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

TL;DR

This study investigates how replacing oxygen isotopes in a cobalt oxide affects its spin state transition temperature, revealing a significant isotope effect linked to electron-phonon interactions and changes in phonon frequency and electron effective mass.

Contribution

It demonstrates the impact of oxygen isotope substitution on the spin state transition temperature and elucidates the underlying electron-phonon coupling mechanisms in the material.

Findings

Spin transition temperature up-shifts by 6.8 K with isotope substitution.

Oxygen isotope exponent ($\alpha_S$) is approximately -0.8.

Isotope substitution decreases phonon frequency and increases electron effective mass.

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.