X-ray and neutron scattering measurements directly demonstrate the existence of polarons in the paramagnetic phase of optimally-doped colossal magnetoresistive oxides. The polarons exhibit short-range correlations that grow with decreasing temperature, but disappear abruptly at the ferromagnetic transition because of the sudden charge delocalization. The "melting" of the charge ordering as we cool through TC occurs with the collapse of the quasi-static polaron scattering, and provides important new insights into the relation of polarons to colossal magnetoresistance.PACS numbers: 75.30. Vn, 75.30.Et, 71.30.+h, 71.38.+i Manganese oxides have attracted tremendous interest because they exhibit colossal magnetoresistance (CMR) -a dramatic increase in the electrical conductivity when they order ferromagnetically. The basic relationship between ferromagnetism and conductivity in doped manganese oxides has been understood in terms of the doubleexchange mechanism [1,2], where an itinerant e g electron hops between Mn 4+ ions, providing both the ferromagnetic exchange and electrical conduction. In addition, an important aspect of the physics of manganese oxides is the unusually strong coupling among spin, charge, and lattice degrees of freedom [2,3]. These couplings can be tuned by varying the electronic doping, electronic bandwidth, and disorder, giving rise to a complex phase diagram in which structural, magnetic, and transport properties are intimately intertwined. The charge-ordered phases represent one of the most intriguing results of balancing these couplings, and have been observed at low temperature in insulating, antiferromagnetically ordered manganites, but are incompatible with double exchangemediated ferromagnetism seen in optimally-doped CMR systems.In comparison to the cubic manganites such as La 1−x A x MnO 3 (A=Sr, Ca, Ba), the two-layer Ruddlesden-Popper compounds La 2−2x Sr 1+2x Mn 2 O 7 [4], where x is the nominal hole concentration, are advantageous to study because the reduced dimensionality strongly enhances the spin and charge fluctuations. The crystal structure is body-centered tetragonal (space group I4/mmm) [5] with a ≃ 3.87Å and c ≃ 20.15 A, and consists of MnO 2 bilayers separated by (La,Sr)O sheets. In the intermediate doping regime (0.32 ≤ x < 0.42), the ground state is a ferromagnetic metal, and the magnetoresistance is found to be strongly enhanced near the combined metal-insulator and Curie transition at T C (112 K for the x=0.4 system of present interest [6]). The present results reveal diffuse scattering associated with lattice distortions around localized charges, i.e. polarons, in the paramagnetic phase. The formation of lattice polarons above the ferromagnetic transition temperature T C has been inferred from a variety of measurements [7], but detailed observation via diffuse x-ray or neutron scattering in single crystals has been lacking until now [8]. Through such measurements, we have observed the collapse of quasi-static polaron scattering when the metallic, ferromagnetic sta...

Neutron scattering has been used to study the structure and spin dynamics of La0.85Sr0.15MnO3. The magnetic structure of this system is ferromagnetic below TC ≃ 235 K. We see anomalies in the Bragg peak intensities and new superlattice peaks consistent with the onset of a spin-canted phase below TCA ∼ 205 K, which appears to be associated with a gap at q = (0, 0, 0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a concomitant lattice distortion. The long-wavelength magnetic excitations are found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic dispersion relation E = Dq 2 . The spin stiffness constant D has a T 5/2 dependence at low T , and the damping at small q follows q 4 T 2 . An anomalously strong quasielastic component, however, develops at small wave vector above ∼ 200 K and dominates the fluctuation spectrum as T → TC. At larger q, on the other hand, the magnetic excitations become heavily damped at low temperatures, indicating that spin waves in this regime are not eigenstates of the system, while raising the temperature dramatically increases the damping. The strength of the spin-wave damping also depends strongly on the symmetry direction in the crystal. These anomalous damping effects are likely due to the itinerant character of the eg electrons.