Chemically Induced Nanoscale Josephson Effects in Non-stoichiometric High-tc Superconductors

Both granular superconductors and artificially prepared arrays of Josephson junctions (JJAs) proved useful in studying the numerous quantum (charging) effects in these interesting systems, including blockade of Cooper pair tunneling [1], Bloch oscillations [2], propagation of quantum ballistic vortices [3], spin-tunneling related effects using specially designed SFS-type junctions [4, 5], novel Coulomb effects in SINIStype nanoscale junctions [6], dynamical AC reentrance [7] and geometric quantization phenomena [8] in 2D JJAs (see, e.g., Ref. [9] for the recent review on charge and spin effects in mesoscopic 2D Josephson junctions). More recently, it was realized that JJAs can be also used as quantum channels to transfer quantum information between distant sites [10, 11, 12] through the implementation of the so-called superconducting qubits which take advantage of both charge and phase degrees of freedom (see, e.g., Ref. [13] for a review on quantum-state engineering with Josephson-junction devices). Recent imaging of the granular structure in underdoped Bi2Sr2CaCu2O8+δ crystals [14], revealed an apparent segregation of its electronic structure into superconducting domains (of the order of a few nanometers) located in an electronically distinct background. In particular, it was found that at low levels of hole doping (δ < 0.2), the holes become concentrated at certain hole-rich domains. Tunneling between such domains leads to intrinsic granular superconductivity (GS) in high-Tc superconductors (HTS). Probably one of the first examples of GS was observed in Y Ba2Cu3O7−δ single crystals in the form of the so-called ”fishtail” anomaly of magnetization [15]. The granular behavior has been related to the 2D clusters of oxygen defects forming twin boundaries (TBs) or dislocation walls within CuO plane that restrict supercurrent flow and allow excess flux to enter the crystal. Indeed, there are serious arguments to consider the TB in HTS as insulating regions of the Josephson SIS-type structure. An average distance between boundaries is essentially less than the grain size. In particu-