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_2Cu_3O_(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_2Sr_2CaCu_2O_(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_BT_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.

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