ab-plane tunneling and Andreev spectroscopy of superconducting gap an pseudogap in

Анотація

We have measured the temperature dependence of gap features revealed by Andreev reflection (D_s) and by tunneling (D) in the ab-plane of optimally and slightly overdoped microcrystals of (BiPb)₂Sr₂Ca₂Cu₃O_10+d (Bi2223) with critical temperature T_c = 110-115 K, and Bi₂Sr₂CaCu₂O_8+d (Bi2212) with T_c = 80-84 K. The tunneling conductance of a Bi2223-insulator-Bi2223 junction shows peaks at the 2D gap voltage, as well as dips and broad humps at other voltages. In Bi2223, similarly to the well-known Bi2212 spectra, the energies corresponding to 2D, to the dip, and to the hump structure are in the ratio 2:3:4. This confirms that the dip and hump features are generic to the high-temperature superconductors, irrespective of the number of CuO₂ layers or the BiO superstructure. On the other hand, in both compounds the D(T) and D_s(T) dependences are completely different, and we conclude that the two entities are of different natures.