The far-infrared dielectric response of superlattices (SL) composed of superconducting YBa$_{ 2}$Cu$_{ 3}$O$_{ 7}$ (YBCO) and ferromagnetic La$_{ 0.67}$% Ca$_{ 0.33}$MnO$_{ 3}$ (LCMO) has been investigated by ellipsometry. A drastic decrease of the free carrier response is observed which involves an unusually large length scale of d$^{ crit}\approx $20 nm in YBCO and d$^{ crit}\approx $10 nm in LCMO. A corresponding suppression of metallicity is not observed in SLs where LCMO is replaced by the paramagnetic metal LaNiO$_{ 3}$. Our data suggest that either a long range charge transfer from the YBCO to the LCMO layers or alternatively a strong coupling of the charge carriers to the different and competitive kind of magnetic correlations in the LCMO and YBCO layers are at the heart of the observed metal/insulator transition. The low free carrier response observed in the far-infrared dielectric response of the magnetic superconductor RuSr$_{ 2}$GdCu$_{ 2}$O$_{ 8}$ is possibly related to this effect.
Categories: cond-mat.supr-con cond-mat.str-el