Direct observation of double exchange in ferromagneticLa0.7Sr0.3CoO3by broadband ellipsometry

We present results of our broadband ellipsometry measurements of the optical response of ferromagnetic ${\mathrm{La}}_0.7{\mathrm{Sr}}_0.3{\mathrm{CoO}}₃$. 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 ${k}_{\mathrm{B}}{T}_{\mathrm{c}}$, 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.