Vibronic phenomena at localized-itinerant and Mott-Hubbard transitions

摘要

It is argued from the pressure dependence of the transport properties that the Mott-Hubbard transition in the AMO{sub}3 perovskites is first-order with a dynamic phase segregation appearing in the metallic phase on the approach to the transition from the itinerant-electron side. A transition with decreasing bandwidth from a strongly enhanced Pauli paramagnetism toward a Curie-Wiess law is observed in the metallic CuO{sub}3 array of La{sub}(1-x)Nd{sub}xCuO{sub}3, 0 < x < 0.6, and in the metallic phase of the compounds LnNiO{sub}3, Ln = La, Pr, Nd, Sm{sub}0.5Nd{sub}0.5. The LnNiO{sub}3 family undergoes an antiferromagnetic-insulator to metal transition at a temperature T{sub}t that is sensitive to 18{sup left}O/16{sup left}O isotope exchange and disappears in LaNiO{sub}3 and PrNiO{sub}3 under 15 kbar hydrostatic pressure. We suggest a bandwidth of the form W = W{sub}b exp (-λε{sub}(sc)/hω{sub}o) where λ～ε{sub}(sc)/W{sub}b, W{sub}b is the tight-binding bandwidth, and a pressure-sensitive ω{sub}o{sup}(-1) is the period of the locally cooperative oxygen displacements that define strong-correlation fluctuations stabilized by an energy ε{sub}(sc). LaMnO{sub}3 exhibits an insulator-conductive electronic transition at a cooperative Jahn -Teller orbital ordering below T{sub}(JT); the magnetic susceptibility obeys a Curie-Weiss law in which θ, but not C, changes discontinuously at T{sub}(JT). We propose a double-exchange coupling involving vibrons.