d-wave superconductivity of the hole-doped (SrCa)(10)Cu17O29 ladder compound

We performed tunneling experiments on single crystals of hole-doped two-leg ladders (SrCa)(10)Cu17O29 with T(c)congruent to 75 K and, for reference pu rposes, on textured bulk samples of Bi2Sr2CaCu2O8 (Bi2212). Point-contact a nd break-junction (Bi2212 only) junction configurations were used. The poin t-contact junctions, both superconductor-insulator-normal-metal and superco nductor-normal-metal, were using Ag counterelectrodes. In all experiments t he tunneling direction was along CU2O3 planes (for the ladders) and CuO2 pl anes (for Bi2212). The recorded tunneling conductance spectra were reproduc ible with sharp, well-defined energy-gap structures. All junctions exhibite d a pronounced zero-bias conductance peak disappearing at the critical temp erature. Experimental results an consistent with the formation of zero-ener gy Andreev bound states at the surface of the superconductors and d(x2-y2)- wave symmetry of the order parameter Delta in both compounds.