The cuprate superconductor problem is approached from the conventional metallic standpoint, valid at dopings near the Tc maximum and beyond. There is strong evidence that the Tc maximum corresponds to EF lying at a van Hove singularity, a special situation leading to Marginal Fermi Liquid behavior, and also to a minimum in the isotope shift, as observed. The superconducting properties are discussed in the light of phononic and electronic pairing models.

In the mixed state of high-temperature superconductors the dominant part of the Seebeck and Nernst effect is due to the thermal diffusion of quasiparticles and vortices, respectively. Our understanding of the Seebeck effect is based on the two-fluid counterflow model of Ginzburg and its extension to the mixed state with the presence of vortices. From the Nernst effect the transport entropy of the vortices is obtained. We summarize the recent thermoelectric and thermomagnetic experiments, paying also attention to the role of the Magnus force (Hall effect) and to the thermal fluctuation effects near TC.

Magnetization studies provide a facile and convenient means for investigating high temperature superconductors. This work provides an overview and some specific examples of the types of information attainable through magnetic experiments. Regimes to be discussed include the Meissner region with low applied magnetic fields; the mixed state in higher fields at high temperatures, where equilibrium superconductive properties are accessible; and the magnetically irreversible mixed state at lower temperatures, often with large critical current densities Jc. Generally, the critical currents in the last region are metastable. The associated flux creep phenomenon also provides insight into the materials problem of optimizing the pinning of vortices, in order to maximize Jc.

There has been considerable discussion on the exact nature of the pinning defects in the high temperature ceramic superconductors, and the YBa2Cu3O7-δ (Y123) structure in particular. High resolution parallel electron energy loss spectrometry (PEELS) allows for fine spatial resolution (< 17 nm) and fast acquisition times to probe the oxygen content of defects (i.e., dislocations, stacking faults, twin boundaries and grain boundaries), all which have been proposed as potential pinning sites or weak-links. Oxygen deficiencies up to 13% were measured at twin boundaries, and 20% at grain boundaries while stacking faults showed an increase in oxygen and copper content. Regions of low oxygen (0.5 < δ < 1) were usually confined to an area within approximately 50 nm of the defect, which is within the spatial resolution of the measurements. Examination of the fine structure of the oxygen K-edge suggests that the oxygen holes are absent at the twin and grain boundaries.

Single phase Tl-2223 samples were prepared in the TI2-x-zBa2Ca2+xCu3O10-y (z represent Tl vacancies in the range 0.22 – 0.27 and y represent oxygen vacancies in the range 0.17 – 0.33) system with x between 0 and 0.4. A small impurity phase was found at x = 0.6. All samples were found to be tetragonal. The Tc varied between 112 and 118 K for the “as-synthesized” samples. After post-annealing in a sealed quartz tube under vacuum at 750*deg;C for 10 days, the Tc increased from 116 to 122 K for x = 0.3 and from 118 to 120 K for x = 0,4 samples. Although x = 0.3 sample had small impurity phases after post-annealing, a large amount of impurity phases were found for x = 0.4 sample after post-annealing. Wet chemical analysis showed that all samples were both Tl and oxygen deficient. The Tc was found to increase with increasing hole concentration. The origin of the holes for all these samples is due to Ca2+ substitution on the TI3+ sites and overlap of Tl-6s band with the conduction band of the CuO2 sheets.

Ultrathin YBa2CU3O7 (YBCO) films were epitaxially grown on non-superconducting PrBa2CU3O7 (PrBCO) buffer layer by reactive evaporation, in which the crystal growth was controlled through the monitoring of reflection high energy electron diffraction (RHEED) specular intensity oscillations. Atomic force microscope (AFM) observation reveals that the steps in the surface of ultrathin YBCO films (∼100Aå) have one-unit-cell height, implying a unit cell-by-unit cell growth manner. One-unit-cell thick (1-UCT) YBCO layer exhibits superconductivity when charge reservoir layers of BaO-CuO-BaO are located above and below the CuO2 bilayer interposed with an Y layer. Essential transport properties of the normal state in bulk YBCO, such as T-linear dependence of resistivity and T-1 dependence Of Hall resistivity, are found to be inherent in the individual CuO2 bilayer. Study of diagonal and Hall resistivities provides evidence of Kosterlitz-Thouless transition to be occurred in 1-UCT YBCO.

Thin films of PrBa2Cu3O7 and in-situ superconducting YBa2Cu3O7 were grown on (100)-oriented SrTiO3 substrates by laser ablation under molecular beam epitaxial condition. The unit-cell by unit-cell growth of these high-Tc cuprate materials has been indicated by RHEED-oscillations and further confirmed by cross-sectional TEM (X-TEM) and small angle X-ray. Superconductivity with Tc (R = 0) at 35 K was observed for samples with one unit-cell YBa2Cu3O7 layers sandwiched between PrBa2Cu3O7 layers. TC varies with the number of YBa2Cu3O7 unit-cell layers. An eight unit-cell thick YBa2Cu3O7 film is superconducting at 86 K. Substrate temperature is found to be the most crucial parameter in determining the growth mode and superconducting properties.

Thin films of various Bi-Sr-Ca-Cu-O (2201, 2212,…, 2–2–11–12) compounds have been synthesized using atomic-layer-by-layer molecular beam epitaxy (ALL-MBE). Single-crystal films with excellent transport properties, smooth surfaces and atomically sharp interfaces in multilayer structures have been obtained. That made it possible to deposit virtually perfect ultrathin layers and superlattices, and fabricate hysteretic Josephson junctions from SIS multilayers.

To make Bi2Sr2CaCu2O8 superconducting films with top insulating SrTiO3, we use the molecular beam epitaxy technique (MBE) with in situ monitoring by reflection high-energy electron diffraction (RHEED). A new (RHEED)image enhancing technique, difference reflection high-energy electron diffraction (DRHEED) gave striking information on the growth process of each layer in the Bi:2212 compound as well as the rough and flat transition occurring during co-evaporated deposition of SrTi03.

Non-resonant microwave absorption is employed to probe YBCO/PrBCO superlattices and compare the response to that of a YBCO film. Near the transition temperatures, the response of the superlattice samples and the YBCO film have similar amplitudes and orientation dependencies. At lower temperatures, the response of the superlattices is much stronger than that of the YBCO film and, while both responses are hysteretic at low temperatures, the widths of the hysteresis have opposite orientation dependencies, which we attribute to the role of the PrBCO layers.

Epitaxial B1-structure TiN/NbN superlattices have been grown by reactive magnetron sputtering On MgO(001). X-ray diffraction and transmission electron microscopy (TEM) diffraction spectra exhibited up to nine orders of superlattice reflections, indicating that the superlattice interfaces were relatively sharp. TEM images also showed well-defined layers. The superlattice wavelength (∧) dependence of the superconducting transition temperature (Tc), critical Current density (Jc), and electrical resistivity (ρ) have been investigated. Tc values increased from 12 K to 17 K with increasing ∧. Jc in a magnetic field perpendicular to the film surface ranged from 104 to 106 A/cm2, increasing with increasing wavelength and decreasing with increasing applied magnetic field. Jc in a field parallel to the film surface was > 10 times higher, ≈ 107 A/cm2. The resistivity exhibited different ∧ dependencies in three different A ranges.

Reflection high-energy electron diffraction (RHEED) has been used to monitor the homoepitaxial growth of SrTiO3 buffer layer, and subsequent heteroepitaxial growth of YBa2CU3O7-δ (YBCO) films, on (100) SrTiO3 by pulsed laser deposition (PLD). The RHEED pattern during initial growth of SrTiO3 becomes sharper and increases in intensity, suggesting that the smoothness of the substrate is improved by growth of the buffer layer. On the smooth surface, the growth of SrTiO3 occurs in a layer-by-layer mode as indicated by the intensity oscillations of the specular beam. YBCO films have been subsequently grown on the buffer layer using a combination of pulsed molecular oxygen and a continuous source of atomic oxygen, with the average background pressure maintained as low as 1 mTorr. A sharp streaky pattern, and intensity oscillations are also observed during growth of YBCO. The YBCO films after cooling down in 1 atm of O2 are superconducting, with zero-resistance temperature of 80 K.

The Tc dependence on oxygen content was measured for YBa2Cu3O7-δ films grown with a variety of techniques (solid phase epitaxy, laser ablation, off-axis sputtering, co-evaporation) at oxygen pressures p(O2) ranging from 1.0 atm to 0.1 mTorr. Dissimilar dependences resulted for each film type, with Tc either increasing or decreasing for small increments in δ from maximum oxygen occupancy. Varying systematically with p(O2) during growth, the deviations are attributed to competing effects from hole-doping lattice defects (most likely on the Y-site) on the carrier density of the CuO2 planes and basal plane oxygen capacity, respectively, giving rise to overdoping or underdoping after low temperature oxidation in 1.0 atm of oxygen.

Single crystalline YSZ and Y2O3 thin films are grown on Si(100) by e-beam evaporation. The amorphous S1O2 surface layer is removed in-situ by initially growing at low oxygen partial pressures in the case of YSZ or by first evaporating metallic Y for the growth of Y2O3. Epitaxy occurs by a solid state reaction after the SiO2 has been reduced by metallic Zr or Y. For Si/YSZ/Y2O3 the growth is cube on cube while in the case of Si/Y2O3/YSZ the oxide layers grow twinned in (110) orientation. XPS analysis and AES depth profiles reveal the reoxidation of the Si during further growth. Critical temperatures of 90 K and high current densities of 3.2×106 A/cm2 are measured on 150 nm thick YBCO films on SOS/YSZ/Y2O3 proving the excellent quality of the YBCO and the underlying buffer layers.

PrBa2Cu2NbO8 (PrBCNO) is an insulating analog to the YB2CU3O7 (YBCO) system, which is interesting as a thin film buffer layer in YBCO/PrBCNO/YBCO tunnel junctions as well as a potential high Tc superconductor if proper doping could be achieved. PrBCNO has a similar structure to PrBCO, with NbO2 planes replacing the CuO chains. Single phase polycrystalline samples of RBCNO have been synthesized with R = La, Pr and Nd. Attempts to synthesize this structure with the smaller ions Y and Gd have been unsuccessful. The PrBCNO samples show a signature in the magnetization of a magnetic ordering at 12 K. No such magnetic phase transition is observed down to 2 K in the NdBCNO material.

Ion beam sputter-deposition has been used to produce high temperature superconducting (HTSC) thin films with controlled orientation. Room temperature scanning tunneling microscopy (STM) studies of ion beam sputter-deposited Y-Ba-Cu-O thin films indicate that the growth mode depends on whether the films are a- or c-axis oriented. The c-axis oriented films appear to grow by a screw dislocation mechanism, producing layered spirals similar to those observed in films grown by plasma sputtering and laser ablation-deposition. STM images of the a-axis oriented films show a growth mode which appears to produce layered structures perpendicular to the substrate with no spirals. Scanning tunneling spectroscopy (STS) studies of the a- and c-axis oriented films tend to reflect the anisotropy of the Y-Ba-Cu-O structure. Both the c-axis and the a-axis oriented films have semiconducting characteristics, possibly due to a native oxide, with a band gap estimated to be 1.4 eV. The c-axis oriented film, however, exhibits more fine structure in its density of states. This apparent anisotropie band structure reflects the anisotropie Y-Ba-Cu-O microstructure and superconducting characteristics. Investigations with x-ray photcelectron spectroscopy (XPS) establish a substantial chemical difference between the two surfaces inferring more substantial native oxides and air-induced by-products on the a-axis oriented film.

Epitaxial films of Ba2YCu3O7-δ (BYCO) as thin as 250 å A and with Jc's approaching those of the best in situ grown films can be formed by co-evaporating BaF2, Y, and Cu followed by a two-stage anneal. These results extend the work on films > 2000 Å thick by R. Feenstra et al. [J. Appl. Phys. 69, 6569 (1991)]. High quality films of these thicknesses become possible if low oxygen partial pressure [p(O2) = 4.3 Torr] is used during the high temperature portion cf the anneal (Ta). The BYCO melt line is the upper limit for Ta. The use of low p(O2) shifts the window for stable BYCO film growth to lower temperature, which allows the formation of smooth films with greater microstructural disorder than is found in films grown in p(O2) = 740 Torr at higher Ta. The best films annealed in p(O2)=4.3 Torr have Jc values a factor of four higher than do comparable films annealed in P2=740 Torr. The relationship between the T required to grow films with the strongest pinning force and p(O2) is log independent of growth method (in situ or situ) over a range of five orders of magnitude of P(O2).

The deposition of in-situ YBa2CU3O7-δ Superconducting films on polycrystalline diamond thin films has been demonstrated for the first time. Three different composite buffer layer systems have been explored for this purpose: (1) Diamond/Zr/YSZ/YBCO, (2) Diamond/Si3N4/YSZ/YBCO, and (3) Diamond/SiO2/YSZ/YBCO. The Zr was deposited by dc sputtering on the diamond films at 450 to 820 °C. The YSZ was deposited by reactive on-axis rf sputtering at 680 to 750 °C. The Si3N4 and SiO2 were also deposited by on-axis rf sputtering at 400 to 700 °C. YBCO films were grown on the buffer layers by off-axis rf sputtering at substrate temperatures between 690 °C and 750 °C. In all cases, the as-deposited YBCO films were superconducting above 77 K. This demonstration enables the fabrication of low heat capacity, fast response time bolometric IR detectors and paves the way for the use of HTSC on diamond for interconnect layers in multichip modules.

Epitaxial Fe-substituted YBCO films on LaAIO3 substrates prepared by pulsed laser deposition exhibit a large variation in Tc with the substrate temperature and the cool-down rate of the films in an O2 environment, immediately after deposition. For 9% Fe films deposited at 700°C, a rapid cool-down yields a Tc of 60 K while a slow cool-down of 10°C/minute gives 18 K; Tc goes to 0 at 11% Fe for the slow cool-down films. For 9% Fe films deposited at 770°C, Tc > 80K and is much less sensitive to the cool-down rate or additional anneals.

The epitaxial growth of YBa2Cu3Ox films on sapphire for microwave applications requires epitaxial buffer layers hindering interdiffusion. Therefore the epitaxial growth of YSZ, the layer sequence Y2O3/YSZ, and Y2O3 on (012) sapphire was examined. In addition the growth of MgO and srTiO3 on sapphire substrates and the growth of MgO and CeO2 on SrTiO3 substrates was studied by X-ray diffraction, X-ray pole figure measurement and RBS-ion channeling. With a combination of these buffer layers bi-epitaxial grain boundaries in YBa2Cu3Ox films can be fabricated in future.