The discovery of a common thread among the cuprate superconductors could lead to a theory of high-temperature superconductivity – a goal that has eluded scientists since the materials were discovered in 1986. An international team led by Bernhard Keimer of the Max Planck Institute for Solid State Physics in Stuttgart has seen – for the first time – a magnetic resonance in a single-layer cuprate superconductor that has previously only been observed in multilayer compounds (H He et al 2002 Science to appear).
Superconductors are compounds that lose their electrical resistance below a certain ‘transition temperature’. In many superconductors, this happens because electrons pair up – overcoming their mutual repulsion – by interacting with vibrations of the crystal lattice known as phonons. But scientists disagree on the mechanisms that form electron pairs in high-temperature superconductors, which have transition temperatures up to 138 kelvin.
High-temperature cuprate superconductors consist of metal atoms separated by layers of copper oxide. Since the super-current flows through these layers, they have been the focus of many attempts to establish the mechanism that underpins high-temperature superconductivity. Neutron scattering has been widely used in these studies because it can reveal the magnetic spins of electrons in these layers.
Early neutron scattering studies showed that – in many high-temperature superconductors –the electrons in the copper oxide layers are excited into a ‘magnetic resonant mode’. This strongly suggested that magnetic spin played a central role in the cuprate superconductors.
But this effect was only found in compounds with double or triple layers of copper oxide, and could not explain why superconductivity existed in materials with single layers. This apparent anomaly left theorists unable to account for the role of spin alignment in the high-temperature superconductors.
Now Keimer and colleagues at the Russian Academy of Science, the Laboratoire Léon Brillouin and CEA Grenoble, both in France, have seen this alignment of spins in the single-layer compound thallium barium copper oxide. The copper oxide layers in this material are flat and uniform, which means that the spin alignment is unlikely to arise from any rogue structural effects. This led the team to believe that the effect is probably common to all single-layer copper oxide compounds – and therefore to all high-temperature superconductors.
‘Our results highlight the central role of magnetism in the mechanism of high-temperature superconductivity’, Bernhard told PhysicsWeb. ‘This magnetic resonant mode appears below the transition temperature and could therefore be a fingerprint of the electron pairing mechanism.’
