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Abstract:
The far-infrared dielectric response of superlattices (SL) composed of
superconducting YBa2Cu3O7 (YBCO) and ferromagnetic La0.67Ca0.33MnO3
(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)approximate to20 nm in YBCO and
d(crit)approximate to10 nm in LCMO. A corresponding suppression of
metallicity is not observed in SL's where LCMO is replaced by the
paramagnetic metal LaNiO3. 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 RuSr2GdCu2O8 is possibly related to this effect.
