Oxygen Isotope Effect on the Spin State Transition in (Pr${}_{0.7}$Sm${}_{0.3}$)${}_{0.7}$Ca${}_{0.3}$CoO${}_3$

Abstract

Oxygen isotope substitution is performed in the perovskite cobalt oxide (Pr${}_{0.7}$Sm${}_{0.3}$)${}_{0.7}$Ca${}_{0.3}$CoO${}_3$ which shows a sharp spin state transition from the intermediate spin (IS) state to the low spin (LS) state at a certain temperature. The transition temperature of the spin state up-shifts with the substitution of ${}^{16}O$ by ${}^{18}$O from the resistivity and magnetic susceptibility measurements. The up-shift value is 6.8 K and an oxygen isotope exponent (${\alpha }_S$) is about -0.8. The large oxygen isotope effect indicates strong electron-phonon coupling in this material. The substitution of ${}^{16}$O by ${}^{18}$O leads to a decrease in the frequency of phonon and an increase in the effective mass of electron ($m$${}^{\ast }$), so that the bandwidth W is decreased and the energy difference between the different spin states is increased. This is the reason why the $T_s$ is shifted to high temperature with oxygen isotopic exchange.

4 pages, 3 figures