Local Atomic Structure and Superconductivity of NEODYMIUM(2-X) Cerium(x) COPPEROXIDE(4-Y): a Pair-Distribution Study

We have carried out a detailed Pair Distribution Function Analysis of powder diffraction data from samples of Nd_{2-x}Ce _{x}CuO _{4-y} with x = 0.165 and x = 0.2. The former is superconducting with an almost full Meissner fraction, and T_{c} = 22K. The latter is isostructural, but exhibits no superconductivity. A "real space refinement" of the PDF has been carried out to extract quantitative information about the local atomic structure of the material. Two main observations were made. The local structure is significantly different from the average crystal structure (T ^' structure) of this material. The differences are explained by displacements of oxygen. The second result was that the local structure changes with the appearance of superconductivity. This was the first observation of such a change in this material. These changes are explained by an increase in the degree of local order of the above mentioned oxygen displacements. The displacements are inhomogeneously distributed throughout the sample and interspersed with regions of undistorted crystal. We argue that this is best explained as the direct observation of the presence of polarons, or bipolarons in the structure. We speculate that the driving force that could give rise to these polarons is a strong coupling of the lattice with the O p_{z} states which lie just below the Fermi surface in the undistorted material. The effective mass of the (bi)polarons, calculated using values estimated from the experiment, was such as to give the bipolarons a condensation temperature of 17K. This is the same order of magnitude as the observed T _{c}. Thus it is plausible that the superconductivity is due to a bipolaronic condensation. The density of polarons is too small to account for all of the chemically doped carriers. Thus it appears that they form the minority of carriers. They coexist with a majority of high mobility, wide band, electronic states, of sigma character.