Scaling of the superconducting order parameter in Bi cuprates with Tc

Abstract

Tunneling measurements have been carried out by the point-contact technique on four members of the Bi cuprates: on the (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ x (2223), Bi 2Y 0.1Sr 1.9CaCu 2O 8+ x (Y2212), Bi 2.1Sr 1.9CaCu 2O 8+ x (2212) superconductors at 4.2 K and on Bi 2.07Sr 1.8CuO 6+ x (2201) at 2 K. The polycrystalline samples have the critical temperature T c = 108 K, 94 K, 85 K and 7 K, respectively. All measured tunneling spectra (tunneling conductance versus the bias voltage) reveal a significant reduction below a certain voltage indicating the superconducting gap edge. The BCS-like formula for the superconducting density of states accounting for some additional smearing can successfully fit the experimental data, particularly at the gap position. Discrepancies found at lower voltages inside the gap may indicate a possible anisotropy of the order parameter, but an extrinsic origin (like inhomogenity in the sample, irregularities in the tunnel barrier) is possible as well. The obtained values of the reduced superconducting energy gap 2 Δ/ k B T c amount to 6–7 for all four systems. Consequently, our results found the gap Δ scaling with the transition temperature T c in the Bi cuprates.