A novel magnetic excitation discovered in two different cuprate superconductors could lead physicists towards a theory of high-temperature superconductivity. Two separate teams of physicists have used the new MAPS spectrometer at the ISIS neutron source in the UK to observe the excitation. Theorists have been trying to explain high-temperature superconductivity, without success, since it was discovered in 1986.
Superconductors are materials that lose their electrical resistance when they are cooled below a certain transition temperature. Superconductivity occurs when electrons overcome their mutual repulsion to form Cooper pairs, which then all condense into a quantum state that does not experience electrical resistance. The Bardeen-Cooper-Schrieffer (BCS) theory explained that low-temperature superconductivity occurs when electrons form pairs as a result of interactions with vibrations of the crystal lattice known as phonons. However, the pairing mechanism in high-temperature superconductors has remained a mystery
All high-temperature superconductors consist of parallel planes of copper oxide. The copper atoms lie on a square lattice and the charge is carried by holes sitting on oxygen sites. Each copper atom has an unpaired electron, and hence a magnetic moment or “spin”, and the arrangements of these spins can be probed in neutron scattering experiments.
Previous neutron-scattering experiments showed that the electrons were excited into a “magnetic resonance mode”, which suggested that magnetic spins played an important role in these materials. However, the effect was only found in some high-temperature superconductors and not in others.
Stephen Hayden from Bristol University in the UK and colleagues at ISIS, Oak Ridge, Tennessee and Missouri-Rolla studied yttrium barium copper oxide (YBCO). They found that when the sample was excited with neutrons, the copper spins responded as a group rather than individually (Nature 429 531). According to Hayden, this so-called collective magnetic excitation means the spins are strongly interacting, and that these interactions could provide the “glue” that is responsible for holding the Cooper pairs together in the material.
Meanwhile, John Tranquada of the Brookhaven National Laboratory in the US and co-workers at Brookhaven, ISIS and Tohoku University in Japan found a similar pattern of magnetic excitations in lanthanum barium copper oxide (LBCO). This material is known to contain “stripes” of charge — regions with a high density of holes — that lie between non-conducting regions with low hole-density (Nature 429 534).
“The magnetic excitations are like fingerprints that say these things [LBCO and YBCO] must be similar in some way,” said Tranquada. “In fact, our results support the concept that stripe correlations might be essential to high-temperature superconductivity.” However, Tranquada admits that his team’s results could be controversial because many physicists believe that charge stripes can only compete with superconductivity in the cuprates.