Vibronic Phenomena At Localized-Itinerant and Mott-Hubbard Transitions

Abstract

It is argued from the pressure dependence of the transport properties that the Mott-Hubbard transition in the AMO₃ 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 CuO₃ array of La_1-xNd_xCuO₃,0 ≤ x ≤ 0.6, and in the metallic phase of the compounds LnNiO₃, Ln = La, Pr, Nd, Sm_0.5Nd_0.5. The LnNiO₃ family undergoes an antiferromagnetic-insulator to metal transition at a temperature T_t that is sensitive to ¹⁸O/¹⁶O isotope exchange and disappears in LaNiO₃ and PrNiO₃ under 15 kbar hydrostatic pressure. We suggest a bandwidth of the form

$$ W = W_b exp( - \lambda \varepsilon _{sc} /h\omega _O ) $$

where λ ∼ ɛ_SC/W_b, W_b is the tight-binding bandwidth, and a pressure-sensitive ω⁻¹ ₀ is the period of the locally cooperative oxygen displacements that define strong-correlation fluctuations stabilized by an energy ɛ_sc. LaMnO₃ exhibits an insulator-conductive electronic transition at a cooperative Jahn-Teller orbital ordering below T_JT; the magnetic susceptibility obeys a Curie-Weiss law in which ?, but not C, changes discontinuously at T_JT. We propose a double-exchange coupling involving vibrons.