Proximity induced metal-insulator transition in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/{\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ superlattices

The far-infrared dielectric response of superlattices (SL) composed of superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO) and ferromagnetic ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ (LCMO) has been investigated by ellipsometry. A drastic decrease of the free-carrier response is observed which involves an unusually large length scale of $d^{\mathrm{crit}}\approx 20\mathrm{nm}$ in YBCO and $d^{\mathrm{crit}}\approx 10\mathrm{nm}$ in LCMO. A corresponding suppression of metallicity is not observed in SL’s where LCMO is replaced by the paramagnetic metal ${\mathrm{LaNiO}}_3.$ 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 is 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 ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$ is possibly related to this effect.