Direct Observation and Calipering of the “Webbing” Fermi Surface of Yttrium

The first measurement of both the size and shape of the region of the Fermi surface of yttrium known as the ``webbing'' is reported. This particular Fermi surface feature is of considerable interest because it is very similar to that found in a number of the heavier rare earth metals, where it is believed to play a vital role in driving the exotic magnetic structures found therein. In this positron study, two-dimensional angular correlation measurements combined with three-dimensional reconstruction provide a direct image of this part of the yttrium Fermi surface.