  The discovery of the Fe pnictide superconductors generated great interest as
the structure consists of planes of a magnetic material quite similar to the
cuprate superconductors. Fe(Te0.5Se0.5) is a particularly simple system whose
planes are isostructural to the FeAs layers found in the originally discovered
superconductors of this type. We report here neutron scattering measurements on
this material that provide an understanding of the superconductivity. Since the
information about the Fermi surface is available both from photoemission and
band structure calculations for FeTe, FeSe and other related materials the
neutron spectra can be used to see if the itinerant electron picture is valid.
The results are consistent with a picture where there are both electron and
hole Fermi surfaces that make exact (pi,pi) transitions possible. This would
normally favor either a spin or charge density wave state. However, our
measurements show the extent of the region where (pi,pi) transitions take place
and demonstrate that there are a much larger number of transitions near pi,pi).
The near (pi,pi) transitions are observed both above and below Tc and are
expected to be strongly pairing. The superconductivity can be attributed to
these excitations while the exactly (pi,pi) transitions produce the narrow
resonance excitation that appears below Tc.
