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YBa2Cu3O7 bridges with widths ranging from 50nm to 3 micrometers were made using electron beam lithography and focused ion beam milling. The current voltage characteristics of the nanobridges show, under microwave irradiation, pronounced Shapiro steps up to the transition temperature. SQUIDs, using these nanobridges, have been made and flux to voltage modulation up to 85K was observed, with a maximum modulation depth of 8 (mu) V at 77K. An unusual temperature dependence of the modulation is observed, which can be explained by assuming an exponential spatial distribution of the superconducting properties near the edge of a superconductor.

We report scanning tunneling spectroscopic studies of the effects of quantum impurities on cuprate superconductors. The samples include p-type YBa 2 Cu 3 O 7-δ single crystals with spinless impurities of Zn 2+ and Mg 2+ ((Zn,Mg)-YBCO) and n-type infinite-layer system Sr 0.9 La 0.1 CuO 2 with 1% magnetic Ni 2+ - or 1% non-magnetic Zn 2+ -impurities that substitute the Cu 2+ in the CuO 2 plane. The local effects of spinless impurities on the quasiparticles spectra of (Zn,Mg)-YBCO are analogous to those of Zn-substituted Bi 2 Sr 2 CaCu 2 O 8+x , and the global effect is manifested by the suppression of the pairing potential Δd and of the spin excitation energy. In contrast, spectroscopic studies of Sr 0.9 La 0.1 CuO 2 reveal momentum-independent spectra and superconducting gap Δ , with (2Δ/k B T c ) ~ 7 for T c = 43 K and no pseudogap above T c . The global response of Sr 0.9 La 0.1 CuO 2 to quantum impurities is similar to that of s-wave superconductors, being insensitive to small concentrations of spinless impurities (Zn) while showing rapid degradation in T c with increasing magnetic impurities (Ni). Moreover, the spectra of the Ni-substituted Sr 0.9 La 0.1 CuO 2 reveal strong electron-hole asymmetry and long-range impurity effects, in contrast to the localized impurity effects in the p-type cuprates, and the introduction of Zn yield no reduction in either Δ or T c . The physical implications of these findings are discussed.

The pairing symmetry and the superconducting gap in high- temperature superconducting cuprates are investigated as a function of the hole doping level (x) and temperature (T), using directional scanning tunneling spectroscopy (STS). It is found that the predominant pairing symmetry is (Formula available in paper), which is insensitive to the variations in T and x. In contrast, the maximum superconducting gap ((Delta) d) in YBa2Cu3O7-(delta ) and La2-xSrxCuO4-(delta ) scales with the superconducting transition temperature (Tc), and the ratio of (2 (Delta) d/kBTc) increases with decreasing doping level. The dominance of dx2-y2 pairing is consistent with strong spatial variations in the local quasiparticle spectra near non-magnetic impurities such as Zn and Mg in a (Zn,Mg)-doped YBa2Cu3O7-(delta ) single crystal. To further elucidate the nature of the pairing state, the c-axis spin- polarized quasiparticle transport in the superconducting state of YBa2Cu3O7-(delta ) is investigated by studying the critical currents and STS under the injection of electrical currents from the underlying ferromagnetic La0.7Sr0.3MnO3 layer in various ferromagnet-insulator-superconductor (F-I-S) heterostructures. The temperature dependent spin diffusion length ((delta) s) and signatures of nonequilibrium quasiparticle distribution under spin injection in d-wave superconductors are determined for the first time.

Over twenty five years ago, we predicted the occurrence of photoinduced superconducting phase transition in some complex systems. In a series of papers, conditions and constraints were determined for the realisation of a photoinduced non-equilibrium superconducting state under transient or metastable situations. Owing to the involvement of two boson modes (one with photons or photoexcitations and the other the virtual mode of the system, namely phonon or any other electronic boson mode) in the interaction mechanism, the transition temperature $(T_c)$ is found to depend on the photon density or photoexcitation dosage impressed on the sample within the absorption depth. Thus $T_c$ can be controlled and tuned as required. After giving an overview of the theoretical concepts and the current status of experiments in this field, we present an extended model in which both effects of intraband and interband transitions caused by the appropriate boson modes are taken into account. The intraband pairing thus obtained consists of two parts, the usual single boson induced pairing and the new radiation induced two boson pairing. The latter leads to the enhancement of $T_c$. The dual role of the radiation field quanta is emphasized. The recent experimental discovery of photo-induced superconductivity in some complex oxide (cuprate) systems in transient or metastable state are in accord with the predicted phenomena. These aspects along with some possible applications are discussed