THE TRANSITION FROM POLARONIC TO ITINERANT ELECTRONS IN THE ORTHOMANGANITES. John B. Goodenough and J.-S. Zhou, Center for Materials Science & Engineering, ETC 9.102 University of Texas at Austin, Austin, TX 78712-1063
The AMO3 perovskites containing transition-metal atoms have an MO3 array with M-O-M bond angles 180deg. - [[phi]] in which the angle [[phi]] increases with the mismatch of the A-O and M-O equilibrium bond lengths. The tight-binding bandwidths for the [[pi]]-bonding t and [[sigma]] bonding e orbitals of d-orbital parentage are W[[pi]] < W[[sigma]]. In the orthomanganites Lnl-xAxMnO3 with A an alkaline earth, the octahedral site high-spin Mn3+ ions have a d-electron configuration t3el:5Eg that is orbitally twofold degenerate, and the on-site electron-electron coulomb energy for adding a fifth d electron is U[[sigma]] [[circleplus]] W[[sigma]] > W[[pi]]. Consequently the equilibrium reaction t3el = t3[[sigma]]*1 for a first-order transition from localized-e to itinerant-[[sigma]]* electrons is shifted to the right as the spin-dependent integral tij = [[epsilon]][[sigma]][[lambda]][[sigma]]2cos[[ph]]cos([[theta]]ij/2) is increased by decreasing [[phi]], by increasing [[lambda]][[sigma]] with hydrostatic pressure, or by reducing the angle [[theta]]ij between spins on neighboring Mn atoms with an external magnetic field. The evolution from polaronic behavior (tij < h[[nu]]R) to narrow-band behavior (tij > h[[nu]]R) is shown to pass through a novel vibronic state in which the electrons of e-orbital parentage are strongly coupled to a cooperative, dynamic Jahn-Teller deformation mode of frequency [[nu]]R. A first-order vibronic-polaronic transition occurs at a Curie temperature Tc. The "colossal" negative magnetoresistance is associated with an increase in Tc with magnetic field, but a giant magnetoresistance below Tc occurs where an applied magnetic field either transfers Anderson localized holes to the dispersionless vibronic band or, at x [[circleplus]] 0.5, shifts a first-order transition from a charge-ordered, antiferromagnetic phase to a ferromagnetic, metallic phase.