Direct observation of double exchange in ferromagnetic La${}_{0.7}$Sr${}_{0.3}$CoO${}_3$ by broadband ellipsometry

We present results of our broadband ellipsometry measurements of the optical response of ferromagnetic La${}_{0.7}$Sr${}_{0.3}$CoO${}_3$. Our data show that the ferromagnetic transition is accompanied by a transfer of optical spectral weight from an absorption band centered at 1.5 eV to a narrow component of the Drude-like peak. The associated reduction of the intraband kinetic energy is significantly larger than $k_{\mathrm{B}}{T}_c$, confirming that the double exchange plays a major role in the ferromagnetism of doped cobaltites. In conjunction with results of recent theoretical studies, the temperature dependence of the Drude-like peak suggests that the double exchange is mediated by $t_{2g}$ orbitals.