Break junction tunnelling measurements were performed on YBa 2 Cu 3 O 7– and Bi 2 Sr 2 CaCu 2 O 8–single crystals and on polycrystalline ( Bi , Pb )2 Sr 2 Ca 2 Cu 3 O 10–samples. In the case of Bi 2 Sr 2 CaCu 2 O 8 the superconducting gap as evaluated from the peak-to-peak distance is Δab = (21.2 ± 2.8) meV , i.e. 2∆ab/kBTc = (5.5 ± 0.8). The differential conductance curves are similar to those of conventional strong coupling superconductors but with a much larger smearing, that cannot be explained with any of the theoretical models proposed so far. The measurements on YBa 2 Cu 3 O 7 show a strong linear background in the conductance. After subtraction of the background, peaks are discernible at ±21 meV, which yields 2∆ab/kBTc ≈ 2.8 if SIS tunnelling is assumed. The conductance curves of the ( Bi , Pb )2 Sr 2 Ca 2 Cu 3 O 10–polycrystals are dominated by the appearance of sharp minima below the superconducting transition temperature. The temperature dependence of these minima can be fitted well by the BCS behaviour.
Relationships between the superconducting gap, pseudogap and transition temperature in high-Tc superconductors