Many recent experiments of angular resolved photoemission spectroscopy (ARPES) have confirmed the existence of saddle points (van Hove singularity or vHs.) close to the Fermi level in five different copper oxide compounds by three different groups, in Stanford', in Argonne2 and in Wisconsin3. These observations have been made in the following compounds : Bi2Sr2CuO6 (Bi 2201), Bi2Sr2CaCu2O8 (Bi 2212), YBa2Cu3O7 (Y123), YBa2Cu4O8 (Y124) and Nd2Ce,CuO4÷ (NCCO). These experiments establish a general feature : in very high T superconductors cuprates (T 90 K) van Hove singularities are present close to the Fermi level. This is probably not purely accidental and we think that any theoretical model must take into account these experimental facts. The origin of high T in the cuprates is still controversial and the role of these singularities in the mechanism of high T superconductivity is not yet established, but we want to stress that the model of 2D itinerant electrons in presence of v.H. singularities in the band structure has already explained a certain number of experimental facts, i.e. high L's, anomalous isotope effect4, marginal Fermi liquid effects5 and the very small values of the coherence length6. It was also been shown that the singularity is in the middle of a wide band and that in these circumstances, the Coulomb repulsion j.t is renormalized and .t is replaced by a smaller number, the effective electronphonon coupling is Xeff l.L and remains positive7. We think that this fact explains the very low T observed in Sr2RuO4, where a very narrow band has been determined by ARPES

Josephson plasma resonance has been introduced recently as a powerful tool to probe interlayer Josephson coupling in different regions of vortex phase diagram. In the pancake liquid plasma resonance frequency (omega) _p(B,T) as function of the magnetic field B was obtained previously using the high temperature expansion. We further develop this approach and derive a general relation connecting (omega) _p(B) with the density correlation function of pancake liquid. The relation provides unique opportunity to extract quantitative information on the c-axis correlations of pancake liquid in crystals with weak and strong pinning from the dependence of plasma resonance on the ab-component of magnetic field at fixed c component. We discuss c-axis correlations in crystals with weak disorder and crystals with columnar defects produced by irradiation.

We studied theoretically the c-axis Josephson critical current for bicrystals of high temperature superconductors twisted an angle (phi) 0 about the c-axis with respect to eachother. We used the effective Lawrence-Doniach models appropriate for the cases or pure s-wave or dx2-y2-wave order parameters, and of a dominant dx2-y2-wave order parameter combined with a subdominant one of either s-wave or d_xy-wave form, as a function of the temperature and (phi) 0. Our results demonstrate that this new phase- sensitive experiment can serve as a very useful test of order parameter symmetry. In particular, the recent zero- field experiments in the vicinity of Tc by Li et al. on c- axis twist junctions of Bi₂Sr₂CaCu₂O_8+(delta ) are very difficult to explain with a dominant dx2-y2-wave order parameter.

A pseudo gap kT* has been found in various observations with cuprate superconductors. Here, as new kT* explanation the Coulomb gap (Delta) U* of blocking layer intermediate state tunneling is analyzed. kT* is congruent to (Delta) U* presents as function of doping and blocking layer width an excellent fit to the data if the counteraction of overlap and charging energies is taken into account. In addition, states at E_F of the blocking layer with their (Delta) U*-values explain various spectroscopic observations reaching from specific heat to (rho) ^{(perpendicular})(T, (omega) ) or (rho) ^(parallel)(T, (omega) ) and to electron spectroscopies, like STS and ARPES, sampling mainly the surface blocking layer. For YBCO, quasi- localized holes in the blocking layer resonant with the extended CuO₂-states are found essential for the superconductivity. This hints to (Delta) U* interface plasmon exchanges between both charge system as one possible cause for the superconducting coherence.

The design and application of Josephson-coupled multilayers (JCMs) as active switching devices in superconducting electronics requires an understanding of the transient dynamics of these structures. We discuss the theoretical issues surrounding the switching dynamics of JCMs, and describe our own theoretical model which includes the high frequency behavior of the superconducting layers through a nonlocal treatment involving the charge-current conservation equations. We cast our discussion in the context of materials system, from that of conventional low T_c engineered JCMs to that of intrinsic high T_c compounds. The possibility of coherent stable state switching in JCM structures is explored.

We discuss a new class of phenomena based on strong interaction between magnetic superstructures and vortices in superconductors. A local variation of the magnetic moment can pin and create vortices spontaneously, changing drastically properties of the superconductor. At the same time, magnetic moments can be changed and strongly interact via coupling with vortices. This highly inhomogeneous mechanism of interaction between superconductivity and magnetism is totally different from the traditional mechanism based on competition of homogeneous order parameters. This mechanism can be realized in a broad class of systems. We discuss the case of superconducting films with magnetic nanoscale dots and recent experiments with rare-earth-nickel-borocarbide magnetic superconductors.

The I-V characteristics of a 1D Josephson-junction array with charging effect is numerically investigated. The hysteresis loops in the I-V curves show the multiple branch structure. When the coupling between junctions is relatively weak and the Josephson critical current j_c is weakly dependent on the junction site, the number of junctions showing the AC Josephson effect changes by one at the transitions to the nearest-neighbor branches. In this case we obtain the I-V curves quite similar to those observe din single crystals of high-Tc cuprates.

The coupling of Josephson oscillations in layered superconductors is studied with help of a tunneling Hamiltonian formalism. The general form of the current density across the barriers between the superconducting layers is derived. The induced charge fluctuations on the superconducting layers lead to a coupling of the Josephson oscillations in different junctions. A simplified set of equations is then used to study the non-linear dynamics of the system. In particular the influence of the coupling on the current-voltage characteristics is investigated and upper limits for the coupling strength are estimated from a comparison with experiments on cuprate superconductors.

Intrinsic Josephson junctions prepared in bulk materials were fabricate in thin films. We selected Bi-2223 instead of often sued Bi-2212 as the superconducting materials in which insulating and superconducting layers are stacked along c- axis. Single crystal fragments having its high crystallinity and flat surfaces were chose carefully among many grains in thin films and Mesa structures were formed by photolithographic technique. Superconducting gap structure and branching structure peculiar to stacked S-I-S junctions were clearly observed on current-voltage characteristics. On the structure stacked by plural intrinsic junctions, the interaction between different intrinsic junctions clearly appeared in decrease of values of Josephson critical current with increasing the number of junctions in the resistive state, and collective switching was observed. The Ic decrease leading to the collective behavior was explained by taking it effective heat diffusion into account. With reducing the number of the junctions, the gap value, which is suppressed by heat generation, increased. This response of the quasi-particle current was evaluated using a single intrinsic junction and confirmed to be able to explained by assuming d-symmetry for the superconducting order parameter without the term of the gap suppression.

We report on the behavior of 123 superconducting superlattices grown on cubic substrates, with the CuO₂ planes perpendicular to the substrate. These multilayers are the ideal system to study the competition between the intrinsic anisotropy, due to the natural anisotropy and the artificially induced anisotropy. The samples are grown by DC magnetron sputtering technique. The superlattices are characterized by the refinement of the structure from x-ray diffraction spectra. The angular dependence of the resistivity and critical current, in the mixed state, are used to study the anisotropy of this system. Two different regimes have been observed, with a crossover from one to another driven by the direction of the applied magnetic field. One of them occurs with the applied magnetic field close to the CuO₂ planes and another with the applied magnetic field close to the substrate. In the former regime, the anisotropy behavior could not be explained taking into account only the intrinsic pinning mechanisms, In this regime,the critical current and the resistivity follow scaling laws, using reduced magnetic field, due to the H_c2 2D and 3D anisotropy models. In the latter regimen, the insulating layers act as strong pinning centers and a break- down of the H_c2 scaling laws is observed.

Ultrathin YBa₂Cu₃O₇ films and [YBa₂Cu₃O₇(12-24 angstrom)/PrBa₂Cu₃O₇(12-96 angstrom)]_n multilayers have been prepared on (100)SrTiO₃ buffer layer by magnetron sputtering for dimensionality investigation of superconductivity. For developing fundamental research and application of perovskite high-Tc superconductor and ferroelectric the heterostructure Pb(Zr, Ti)O₃/YBa₂Cu₃O₇ and Pb(Zr, Ti)O₃/SiO₂/Si thin films also have been prepared and investigated.

In order to study the interplay between superconductivity and magnetism in proximity systems, we measured the critical temperature Tc decreases continuously with increasing t_Fe up to t_F3 equals 15 angstrom. At higher t_Fe values Tc suddenly drops due to a larger pair breaking effect when the Fe layers change from the nonmagnetic to the ferromagnetic to the ferromagnetic regime. Measurements of H_c2(T) show that the superconducting Nb layers are already decoupled for t_Fe equals 12 angstrom; magnetization measurements however indicate that the Fe layers are only ferromagnetic for t_Fe > 15 angstrom. Critical current measurements indicate the presence of a matching effect for fields applied parallel to the layer due to the penetration of a row of vortices in the middle of the multilayer. The interaction of flux lines with magnetic dots is studied in more detail in Pb films with a lateral superlattice of magnetic Co dots. Matching anomalies in the magnetization curves indicate the stabilization of specific stable vortex lattices at integer and rational multiples of the first matching field. The influence of the stray field of the magnetic dots on the pinning properties will be discussed.

We summarize some recent activities of our group involving e-beam and optically patterned, Nb-based, superconducting nanostructures, as well as related computer modeling studies. The devices described are i) Fiske cavities buried in a Nb base layer and driven by an overlying Nb/AlO_x/Nb Josephson oscillator junction, ii) Nb/AlO_x/Al/AlO_x/Nb five-layer double junctions which Maezava and Shoji have observed to perform as nonhysteretic SNS Josephson single junctions, and iii) critical current measurements on e-beam patterned, 2D arrays of periodic and aperiodic ferromagnetic Ni 'dots' embedded in a Nb film. We also describe a method of 'base-electrode planarization' leading to more uniform junction formation. The computer modeling studies involve the stable and unstable solution states in a static magnetic field of current-biased single and double Josephson junctions.

Nb/CuMn multilayers, with different CuMn layer thicknesses and Mn concentration have been realized by sputtering. On the patterned superconducting samples the transport properties have been measured at different temperatures and external parallel and perpendicular magnetic fields. The pinning force curves obtained in the case of perpendicular magnetic fields, clearly present a temperature scaling indicating a common pining mechanism active at all the investigated temperatures, while in the case of parallel magnetic fields such scaling was not observed. The dependence of the pinning forces upon the Mn concentration has also been investigated. Preliminary measurements on Nb/CuMn samples with a regular array of artificial dots are also presented.

DC reactive magnetron sputtering has been used to prepare niobium nitride/aluminium nitride superconducting/insulator superlattices. Deposition conditions were optimized in order that multilayer fabrication could be achieved by simple computer control of the substrate positioning beneath each sputtering target.

The superconducting critical temperature Tc of Nb/CuMn multilayers has been measured in a series of samples with fixed Nb layer thickness (d_CuMn) and varying CuMn layer thickness and vice versa, where the CuMn composition and the layer thicknesses were systematically varied. An oscillating behavior has been observed for Tc(d_CuMn) strongly dependent on the Mn concentration values. Different explanations are discussed and the most plausible seems to rely on the presence of a d_CuMn-dependent phase-shift value in the range (0, (pi) ) between adjacent superconducting layers.

Commensurability effects between the superconducting flux line lattice and a square lattice of submicron holes in 1500 angstrom vanadium films were studied by atomic force microscopy, DC magnetization, AC susceptibility, magnetoresistivity and I-V measurements. Peaks in the magnetization and critical current at matching fields are found to depend nonlinearly upon the value of external AC field or current, as well as the IR symmetry of the flux line lattice.

The c-axis optical properties of cuprate superconductors have been studied by grazing angle reflectivity technique. We tested the interlayer tunneling model of high temperature superconductivity and showed that the basic relation between the condensation energy of the superconducting state and the interlayer Josephson coupling does not hold for Tl₂Ba₂CuO₆. Measuring the reflectivity at an oblique angle of incidence on the ab-surface of Tl₂Ba₂CuO₆, we found the c-axis plasma resonance at 28 cm^-1. The corresponding Josephson coupling energy is at least an order of magnitude lower than the condensation energy determined for the specific heat measurements. In the double layer compound Tl₂Ba₂CaCu₂O₈ the c-axis plasma resonance was observed at 26 cm^-1. The c-axis penetration depth and the c-axis conductivity follow the Ambegaokar-Baratoff or 'dirty limit' superconductor relation if a very small energy gap is assumed.

The non-linear, c-axis, inter-layer electrical properties of Bi₂Sr₂CaCu₂O₈ single crystals in the normal and superconducting states have been investigated. Measurements of the IV characteristics and dynamic conductivity across lithographically defined stacks of intrinsic junctions on the surface of single crystals of various doping are reported. We demonstrate that the increase in c-axis resistance at low temperatures is related to the emergence of a normal state pseudo-gap. At T_c a zero bias Josephson tunneling conductance peak emerges, along with additional voltage dependent features, consistent with an energy gap in the tunneling density of states. The additional structure emerges at an already well-defined energy, implying the condensation of pre-formed pairs. Both the normal and superconducting state derived energy gaps are significantly smaller than obtained from ARPES and surface tunneling measurements. The absence of significant thermal broadening suggests coherent c-axis tunneling between adjacent CuO₂ bi-layers, at variance with generally accepted views. Well into the superconducting state, the IV characteristics become strongly re-entrant with a functional from that suggests non-equilibrium tunneling effects play an important role at large biases.

Thin films of CeRu₂ have been prepared with sharp superconducting transitions at temperatures of about 5.6K at large thickness, slowly decreasing to 4K around 50 nm. Below 50 nm the superconducting transition temperature decreases abruptly. The flux pinning properties of the thicker films are very anomalous. At low fields the pinning force F_p is very high, of the order of 10⁹N/m³, and probably due to the strong disorder, but F_p drops to zero at unusually low reduced fields. This leads to a field- temperature phase diagram with an upper critical field line quite similar to the one for bulk crystals, but with an irreversibility line which lies much lower; it cannot be described with conventional theories for either 2D or 3D vortex lattice melting.

We analyzed the temperature, the magnetic field and the normal layer thickness dependences of the pinning force F_p in superconductor (S)-normal metal (N)-superconductor (S) (SNS) multilayers in the framework of the Ginzburg- Landau (GL) theory with the step-like parameters. In particular we studied the temperature dependence of the magnetic field H_peak which corresponds to the pinning force maximum F_pmax in the F_p(H) dependence. It was shown that for temperature well below the critical value the magnetic field of the periodic solution of the GL equation H_D corresponds to the field H_peak and is function of the temperature. Our results are in good agreement with the recent experiments on Nb/Pd multilayers.

We derive a classical equation of motion on the superconducting phase in Josephson coupled layered High-Tc superconductors from the effective action obtained by using the path-integral method for the BCS microscopic theory considering the Josephson coupling between neighboring superconducting layers. The equation can describe the dynamics of the superconducting phase in the presence of the ab-plane parallel magnetic field. We perform numerical simulations on the equation, and study the dynamics of the superconducting phase under the collective motions of the Josephson vortices. In the simulations, we employ the open boundary condition in the 2D rectangular computational region, and measure the local electric field at the edge from interests about the emission of the electromagnetic wave by the flux flow. It is found that in the low field region, chaotic dynamics of the electric field is observed by reflecting non-regular motions of weakly correlated vortices. On the other hand, in the high field region, regular oscillations of the electric field can be detected as long as the stability of the driven vortex lattice is maintained, while further increasing the transport current results in instabilities of the driven vortex lattice and chaotic oscillations of the electric field appear.

A series of c-axis oriented high temperature superconductor superlattices YBa₂Cu₃O_7-(delta )/PrBa₂Cu₃O_7-(delta ) were synthesized using dc magnetron sputtering. We varied the thickness of intervening PrBa₂Cu₃O_7-(delta ) layers as two, four, and eight unit cells with each individual YBa₂Cu₃O_7(delta ) layer fixed to eight-unit-cell thick. X-ray diffraction analysis indicates that these specimens were grown epitaxially having a rather satisfactory superlattice modulation along the c-axis. Furthermore, atomic force microscope (AFM) images show that surfaces are very smooth. Shaped into a hall bar geometry, we have investigated its transport properties, including resistivity, current-voltage characteristics, and noise. In the superconducting transition region, the electrical conduction of these specimens was found to be influenced by dissociated free vortices. This was evident from the resistive transition and current-voltage characteristics, which are well explained by Bardeen-Stephen model and Kosterlitz-Thouless (KT) transition theory, respectively. KT transition temperatures determined from the resistance fittings and current-voltage characteristics were in good agreement. Somewhat complex features were observed in the noise measurement with the variation of the transport current and the external magnetic field. There appeared a single or multiple peaks near the resistive transition tail. While noise peaks were suppressed monotonically as the magnetic field increased, noise peaks were enhanced at first and then suppressed as the bias current increased. We explain these behaviors using the flux flow model.

We have performed a detailed study of the effect of the HTS degraded surface layer on conductance spectra of HTS/metal junctions. It has been found that the corresponding resistance changes by applying an external bias voltage can be explained as an alternation of the oxygen content near the interface. To take into account the spatially inhomogeneous nature of a few top HTS unit cells, we have examined theoretically the influence of a nonuniform normal covering on the subgap tunnelling characteristics of a superconductor. A simple approach to coherent charge transport through a planar double-barrier structure has been generalized to the cause of an anisotropic order parameter, non-uniformity of the middle non-superconducting layer, and magnetic field effect. In particular, we show that, under suitable circumstances, the conductance peak at zero bias voltage and its splitting at low temperatures often observed for HTS/metal junctions can be interpreted within the s-wave scenario, too, if above mentioned inhomogeneity is considered.

In this paper we show that (BaCuO₂)_m/((Ca,Sr)CuO₂)_n superlattices with good crystallographic quality can be engineered by Pulsed Laser Deposition in a wide range of composition, namely for m equals 2 and n ranging form 1 to 6. The temperature dependence of resistivity of the superlattices grown at relatively high oxygen pressure and temperatures above 570 degrees C is metallic with a full transition to a superconductive state. For optimized growth conditions, the critical temperature can reach a value of about 80 K. We investigated some of the electrical transport properties of these superlattices. The result of these measurements indicate, for these artificial structures, an intermediate degree of anisotropy between the highly anisotropic BSCCO and the relatively isotropic YBCO. Moreover, the dependence of T_c on the number of CaCuO₂ layers was investigated and a maximum was found for n equals 2-3.

Bulk Bi₂Sr₂CaCu₂O_{8 + (delta} ) (Bi2212) bicrystals containing a single high quality twist grain boundary junction were prepared in order to investigate the orbital symmetry of the superconducting order parameter in highly anisotropic Bi-based high temperature superconductors. The misorientation angles of the bicrystals ranged from 0 degrees to 180 degrees. The microstructure in the vicinity of the junction was characterized using high- resolution, nano-probe analytical microscopy. We fond that some high angle twist junctions were able to carry a critical current density similar to their constituent single crystals. These results cannot be interpreted in terms of a pure d_x2-y2-wave order parameter for superconducting Bi2212.