Can spin fluctuations explain superconductivity?
Sep 24, 1999
The mechanism responsible for high-temperature superconductivity is one of the biggest unsolved problems in physics. Superconductors lose all resistance to electric current below their superconducting transition temperature, which can be as high as 150 Kelvin for high-temperature superconductors. Now three physicists are claiming that interactions between the charge carriers in the material and fluctuations in the spins of the atoms play a major role in high-temperature superconductivity (Nature 401 354).
Superconductivity happens when the charge carriers overcome their mutual repulsion and bind together into Cooper pairs. In low-temperature superconductors phonons - quantized vibrations of the crystal lattice - are responsible for this pairing. Jules Carbotte from McMaster University in Canada, Ewald Schachinger from the Technical University of Graz in Austria, and Dimitri Basov from the University of California at San Diego believe that spin fluctuations play a similar role in high-temperature superconductors.
Carbotte and co-workers reached this conclusion by combining data from infrared spectroscopy experiments and neutron scattering measurements on yttrium barium copper oxide (YBCO), which is probably the most widely studied high-temperature superconductor. Infrared spectroscopy provides information on the charge carriers, while neutron scattering probes the spin fluctuations in the material. Neutron scattering experiments with YBCO have revealed a peak in the spin fluctuation spectrum at an energy of 41 meV. Carbotte and co-workers found that the coupling strength between the charge carriers and peaks in the fluctuation spectrum was strong enough to explain the high transition temperatures observed in materials such as YBCO. However, the origin of the 41 meV peak remains an mystery, and only time will tell if the community accepts that spin fluctuations are the cause of high-temperature superconductivity.