  Colossal magnetoresistance (CMR) is associated with the phase transition from
a metallic ferromagnetic to insulating paramagnetic phase, which can be
controlled by an applied magnetic field. The insulating phase occurs due to
trapping of the charge carriers by polaronic lattice distortions, which raise
the resistivity. Theories based on local physics predict that the magnitude of
the resistivity jump at Tc is determined by how much, on average, the amplitude
of these distortions increases at the phase transition. Using neutron
scattering, we measured the average distortion amplitude in La0.7Sr0.3MnO3.
Surprisingly, its increase from below to above Tc is just as large as in other
manganites, which have a much larger resistivity jump. This result suggests
that the strength of CMR is determined not by the size of distortions, but by
their cooperative nature specific to each compound. Existing theories need to
be extended to include correlations between different unit cells to explain and
predict the strength of CMR.
