Local and global effects of quantum impurities on the quasi-particle tunneling spectra of p-type and n-type cuprate superconductors

Abstract

We report scanning tunneling spectroscopic studies of the effects of quantum impurities on cuprate superconductors. The samples include p-type YBa 2 Cu 3 O 7-δ single crystals with spinless impurities of Zn 2+ and Mg 2+ ((Zn,Mg)-YBCO) and n-type infinite-layer system Sr 0.9 La 0.1 CuO 2 with 1% magnetic Ni 2+ - or 1% non-magnetic Zn 2+ -impurities that substitute the Cu 2+ in the CuO 2 plane. The local effects of spinless impurities on the quasiparticles spectra of (Zn,Mg)-YBCO are analogous to those of Zn-substituted Bi 2 Sr 2 CaCu 2 O 8+x , and the global effect is manifested by the suppression of the pairing potential Δd and of the spin excitation energy. In contrast, spectroscopic studies of Sr 0.9 La 0.1 CuO 2 reveal momentum-independent spectra and superconducting gap Δ , with (2Δ/k B T c ) ~ 7 for T c = 43 K and no pseudogap above T c . The global response of Sr 0.9 La 0.1 CuO 2 to quantum impurities is similar to that of s-wave superconductors, being insensitive to small concentrations of spinless impurities (Zn) while showing rapid degradation in T c with increasing magnetic impurities (Ni). Moreover, the spectra of the Ni-substituted Sr 0.9 La 0.1 CuO 2 reveal strong electron-hole asymmetry and long-range impurity effects, in contrast to the localized impurity effects in the p-type cuprates, and the introduction of Zn yield no reduction in either Δ or T c . The physical implications of these findings are discussed.