A study of bond-length fluctuations in transition metal oxides

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Yan, Jiaqiang

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Bond-length fluctuations with different origins have been investigated by thermal conductivity measurement performed on La1.60-xNd0.40SrxCuO4, RCoO3, and RVO3 single crystals grown by floating zone method. Thermal conductivity has been proved to be a sensitive probe to bond-length fluctuations in stronglycorrelated transition-metal oxides. Superconductivity in cuprates occurs at a crossover from localized to itinerant electronic behavior. The segregation of localized spins and delocalized holes into hole-poor and hole-rich regions in La2-xSrxCuO4 induces bond-length fluctuations via a strong electron-lattice coupling. This bond-length fluctuation suppresses in-plane thermal conductivity due to charge fluctuations in this quasi- 2D system. In the La1.60-xNd0.40SrxCuO4 system, the low-temperature orthorhombic (LTO) phase transforms into a low-temperature-tetragonal (LTT) phase with decreasing temperature. The hole-rich regions order into static stripes in the LTT phase of La2-x-yNdySrxCuO4; this charge order revives the phonon contribution to the thermal conductivity. The phonon thermal conductivity in the normal state of LTT phase and the LTO phase of some underdoped compositions of LSCO calls for reconsideration of the role of bond-length fluctuations on superconducting pairing in different structures. Suppression of the phonon thermal conductivity in the Mott-Hubbard insulator RCoO3 is interpreted to be caused by the spin-state transition from the low-spin t6 e 0 ground state to a higher spin-state, either intermediate-spin t5 e 1 or high-spin t4 e 2 , with increasing temperature. RVO3 offers us a unique chance to study the bond-length fluctuations caused by strong spinorbital-lattice coupling. An unusually strong orbital-lattice and spin-lattice coupling has been clearly demonstrated.