Implications of Tunneling Studies on High-Tc Cuprates: Superconducting Gap and Pseudogap

Tunneling spectra have been measured on high-Tc cuprates including single crystals Bi2Sr2-xLaxCuO6+δ (Bi2201) and Bi2Sr2CaCu2O8+δ (Bi2212) using superconductor-insulator-normal metal point contact or superconductor-insulator-superconductor break junction methods. The doping dependence of the energy gap parameter is similar in both Bi2212 and Bi2201, increasing monotonically to very large values in the underdoped regime even as Tc decreases. This doping dependence of superconducting gap is similar to that of pseudogap temperature, T*, indicating this is consistent with the scenario whereby the low-energy pseudogap is due to some type of precursor of superconductivity. The high-energy feature observed as the hump structure may be another kind of pseudogap whose energy scale is much larger than superconducting gap, and it may be magnetic in origin.