Quasiparticle States of HIGH-TC Oxides Observed by a Zeeman Magnetic Field Response

Spin polarized tunneling spectroscopy was applied to optimally doped YBa₂Cu₃O_7-δ (YBCO) in order to detect the quasiparticle properties of d-wave superconducting phase. YBCO/La_0.67Sr_0.33MnO₃ junctions were fabricated by PLD and photolithographic process and a Zeeman magnetic field was applied to the direction parallel to the ab-plane. Expected Zeeman splitting of conductance spectrum was not detected for the field up to 14T. Instead, unpredictably large gap suppression due to the applied field was observed. Since both of these responses are inconsistent with weak coupling BCS model which takes into account of d-wave symmetry, the present results reflect the anomalous quasiparticle states in high-T_c oxides.