Tunneling spectroscopy with intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ and Tl2Ba2Ca2Cu3O10+δ

The paper presents a detailed discussion of the current-voltage characteristic of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+delta and Tl2Ba2Ca2Cu3O10+delta. In these materials Josephson tunnel junctions are formed naturally between adjacent superconducting CuO2 bilayers or trilayers. A typical sample consists of a stack of Josephson junctions. We explicitly show that all junctions inside a given sample have identical tunneling characteristics. We discuss the shape (general curvature) of the current-voltage characteristic in terms of a superconducting order parameter that has a predominant d(x2-y2) symmetry. The IcRn product of the intrinsic Josephson junctions turns out to be 2-3 mV, about 10% of the maximum energy gap Delta(0)/e. The current-voltage characteristic of every individual junction exhibits pronounced structures in the subgap regime. They are best explained by a recently proposed resonant coupling mechanism between infrared active optical c-axis phonons and oscillating Josephson currents.