Dependence of the critical temperature of YBCO thin films on spin-polarized quasiparticle injection

Non-equilibrium effects in cuprate superconductors caused by quasiparticle (QP) injection are investigated by electrical transport measurements. We explore tile reduction of T-c in YBa2Cu3O7 thin films by highly spin-polarized QP injection and compare the results with weakly polarized as well as ordinary QP injection. Heterostructures consisting of ferromagnetic La0.67Ca0.33MnO3 and superconducting YBa2Cu3O7 layers were fabricated by pulsed laser deposition techniques in conjunction with shadow mask methods to define an appropriate sample geometry. Standard four probe measurement techniques have been applied for probing the transition to superconductivity of YBCO thin films with and without quasiparticle injection. We observe a NO decrease of T-c for injection current densities of 5 x 10(-2) A/cm(2) in the case of unpolarized QPs whereas highly spin-polarized QPs show a much stronger decrease of 14%. This result clearly demonstrates that the spin plays a crucial role in the T-c reduction. Using the theoretical description of QP injection effects of Bhattacharjee and Sardar based on the Owen-Scalapino approach for the description of non-equilibrium effects in superconductors we find a surprisingly good agreement of the experimental data with the model for interlayer tunneling. (c) 2007 Elsevier B.V. All rights reserved.