The paper describes the magnetic fluctuations in an optimally doped high-temperature superconductor. We discovered that when a magnetic field is applied, and the system enters its vortex phase, some of these fluctuations slow down. We interpreted this signal as coming from the vortex cores of the material, i.e. the regions where the magnetic field is concentrated and the material is normal instead of superconducting. This indicates that magnetism exists inside the vortices of a high-temperature superconductor. This is a most important clue for the ongoing effort of trying to understand the nature of high-temperature superconductivity—one of the most important questions in modern condensed matter physics. Prior to this experiment there had been a great deal of theoretical speculation on the nature of the vortex cores in high-Tc's. An extrapolation of our results would be that a less doped (underdoped) superconductor would exhibit static magnetic order upon application of a magnetic field. This was indeed found by us in a subsequent paper (Lake et al. Nature 415, 299, 2002), and these results confirm our interpretation of the first paper. Since the publication of the second paper, the two papers are almost always cited together.

Our results are the probably the clearest evidence yet that magnetism exists in the vortex cores of a high-temperature superconductor. This has certainly influenced the way theoreticians think about this problem.

This was as a result of a long-term scientific collaboration between our group at Riso, Bella Lake from the University of Oxford, and Gabriel Aeppli, then at NEC. One of the most important aspects was that we had developed, in collaboration with Gabriel, Bella, Thom Mason from SNS, and others, a new type of neutron spectrometer called RITA. RITA was specifically optimized for looking at the magnetic fluctuations in high-Tc materials. As soon as it was ready we started to revisit the problem of understanding the role played by magnetic fluctuations in LSCO, first in zero field (Lake et al., Nature (400, 43, 1999), before moving on to studying the magnetic dynamics in the vortex phase. All in all, the experiments were very demanding as we were learning how to use the spectrometer and debugging it at the same time. Bella deserves most of the credit for this project turning out to be successful.