  Charge carriers in low-doped semiconductors may distort the atomic lattice
and trap themselves forming so-called small polarons. High carrier
concentrations can lead to short range ordered polarons (large polarons) and
even to long range charge and orbital order. Both systems should be insulating
with a large electrical resistivity, which decreases with increasing
temperature. However, photoemission measurements recently found a polaronic
pseudogap, in a metallic phase of La2-2xSr1+2xMn2O7. This layered manganite is
famous for colossal magnetoresistance (CMR) associated with a phase transition
from this low-temperature metallic phase to a high temperature insulating
phase. Broad charge order peaks due to large polarons observed by neutron and
x-ray scattering in the insulating phase disappear when La2-2xSr1+2xMn2O7
becomes metallic. We report results of inelastic neutron scattering
measurements showing that polarons remain inside the metallic phase as
fluctuations that strongly broaden and soften certain phonons near the wave
vectors where the charge order peaks appeared in the insulating phase. Our
findings imply that polaronic signatures in metals may generally come from a
competing insulating charge-ordered phase. It is highly relevant to cuprate
superconductors with both a pseudogap, and a similar phonon effect associated
with a competing stripe order.
