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...

By providing the capabilities for high-resolution, high-energy and time-resolved powder X-ray diffraction, beamline P02.1 is a versatile tool to tackle various problems in materials science, crystallography and chemistry.

Thin films of PbTiO3, a classical ferroelectric, have been grown under tensile strain on single-crystal substrates of DyScO3. The films, of only 5 nm thickness, grow fully coherent with the substrate, as evidenced by synchrotron x-ray diffraction. A mapping of the reciprocal space reveals intensity modulations (satellites) due to regularly spaced polar domains in which the polarization appears rotated away from the substrate normal, characterizing a low-symmetry phase not observed in the bulk material. This could have important practical implications since these phases are known to be responsible for ultrahigh piezoelectric responses in complex systems.

In this paper it is shown that diffuse-scattering experiments within the region of total external reAection can be explained quantitatively using the distorted-wave Born approximation for layer systems. Three Si/Ge samples with different degrees of complexity were investigated. The simultaneous analysis of the specular rejected intensity and the diffuse scattering leads to one consistent set of interface and layer parameters, which is able to fit both the shapes and the locations of all dynamic peaks in the off-specular scans and the characteristics of the rejected intensity. Therefore the distorted-wave Born approximation seems to give a correct and complete description of the diffuse scattering in the region of total external reflection.

The structural investigation of poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) (PF2/6) in aligned thin films is presented. Formation of a thickness dependent triaxial texturing is identified in thermotropically aligned films. X-ray reflectivity measurements reveal good macroscopic quality, and polarized photoluminescence and dichroic ratios in absorption indicate clear axial alignment. Grazing-incidence X-ray diffraction shows axially aligned mesomorphic structure with a distinct arrangement of helices and large correlation lengths, indicating a high local lateral order. Theoretical models produced using molecular mechanics methods suggest 5/2-helicity. The polymer chains are parallel to the substrate in the c direction. In particular, the hexagonal-like cells are flattened in the direction of the surface normal and reveal two kinds of coexistent crystallites, a multiple orientation where the greater proportion of the crystallites have one crystal axis a perpendicular to the substrate surface, whereas a smaller proportion is aligned with the crystal axis a parallel to the surface. In thinner films the former class of orientation is usually dominant, while the proportion of the parallel orientation type increases with prolonged annealing.