Charge density studies utilizing powder diffraction and MEM. Exploring of high Tc superconductors, C₆₀ 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 (PrBa₂Cu₃O_7-δ), YBCO (YBa₂Cu₃O_7-δ), C₆₀ 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_π) 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, C₆₀ 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 C₆₀ 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 dx²- y² 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.