Observation of a square flux-line lattice in the unconventional superconductor Sr2RuO4 (vol 396, pg 242, 1998)

The phenomenon of superconductivity continues to be of considerable scienti fic and practical interest. Underlying this phenomenon is the formation of electron pairs, which in conventional superconductors do not rotate about t heir centre of mass ('s-wave' pairing; refs 1, 2). This contrasts with the situation in high-temperature superconductors, where the electrons in a pai r are believed to have two units of relative angular momentum ('d-wave' pai ring; ref. 3 and references therein). Here we report small-angle neutron-sc attering measurements of magnetic flux lines in the perovskite superconduct or Sr2RuO4 (ref. 4), which is a candidate for another unconventional paired electron state-'p-wave' pairing, which has one unit of angular momentum(5- 7). We find that the magnetic flux lines form a square lattice over a wide range of fields and temperatures, which is the result predicted by a recent theory(8,9) of p-wave superconductivity in Sr2RuO4. This theory also indic ates that only a fraction of the electrons are strongly paired and that the orientation of the square flux lattice relative to the crystal lattice wil l determine which parts of the three-sheet Fermi surface of this material a re responsible for superconductivity. Our results suggest that superconduct ivity resides mainly on the 'gamma' sheet(9).