconductivity.
In order to observe the excitation, it was essential for the scattering experiment to be performed using polarized neutrons. “This is necessary to separate the magnetic signal from other non-magnetic ones, and from the background noise,” explains Dr Paul Steffens, a scientist at the ILL. “In a standard experiment without polarization analysis, it is practically impossible to prove its magnetic character and, because it is so weak compared to the background, to see it in the first place.”
Professor Chandra Varma from the University of California at Riverside, the original proponent of the hidden magnetic order picture for the pseudo-gap phase, considers the new experimental findings “significant”. He points out that they add to a growing list of previous experiments consistent with his theory. However, he also cautions that substantial questions still remain open, and that a lot of experiments are still required to address these.
A crucial issue being investigated already is whether or not the magnetic excitation is a general signature of high-temperature superconductivity. So far it has only been observed in HgBa2CuO4+δ, which has one of the simplest crystal structures of any of the copper oxide families. Steffens tells Materials Today his team is currently extending the scope of its work to include other, less structurally ideal copper oxides.