Berlin 2005 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 2: Correlated Electrons - Metal Insulator Transition

TT 2.8: Vortrag

Freitag, 4. März 2005, 12:00–12:15, TU H2053

Thermal Conductivity, Thermopower, and Figure of Merit of La_1−xSr_xCoO₃ — •M. Kriener, K. Berggold, I. Klassen, T. Lorenz, C. Zobel, and A. Freimuth — II. Physikalisches Institut, Universität zu Köln, 50937 Köln

Cobaltates attract strong interest because Co ions have the possibility to occur in different spin states. Especially, LaCoO₃ shows a temperature-induced spin-state transition whose microscopic origin is still discussed controversially. Recently, the layered cobaltate Na_xCoO₂· y H₂O attracted much interest due to its superconductivity and the water-free compound Na_xCoO₂ due to its possible relevance for thermoelectric cooling. Na_xCoO₂ has a low electrical resistivity ρ, a low thermal conductivity κ, and a large thermopower S, leading to a rather large thermoelectric figure of merit Z T=S²T / κρ which should approach unity for an effective thermoelectric cooling. In this talk we present a systematic study of the figure of merit on a series of single crystals of La_1−xSr_xCoO₃. Substituting La³⁺ in LaCoO₃ by Sr²⁺ drives the compound from a nonmagnetic insulator through a spin-glass phase (x ≥ 0.04) to a ferromagnetic metal for x≥ 0.18 [1]. The entire series of La_1−xSr_xCoO₃ has rather low κ values, whereas S strongly decreases with increasing x. For intermediate Sr concentrations in the spin-glass region we find notably large values of Z indicating that Co-based materials could be promising candidates for thermoelectric cooling.

[1] M. Kriener et al., Phys. Rev. B 69, 094417 (2004) Supported by the DFG through SFB 608.