Since the discovery of superconductivity in 1911, most of the tailoring of new superconducting materials have been based on empirical criteria such as educated substitutions inspired by the periodic table. This task has become very complicated with the advent of high Tc oxide superconductors. In particular, it would be difficult to envisage new superconducting systems different from cuprates and bismuthates. By analyzing as to how some superconducting materials were discovered, four strategies are proposed. These are based on: (1) band structure calculations, which may lead to systems different from known ones; (2) high resolution electron microscopy studies, which may lead to different structural arrangements within the same systems; (3) systematic studies of phase diagrams; (4) in-situ high pressure studies of the critical temperature of known systems, which may lead to cation or anion substitutions simulating at ambient the structure stable under pressure and responsible for the higher T_c. An example of the latter strategy is discussed in detail.

The scaling behavior of the normal state transport and magnetic properties of the underdoped cuprates as a function of T/T₀ is now interpreted as an evidence for the opening of a pseudogap at T* approximately T₀. To investigate the origin of the pseudogap, we have analyzed first the temperature dependence of the resistivity (rho) (T) of the novel Sr_2.5Ca_11.5Cu₂₄O₄₁ spin-ladder compound under hydrostatic pressure of up to 8 GPa. This (rho) (T) dependence has been successfully explained by assuming that the relevant length scale for electrical transport -- the inelastic length -- is given by the magnetic correlation length related to the opening of a spin-gap in a one- dimensional (1D) even-chain spin-ladder (1D-SL). As a next step, we compared the (rho) (T) curves of the SL compounds and underdoped cuprates. We have found that the (rho) (T) dependences of YBa₂Cu₄O₈ and underdoped YBa₂Cu₃O_x demonstrate a remarkable scaling with the (rho) (T) of the 1D-SL compound Sr_2.5Ca_11.5Cu₂₄O₄₁. This scaling implies that the pseudo-gap below T* in underdoped YBa₂Cu₄O₈ and YBa₂Cu₃O_x is the spin-gap in the even-chain 1D-SL formed at T less than T* in these materials. Therefore, at temperatures T_c(x) less than T less than T*(x), underdoped cuprates are effectively in a 1D ('stripe') regime. A Josephson-like coupling of these stripes leads to a recovery of a 2D regime at low temperatures T approximately T_c(x).

Important features of electrical transport in cuprate superconductors are discussed with emphasis on the observations of two distinct dissipative processes in the normal state. The significance of the Matthiessen's rule behavior of both scattering rates is highlighted. Recent evidence that the thermoelectric power is governed by both relaxation processes indicates that the second scattering rate is not related to cyclotron motion as previously conjectured. Transport measurements in the normal state of high T_c cuprate superconductors present a host of tantalizing anomalies. Most experimental probes of electrical and thermal transport properties, including the resistivity, Hall effect, and the thermoelectric power, show intriguing deviations from the behavior expected from a usual Fermi liquid metal. What makes the problem of normal state transport so compelling is that the peculiar features of these transport properties -- their deviations from Fermi liquid behavior -- all show simple systematics. The difficulty is that when each transport property is studied in isolation, its systematics are obvious, but there is no obvious relation between different experiments. What is needed is a unifying picture -- a single transport phenomenology which can reproduce such seemingly unrelated mysteries as the temperature dependence of the Hall effect and the two-component thermopower. The goal of this article is to discuss a number of experimental guides and limitations for the development of a normal-state transport phenomenology. Some features of experimental data, such as the observation of Matthiessen's rule are so simple that they are commonly overlooked. Others, such as the appearance of two distinct lifetimes in the thermoelectric power are quite new and have important consequences. It seems useful to assemble them here.

To answer the question of the main title, which concerns central aspects of the vortex behavior in multi-layered high- temperature copper-oxide superconductors (HTSC), in this paper we review some of our recent measurements of the thermal fluctuation effects on the magnetization around the superconducting transition of high-quality single-crystals of different HTSC families, with a different number, N, of superconducting CuO₂ layers per layering periodicity length, s. These results suggest that in highly-anisotropic multilayered HTSC the N successive pancakes in the closest N layers in s are strongly magnetically coupled but they are uncorrelated at longer distances. Therefore, they may be seen as an unique 'magnetic mille-feuilles' of N pancakes, with an effective thickness of the order of (N-1)-times the interlayer distance between the closest CuO₂ layers in s. In contrast, in these highly-anisotropic HTSC the Josephson coupling between the CuO₂ superconducting layers is very weak and the fluctuations of the Cooper pairs above T_c are strongly located in each CuO₂ superconducting layer.

The effect of spin-polarized injection on the superconductivity order parameter is investigated in a device consisting of YBa₂Cu₃O_7-(delta )/Au/Ni_0.8Fe_0.2 layers. A non-equilibrium theory which qualitatively agrees with the results of measurements made on superconductor/insulator/ferromagnet structures is presented. A quantitative analysis shows that this theory predicts injection currents that are several orders of magnitude too large. Recent results suggest that superconductivity in thin films can be strongly influenced by the injection of a spin- polarized current from a ferromagnetic material. The effect has been found to occur in both low T_c (Sn) and high T_c (YBa₂Cu₃O_7-(delta )) superconductors when either a conventional ferromagnetic metal, permalloy (Ni_0.8Fe_0.2), or a colossal magnetoresistive material were used as the source of spin polarization. Control experiments showed that unpolarized current from a nonmagnetic metal had comparatively little effect on the same superconductors. A phenomenological model, in which the energy gap of the superconductor is perturbed by the presence of excess spin polarized electrons, has been shown to qualitatively mimic the experimental results. However, an estimate of the current needed to significantly suppress the gap is shown to be several orders of magnitude larger than is observed.

The onset of dissipation in thin, tape shaped form of high-T_c superconductors has been studied by the use of differential resistance characteristics in absence or in small magnetic fields. The studies reveal two characteristic currents: the onset current defining a sharp onset of dissipation and the critical current, a parameter of power-law dependencies. A broad range of intermediate currents is characterized by an anomalous saturation at the very low level of dissipation. The observations are interpreted inside the percolation model for the onset of dissipation as a crossover from the range dominated by finiteness of the sample on the scale of current-dependent correlation length to the range of power laws that corresponds to scaling regime of critical phenomena.

The emphasis is on the study of the relationship between oxygen profiles, crystallographic structure and electrical and physical properties of the YBaCuO thin films formed in situ at the same and optimized conditions of sample deposition for the microwave applications and further submitted to the different conditions of sample cooling. Particularly interesting results are obtained by sample cooling at low oxygen pressure in the presence of oxygen plasma. The studies of the oxygen depth concentration profiles and of the oxygen contents are carried out using the Nuclear Reaction Analysis (NRA). These results are correlated, on one hand, with the measurements of the atomic composition and structure by RBS, XRD and TEM and, on the other hand, by the measurements of the electrical and physical properties Tc, Jc and microwave surface resistance Rs. All results are consistent with the idea that the fully oxygenated films are formed during in situ growth at T approximately 700 degrees Celsius. During the sample cooling the oxygen loss or uptake take place as a function of oxygen chemical potential. A huge oxygen overdoping is evidenced in the samples cooling down in the presence of the oxygen plasma. Moreover, the presence of 'easy paths' for oxygen diffusion in c-axis oriented thin films is evidenced. Their oxidation state strongly affects Jc and Rs properties in opposite to Tc one and is sensitive to the room temperature corrosion. The fundamental and applied consequences of these findings for the mechanism of thin film growth and for the properties of the films in the presence of cathodic plasma or laser plume are analyzed.

Measurements by our group (and others) of the thermoelectric power in the normal state of cuprate superconductors show that thermopower, like the resistivity and Hall angle, shows a systematic pattern intimately related to the superconductivity. In the strongly overdoped regime, the transport properties show relatively conventional metallic behavior: for example, thermopower is often approximately proportional to temperature. As the hole concentration is then decreased and T_c increases to its maximum value, the in- plane thermopower shows an approximately temperature-indecent shift upwards to positive values, retaining a similar temperature coefficient of around -0.03 (mu) V/K² for a wide range of cuprates. This continuity in the transition from more conventional behavior is easier to explain in the Van Hove scenario and by electron-phonon effects than by some exotic mechanisms for the origin of cuprate superconductivity. We also note that thermopower patterns surprisingly similar to that in the cuprates are seen in non-cuprate superconductors such as NbN, and also in the nonmagnetic cubic perovskite superconductor Ba_1-xK_xBiO₃ (which, however, shows a nonlinearity at around 150 K). The thermopower of fullerene, Chevrel-phase compounds, NbN and disordered metal superconductors shows a low-temperature change of slope in good agreement with electron-phonon renormalization, demonstrating that thermopower can in many cases be used as a probe of the electron-phonon interaction.

We have investigated a temperature dependence of the critical current density J_c(T) and a dependence of the energy barrier for vortex motion on current density U_eff(J) in several YBCO and TBCCO thin films. The measurements have been done using rings in a persistent mode and a scanning Hall probe to record J_c(T) and U_eff(J). The results revealed that both quantities exhibit a universal behavior governed by an intrinsic property of these superconductors; Josephson nanostructures in the a-b planes of YBCO and TBCCO.

Both the mathematical foundations and the connections with experiment of the filamentary model of high temperature cuprate superconductivity have improved greatly in the last two years. The filamentary paths are variationally optimized conductivity paths for electrons near the Fermi energy in the normal state. These paths, and the electronic basis states connected with them, account not only for the anomalously linear temperature dependence of the normal-state resistivities and Hall numbers, but also for the astonishing compositional coincidence of these linearities with the maximum in T_c. These anomalies appear to coincide with the metal-insulator transition. To understand the latter, the author has, for the first time, solved its simplest case, which occurs in semiconductor impurity bands (such as Si:P). There a careful analysis of the experimental data (some of which have been 'lost' for twenty years uncovered a phase diagram with a new intermediate 'X' phase which has properties very similar to those of the cuprate 'non-Fermi liquid' normal state. In the X phase the dopant-electron-phonon interaction is approximately 25 times larger than in normal metallic Fermi liquids. Together with the phase-diagram similarity this large factor strongly suggests that the filamentary model is the correct starting point for a non-effective medium percolative model of high temperature cuprate superconductivity.

Strongly correlated electrons in copper oxide planes are modeled by a random tiling of CuO₄ molecules at finite temperatures. This model is a non-perturbative extension of Gutzwiller's variational assumption. An effective one-particle theory is constructed through the use of a combinatorial transform to express the problem in momentum space, without averaging over occupation numbers in real space. Temporal correlations are lost, because of the Gutzwiller approximation, implemented by taking one kind of spins to be static. Thermodynamic functions can be computed at any temperature and filling. A Mott-Hubbard transition is found in doping, but cannot be crossed in temperature. The effective Fermi liquid can be strongly renormalized, though it does not break down. In the derivation of the model, a formal connection between projected hopping and pair confinement is established.

The quasiparticle excitation spectrum of an isolated vortex in a clean layered d-wave superconductor is calculated, using both the Bogoliubov-de Gennes and the quasiclassical Eilenberger equations. Analytical solutions are obtained within the model of step-variation of the gap function, adjusting the Caroli, de Gennes and Matricon approach for low- lying excitations in cuprates. A large peak in the density of states in the 'pancake' vortex core is found, caused by the two-dimensional and strong coupling nature of high-temperature superconductivity.

Self-limiting function of Bi element in connection with experimental results of sticking coefficients of constituent atoms of BSCCO is described. Molecular beam epitaxial growth of Bi₂Sr₂CuO_x (Bi2201) and Bi₄Ti₃O₁₂ (BIT) is demonstrated using this function. A further advanced method and its growth results for Bi₂Sr₂CaCu₂O_x (Bi2212) and Bi₂Sr₂Ca₂Cu₃O_x (Bi2223) growth are shown, where a chemical reaction between Bi2201 molecular layer and one or two monolayers of Ca and Cu deposited on Bi2201 is used. Estimation by means of X-ray diffraction, reflection high-energy electron diffraction, and atomic force microscopy shows excellent quality of the films obtained. Heteroepitaxial growth of BIT/Bi2201 using the self- limiting function of Bi is also done.

Two novel experiments, one concerned with the fast optical response related to Cooper pair breaking using femto-second laser pulses in YBCO films and the second related to uncorrelated dynamic distortions as a function of oxygen concentration for YBCO single crystals by ion channeling, have provided information that may be relevant for understanding the mechanism of superconductivity in high T_c superconductors. In the first case a dramatic resonant photon energy dependence around 1.5 eV in the pair breaking rate has been observed, suggesting that an intermediate excitation may be playing an important role in the depairing process. In the second experiment, ion channeling investigations in YBa₂Cu₃O_7-(delta ) ((delta) equals 0.05 to 0.8) crystals have clearly shown an evolution of the lattice from that of a Debye solid for (delta) equals 0.8 to one exhibiting significant lattice coherence even at much higher temperatures than T_c, with discontinuities which track T_c. These two experimental results are discussed in the light of different mechanisms including the formation of Zhang-Rice singlet or other mechanisms that lead to a non local hole.

Continuous single crystal (001)PbTiO₃ thin films under a step-flow growth mode were epitaxially grown on the miscut (001)SrTiO₃ substrate with miscut angle of 1.7 degrees. A planar magnetron sputtering system was used for the epitaxial growth. The film thickness was ranged from 5 to 250 nm. The surface of the sputtered PbTiO₃ thin films comprised periodic striped patterns with step lines and atomically flat terraces. The typical step height and terrace width were 3 nm and 200 nm, respectively. The partial oxygen pressure during sputtering deposition affected the stability of the step-flow growth mode. Lower partial oxygen pressure stabilized the step-flow growth mode resulting in a layer growth. The PT thin films include the stress due to the tetragonal lattice deformation.

We discuss technology issues in deposition of high temperature superconducting YBCO thin films by reactive evaporation. Several key components are necessary to make this a robust manufacturing process. The oxygen pocket heater is the most significant part and determines most of the repeatability and uniformity issues. Additionally, a reliable and repeatable measurement of the atomic beams is necessary for good control of the cation stoichiometry. Quartz crystal monitors (QCM) are currently used for rate control, although their current implementation limits them to rate control at the level of 5%. A tunable-diode laser sensor has been compared directly against a QCM and demonstrated to agree to 1% of the QCM flux measurements. Because of the ability of these new laser senors to measure the deposition flux in the sample position, they seem the most promising technology for vapor flux measurement and control.

Pulsed Laser Deposition (PLD) has developed from a fast but limited preparation tool towards a competitive thin film deposition technique. One of the advantages above other techniques is the possibility to growth at relative high background pressure, with a large freedom in choosing the kind of gas. In a number of applications, the gaseous species in the background pressure even are part of the elements to be grown. An evidence example is oxygen in the case of high Tc superconductors and giant magnetic resistors. However, the benefit of relative high pressures hampers the use of standard diagnostics and monitor techniques, e.g. Reflective High Energy Electron Diffraction (RHEED), used for thin film growth. With the possibility to use RHEED at standard PLD pressures it became possible to study the growth of oxide materials under different oxygen and temperature conditions. Here, we present the results on applying this technique on SrTiO₃, which can be grown in different growth modes, depending on temperature and oxygen pressure, during growth. Applying a modified etch technique to SrTiO₃ single crystals, we were able to grow homo-epitaxial SrTiO₃ in a real 2D growth mode. Additional to the usual information obtained with RHEED, another phenomena can be observed. The pulsed way of deposition, characteristic for PLD, leads to relaxations in the intensity of the diffracted pattern. This is caused by the mobility of the deposited material from a disordered distribution till an ordered one. These relaxation times give extra information about relaxation, crystallization, and nucleation of the deposited material. The results obtained from the intensity oscillations as well as relaxations, has led to a different approach to deposit these complex oxide materials, so-called interval deposition. In this contribution first results on this interval deposition will be presented.

YBa₂Cu₃O_7-x (YBCO) thin films have been grown on vicinal cut SrTiO₃ (001) single crystals. The films show a pronounced anisotropic resistivity and flux pinning properties in the substrate plane. UHV scanning tunneling microscopy and transmission electron microscopy have been used to investigate the substrate surface, film morphology and the growth induced defect structure. The anisotropy is caused by planar defects generated via self-organization of the YBCO which lead to an exceptionally large critical current density up to 8 X 10¹¹ A/m² at 4.2 K. The films are partially detwinned as a consequence of the modification of the growth mode by the terrace-type surface morphology due to the vicinal cut of the substrates.

This paper describes our recent progress in MBE growth and the surface/interface investigation of high-T_c superconducting films. The careful XPS analyses of the in-situ surface and interface reaction for MBE grown (Nd,Ce)₂CuO₄ films demonstrate that correct oxygen stoichiometry is the most crucial issue in preparing high-quality films and improving the characteristics of tunnel junctions. In other words, oxygen nonstoichiometry easily occurs at the surface or interface in cuprates. By utilizing this fact in a positive way, we have succeeded in preparing of the highest quality La₂CuO_4+(delta ) films ever and have discovered a new superconducting Ba₂CuO_4-(delta ) film by a simple and efficient oxidation process using ozone in a vacuum chamber.

Recent findings in the experiments on high T_c superconductors by angle resolved photoemission are discussed. Detailed temperature and doping dependent lineshape studies on cuprates lead to a new interpretation of the photoemission data. The broad photoemission peaks which often extend to the Fermi level when they are a few hundred meV away from the Fermi level are not single states with very large broadening. Rather, the broad structure should be interpreted as a collection of states. It is found that the low energy part of this structure is involved in the pairing and, contrary to the conventional superconductors, the pairing occurs not only near the so called Fermi surface but also away from it.

Experiments are enlightening that the high T_c superconductivity in cuprate perovskites takes place in a particular phase showing segregation of localized and itinerant charge carriers. We have studied topological features of the Fermi surface of Bi₂Sr₂CaCu₂O₈+d (Bi2212) superconductor to investigate implication of charge segregation in the system. For the purpose we have used constant initial state angle-scanning photoemission supported by the energy distribution curves. The resulting Fermi surface measured over an extended Brillouin zone shows broken segments with partial gaps around ((pi) ,O) and shadow bands around (0.5(pi) ,0.5(pi) ) and equivalent locations. In addition data provide an evidence for new electronic states at the Fermi surface due to a one dimensional band with a small dispersion and small k_F along the ((pi) ,O) direction. We argue that the depression of photointensity around ((pi) ,O) and equivalent locations with well-defined k_F and one dimensional band are related with charge ordering in the CuO₂ plane along the (-(pi) ,(pi) ) direction and spin fluctuations along the ((pi) ,O) direction.

We report on ferroelectric field effect experiments in epitaxial oxide heterostructures consisting of the ferroelectric oxide Pb(Zr,Ti)O₃ and the metallic oxides GdBa₂Cu₃O₇ and SrRuO₃. To perform the experiments, we used conventional capacitor structures, as well as a scanning probe approach that allows one to control the local ferroelectric polarization without the use of permanent electrical contacts. In the case of the scanning probe approach, nanometer scale control of the ferroelectric domain structure can be achieved over large areas of up to 2500 micrometer². Nonvolatile, reversible electronic nanofeatures were written in Pb(Zr_0.52Ti_0.48)O₃/SrRuO₃ heterostructures by switching the local polarization field of the ferroelectric layer, inducing a field effect in the thin (30 angstrom) SrRuO₃ layer that changes its sheet resistance by 7%. This doping technique permits one to write reversible, nonvolatile electronic structures without requiring traditional lithographic processing or permanent electrical contacts.

We have grown epitaxial films of Sr₂RuO₄, Ba₂RuO₄, and Sr₂TiO₄ and studied their structural and transport properties. These phases are all layered perovskites with the K₂NiF₄ structure. Sr₂RuO₄ is the only known superconducting layered perovskite that is free of copper (T_c equals 1.35 K in bulk single crystals). In light of the observation that hydrostatic pressure reduces the T_c of Sr₂RuO₄, we have investigated the effect of replacing Sr²⁺ with the larger Ba²⁺ ion. In bulk samples, Ba₂RuO₄ is not isostructural with Sr₂RuO₄ when synthesized at atmospheric pressure. However, using pressures of 65,000 atm., the synthesis of a tetragonal form of Ba₂RuO₄ (with the K₂NiF₄ structure) has been reported. We have grown epitaxial films of the tetragonal form of Ba₂RuO₄ through the use of epitaxial stabilization. In addition to studying these ruthenates that are isostructural to the high-T_c superconductor (La,Sr)₂CuO₄, we have also investigated La-doped Sr₂TiO₄. Doped SrTiO₃ is a well-known superconductor, but the effect of reducing the dimensionality of SrTiO₃ from 3-D to 2-D (i.e., Sr₂TiO₄) has never been reported. The Sr₂RuO₄ and Ba₂RuO₄ films have been grown by pulsed laser deposition (PLD); Sr₂TiO₄ films have been grown by reactive molecular beam epitaxy (MBE). The growth conditions yielding phase-pure films have been mapped out. The orientation of the epitaxial films has been controlled by choosing appropriate substrates. Resistivity versus temperature measurements show that all three materials exhibit metallic conductivity. A resistivity, (rho) varies direct as T² has been observed in many samples at low temperatures, T. However, superconductivity has not been observed in any samples. In addition to growing Sr₂TiO₄, the n equals 1 member of the Sr_n+1Ti_nO_3n+1 Ruddlesden-Popper homologous series, we also used the atomic layering capability of MBE to grow phase-pure, epitaxial, thin films of the n equals 2 - 5 members of this homologous series (i.e., Sr₃Ti₂O₇, Sr₄Ti₃O₁₀, Sr₅Ti₄O₁₃, and Sr₆Ti₅O₁₆).

We have investigated the differences in (magneto-)resistance, magnetism and morphology of thin films of La_0.73Ca_0.27MnO₃, grown by sputter deposition in high oxygen pressure on substrates of either SrTiO₃ or LaAlO₃. We find that growth on SrTiO₃ is epitaxial and sooth, in contrast to growth on LaAlO₃, where islands are formed at low coverage, leading to rough and disordered films at higher coverage. This type of deposition appears to lead to different film morphology than films grown by laser ablation.

The complex ac sheet impedance of c-axis oriented YBa₂Cu₃O₇ thin films was measured as a function of temperature in the frequency range 10²Hz - 10⁵ Hz for film thicknesses varying between 24 Angstrom and 1100 Angstrom. We present the measurements on the thinnest films and examine their consistency with the predictions from various models for the superconducting transition, in particular the Kosterlitz- Thouless (KT) theory for the vortex-antivortex unbinding phase transition.

Recent results showed that a relationship exists between a large modulation of the critical temperature and small variations in the lattice parameters for strontium-doped La₂CuO₄ '214' thin films grown by molecular beam epitaxy under compressive or tensile strain. In this paper we search for the subtle deviation of the atom positions responsible for the observed behavior. We compare our findings regarding charge transfer, anisotropy, in-plane resistivity, and superfluid density with trends observed among the family of the one-CuO₂-layer compounds. This systematics leads us to the following conclusions: (1) increasing the distance between the charge reservoir and the CuO₂ plane is essential to increase T_c, (2) the electrical coupling from one CuO₂ plane to the next CuO₂ plane may not be essential for the mechanism, (3) a sufficient density of carriers is present in the underdoped state but the appearance of superconductivity is hindered by a large scattering rate, and (4) this scattering rate is determined by the two-dimensional confinement of the carriers in the CuO₂ planes.

YBaCuO thin film surfaces have been studied with an original laboratory-made attachment associated with a commercial atomic force microscope. Using a doped silicon probe coated with doped diamond, we have obtained simultaneously topographical and local contact resistance surface images within a given area of the sample. YBaCuO films on various types of substrates were observed: polycrystalline yttria-doped zirconia (PYSZ), and MgO or SrTiO₃ single crystals. For YBaCuO films grown on PYSZ, the electrical image has clearly revealed the presence of electrical disconnection zones between grains, which correspond to grain boundary areas observed on the topographical image. The presence of such defects can explain the modest critical current density (J_c approximately equals 3 X 10⁴ A/cm² at 77 K) measured on these granular films. On the opposite, for films grown on single-crystal substrates, the electrically connected areas between grains are visible on the electrical images, that can be correlated to better electrical transport properties of the films. Moreover, island-shaped grains exhibiting terraces of one unit cell vertical height could be seen, on close inspection of films grown on PYSZ and MgO substrates. For YBaCuO elaborated on SrTiO₃ substrates, the observed grain structure rather exhibited a spiral shape.

The crystallinity and physical properties of complex oxide films are strongly influenced by the quality of the substrate. This is determined by the bulk microstructure (e.g. grains, twin boundaries, vacancies) and by the surface characteristics. In the case of single-crystal films grown on single-crystal substrates, the latter point becomes extremely important and has to be studied in detail at the nanometer scale. In the case of SrTiO₃ (001) substrates, the fabrication process usually leads to samples with different surfaces. The topography itself is very sensitive to process parameters, and has exhaustively been studied in the past few years. The terminating layer can be a mixture of both SrO and TiO₂ planes. Only few techniques can determine this surface chemical composition, and so far only on a macroscopic scale. We report here, for the first time, the chemical characterization of such a surface, for which we use a combination of annealing and scanning probe microscopy. We then applied the same technique to the characterization of the surface LaAlO₃ (001) and SrLaAlO₄ (001) substrates. So far, no friction contrast has been observed on these latter surfaces. In the case of LaAlO₃, the characteristic twin structure is clearly revealed.

A model based on the diffusion of the vortices is proposed in order to describe the flux creep and the flux flow regimes in high Tc films in which a bias current flows when no large magnetic field is applied. We show how it is possible, from the results of the model, to estimate the effective superconductive thickness of the samples and the main quantities characterizing the flux creep regime. In the flux flow regime, the model accounts for the non-linear current- voltage curves which are actually measured.

Superconducting HgBa₂CaCu₂O_x thin films were epitaxially grown on (100) LaAlO₃ substrates with high phase purity and predominant a-axis orientation by conventional two-step method with modification. Control experiments of crystal orientation of Hg-1212 thin films were carried out by varying the quenching temperatures; 800, 700, 600, and 500 degrees Celsius, during cooling cycle. Optimal quenching temperature was found to be 700 degrees Celsius for the epitaxial a-axis oriented Hg-1212 films. The films quenched from 800 degrees Celsius reveal a mixture of a- and c-oriented grains in approximately equal amounts. However, the highly a-axis oriented films were acquired by quenching at 700 degrees Celsius. With lowering of quenching temperature to 600 degrees Celsius, the decomposition of a-axis grains was observed. Upon further decreasing the quenching temperature below 500 degrees Celsius, the predominantly c-axis oriented films were obtained. The films quenched at 700 degrees Celsius exhibit a zero-resistance superconducting transition temperature greater than 120 K which is comparable to epitaxial c-axis oriented films. In-plane epitaxy was confirmed by x-ray diffraction pole figures.

We compare the tunneling behavior of artificial Y(Ho)BaCuO- based HTS-I-HTS trilayers with our previous data obtained in YBaCuO/Pb junctions with natural barriers. Y₂O₃, SrTiO₃ and PrBa₂Cu₃O_7-(delta ) have been used as insulating layers in the HTS-based heterostructures that, investigated by X-ray and HREM TEM analysis, all showed good structural properties. However, we cannot exclude spurious effects due to the critical interface formation with the Y(Ho)BaCuO superconducting electrodes in the tunneling characteristics of the Y₂O₃, SrTiO₃ trilayers. For the YBa₂Cu₃O_7-(delta )/PrBa₂Cu₃O_{7- (delta} )/HoBa₂Cu₃O_7-(delta ) junctions, we have found quasi-ideal conductance characteristics with maxima at V equals plus or minus 40 mV. These values well relate to the plus or minus 20 mV structures measured in YBaCuO/Pb junctions with natural barriers. The low values of the G(O)/G(100 mV) approximately equals 0.05 and the flat background conductances measured in the artificial junctions, seem to confirm better defined interfaces in these structures with CuO₂ planes facing the barrier insulating layer.

The use of silicon as substrate for thin film devices based on high temperature superconducting oxides requires additional buffer layers to prevent interdiffusion, lattice mismatch, and internal stress by different thermal expansion coefficients. We tested different materials like yttrium-stabilized zirconia (YSZ), CeO₂, and CoSi₂. Laser deposition of a double buffer system YSZ/CeO₂ gives best results for silicon substrates up to 2 inch wafers. In this way the superconducting YBa₂Cu₃O_7-x (YBCO) films can reach a zero resistance temperature near 89 K and critical current densities at 77 K of up to 7(DOT)10⁶ A/cm². Additionally a nonsuperconducting but crystalline phase with the same stoichiometry (YBCO*) is used as passivation layer. Based on this technology we realized and investigated step- edge as well as new silicon bicrystal Josephson junctions, superconducting quantum interference devices (SQUIDs), bolometers using different compensation principles, and a new hybrid magnetometer. The hybrid magnetometer based on a simple Hall sensor was integrated with a superconducting antenna loop on the same chip.

Due to the non-steady state nature of film growth by laser MBE, as well as by the conventional pulsed laser deposition (PLD), we could find an advantage of the method for controlling the film growth. The independent optimization of nucleation and growth processes is possible by pulse sequence and laser energy density. Another critical factor for controlling the growth mode was revealed to exist in the surface state of the substrate as the stating point of film growth. We have developed a wet etching method for automatically finishing SrTiO₃ substrate surface to facilitate the layer by layer growth of high Tc and other oxide thin films. The sequential deposition of SrO and BaO monolayers on the treated SrTiO₃ substrate greatly reduced the nucleation of precipitates in the YBa₂Cu₃O₇ (YBCO) thin films. Thus, PLD growth of YBCO thin film could be controlled on an atomic scale. Temperature gradient deposition achieved the orientational control of YBCO thin films on (100) and (110) SrTiO₃ substrates. Laser MBE successfully produced high quality epitaxial oxide films by 2-dimensional layer by layer manner as well as by step-flow mode, as verified by the sharp RHEED intensity oscillation. This dimension control epitaxy enabled us to fabricate oxide superlattices and quantum wires. With the substrate treatment and buffer layer techniques, together with suitable choice of insulating layer material, we have verified the fabrication of superconducting-insulating-superconducting trilayer which is the prototype for tunneling junctions. A possibility of using (110) oriented YBCO thin films for Josephson THz laser is also discussed.

High-Tc Josephson junctions have been fabricated by a focused Ga-ion beam (FIB) with a full width of half maximum of 10 nm. Using two functions of etching and deposition, two types of the junction structures have been made on MgO and SrTiO₃ substrates. The first one is a grain boundary junction formed on the narrow modified area which is ion-irradiated on the substrate prior to film deposition. For this fabrication process, RSJ-type I-V characteristics are observed only on the MgO substrate. The Ic spread of working 30 junctions, (sigma) , is plus or minus 59% on the substrate of 1 cm². The second one is a proximity-effect junction with Au barrier layer. The tunneling area has been formed by etching and deposition technique of high-resolution FIB after film deposition. This junction also shows RSJ-type properties. The first type junctions are applied for some integrated circuits, using advantage of freedom of choice in the position of each junction with the high accuracy of the ion-beam control.

The properties of YBa₂Cu₃O_7-x grain boundary Josephson junctions have been reproducibly modified by a focused electron beam irradiation. The original junctions were fabricated by using the property of YBa₂Cu₃O_7-x film to grow (103)-oriented on the bare (110) SrTiO₃ substrate and (001)-oriented on the part of the substrate with the MgO seed layer. The junction parameters can be adjusted controllably by applying an appropriate irradiation dose. Electron irradiation reduced the critical current of the junctions I_C and increased the normal state specific resistivity. The shift of the voltage position of the Fiske steps was also observed.Isothermal annealing partly restores the original junction properties. A correlation between the transport properties and the microstructure was obtained by determining the ratio of a barrier thickness to the dielectric constant of the junctions with different barriers. These results give evidence of the role of the oxygen content and the dielectric constant of the interface region in transport phenomena. The experiment also demonstrates frequency tunability in a resonant soliton oscillator.

Josephson tunnel junctions between a conventional superconductor (Pb) and a High Tc material YBa₂Cu₃O_7-(delta ) (YBCO) have been reproducibly obtained in a film based planar geometry. Using a quasi-MBE method to grow high quality thin films, and making in-situ Pb/YBCO junctions along different crystallographic orientations, we studied the Josephson coupling along the (001) and (103) directions, together with the excitation spectra through quasi-particle tunneling. We evidenced that the coupling along the c- direction is small but finite, compatible with a small s-wave component in YBCO, and that a Zero Bias Conductance Peak is present when tunneling in the ab-plane, as expected for an Andreev Bound State (ABS) at a surface of a dominantly d-wave superconductor. The role of twin boundaries in c-axis experiments is discussed, and the coupling between the ABS with the Josephson current as well.

The discovery of high-temperature superconductors (HTS) and associated expectations of application of these materials in ultrafast electronics and optoelectronics has created an urgent need for a better understanding of carrier dynamics in HTS, including the HTS electrical and optical properties and their response to pulsed, external electromagnetic perturbations. The above goals were accomplished via comprehensive transient photoexcitation measurements of light- induced nonequilibrium phenomena in high-quality, epitaxial YBa₂Cu₃O_7-x (YBCO) thin-film microbridge samples. The photoresponse from less than 100-fs laser pulses (390-nm wavelength) was measured using a subpicosecond electro-optic sampling system, in both the microbridge superconducting (flux-flow) and the resistive (switched) states, at temperatures ranging from 20 K to 80 K. The physical origin of the signal was attributed to the nonequilibrium electron heating effect, in which only electron states are perturbed by laser radiation, while the film phonons remain in thermal equilibrium. From the observed single-picosecond electrical transients, we were able to extract the characteristic electron thermalization and electron-phonon relaxation time constants to be 0.56 ps and 1.1. ps, respectively. The above quantities were essentially independent of temperature within our temperature-testing range, in agreement with the two- temperature model. The ratio between phonon and electron specific heats in YBCO was determined to be 38. The nonequilibrium kinetic-inductive response was also measured, fitted into both the two-temperature and Rothwarf-Taylor models, and compared to the predictions of s- and d-wave pairing mechanism models. No phonon trapping effect (typical for low-temperature superconductors) was observed in YBCO; thus, the quasiparticle lifetime was given by the quasiparticle recombination time and estimated from the Rothwarf-Taylor equations to be unphysical low, far below 1 ps, and approximately 2.5 ps from the two-temperature model. All the presented characteristic time constants must be regarded as the intrinsic response of a YBCO superconductor; thus, hot-electron HTS photodetectors should exhibit intrinsic bit rates exceeding 100 Gbit/s, making them one of the fastest optoelectronic switches, well suited for digital and communication applications.

Small volume high-T_c super-conducting YBa₂Cu₃O₇ (YBCO) thin films are used as low power, very wide bandwidth mixers in the frequency range of 75 GHz to 2.5 THz. The YBCO films are patterned into lattice-cooled hot-electron bolometers (HEB) coupled to an integrated Au thin-film antenna and transmission line. Near 77 K, these mixers have responsivity as high as 780 V/W using only 8 nW of local oscillator (LO) power at 585 GHz. The responsivity can be shown to be truly bolometric. Direct heterodyne and homodyne down-conversion mixing using local-oscillator frequencies of 75 GHz and 585 GHz show overall conversion gains of -35 dB, which includes a -18 dB coupling loss, using only approximately 1 (mu) W of LO power. The gain bandwidth shows a simple Lorentzian roll-off with -3 dB point of 5 to 8 GHz. The large gain bandwidth and small power requirements make these high-T_c superconducting mixers an attractive alternative to existing Schottky diode and conventional superconducting receiver technologies.

Among the two categories of superconducting detectors that are actually operational for competitive performance in the submillimeter-wave/terahertz range, SIS devices used as heterodyne mixers offer very low noise level, but are frequency limited by the superconducting energy gap. Bolometric detectors on the other hand, although of lesser performance a priori, have no intrinsic frequency limitation due to their purely thermal sensing principle. Moreover, their inherent slow response can be overcome by developing hot- electron bolometer technologies based on superconducting nanostructures, that allow promising output frequencies of several GHz. In order to implement competitive submillimeter wave detectors, wideband planar antennas are preferred to improve the radiation to device coupling, rather than conventional bolometric structures (of either monolithic or composite type) that do not allow both sensitive and fast detector operation. All these aspects are commented and many realizations covering a large number of technologies, in view of both homodyne and heterodyne detection, are reviewed, commented and discussed.

Superconducting film and substrate interface effects on the response of superconductive edge-transition bolometers are modeled with a one dimensional thermal model in closed form, for samples with large area patterns compared to the substrate thickness. The results from the model agree with experimental results on samples made of meander line patterned granular YBCO films on crystalline substrates, in both the magnitude and phase of the response versus modulation frequency up to about 100 KHz, the limit of the characterization setup. Using the fit of the calculated frequency response curves obtained from the model to the measured ones, values of the film- substrate and substrate-holder thermal boundary resistance, heat capacity of the superconducting film, and the thermal parameters of the substrate materials could be investigated. While the calculated magnitude and phase of the response of the SrTiO₃ substrate samples obtained from the model is in a very good agreement with the measured values, the calculated response of the LaAlO₃ and MgO substrate samples deviate slightly from the measured values at very low frequencies, increasing with an increase in the thermal conductivity of the substrate material. Using the fit of the calculated response to the measured values, film-substrate thermal boundary resistances in the range of 4.4*10^-3 to 4.4*10^-2 K-cm^-2-w^-1 are obtained for different substrate materials. The effect of substrate optical absorption in the response of the samples is also investigated.

The microstructure of La_1.9Sr_0.1CuO₄ thin films grown by molecular beam epitaxy on SrLaAlO₄ and SrTiO₃ is investigated by transmission electron microscopy. Using SrLaAlO₄ as substrate material, compressive strain is induced, which leads to a drastic increase of T_c. In contrast, SrTiO₃ yields a tensile strain and a decrease of T_c. The film on SrLaAlO₄ has a low defect density. Only misfit dislocations with an average spacing of 200 nm are found, which are more or less irregularly distributed. For SrTiO₃, a periodic array of misfit dislocations is present with an average distance of 16 nm. At the interface, as derived from the comparison with simulated images, (LaSr)₂CuAlO_6-x and (LaSr)TiO_3-x are formed on SrLaAlO₄ and SrTiO₃, respectively. These intermediate layers are found to increase the corresponding compressive and tensile strain further. The lattice deformation is determined based on lattice-image analysis. Here, the distance of CuO₂ planes can be measured locally. It turns out that applying a compressive or tensile strain increases or decreases the distance between CuO₂ planes. Accordingly, direct evidence is presented that a decoupling of CuO₂ planes leads to an increase of T_c, which is in contradiction to recent theoretical predictions.

In the field of high-temperature superconductivity there has been no increase in the critical temperature T_c of bulk compounds since 1993. However, an analysis of the uniaxial strain or pressure derivatives of T_c in the cuprate superconductors allows us to predict that under a compressive epitaxial strain, a large increase of T_c should be possible. We demonstrate the experimental feasibility of this approach for La_2-xSr_xCuO_{4+/- (delta} ) ('214') thin films deposited on (001)-oriented SrLaAlO₄ substrates for different Sr content (0.045 less than or equal to x less than or equal to 0.11). Under epitaxial strain, a large jump of the critical temperature T_c is observed, as well as a drastic change in the resistive behavior from insulating to metallic at low temperatures.

In the La-Ti-O system many phases with different structures exist depending on the actual oxygen content. Starting from LaTiO₃, which has an orthorhombic unit cell, to the monoclinic La₂Ti₂O₇ structure, first semiconducting, then metallic and finally ferroelectric characteristics can be obtained at room temperature. Recent transmission electron microscopy studies of the bulk materials showed that the transformation of the LaTiO₃ structure to La₂Ti₂O₇ is based on the insertion of an additional oxygen plane in the {110} perovskite planes. A controlled insertion of this oxygen plane would allow heterostructures to be grown that exhibit different physical properties but have perfect chemical compatibility. Here, we report on our results of epitaxially grown thin films of La₂Ti₂O₇ on (110)-oriented SrTiO₃ and LaAlO₃ substrates using sequential deposition of La and Ti under a beam of atomic oxygen. The epitaxial behavior and the microstructural properties of these films were investigated by transmission electron microscopy.

Phase formation of Bi-Pb-Sr-Ca-Cu-O (2223) superconducting materials has been studied for samples produced by the thermal co-decomposition and freeze-drying methods under ambient atmosphere. Phase development and effect of doping were investigated by x-ray diffraction and electron microscopy. It was found that the addition of Vanadium enhances 2223 phase substantially depending on the doping amount.

Spectra of thermo- and X-ray-luminescence (TL and RL) were studied in the samples of YBa₂Cu₃O_7-x ceramics with different initial defects subjected to high temperature treatment in vacuum at T equals 200 - 900 degrees Celsius and to the high dose (gamma) - and n-(gamma) -irradiations. The unirradiated samples of the single phase YBaCuO-ceramics containing only orthophase do not show the RL and TL. After thermal treatment of these samples at T equals 500 degrees Celsius the RL with maximum at 500 - 560 nm appears and after annealing at T equals 850 degrees Celsius two TL peaks with T_max equals 90 and 130 - 170 K are found, both coinciding with maxima of the RL and TL observed in the multiphase YBaCuO-ceramics, containing the thetra- and impurity dielectric phases. It was established that the luminescence centers are not created in the superconducting (SC) orthophase of these ceramics and are due to both native oxygen vacancies and those generated by thermal treatment and (gamma) - and n- (gamma) -irradiations which are localized in the thetra- and impurity phases. The contribution of the n-generated defects into luminescence centers formation prevails at F greater than or equal to 10¹⁷ neutrons/cm². It was shown that the (gamma) -component of the n-(gamma) -radiation plays the dominant role in the defect formation at F less than or equal to 10¹⁶ neutrons/cm² and the neutron component does at F less than 10¹⁶ neutrons/cm².

We have studied electron diffusion in thin single-crystal films using the Kubo formalism, at the Debye and room temperatures. We show that the diffusion coefficient in very thin films is half of the one found in crystals. This result is in good agreement with the experimental results particularly for aluminum. In our numerical calculations, the parameters of metal structure are fitted to obtain the formula for the diffusion coefficient. More specifically, these calculations show that increase in the crystal thickness causes increase of the coefficient of electron diffusion, and for the thickness of few hundred atomic monolayers reaches the value in bulk crystals. For example, the diffusion coefficient of a three-layer-thick film is 0.706(DOT)10^-7 cm²/s, for five-layer film it amounts to 0.872(DOT)10^-7 cm²/s, while for a thousand-layer film it is 1.253(DOT)10^-7 cm²/s. The last value is rather close to the one we found for the bulk structure, 1.255(DOT)10^-7 cm²/s.

We describe a new pled laser deposition (PLD) system that is linked to an angle-resolved photoemission (ARPES) chamber at the Synchrotron Radiation Center (SRC) in Wisconsin, USA. We also discuss our first results on epitaxially grown YBa₂Cu₃O_7-(delta ) (YBCO) films. The core level photoemission data indicate that a Ba-oxide layer is the dominant surface layer. We were not able to reproducibly detect a sharp fermi edge in the photoemission spectra and thus conclude that the surface layer is non-metallic, probably due to oxygen loss at the surface. The absence of screening of the Y and Ba core levels is a further argument for this conclusion. Further experiments with ozone treated film surfaces are currently under way.

The coupling between a HTSC thin film (S) and a normal conductor (N) in proximity is still not well understood. Most of the investigations done so far look at the penetration of superconductivity into N. We have extended this type of investigation to look at the other side of the coin, namely at the influence of N on S. Here, we report measurements of the critical temperature of S-N bilayer films as a function of the thickness of the YBCO (S) layer using Co doped YBCO as the Normal material. To understand the role of the S-N interface, bilayer having different interface morphologies were prepared using different growth modes of the films. We found that depending on the morphology of the S-N interface, the coupling between S and N layers can be turned on to depress the T_c of S by tens of degrees, or turned off so the layers appear almost totally decoupled. This novel effect is strongly correlated with the presence of different crystalline orientations on the interface. The range of influence of N on S is about 240 angstrom, rather than 20 angstrom expected from the coherence length (xi) s. These observations are explained using a theory of quasiparticle transmission into an anisotropic superconductor.

We have deliberately controlled the initial stage nucleation, growth mechanism and domain structure of epitaxial SrRuO₃ thin films, using substrate etching, miscut and lattice mismatch. The use of buffered HF etched substrates results in more uniform coverage and two-dimensional growth of ultrathin SrRuO₃ films, as compared to polished substrates. For thicker films, the use of exact (001) SrTiO₃ substrates results in growth by two-dimensional nucleation, leading to two 90 degree domains in the plane. As the miscut angle of vicinal (001) SrTiO₃ substrates is increased, step flow growth occurs leading to single domain films. An incoherent three dimensional island growth occurs on (001) LaAlO₃ substrates, resulting in multi domain films.

Ambient observation of magnetic domain structures by magnetic force microscopy (MFM) in La_0.67Sr_0.33MnO₃ films has not been clearly correlated with stresses induced by kinetic growth processes or the compressive (LaAlO₃) or tensile (SrTiO₃) nature of the film-substrate lattice mismatch. Although domain-like magnetic structures have been seen for some as-grown films and related to substrate-induced stress and film thickness, no magnetic structure has been seen for other films grown under similar conditions on the same pair of substrates. In this study we have grown films over a range of temperatures by pulsed-laser deposition, using the above substrates, to determine the relationship between growth and stress-induced magnetic structures. Results from scanning tunneling, atomic force, and magnetic force microscopies, measurements of temperature-dependent magnetization and structure-dependent coercivity show the relationship between growth and magnetic properties. Maze-like domain structures, with separations between 150 nm and 200 nm, were only observed for the thicker films grown at the highest temperature, 800 degrees Celsius. Application of an in-plane magnetic field converted these domain structures to stripe-like domains whose spacing and out of plane component decreased as the field was increased.

The dynamics of the optical response of BSCCO-2212 measured at 1.5 eV has been investigated using femtosecond spectroscopy. The temperature dependence of the response shows three distinct regimes: (1) T less than T_c: with increasing temperature the differential reflectance, (Delta) R/R greater than 0, decreases to zero at T_c, similar to the behavior of the BCS gap function, and the relaxation rate increases linearly with temperature. (2) T_c less than T less than T* (T* - T_c approximately 35 K): (Delta) R/R less than 0. (3) T greater than T*: a weak, positive (Delta) R/R response is measured in the metallic phase. The intermediate regime observed here coincides with the pseudo gap phase detected in ARPES measurements.