Tunneling spectroscopy study of spin-polarized quasiparticle injection effects in cuprate/manganite heterostructures

Scanning tunneling spectroscopy was performed at 4.2 K on epitaxial thin-film heterostructures comprising YBa2Cu3O7−δ and La0.7Ca0.3MnO3, to study the microscopic effects of spin-polarized quasiparticle injection from the half-metallic ferromagnetic manganite on the high Tc cuprate superconductor. The quasiparticle tunneling characteristics observed were consistent with d-wave pairing symmetry, with a gap-maximum Δ0≈22 meV, up to at least 35 mA (7×103 A/cm2) injection. Spectral smearing observed at higher injections could be fitted to elevated effective quasiparticle temperatures, even though negligible sample heating was detected by in situ thermometry. The overall spectral evolution with the injection current also appears to be nonthermal in character, showing a nonmonotonic change in both the zero-bias tunneling conductance and the area under the conductance spectrum. We discuss general implications of these results for the scenario of dynamic pair breaking by a nonequilibrium distribution of spin-polarized quasiparticles.