TUNNELLING SPECTROSCOPY OF HIGH-TC SUPERCONDUCTORS

Abstract

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.