Twenty years ago, a discovery was made that revolutionised our understanding of an unusual type of electronic behaviour called superconductivity – electrical conduction without resistance. Researchers found that some copper-oxide compounds (cuprates) became superconducting at liquid-nitrogen temperatures; until then, the phenomenon had been seen only at very low temperatures. The materials caused enormous excitement because of their technological potential in efficient power transmission and other applications.
Despite a great deal of study, there is still much we do not know about how superconductivity works in the cuprates. In conventional superconducting materials, vibrations of the crystal lattice couple with electrons, creating a new state whereby pairs of electrons with opposite spins collectively surf through the lattice unimpeded. Although the spin-pairing is still the key to the onset of superconductivity in the cuprates, lattice vibrations appear to play a lesser role. What is more significant is how the electron spins interact collectively – in other words, the materials’ magnetic properties.
Until a few years ago, it was difficult to probe the magnetic behaviour of the cuprates experimentally in detail. Inelastic neutron scattering can provide a measure of the tiny energy fluctuations (‘excitations’) associated with the electron spin interactions, but the instrumentation had not been available to probe the higher-energy magnetic excitations that might reveal the secret of high temperature superconductivity.
Mapping magnetic properties
Today, the Multi-Angle Position Sensitive (MAPS) spectrometer, installed at ISIS in 2001, can uniquely record the spin fluctuations over a wide spectrum of energies. Recently, international collaborations have studied two of the cuprates – one containing yttrium and barium (YBa2Cu3O6+x), and the other lanthanum and strontium (La2-xSrxCuO4), using MAPS.
By measuring the energy changes in the compounds at various neutron energies, over different temperatures and varying compositions (different proportions for x), the researchers were able to map the entire spectrum of magnetic excitations, accessing those at higher energies not measured before. This allowed them to obtain a comprehensive picture of their magnetic behaviour. They were able to calculate the change in magnetic energy in going from the normal to the superconducting state, and show that it was more than enough to cause the collective pairing that is the signature of superconductivity.
In the case of the lanthanum strontium cuprate, the team showed that there were two sets of fluctuations associated with the collective magnetic behaviour. The higher-energy set matched features seen in a different type of experiment using ultraviolet light to explore the electron energies, which had been linked to the formation of a superconducting state in these puzzling materials.
Prof. Steve Hayden (University of Bristol) S.Hayden@bristol.ac.uk
Research date: December 2004
The structure of the high-energy spin excitations in a high-transition temperature superconductor, SM Hayden et al., Nature 429 (2004) 531.
Magnetic energy change available to superconducting condensation in optimally doped YBa2Cu3O6.95, H Woo et al., Nature Physics 2 (2006) 600.
Two energy scales in the spin excitations of the high-temperature superconductor La2-xSrxCuO4, B Vignolle et al., Nature Physics 3 (2007) 163.