Charge density studies utilizing powder diffraction and MEM. Exploring of high Tc superconductors, C-60 superconductors and manganites

The recent progress of the accurate charge density studies by the Maximum Entropy Method (MEM) utilizing X-ray powder diffraction is reviewed with some examples. Results for PrBCO (PrBa2Cu3O7-delta), YBCO (YBa2Cu3O7-delta), C-60 superconductors (Rb2CsC60, K2RbC60, Na2RbC60) and the layered manganite, NdSr2Mn2O7, which is well known as colossal magnetoresistive (CMR)-related material, are given. For non-super conductor, PrBCO, it is found in the MEM charge density that there exist directional robes of the charge density from Pr atom toward the O atoms in the CuO2 conduction planes. On the other hand, for a very well known high Tc super conductor, YBCO, appreciable charge densities in the interatomic region around the Y atom is not recognized in the MEM charge density. The distinct difference of the charge densities between PrBCO and YBCO presents clear experimental evidence of the hybridization between Pr(4f)-O(2p(pi)) orbitals which supports the idea that the hole trapping by the hybridized states suppresses the superconductivity in PrBCO. The MEM charge densities of the fullerene superconductors, Rb2CsC60, K2RbC60 and Na2RbC60, show distinct structural differences from that of non-superconductors, C-60 and Li2CsC60, reflecting the superconducting properties. And the charge deficiencies of the doped metal atoms, which should be associated with charge transfer from the metal atoms to the C-60 molecule seems to have strong correlation to the superconducting transition temperature, Tc. As the bigger the charge transfer, the higher the Tc. The accurate MEM charge density of antiferromagnetic manganite, NdSr2Mn2O7, presents the direct imaging of spontaneous ordering of the dx(2)-y(2) orbital indicating anisotropic exchange couplings between the local-spins on the Mn sites, which causes an unique layered-type spin order. The theoretical background of the MEM is also mentioned in some detail.