High-temperature superconductivity in cuprates occurs in CuO₂ planes within a relatively narrow interval of impurity hole concentration p [1]. It is well established that the transition temperature T_c varies with p according to a parabolic law. At the same time there is a conflicting information about a doping dependence of a superconducting gap Δ. In the present investigation we have measured Δ in doped Bi-2212, Bi-2201 and Hg-1201 by tunneling, Josephson and Andreev spectroscopy using a break junction technique. In particular we have studied the intrinsic Josephson effect (IJE) on natural ultrathin steps (with a height from 1.5 to 30 nm) which are always present on cryogenically cleaved surfaces of doped HTSC single crystals. For a stack of n equivalent junctions the value of the gap voltage V_gn corresponding to a sharp increase of a quasi-particle current (J||c) is given by: V_gn = (2Δ/e)n. The experimental dependence V_gn(n) has been used to determine the gap parameter Δ with high accuracy. The unusual sharpness of a gap feature in case of a highly anisotropic gap parameter could be caused by an extended van Hove singularity [1], the presence of which significantly enhances the gap structure in current-voltage characteristics for J||c [2, 3]. We have used the data of tunneling, intrinsic tunneling and Andreev spectroscopies to derive the dependence of the superconducting gap Δ on the impurity hole concentration p in Bi-2212, Bi-2201 and Hg-1201. In all cases the superconducting gap Δ(4.2 K) was found to scale with the transition temperature T_c in the entire doping range.

References
1. A.A. Abrikosov, Physica C 341-348, 97 (2000).
2. J.Y.T. Wei et al., Phys. Rev. B 57, 3650 (1998).
3. P.I. Arseev et al., cond-mat/0304525.

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1. Extended van Hove Singularity, Strong Electron-Phonon Interaction and Superconducting Gap in Doped Bi-2212 Single Crystals 
2. Evidence for strong electron-phonon interaction in doped Bi-2212 from Josephson spectroscopy