The differences between the old (inter-) metallic superconductors and the new oxide superconductors are not limited to the much high values of T_c attainable in the latter. There are many pervasive differences caused directly by oxide chemistry, quasi-perovskite local coordination configurations, and layered metal-semiconductor-metal- superconductor-structures. When these differences are ignored, for instance in theoretical models which make effective medium approximations, many experiments appear to present anomalous results. These anomalies largely disappear when account is taken of the real materials properties of the cuprates and other new oxide superconductors, for instance in theoretical models which treat transport as a partially percolative process. This percolative process directly reflects the fact that the highest values of T_c, as well as the most anomalous normal-state transport properties, occur in materials vicinal to a metal-insulator transition. As the metallic and insulating regions alternate even in single-crystal samples, effective medium models, and most effective-medium parameters, lose their significance. Examples of attempts to measure microscopic properties illustrate the importance of filamentary effects on both normal-state and superconductive properties.

An analysis of the temperature dependence of the upper critical field in several cuprate families leads to the conclusion that magnetic impurities are present even in samples with the maximum observed value of T_c. A new parameter, 'intrinsic' T_c which is its value in the absence of magnetic impurities, is introduced. The maximum value of 'intrinsic' T_c which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160-170K. THis is an upper limit of T_c in the cuprates.

On the basis of experimental studies of specific heat, magnetic properties and thermal expansion it was shown that dielectric-metal phase transition was the consequence of delocalization of heavy holes located on metal-apical oxygen bonds in the high-temperature superconducting (HTSC) oxide systems. The dielectric gaps in electronic spectrum of the oxide systems appear owing to the charge density waves (CDW) of the ordered arrays of such localized (heavy) holes. The CDW is the microscopic reason of the negative thermal expansion observed in dielectric phases of some HTSC systems. The free carriers introduced in the oxide systems by doping can couple through excitations of these localized holes (local bosons). Such free carriers pairing leads to the high-temperature dependence of H_c2(T). The suggested localized holes on the metal-apical oxygen bonds and local bosons in addition to the usual phonons are the basis for the explanation of the experimental data presented in this report.

Hypocharged oxygen, and not hypercharged Cu⁺³ is shown to be the generator of high-temperature superconductivity. Models based on Cu⁺²$ARLRCu⁺³ charge-fluctuations (such as t-J models), are ruled out experimentally. Experimental evidence indicates that the holes of the hypocharged oxygen in the charge-reservoir regions contribute primarily to the superconductivity,contrary to most current models of high- temperature superconductivity, which are based on superconductivity originating in the cuprate-planes. The data suggest that a successful theory of high-temperature superconductivity will be BCS-like and will pair holes through the polarization field, perhaps electronic as well as vibrational polarization.

Structural inhomogeneities in crystals of YBa₂Cu₃O_7-(delta ) are characterized using high resolution x-ray diffraction. Rocking curves and a high resolution specular radial scan show features indicating that the crystals are not truly 'single'. The high resolution radial scan of a crystal that had a superconducting transition width of only one-half Kelvin, as determined by ac susceptibility, and appeared to be free of gross defects, when examined with conventional x-ray diffraction techniques, shows a distribution in the c-axis lattice parameter. These results suggest that crystals with sharp transitions should be carefully examined for the presence of structural anomalies which might influence the superconductive properties.

An overview of recent developments in the understanding of thermal conduction in the cuprates is presented. Anomalous phonon damping underlies an unusual temperature dependence of the in-plane thermal conductivity for the insulating phase of some materials. This damping is sensitive to light oxygen doping in YBa₂Cu₃O_6+x (YBCO), is possibly related to local structural distortions, and may persist in superconducting compositions. New systematics for the superconducting-state enhancement of K_ab are discussed. For YBCO the oxygen-doping dependence of the enhancement correlates with that of the pari density as determined by specific heat jumps. The large enhancements observed for fully-oxygenated specimens are attributed to pair condensation on the CuO chains. The implications of this finding and its possible connection with the emergence of the normal-state pseudogap are examined.

Two different thermomagnetic transport quantities, the electrothermal conductivity and the Nernst effect are shown to be powerful probes of high-temperature superconductors. In the vortex state below T_c, the electrothermal conductivity is independent of both the magnetic field and the cortex viscosity because it is sensitive only to the properties of the vortex normal cores. Some new data from cuprate superconductors show a surprising, low-field anomaly in the dilute vortex limit. Above T_c in the normal state, it is shown how the Nernst effect is a probe of transport anisotropy around the Fermi surface.

By simply plotting the critical temperatures T_c for the major superconductors versus the distance d between a cuprate-plane Cu-site and the nearest oxygen site in a charge-reservoir, it becomes apparent that the superconductivity originates in the charge-reservoirs, not in the cuprate planes. By plotting u_c versus d, where u_c is the amount of impurity Ni or Zn on a Cu-site (presumably in a cuprate-plane) required to suppress T_c to zero, it is evident that pair-breaking requires exponentially more impurities as the cuprate-planes become further from the charge-reservoirs, indicating directly that the superconducting condensate is primarily in the charge- reservoirs, not in the cuprate-planes. The fact that u_c(Ni) < u_c(Zn) for Nd_2-zCe_zCuO₄ indicates that this material is a BCS-like polarization- pairing superconductor, not a spin-fluctuation paired superconductor. The fact that u_c(Ni) approximately equals u_c(Zn) for most other high-temperature superconductors indicates that the primary superconducting condensate is more distant than the range of the exchange interaction from the cuprate- plane Cu-sites where the impurities reside. The data for YBa₂Cu₄O₈ and for La_0.6Ca_0.4Ba_1.35La_0.65Cu₃O_x indicate that the behavior of Ni and Zn in YBa₂Cu₃O_x, u_c(Ni) > u_c(Zn), is an artifact of different solubilities on different sites, and not evidence of spin-fluctuation- pairing. The trends in the data suggest that there is only one mechanism of high-temperature superconductivity, that the mechanism involves some form of polarization-pairing, and that the superconductivity originates primarily in the charge-reservoirs, not in the cuprate-planes.

A number of high-Tc superconducting cuprate phases related with YBCO were studied by means of high resolution electron microscopy and electron diffraction. The lower symmetry superstructures were widely detected. The superstructures are due to cation and/or oxygen atom ordering mainly in the MO_1-x 'chain' layer, while the remaining part of the structure is hardly affected. The superlattice based on the 2 (root) 2a_p X (root) 2a_p X c_p orthorhombic cell is found in the Co-1212 and Ga-12n2 compounds; a commensurate superstructure based on the 4a_p X 2a_p X c_p orthorhombic cell in the Al-1222 compound, and an incommensurate modulated structure on the monoclinic cell 3.43a_p X a_p X 1.15c_p in the S-1212 compound. A rock-salt layer ordering on the two-dim 2a_p X 2a_p superlattice was observed in the Ga-1222 compound; it appears as a consequence of partial replacement of Ba for Sr in the (Sr/Ba)O-GaO-(Sr/Ba)O lamella. A two-dim modulation in the CuO₂ planes on a 2 (root) 2a_p X 2 (root) 2a_p superlattice was detected in the vacuum heated YBCO (Cu-1212), Al-1222, and Bi-2212 compounds. All these studies support the concept of cuprate superconductor structure as an alternating stack of MO-'chain' layers, whose structure determines the overall symmetry, and the CuO₂-'block'-layers, which tends to preserve tetragonal symmetry regardless of the number of constituting perovskite, rock-salt, or fluoruite type layers.

The specific role of disorder in the transport properties of YBCO has been investigated, using both light-ion irradiation of thin films to finely tune the amount of atomic disorder, and ultra-thin films grown to study possible dimensional effects. For weak disorder, the samples display a resistive transition typical of the mean-field paraconductive regime of an homogeneous media, well described by the Lawrence and Doniach model for layered superconductors. As the disorder increases, two effects take place. First, the c-axis coherence length becomes shorter, leading to a more anisotropic material, as shown by the excess conductivity above Tc. Second, an incipient granularity is revealed, leading to a less sharper transition, which is analyzed within the random 3D XY critical model for the paracoherence transition. Two main results are derived: and experimental test of the Ginzburg criteria for the paracoherence transition, and a new fluctuation regime in a nanometric grain size superconductors.

It's shown in the framework ofthe multiband p-d model that formal Ni2 ion state is not a S1 d8 ion but is a mixture of d9 5=1/2 and d'° 5=0 configurations. High spin contribution is eliminated by strong electron correlations. We assume that for both Zn and Ni substitution the concentration dependence of TN and T is determined by the diamagnetic contribution to the impurity state and this assumption is in a good agreement with experimental data on 1 :2:3 and 2: 1 :4 systems.

The number and variety of thallium based materials that can be made by in situ methods have grown consistently since the first report of successful thallium cuprate deposition by Face and Nestlerode in 1992. Processes for the deposition of superconductors, normal metals, and insulators have been developed. Most work to date has been done on the Tl-1212 phases, TlBa₂CaCu₂O₇ and (Tl,Pb)Sr₂CaCu₂O₇. REcently however, the in situ thallium technique has been extended to other materials. For example, epitaxial thin films of thallium tantalate, an insulator of the pyrochlore structure and a potential buffer layer for thallium cuprate films, have been grown. Multilayers, important in the fabrication of Josephson junctions, have been demonstrated with the thallium lead cuprates. This paper reviews progress in the area of in situ thallium deposition technology which will make more complex thallium cuprate multilayer structures and devices possible.

In this article, we describe some recent progress in understanding nucleation of high T_c superconducting cuprates during epitaxial film growth. A brief review is given of a number of film characterization techniques which can provide useful information to this end. Based on data from some of these techniques a nucleation model is discussed for the RBA₂Cu₃O_y (R equals Y or rare earth) compound. For growth on TiO₂ terminated (100) SrTiO₃, our nucleation model suggests a 1:3:3 cation stoichiometry for the c-axis nuclei. Various other substrates are also discussed.

Thin films and heterostructures of a new family of copper- oxide-based isotropic metallic oxides such as La_6.4Sr_1.6Cu₈O₂₀, La₅BaCu₆O₁₃ and La₆BaYCu₈O₂₀ have been investigated. These metallic oxides are oxygen deficient pseudo-cubic perovskites that exhibits Pauli paramagnetism, which could be ideal normal metals for SNS junctions in high T_c superconducting devices. We have grown epitaxial thin films of these metallic oxides and SNS superconducting heterostructures in situ by 90 degrees off-axis sputtering. X-ray diffraction and cross-sectional transmission electron microscopy reveal these heterostructures to have high crystalline quality and clean interfaces. This material will facilitate fabrication of ideal SNS Josephson junctions with low boundary resistance due to its excellent chemical compatibility and lattice match with cuprate superconductors and will be useful for determining the source of interface resistance in such heterostructures.

Large area deposition of YBa₂Cu₃O_7-(delta ) is desirable for cost-effective production of thin superconducting films at larger scale. Our technique of thermal co-evaporation should be particularly well suited for this goal because it is intrinsically homogenous. We have achieved large area deposition by using a rotating disk heater with an oxygen pocket. This arrangement allows for intermittent metal deposition and oxidation scheme with which we fabricate separated zones.Here we present a improved version of this deposition scheme with which we fabricate high quality YBCO films on an area which is 9 inches in diameter. The area is used for simultaneous deposition on smaller wafers, e.g. 12 wafers of 2 inches, or for large sapphire wafers of 8 inches.

The correlation between defect formation and carrier doping in epitaxial films of the infinite layer compound SrCuO₂ has been studied via molecular beam epitaxy controlled layer-by-layer growth experiments, chemically resolved scanning transmission electron microscopy,scanning tunneling microscopy, x-ray diffraction, electrical transport measurements, and post-growth oxidation-reduction annealing. Based on the complementary information provided by these experiments, it is concluded that the carrier doping is dominated by the formation of an electro-doped, Sr and O deficient matrix under mildly oxidizing growth conditions. Hole-doping is induced by extended defects containing excess Sr atoms and may lead to superconductivity after high- temperature oxidation.

We investigated the current-voltage characteristics and critical scaling behavior in c-axis oriented YBa₂Cu₃O_7-(delta ) (YBCO) films whose thicknesses are from 18 nm to 230 nm, in magnetic fields applied perpendicular to the film surface. The vortex-glass transition temperature (T_g) decreased as the thickness decreased, and wide temperature critical scaling region was observed in thinner films. We note that the reduction of T_g is similar to the reported result of the vortex-glass transition of narrow YBCO strip lines, and that wide temperature critical scaling region was also observed in the vortex-glass transition of Bi₂Sr₂Ca₂Cu₃O_x (Bi-2223) films. From the analogy to the study of Bi-2223 films, we suggest that, in the thinner YBCO films, the vortex-glass correlation length parallel to the c-axis (xi) _g// is limited by the film thickness, and that wide critical scaling region originates from the enhanced thermal fluctuation effect brought by the reduced correlation volume through the interruption of vortex correlation along c-axis.

The heteroepitaxial growth of high temperature superconductor thin films on single crystal substrates produces strained heterostructures when the mismatch is small and the thickness of the epilayer is not large. ALthough a large number of studies have been carried out in the case of semiconductor epitaxy, only a few papers report models or experimental results dealing with the relaxation of the elastic strain in cuprate heterostructures. We apply here the calculations performed for semiconductor epitaxial layers and pseudomorphic superlattices to estimate the critical layer thickness of cuprate thin films, heterostructures and superlattices. The result of these calculations is discussed with respect to the previously reported data and also to our results in the case of YBaCuO heterostructures.

The density of states (DOS) near the Fermi level has ben observed by scanning tunneling spectroscopy of La_2-xSr_xCuO₄ and La_2-xSr_xNiO₄ thin films. In La_2-xSr_xCuO₄, the DOS around E_F gradually appears with tailing both from the conduction band and the valence band edges accompanied with the increase of carrier concentration. Compared with cuprate thin films, the DOS variation of La_2-xSR_xNiO₄ with carrier increase is slow and no clear differences in the STS are observed in the region with 0.15 < x < 0.5 in the nickelate. However, the DOS of heavily doped La_2-xSr_xNiO₄ with x > 1.0 is quite similar to that of doped cuprates. Systematic investigation of superconducting transition temperatures (Tc) has been performed on the Bi₂O₂/SrO-inserted infinite-layer lattices, Bi₂O₂/SrO+(Ca_1-xSr_xCuO₂)_n with n equals 2 to 10. The n dependence of Tc varies depending upon the Sr/Ca ratio at alkaline earth metal sites. The observed results are explained by the prediction based on inter-layer effect between two-dimensional resonating valence bond sheets.

In situ accurate flux monitoring technique utilizing atomic absorption spectroscopy (AAS) for the molecular beam epitaxy (MBE) growth is described. The AAS signal data are calibrated using the inductively coupled plasma spectroscopic technology. The calibration curves are not influenced by variations of both the substrate temperature and ozone flux rate within the range of MBE growth conditions. SrO and CaO are epitaxially grown on SrTiO₃ substrates with RHEED intensity oscillations. This fact becomes another convenient and practically important method for calibrating the AAS data. A method for estimating the ozone flux is also discussed. A gas-laser-etching of Bi-Sr- Ca-Cu-O thin films is carried out by digitally counting an etch process loop. Laser irradiation and NF₃ gas-supply are separately made. At a laser fluence range the etch depth is independent of the laser fluence, suggesting that the etching is limited by the amounts of absorption gas molecules. The way how the etch depth depends on the etching loop count is intensively investigated. The depth is saturated after etching of a few loops. The surfaces of the etched area are flattened and smoothened. Possible models about the etch depth saturation and surface flattening are discussed.

Molecular beam epitaxial growth of cuprate oxide superconducting thin films using a mass separated, low energy, O⁺ beam source, is discussed. The mono-valent O⁺ ion is chemically, highly reactive, and its kinetic energy at 10's of electron volts, is significant. It allows the growth of REBa₂Cu₃O_7-(delta ) (REBCO) thin films at low pressures and temperatures. The effective over-pressure of the O⁺ ions at the substrate being 2 X 10^-7 Torr, and the optimum growth temperature 500 degrees C. These conditions are below the currently accepted stability line for the growth of REBCO thin films. We characterize the physical and chemical properties of the O⁺ ion beam, and its effect on superconducting thin films. We have grown highly crystalline BaO and EuBa₂Cu₃O_7-(delta ) (EBCO) thin films on SrTiO₃ substrates. The full width at half maximum of the rocking curves for BaO(002) equals 0.07 degrees and that for the EBCO (005) peak equals 0.05 degrees. Also, we found that BaO is a good insulating material (1.7 X 10¹³ (Omega) m at 4K), with an excellent lattice match to EBCO, therefore a suitable candidate as an insulating layer in multilayer structures. The results of the first growth studies of BaO/EBCO multilayers are discussed.

We have studied the flux-line confinement by a triangular and square array of submicron holes (antidots) in superconducting films (Pb, WGe) and multilayers (Pb/Ge). For large antidots, sharp cusp-like magnetization anomalies appear at the matching fields H_m, caused by the formation of multi-quanta vortices at each subsequent H_m. Critical current density j_c and pinning force f_p are strongly enhanced due to the pinning of single- or multiple-quanta vortices by the antidots. For relatively small antidots, new composite flux-line lattices, consisting of single- or multi-quanta vortices at antidots and single vortices at the interstices are observed at temperatures close to the critical temperature. The measurements of j_c and f_p for several antidot radii demonstrate that pinning centers with a size considerably larger than the coherence length are very efficient.

Heteroepitaxial growth has been studied with copper- and cobalt-based perovskite oxides using molecular beam epitaxy with purified ozone. At firs, oxidation of Co metal is investigated and its phase diagram under ozone ambient is experimentally determined. Next, we have tried to fabricate superlattices with two different combinations: Bi₂Sr₂CuO_6+(delta )/Bi₂Sr₂CoO_6.25 and SrCuO_2+(delta )/SrCoO_2.5. The former is revealed to be unsuitable for superlattice fabrication because of interdiffusion between Cu and Co, which is also confirmed in thicker bilayer films of Bi₂Sr₂Ca_n-1Cu_nO_2n+4+(delta )(n equals 1, 2, and 3) / Bi_{2$Sr(subscript m+1}Co_mO_y (m equals 1 and 2). In contrast, we can arrange B-site cations (Cu or Co) on an atomic-layer scale in the latter combination, in which we can change the B-site cations in every two layers.

Pulsed-laser deposition and epitaxial stabilization have been effectively used to engineer artificially-layered thin- film materials. Novel cuprate compounds have been synthesized using the constraint of epitaxy to stabilize (Ca,Sr)CuO₂/(Ba,Ca,Sr)CuO₂ superconducting superlattices in the infinite layer structure. Superlattice chemical modulation can be observed from the x-ray diffraction patterns for structures with SrCuO₂ and (Ca,Sr)CuO₂ layers as thin as a single unit cell. X-ray diffraction intensity oscillations, due to the infinite thickness of the film, indicate that (Ca,Sr)CuO₂ films grown by pulsed-laser deposition are extremely flat with a thickness variation of only approximately 20 angstrom over a length scale of several thousand angstroms. This enables the unit-cell control of (Ca,Sr)CuO₂ film growth in an oxygen pressure regime in which in situ surface analysis using electron diffraction is not possible. With the incorporation of BaCuO₂ layers, superlattice structures have been synthesized which superconduct at temperatures as high as 70 K. Dc transport measurements indicate that (Ca,Sr)CuO₂/BaCuO₂ superlattices are two dimensional superconductors with the superconducting transition primarily associated with the BaCuO₂ layers. Superconductivity is observed only for structures with BaCuO₂ layers at least two unit cells thick with T_c decreasing as the (Ca,Sr)CuO₂ layer thickness increases. Normalized resistance in the superconducting region collapse to the Ginzburg-Landau Coulomb gas universal resistance curve consistent with the two-dimensional vortex fluctuation model.

Through intercalation, we have successfully synthesized new materials having layer-by-layer stacking of superconducting and superionic conductor, Ag_xI_yBi₂Sr₂Ca_n-1Cu_nO_2n+4 (n equals 1, 2 and 3). According to powder x-ray diffraction analysis, the lattice expansions along the c-axis upon intercalation of Ag-I are approximately 7.3 angstrom independent on various x values. Superconducting properties were maintained after the intercalation with slight T_c depressions. In order to investigate the evolution of electronic and crystal structures of host compound upon Ag-I intercalation and to determine the intracrystalline structure of the intercalated silver iodide, extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) analyses have been performed. According to the nonlinear curve fitting for the Ag K-edge EXAFS spectra, the coordination number for silver in the Ag-I intercalate has been determined to be 4, which is in good agreement with the previous results for Ag⁺ ionic conductors. This strongly suggests that our new compounds may also have AG⁺ ionic conductivity, which has been measured by the a.c. impedance spectroscopy and the pulsed method. All the intercalates have been proved to be mixed conductors with substantial ionic contributions. For example, the ionic conductivity of Ag_xI_yBi₂Sr₂CaCu₂O₈ (x equals 1.09) is about 10^-1.5 (ohm cm)^-1 at 270 degrees C, which is comparable to those of other Ag⁺ superionic conductors such as AG⁺-(beta) - alumina and Py₅Ag₁₈I₂₃ (Py equals C₅H₅HN), with an ionic transference number of 0.40.

Thin films of YBCO were patterned down to the sub-micrometer scale by means of electro-beam lithography (EBL). A resist system based on amorphous carbon layers was developed, which allows the production of uncovered high-T_c superconductor microstructures. With this method we generated microbridges with a width of minimum 200 nm and a length of up to 5 micrometers. The layered structure of high T_c superconductors enables a further modification of the microbridges on the nm-scale into lateral weak-links by means of a scanning tunneling microscope (STM) using high tunneling currents in the range of nA and fast scanning modes ('etching'). This modification was carried out in N₂-atmosphere after the YBCO microstructures were sputtered in Ne-atmosphere. In UHV we did not observe any etching process. Using atomic layer-by-layer MBE we have prepared BiSrCaCuO thin films and vertical S-N-S junctions on SrTiO₃ substrates at 720 degrees C in 2(DOT)10^-5 mbar ozone pressure. 40 nm thick Bi-2212 films showed an inductively measured T_c of 84 K. Thinner films have T_c values of 64 K and 46 K for 15 nm and 10 nm thick films, respectively. We present in-situ-STM images of the surface topography and TEM investigations of the dependence of the substrate/film interface on the first deposited layer. The S-N-S junction was made with Bi-2201 as barrier material and showed quasiparticle tunneling dI/dU-U characteristic. We estimated 2(Delta) (0)/k_BT_c to 3.5-4 with a non BCS- like linear temperature dependence.

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.

A significant portion of the worldwide effort to develop high-T_c Josephson junctions for electronic applications has been devoted to superconductor-normal-superconductor (SNS) junctions. This paper expands upon our recent review of this subject. Numerous groups have reported nominally-SNS devices with a variety of geometries and normal interlayers. The electrical properties of these junctions have been interpreted almost exclusively with reference to conventional proximity effect theory, which describes the behavior of low-T_c SNS devices very well. In fact, however, almost all high-T_c SNS junctions are better understood by recognizing that transport occurs through unintended pinholes in the normal interlayers. It is only recently that proximity effect theory was successfully applied to any high-T_c device, despite vigorous but unsupported prior claims. The evolution of research on high- T_c SNS junctions is instructive in that it illustrates the danger inherent in assuming that device behavior can be interpreted by simply applying the most obvious theory connected with its intended structure.

The spatial distribution of supercurrent in high-T_c Josephson junction devices has been studied extensively using field modulation measurements of the critical current and microwave absorption. The devices are edge junctions composed of YBa₂Cu₃O₇-YBa₂Co_0.21Cu_2.79O₇-YBa₂Cu₃O₇. The l_c(H) patterns allow a quantitative Fourier transform analysis to obtain a self-consistent spatial supercurrent density distribution, J_s(x). These junctions are found to be more homogeneous than in most other high-T_c Josephson junctions reported to date.

High quality thin films of Bi₂Sr₂Ca₂Cu₃O₁₀ with critical temperatures of 95K were used to prepare grain boundary Josephson junctions on commercial 36.8 degrees SrTiO₃-bicrystal substrates. I_cR_n- products of 50 (mu) V at 77 K and 0.7 mV at 4.2 have been reached. For temperatures higher than 50K the current- voltage curves of the junctions can be well described by the resistively shunted junction model and show no hysteresis. From the hysteretic behavior at low temperature we estimate a junction capacitance of 21 (mu) F/cm². The Fraunhofer pattern of the critical current in an external applied field shows, that the junctions are inhomogeneous on a micrometers scale. The SQUID modulation of a 30 X 40 micrometers ² wide superconducting loop containing two 10 micrometer wide junctions yields a flux-voltage transfer function of 2.7 (mu) V/(Phi) ₀ at 78K.

There is a variety of different types of high-T_c Josephson junctions corresponding to the short coherence length, high anisotropy and some interface problems of the oxide superconductors. Using submicron technologies nanobridges and bridges modified by ion beams in a hundred nanometer region can be fabricated. Depending on preparation parameters the ion beam influence causes implantation, sputtering or a modification of the lattice changing the superconducting properties. The case of modification is discussed in details. It is shown how parameters of the preparation process influence the physical properties of these junctions. The application of such junctions is shown for DC-SQUIDs and gradiometers including a comparison to other junction types like bicrystal or step-edge junctions. Submicron technology is useful for preparation of intrinsic stacked junctions out of thin films. In this case the single junction dimension is determined by the coupling of two copper oxide planes in an atomic scale. A mesa structure acts as a series connection of a number of single junctions corresponding to the stack height. Preparation and physical properties of these types of junction arrays are given in detail. The possible application of such new kind of devices as radiation sources or voltage standard will be discussed.

A c-axis resonance mode has been observed directly in (formula available in paper) single crystal s using ac bolometric technique. The resonance is observed in the 30 to 90 GHz frequency range in the presence of an applied magnetic field H ((parallel)c). Below the irreversibility line T_irr(B), the magnetic field and temperature dependence of the resonance can be summarized as (formula available in paper) and (formula available in paper)are temperature independent but sample dependent parameters. Above (formula available in paper) decreases slowly with T. We report results for measurements carried out in an oblique magnetic field, where an unusual re-entrance cusp is observed for (formula available in paper). This provides firm evidence that it is the Josephson plasma mode. By fitting the data to a recent calculation by Bulaevskii et al., we obtain the anisotropy parameter (gamma) equals 420 and zero field Josephson plasma frequency 5.3 cm^-1.

The large anisotropy and the extremely short coherence lengths of the high-T_c superconductors suggest that the layered crystal structure is mapped onto a periodic modulation of the superconducting order parameter. Even ideal high-T_c single crystals consist of a stacked series of superconducting and non-superconducting layers. As the typical interlayer distance is approximately 15 angstrom, a single crystal of 3 micrometers thickness should behave like a stack of 2000 Josephson junctions. In order to access these atomic scale devices, vertical structures on single crystal surfaces and epitaxial thin films were fabricated. We present results of mesa structures and vertical step-edges.

In relation to our research for the fabrication of high-T_c Josephson tunnel junction composed of YBCO(S)/oxide insulator(I)/YBCO layers, two crystal engineering issues are presented and discussed on pulsed laser processing of oxide thin films. One is the epitaxial growth of highly crystalline and orientation-controlled YBCO films and the other is the molecular layer epitaxy of perovskite and rock salt oxides films. Quantitative results are presented on the crystal quality, surface atomic layers and morphology, and electronic properties of the films and junctions. Discussion will be made on such problems as the thermodynamics versus kinetics in the film growth, identification and control of the topmost atomic layers of substrates and growing films, and electronic state of high-T_c films based on the scanning tunneling and photoelectron yield spectra.

Stacked series arrays of intrinsic Josephson (IJ) tunnel junctions fabricated on the surfaces of Bi₂Sr₂CaCu₂O_8+x (Bi2212) single crystals by photolithography have been studied. Advanced technology with in-situ control of stack heights allows us to make a specified number (3- 2000) of junctions in the stack. The quasi-particle branch of an individual IJ junction has a well-developed gap structure. The gap value is approximately 12-13 meV at low temperature, which is approximately two times smaller than reported elsewhere. The temperature dependence of the gap deviates strongly from the BCS one. Proximity induced superconductivity of the Vi-O layers is one probable explanation for the reduced gap observed in the experiments. True superconducting contact obtained between the outermost Bi-O layer of a freshly cleaved Bi2212 single crystal and a Pb thin film is a supporting evidence for the Bi-O layers being superconducting. Multiple peaks are seen in the first derivative of the current-voltage characteristics at sub-gap voltages for all samples studied. The peak positions in voltage do not change with temperature and do not depend on the dimensions of the stack. Dynamic modulation of the tunnel density of states by low energy optical phonons and/or resonant tunneling through localized electronic states of the Bi-O and Sr-O layers are possible reasons for these observations.

In addition to the conducting CuO₂ (S) layers, most high-T_c superconductors also contain other conducting (N) layers, which are only superconducting due to the proximity effect. The combination of S and N layers can give rise to complicated electronic densities of states, leading to quasilinear penetration depth and NMR relaxation rate behavior at low temperatures. Surface states can also complicate the analysis of tunneling and photoemission measurements. Moreover, geometrical considerations and inhomogeneously trapped flux are possible explanations of the paramagnetic Meissner effect and of corner and ring SQUID experiments. Hence, all of the above experiments could be consistent with isotropic s-wave superconductivity within the S-layers.

MOS and MOSFET structures were constructed with a TiO₂ single crystal as a substrate. It was demonstrated that the n-type carriers injected by the applied gate field have a much higher mobility than the chemically doped carriers, by nearly two orders of magnitude. This result suggests that the intrinsic carrier mobility in TiO₂ may be substantially higher than usually assumed. Other MOSFET effects including the nonlinear optical effects are discussed.

We have studied the static I-V characteristics and vortex dynamics in stacked Nb/AlO_x/Nb Josephson junctions. In josephson junction stacks consisting of two junctions having identical maximum Josephson supercurrent I_c,I_c versus H characteristics deviating from the Fraunhofer pattern have been observed, implying that a structural phase transformation to a triangular vortex lattice occurs with increasing H. Interjunction coupling leads to splitting of the Swihart mode; which manifest itself as Fisk steps with different voltage spacings. When subjected to microwave radiation, the I-V curves of the stacked Josephson junctions exhibit new features which are absent in single junctions.

A ferroelectric-superconducting three-terminal device consisting of a YBCO base layer and a PbZr_xTi_1-xO₃ (PZT) gate has been developed. This ferroelectric-superconductor field effect transistor has non-volatility and retention behavior based on the memory effect of the ferroelectric gate. The FSuFET was characterized both by an admittance spectroscopy and by DC I-V measurements after polarizing the PZT gate with both positive and negative pulses. The J_c modulation of the YBCO channel by the gate polarization field has been found greater than 90 percent. The retention time longer than 10⁶ seconds has also been obtained.

Electrochemical techniques are exploited to fabricate conductive polymer/high Tc superconductor bilayer structures. SCanning electron microscopy and electrochemical techniques are utilized to characterize the electrodeposition of polypyrrole layers grown onto YBa₂Cu₃O_7-(delta ) films. In such hybrid polymer/superconductor systems, it is found that when the polymer is oxidized to its conductive state, the transition temperatures (T_c) and critical currents (J_c) of the underlying superconductor film are suppressed. Reversible modulation of the values of the transition temperatures of up to 50K are noted for these structures. Upon reduction of the conductive polymer layer back to its non-conductive form, both T_c and J_c are found to return to values close to those acquired for the underivatized YBa₂Cu₃O_7-(delta ) film. Moreover, measurements as a function of temperature of the polymer/superconductor interface resistance show dramatic decrease in this value at T_c. ALso, estimates of superconducting coherence lengths within the organic conductor samples suggest superconducting properties over macroscopically large distances within the organic materials can be expected. Collectively these results are consistent with the first observation of a conductive polymer proximity effect.

Fabrication and transport properties of three-terminal Josephson junctions are described. The devices are excellent Josephson junctions which can be switched and tuned with an applied gate voltage.

The main phenomena that may be responsible of radiation detecting mechanisms in superconductors are described in a first part of this paper. Several examples are given, ranging from broadband and sensitive bolometric devices to ultrafast nonbolometric infrared detectors. The second part is devoted to the study of various transition edge bolometers designed to be built at low cost. Polycrystalline zirconia substrates have been used, to grow YBCO films by both in situ and ex situ oxygenation after radiofrequency sputtering. The voltage responsivity at 10.6 micrometers wavelength has been studied with respect to the modulation frequency of the incident radiation, both experimentally and theoretically. A 2D thermal model has been developed, allowing to interpret the complex (amplitude and phase) experimental frequency responses of various devices. In particular, the amplitude response can be described as a succession of f^-1 and F^-1/2 segments. Noise measurements show NEP and detectivity values reaching a very satisfactory level for granular films.

On the basis of our discoveries of anomalous photoconductivity of insulators correlated with high-Tc superconductivity, we introduce novel concepts of 'superconductive optoelectronic devices'. We have proposed that one must be able to fabricate a new type of device by combining these photoconductors for the gate region and relevant superconductors for the source and drain regions, both effective below their Tc's. We have been continuing a series of our further experimental studies seeking actual possibilities by utilizing the basic substance Cu₂O for the gate material and superconductive LBCO, LSCO, and YBCO for the source and drain materials, e.g., YBCO/Cu₂O/YBCO. Here, we report an observation of resonant and hybrid emergences of photoconductivity of Cu₂O in the gate region peculiarly in conjugation with the high-Tc superconductivity utilized in the source and drain regions in superconductive optoelectronic devices. Microwave photosignals at 35GHz guarantee a high-speed operation of the device in the n-sec region. We feed these results in a Nano-engineering back to basic physics of oxide superconductor in order to shed a new light on substantial natures of the Cu-O based high-Tc superconductivity.

Au/SrTiO₃/YBa₂Cu₃O_7-x heterostructures have been prepared in situ by pulsed laser deposition onto (100) SrTiO₃ substrates. Electric field controlled devices were patterned by suitable metal masks applied successively during the deposition process. Electrical break-through, polarization and relative dielectric permittivity of the SrTiO₃ layer have been studied as a function of the applied electric field. Resistivity, superconducting transition temperature and critical current density of the YBa₂Cu₃O_7-x layer with a thickness of a few unit cells were strongly affected by the electric field. The device parameters response time, transconductance and voltage gain were investigated. THe response time was dominated by the RC time constant of the device in agreement with the mechanism of direct charge transfer by the electric field, but the devices did not reveal a real voltage gain over one.

Commercialisation of High Temperature Superconducting microwave circuits to communication systems depends on losses of HTS films exhibited at high frequencies. The losses must be much lower that those of conventional metallic films to compensate for the change of technology and need for cooling. Hence accurate measurements of surface resistance have been of primary importance almost since the discovery of High Temperature Superconductors in 1987. Resonant and direct transmission methods, used in the past for metals and Low Temperature Superconductors, have been employed for the microwave characterisation of YBaCuO, B1SrCaCuO and ThBaCaCuO films. After a copper cavity, a parallel plate resonator and a confocal resonator, recently a dielectric resonator has become the most popular technique for loss measurements of HTS films at microwave frequencies. The application of dielectric resonators (DRs) to characterisation of HTS materials is due to possibility of high sensitivity, resolution and big range of surface resistances that can be measured. This application followed use of dielectric resonators for measurements of surface resistance of metals [1-3] and of dielectric materials, benefiting from developments in Low Phase Noise Stable Oscillators and Filters. The first use of DRs to microwave characterisation of HTS materials was proposed (to the knowledge of authors) in 1989 by Fieduszko and Heideman [4]. Since then several groups [5-16] have developed variety of designs for this purpose. For the dielectric resonator technique the surface resistance of superconducting films, Rs, is calculated from the loss equation, similarly as for any other resonator technique, namely from (1) Q Q Qd Qrad where Q0 is the unloaded quality factor and Qd and Qrad are the quality factors related to conductor, dielectric and radiation losses respectively. O-8194-2071-9/96/$6.OO SP!E Vol. 2697 / 77 Conductor losses are directly determined by the surface resistance of superconducting parts and of metallic parts, and dielectric losses by the loss tangent of the dielectric rod according to: ._!:._ = + Rsmet Q A Amet —=pdtanö (2) where A, Amet and Pd are geometrical factors of the superconducting parts, the metallic parts and of the dielectric rod, tanö is the loss tangent of the dielectric and Rsmet is the surface resistance of the metallic parts. The factors A and Amet depend on the design and dimensions of the resonator. The constant Pd for resonators in the trapped state is approximately equal to unity, only weekly depending on dimmensions. The unloaded Quality factor Q0 is determined from the loaded quality factor, calculated from the transmission or reflection coefficients measured around the resonance. In general to achieve high sensitivity and high resolution of the dielectric resonator technique for measurements of surface resistance of HTS films, radiation and dielectric losses of the resonator should be small as compared to losses in conducting parts. Also losses in HTS films under test should be large as compared to losses in metallic parts of the resonator. To ensure small errors in the surface resistance values, Qfactors should be measured with high accuracy and geometrical factors should be known exactly. It is however often not feasible to fulfil all these requirements in practice. Usually a resonator's design, choice of dielectric or computational models used for calculation of geometrical factors may impose significant limitations on accuracy of a system for characterisation of HTS materials. This paper (not pretending to cover fully this complicated issue) discusses some problems related to errors which may be encountered when dielectric resonators are used for surface resistance measurements of superconducting films.

The temperature dependencies of electromagnetic penetration depth )a,b(T) and high-frequency losses ReoT) have been investigated using the radio-frequency (1-20 MHz) and microwave (30 GHz) surface impedance methods on 1 unit cell (UC) and 2UC-thick Y1Ba2Cu3 epitaxial films sandwiched between Pro6Yo.4Ba2Cu3O7_ epitaxial layers. It was observed that the A(T) component exhibits a clear break near the critical temperature T from a linear BCS-like dependence at low temperatures, and a maximum in Rec(T) close to the onset point of L1(T). The break is destroyed in a very small perpendicular magnetic field (' 1 mT). This feature exhibit saturation as a function of the magnetic field in the range 3-5 mT. At the same time, no influence of the same H on the Reo(T) dependence at microwave frequencies was detected. A huge increase in T (70% and 30% for 1UC and 2UC samples, respectively) as a function offrequency within the frequency range 1 MHz —30 GHz was discovered. All these features are explained and discussed in the framework of dynamic theories of BKT transition.

Strain in a superconductor, produced by the normal vortex core, can affect both static and dynamic properties of vortices. It causes an additional vortex-vortex interaction which is long-ranged as compared with finite but much stronger London interaction in the fields far below H_c2. The energy of this magneto-elastic interaction is calculated within London model. The role of strain effects in forming vortex lattice structure is demonstrated for YBa₂Cu₃O₇.

We have investigated the resistive transition of YBCO ultra- thin films (thickness from 5 to 50 nm) grown on MgO(100). The amount of disorder increases as the thickness is reduced, leading to a broad transition that can be described using a 3D weakly-coupled Josephson array. Below a critical thickness, this regime seems to dominate even the fluctuating part of the transition (paraconductive region), when the system undergoes a 3D-0D transition.

We report measurements of surface impedance in YBa₂Cu₃O₇ (YBCO) ultra-thin films and YBCO/PrBa₂Cu_3-xGa_xO₇ superlattices deposited by laser ablation on SrTiO₃ with different thickness d of the YBCO sublayer. We find that the T_c is unchanged in relaxed or strained structures while the width follows two distinct regimes: it is narrow and slightly decreases with d in the former and is broad and follows the law (Delta) T_c approximately (root)d in the latter. We propose a simple argument which explains these two regimes in terms of statistical fluctuations of the number of carriers respectively in single relaxed crystalline domains and in the whole strained sublayer.

We report a combined analysis of resistivity and x-ray diffraction rocking curve measurements on c-axis oriented YBA₂Cu₃O_6.9 films epitaxially grown on (100) SrTiO₃ and LaAlO₃ by ion-beam sputtering. We find that the growth-induced reduction of long-range lattice order in the films begins to depress superconductivity and normal conductivity at a critical value of lattice coherence length of approximately equals 10 and 5 nm for the two above types of substrates respectively. Evidence for disorder-induced localization is given by a deviation from linearity of the temperature-dependence of the resistivity which scales as the reduction of superconducting critical temperature. Similar nonlinear dependence observed in slightly reduced or lightly Co-doped samples suggests that the disorder in our films significantly affects the CuO chains. Our analysis of the paraconductivity term in the films gives evidence for the enhancement of the superconducting fluctuations by the disorder.

Nucleation of the superconducting phase in proximity coupled superconductor-antiferromagnetic (SC/AF) multilayers is studied theoretically. Assuming that both superconducting and antiferromagnetic metals are dirty the superconducting transition temperature T_c and upper critical fields, H_c2(parallel)(T) and H_{c2(perpendicular})(T), as functions from the systems parameters have been calculated. Comparison of the results for the SC/AF structures and for the SC/ferromagnetic multilayers shows that the values of the T_c H_c2(parallel)(T) and H_{c2(perpendicular})(T) are more sensitive to the ferromagnetic exchange field that to the antiferromagnetic one. The main difference in the values of the critical fields is obtained for the structures formed by thin superconducting layers. The finite effect on the superconducting properties of the multilayers does not depend on the strength of the magnetism only, but on the scattering mechanism of the electrons at the interfaces too. The advantages of the antiferromagnetic interaction for nucleation of the SC phase will be lost if nearly all Cooper pairs are destroyed due to the interface scattering.

We report results of measurements on the superconductor/insulator system consisting of alternating layers of amorphous Ta and amorphous Ge. This system is relatively complex owing to a very thin mixed interface layer in which the pairing interaction is stronger than in the Ta itself. Measurements have now been performed using a dilution refrigerator at temperatures down to 50 mK, and using magnetic fields of up to 14 tesla. The phase diagram of the upper critical field versus temperature has been investigated for various layer thicknesses with the field both parallel and perpendicular to the layers. The results are shown to conform with existing theories on layered superconductors. The nonlinear I-V characteristic of these multilayers is also analyzed in zero field for a sample with thin superconducting layers and evidence for a Kosterlitz- Thouless transition is observed.

Recently, superconductive Nd₁Ba₂Cu₃O_y (Nd123) thin films with high superconducting transition temperature (Tc) have been successfully fabricated at our institute employing the standard laser ablation method. In this paper, we report parts of the results of surface characterization of the Nd123 thin films using an ultrahigh vacuum scanning tunneling microscope/spectroscopy (UHV- STM/STS) and an atomic force microscope (AFM) system operated in air. Clear spiral pattern is observed on the surfaces of Nd123 thin films by STM and AFM, suggesting that films are formed by 2D island growth mode at the final growing stage. Contour plots of the spirals show that the step heights of the spirals are not always the integer or half integer numbers of the c-axis parameter of the structure. This implies that the surface natural termination layer of the films may not be unique. Surface atomic images of the as-prepared Nd123 thin films are obtained employing both STM and AFM. STS measurements show that most of the surfaces are semiconductive, or sometimes even metallic. The results of STS measurements together with the fact that we are able to see the surface atomic images using scanning probe microscopes suggest that exposure to air does not cause serious degradation to the as-prepared surfaces of Nd123 thin films.

Universal aspects of inhomogeneous transport was studied in natural Josephson junction networks. The weak link networks of polycrystalline high-T_c superconductors were found perfectly suitable for quantitative investigation of cluster growth phenomena in percolation (or fractal) networks. We report the experimental results of our studies of I-V characteristics of the two high-T_c families, YBCO and BSCCO. The onset of dissipation, represented by these characteristics, is interpreted as a critical phenomenon, i.e., as a current-induced phase transition. The cluster growth which underlies this phase transition is analyzed by the use of an appropriate model. The model links the non- Ohmic weak link network with classical Ohmic percolation networks (e.g., random resistor network). As a result, we were able to determine the values of appropriate dynamical exponents (exponent t with high precision, t equals 2 +/- 0.1, and exponent s approximately, s approximately equals 0.7), as well as to assign the dissipative ranges inside which the corresponding cluster dynamics takes place. It is therefore concluded that the high-T_c weak link networks represent a natural mesoscopic-scale system convenient for systematic investigation of the transport critical phenomena.

The pairing instability induced by either a given attractive interaction or particle-hole excitations is examined by using the perturbational diagram approach. A graphical functional derivative method based on Ward's identity is developed to represent the contributions to the interparticle interactions from different channels. It is shown that pairing instability for the electrons excited from the Fermi sea may happen due to the presence of holes even when the original interaction is repulsive. Thus, in contrast to the Cooper instability caused by a given attractive interaction, the pairing instability can be induced by the particle-hole excitations. The scattering diagrams from this model allows one to evaluate the two particle vertex function and T_c by using the pole condition of the vertex function. It has been found that T_c can reach quite high values and gives rise to the bell shape. The application of the results to a layered 2D Fermi gas is considered.

By means of the perturbative diagram approach the effects of vertex corrections on the charge and spin responses of the Fermi liquid are considered. It is shown that the interaction in the particle-hole channel induces the multipair excitations in both the charge and spin channels. Such excitations lead to the appearance of the anomalous Fermi liquid term Im(chi) approximately - (omega) /T in the charge and spin responses of the Fermi liquid in addition to the normal response approximately - (omega) /v_Fk. Such a contribution of the anomalous term to the charge and spin responses is in agreement with the marginal Fermi-liquid hypothesis proposed by Varma et al. to explain the universal anomalies in the normal state of the cuprate high-Tc superconductors.

We do not find evidence for strong fluctuation effects in the magnetic penetration depth of films of YBCO. While in YBCO crystals it has been found that (lambda) ^-3(T) is linear in T below T_c over a range of temperatures from 5K to 15K, (lambda) ^-2(T) is linear over a range of several degrees K just below T_c in YBCO and SmBCO films with very sharp transitions. At low temperatures, we find a T-linear behavior in (lambda) (T) in pure YBCO and SmBCO films. However, we do not find evidence that the T-linear behavior is due to thermal phase fluctuations which should increase in strength as (lambda) (0) increases.

The quest for new cuprates compounds exhibiting superconducting properties at elevated temperatures was intensified recently. The synthesis under high pressure led first to an increased Tc record of around 160K with Hg compounds, and second to new bulk compounds including Cu, CO₃ and infinite layer families. Meanwhile the results concerning thin films of new cuprates, even grown by atomic layering, were not as convincing. We describe here the growth of infinite layer related compounds with emphasis on the growth mechanisms. The deposition is performed in the range of 500 to 550 degrees C under atomic oxygen, using real time control by RHEED intensity. Various deposition sequences were used leading mainly to two basic families. The first one belongs to the infinite layer family, while the other one seems to belong to the spin ladder Ca_n-1Cu_n+1O_2n family. Transport properties in a wide range of temperatures are presented and discussed.

We have studied theoretically a superconducting system with the order parameter periodic in space. The period of such a superconducting superlattice (SS) should be of the order of the coherence length (xi) (T), so that it is a macroscopic value in comparison with the atomic scale. The spectrum of the condensate in such structures becomes of the band type with a finite number of energy bands. Splitting of the spectrum of the condensate into the allowed and forbidden energy bands starts from a certain threshold level of modulation of the order parameter by the superlattice. In view of periodicity of the system, the current states of the condensate in excited bands at higher values of energy can exist in principle along with the conventional Josephson current, corresponding to the lowermost energy band. The presence of the SS energy bands makes possible the transitions of the condensate as a macroscopic system from one quantum state to another. A set of equations in the framework of the Ginzburg-Landau scheme has been obtained and current-phase relations for a SS have been calculated. The crossover from the Josephson-like behavior of a SS (the high value of the constant (lambda) , characterizing suppression of the superconducting order parameter) to the homogeneous current-carrying state has been investigated. It has been shown, that as (lambda) decreases, the maximum superconducting SS current is displaced to a range of a lower value of the phase difference in the lattice period. We study the band states stability of the SS.