We demonstrate experimentally the existence of a continuous phase transition between a normal and a true superconductiug phase (with zero linear resistivity) in epitaxial films of Y-Ba-Cu-0 in strong magnetic fields, H&&H, I. The nonlinear I-V curves show scaling behavior near the transition and the relevant critical exponents are extracted. These exponents are consistent with values expected for freezing into a superconducting vortex-glass phase.

We report observations of spontaneous formation of magnetic flux lines during a rapid quench of YBa(2)Cu(3)O(7-delta) films through T(c). This effect is predicted according to the Kibble-Zurek mechanism of creation of topological defects of the order parameter during a symmetry-breaking phase transition. Our previous experiment, at a quench rate of 20 K/s, gave null results. In the present experiment, the quench rate was increased to >10(8) K/s. The amount of spontaneous flux increases weakly with the cooling rate.

We describe the voltage-current characteristics of YBa2Cu3O7−δ epitaxial films within the flux creep model in a manner consistent with the resistive transition behavior. The magnitude of the activation energy, and its temperature and magnetic field dependences, are readily derived from the experimentally observed power law characteristics and show a (1−T/Tc)3/2 type of behavior near Tc. The activation energy is a nonlinear function of the current density and it enables the determination of the shape of the flux line potential well.

We describe observations of spontaneous flux generation inside a YBa(2)Cu(3)O(7-delta) loop made of 214 Josephson junctions in series. The flux is generated spontaneously during cooldown into the superconducting state. The experiment is motivated by the Kibble-Zurek scenario of formation of topological defects in condensed matter systems. The transition from decoupled superconducting segments into a coherent loop is determined by the strength of thermal fluctuations in the junctions. Values of the flux measured at the end of each cooldown follow a normal distribution, and are consistent with the instantaneous phase differences across the junctions adding up as the loop becomes coherent.

In the underdoped high temperature superconductors, instead of a complete Fermi surface above Tc, only disconnected Fermi arcs appear, separated by regions that still exhibit an energy gap. We show that in this pseudogap phase, the energy-momentum relation of electronic excitations near EF behaves like the dispersion of a normal metal on the Fermi arcs, but like that of a superconductor in the gapped regions. We argue that this dichotomy in the dispersion is difficult to reconcile with a competing order parameter, but is consistent with pairing without condensation.

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We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the high temperature superconductors (HTS) YBa2Cu3O 7−δ (YBCO), Bi2Sr2CaCu2O 8+δ (BSCCO), La1.85Sr0.15CuO4 (LSCO) and Nd1.85Ce0.15CuO4−y (NCCO). In all experiments the tunneling direction was along the CuO2 planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak (ZBCP) was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.To appear in Physical Review B There is strong evidence that the superconducting order parameter (OP) in the HTS has a dominating d-wave symmetry [1,2]. For this pairing symmetry there is a π-phase shift of the OP in orthogonal k-space directions resulting in a positive and negative sign of the pair potential in those directions. This also means that there are directions with nodes of the pair potential, e. g. for a pure d x 2 −y 2 -symmetry, the nodes are along the [110] direction in the CuO 2 plane. For the tunneling spectra of junctions employing HTS electrode materials with a d-wave symmetry of the OP, a pronounced ZBCP has been predicted originating from mid-gap surface (interface) states or zero energy bound states (ZES) at the Fermi level [3][4][5][6][7][8]. The physical reason for these states originates from the fact that quasiparticles incident and reflecting from the surface propagate through different order parameter fields which leads to Andreev reflection. The constructive interference between incident and Andreev reflected quasiparticles results in bound states. Stable ZES are formed if the scattering induces a change in sign of the OP. For a d x 2 −y 2 -wave symmetry such sign change and, hence, the presence of ZES is possible for all surfaces parallel to the c-axis except for those with the lobe directions perpendicular to the surface, whereas for a s-wave symmetry no ZES are possible. The spectral weight of the ZES for a d x 2 −y 2 -wave symmetry depends on the orientation of the surface with respect to the crystal axis. The maximum spectral weigth is expected for a (110) surface and, hence, a maximum ZBCP is expected for tunneling in the direction of the nodal lines, i. e., the [110] direction. This has been observed recently using low temperature scanning tunneling spectroscopy (LTSTS) [9] and planar type junctions [10]. We note that the ZBCP is sensitive to surface roughness making it difficult to distinguish between the directions in the plane [11][12][13].Initially, the ZBCP in the tunneling spectra...