This paper is mainly concerned with the problem of the origin of high Tc. S vs d-pairing, the isotope effect, and strong coupling of the carriers to low frequency optical phonon modes are discussed. This strong coupling is sufficient to provide high Tc.

We consider a low-density electron-gas (r_s equals 10 - 16) imbedded in a highly dispersive medium ((epsilon) ₀/(epsilon) _(infinity ) > 10). We find that the Fermi velocity is greatly enhanced over the single-particle value. This enhancement occurs only very close to the Fermi surface, within an energy range smaller than (omega) ₀, the characteristic frequency of the dispersion. This theory applies to high-T_c cuprates, as well as to several other systems. It accounts for the highly-anomalous transport properties in the normal state. We believe that it also accounts for the high value of T_c, based on the standard phonon- mediated interaction BCS theory. There exist some experimental indications for a greatly- enhanced Fermi velocity in YBa₂Cu₃O₇.

The recent discovery of superconductivity in HgBa2Cu04+8 below 94Kl) suggested the possibility of considerably enhanced critical temperatures Tc in similar compounds with more than one Cu-0 plane per unit cell of the crystal lattice. Attempts to synthesize the compounds HgBa2Cao-1Cun<<2n+2+a with n = 2, 3 or 4 (Hg-1212, Hg-1223 and Hg-1234) have been successful and maximum values of Tc exceeding 130K have been achieved2.3). Both x-ray and neutron-diffraction techniques have been used to investigate the structural aspects of these materials, determining lattice constants and, at least in some cases, the degree of partial occupation of oxygen-atom sites in the Hg planes and investigating sublattice disorder 4.5). High-resolution electron microscopy revealed the existence of perfectly ordered polytypes, formed by stackin~s of Hg-1212 and Hg-1223 unit cells along the c direction; corresponding c-axis constants of up to 188 A have been evaluated 6).

Various experimental results show that in high temperature superconducting oxides the lattice responds rather anomalously to the onset of superconductivity. This suggests that some unconventional electron-lattice interaction may be at work in these solids which could be important to the mechanism of high temperature superconductivity. We describe our recent experimental observations by neutron and X-ray scattering as well as some theoretical results which support this view, and discuss their implications.

In this short paper, we describe our recent experimental results from high-temperature superconductors. In the normal state, the data reveals interesting features of the Fermi surfaces and low energy excitations near the Fermi level. In the superconducting state, the data shows a very strong anisotropy in the superconducting gap.

We argue that the photoelectric effect in high-temperature superconductors is not only a source of external valuable information, but one of the most important and interesting open problems in today's physics. Even without a complete picture of this phenomenon, very important conclusions can be obtained from an empirical analysis of photoemission data, notably on the parity and on the link between superconductivity and Anderson localization. But a complete theoretical framework is urgently needed. Its development can contribute to the conceptual revolution that might be necessary to understand high-temperature superconductivity, as the Drude-Fermi-Landau revolution was necessary to understand metallic conductivity.

Raman scattering from charge and spin excitation in cuprates is reviewed. The two-magnon resonance Raman profile is used to determine the most probable energy for photon-assisted charge transfer leading to exchange between two spins. The difference between this energy and that for absorption is attributed to spin relaxation (spin polaron) effects. Two-magnon spectra in superconducting cuprates are presented. It is argued that their presence is evidence that the antiferromagnetic correlation length is equal to or greater than three lattice constants. The recently-discovered Raman-active excitons are described and possible assignments of them are discussed. It is pointed out that in highly correlated metals and insulators, the mechanisms for light scattering that are familiar for doped semiconductors or conventional metals are not necessarily the only ones that are operative.

A detailed study of electron Raman scattering (RS) has been carried out in superconducting and insulating YBa₂Cu₃O_6+x single crystals. The spectral redistribution at frequencies (omega) equals 600 cm^-1 in different polarizations indicate that the superconducting gap is strongly anisotropic. In the normal (metallic) phase the behavior of the imagery part of the response function R'((omega) ) in the polarization (x'x') corresponds to the model of a marginal Fermi liquid, and in the polarization (x'y') and (zz), this behavior is independent of the temperature. In insulating crystals, R'((omega) ) is independent of the temperature up to T >= 200 K in both polarizations. The studies of two-magnon RS were carried out on YBa₂Cu₃O_6+x crystals with oxygen content near the metal- insulator transition. It was found that the chain oxygen reordering processes strongly influence the two-magnon RS spectrum.

We report on the tunneling properties of Ba_1-xK_xBiO₃ (BKBO) grain boundaries prepared on bicrystal substrates. We studied symmetric tilt boundaries with 5 degree(s), 24 degree(s), 36.8 degree(s) and 45 degree(s) misorientation on SrTiO₃, and 24 degree(s) on MgO substrates. The three high angle misorientations used yield clear superconductor- insulator-superconductor behavior, with very little conductance below the bias corresponding to twice the BKBO energy gap, and a strong peak at that value. The two samples prepared on MgO yielded somewhat broader tunneling characteristics, but otherwise similar features compared to the samples prepared on SrTiO₃; they showed the lowest junction resistivity and the weakest conductance increase at high bias. The linear conductance background consistently observed in superconductor-insulator, normal metal junctions is absent in the grain boundary junctions. For the 5 degree(s) samples, we observe weak link behavior below T_c, with a critical current density of 65 A/cm². A similar angular dependence is seen in Nd_1.85Ce_0.15CuO_4-(delta ).

We report STM spectroscopy measurements of in-situ cleaved Bi₂Sr₂CaCu₂O₈ single crystals at 4.8 Kelvin, where we achieved strong evidences for true vacuum tunneling. These careful experiments result in very reproducible spectroscopy as a function of position on the surface, and as a function of tip/sample spacing. The characteristic features of the tunneling spectra are a significant filling of the gap region, a large density of states at the gap edges and a weak dip about 70 meV below the Fermi level. Such IV characteristics are not compatible with a single gap BCS-like s-wave theory. Furthermore, we report spatially resolved spectroscopy where we observe regions with two distinct gap values. A double gap structure appears in the tunneling spectra acquired in the vicinity of the boundary between these regions. We believe the double gap structure we observe in this case does not reflect an intrinsic gap anisotropy, but seems rather related to crystalline inhomogeneities. This demonstrates the potential of the STM's spatial resolution to shed some light on the controversy among the tunneling spectroscopy of high temperature superconductors published so far.

Thin bismuth strontium calcium copper oxide (BSCCO) films and BSCCO/insulator/BSCCO trilayers have been prepared on SrTiO₃ and MgO substrates by evaporation from elemental sources in ozone atmosphere. Accurate control of the stoichiometry is achieved through monitoring of the atomic fluxes by use of in situ atomic absorption spectroscopy, as well as by reflection high-energy electron diffraction. Nevertheless, nanometer-scale second- phase precipitates are sometimes observed. These defects and the flat regions around them have been probed by a variety of microanalytical techniques, including Rutherford backscattering spectroscopy, particle-induced x-ray emission, atomic force microscopy, microscopic secondary ion mass spectroscopy and transmission electron microscopy.

The effects of vortex localization at columnar defects on the linear and non-linear dissipation in Tl₂Ba₂CaCu₂O₈ films are studied. The confinement of vortices into columnar defects leads to a minimum in the angular dependence of linear resistivity when the external field is brought into alignment with the defects. This observation provides compelling evidence for non-zero line tension of vortices in this highly anisotropic cuprate. The electric field vs current density isotherms, and the temperature dependence of linear resistivity, both, indicate a Bose glass type of phase transition in this system.

The new results on anomalous IR-transmittance and complex dielectric function are reported. The supposition is made that the new phase is a heterogeneous system with a polaronic mechanism of percolation. The latter is confirmed by numerical simulation of IR-transmittance and dynamic conductivity.

We present a brief review of results on the surface impedance of cuprate superconductors, focusing mainly on YBa₂Cu₃O_7-(delta ) (YBCO) and evidence of d-wave superconductivity in that material. We then discuss our recent results on Ba-K-Bi-O thin films, and the effects of DC electric fields on the surface impedance of YBCO films. A summary of our data on high quality thin films and single crystals of the electron-doped Nd_1.85Ce_0.15CuO_4-(delta ) (NCCO) cuprate superconductor follows. Surprisingly, the measurements on NCCO are consistent with the behavior of an s-wave BCS superconductor, in striking contrast to recent results on YBCO. Finally we discuss some of the interesting potential implications of d-wave superconductivity for microwave applications of the cuprates.

The electric field effects on bi-crystal YBa₂Cu₃O_7-(delta ) grain boundaries were studied. The field effects were examined for an inverted MIS structure sample, in which a channel was arranged across the grain boundary on the SrTiO₃ bicrystal substrate served as an insulator with the thickness of 50 micrometers . The field-induced change in the normal resistance was enhanced not only by an increase in the dielectric constant of gate insulator but also by a reduction in the carrier density nearby the grain boundary. For the sample with the YBCO thickness of 1,000 angstroms, the gate voltage of 80 V corresponding to 2 X 10⁴ V/cm induced the relative changes in the normal resistance up to around 5%. The grain boundary junctions showed hysteretic I-V properties and sub-gap structures caused by the self-excited resonance of ac Josephson effect. Significant field effects on the hysteresis and sub-gap structures were observed. These effects were attributable to the field-induced changes in dielectric properties of the grain boundary. It was apparent that the effective dielectric constant of the grain boundary was several tens and decreased with increasing gate voltages.

The preparation of a hybrid conducting polymer/high-temperature superconductor device consisting of a polypyrrole coated YBa₂Cu₃O_7-(delta ) microbridge is reported. Electrochemical techniques are exploited to alter the oxidation state of the polymer and, in doing so, it is found that superconductivity can be modulated in a controllable and reproducible fashion by the polymer layer. Whereas the neutral (insulating) polypyrrole only slightly influences the electrical properties of the underlying YBa₂Cu₃O_7-(delta ) film, the oxidized (conductive) polymer depresses T_c by up to 50 K. The observed reversible shifts in T_c are the largest reported to date. In a similar fashion, the oxidation state of the polymer is found to reversibly modulate the magnitude of J_c, the superconducting critical current. Thus, the operation of a molecular/superconductor switch for controlling superconductivity is demonstrated.

c-axis oriented YBa₂Cu₃O_x narrow films submitted to pulses of supercritical current show the phenomenon formerly observed on aluminum filaments by Pals et Wolter. The transition to a dissipative state is delayed by a long time t_d which is found to be independent of both the magnetic field (up 5 kG), of the temperature (up to at least 40 K), but to vary strongly with the ratio I/I_c, where I_c is the critical current. Our values of t_d, measured from 1 to 500 nanoseconds, fit approximately the time of extinction of the order parameter expected from the 1D Ginzburg-Landau equation. The rise of voltage at t_d differs from a heating process in several respects. The most striking is the memory effect obtain when applying a subcritical current pulse prior to the supercritical pulse. Another feature of the experiment is the identification of the initial voltage causing the acceleration of the superconducting pairs. The resistance reached after t_d corresponds to an extension of approximately equals 3 micrometers for the phase-slip center. If this can be identified with twice the quasiparticle diffusion length, the inelastic electron lifetime turns out to be a few nanoseconds, in agreement with the electron cooling time measured independently.

With a new aspect in the physics of condensed matters, we have studied temperature dependences with anomalies of the transient photoconductivity in insulators or semiconductors La-Cu-O and Y-Cu-O etc., between 4.2 K and 300 K, especially on the systems close to high- T_c superconductors. Different from submergences in normal photoconductors with decreasing temperature, we observe several unexpected emergences of photoconductivity even for host oxide insulators below temperatures in correspondences with the critical temperatures Tsc of superconductors. The Cu₂O-like part in insulators at photoexcitation exhibits behaviors similar to the doped Cu-O based superconductors in the dark, irrespective of variety of crystal symmetry, dimensionality, and huge difference in carrier concentration. Situations are similar for the Bi-O based superconductors. Thus, we speculate that Cu₂O or Bi₂O₃ single crystals themselves at photoexcitation can be the basic substances equivalent to the Cu-O or Bi-O based oxide superconductors. Finally, we proposed that, by combining these photoconductors for the gate materials and relevant superconductors for the source and drain materials, both become effective simultaneously below Tsc, one can fabricate a novel device such as optically controllable FET-type transistors and accumulate them into high density. Therefore, we believe that our discovery of such phenomena reveals a possibility to open a new field in contemporary science and technology to be called `Superconductive Optoelectronics'.

Over twenty fiber years ago, we predicted the occurrence of photoinduced superconducting phase transition in some complex systems. In a series of papers, conditions and constraints were determined for the realization of photoinduced non-equilibrium superconducting state under transient or metastable situations. Owing to the involvement of two boson modes (one photons or photoexcitations and the other the virtual mode of the system, namely phonon or any other electronic boson mode) in the interaction mechanism, the transition temperature (T_c) is found to depend on the photon density or photoexcitation dosage impressed on the sample within the absorption depth. Thus T_c can be controlled and tuned as required. After giving an overview of the theoretical concepts and the current status of experiments in this field, we present an extended model in which both effects of intraband and interband transitions caused by the appropriate boson modes are taken into account. The intraband pairing thus obtained consists of two parts, the usual single boson induced pairing and the new radiation induced two boson pairing. The latter leads to the enhancement of T_c. The dual role of the radiation field quanta is emphasized. The recent experimental discovery of photo- induced superconductivity in some complex oxide (cuprate) systems in transient or metastable state are in accord with the predicted phenomena. These aspects along with some possible applications are discussed.

Superconductivity and metal-insulator phase transition in a layered 2D electron liquid, taking into account electron-electron correlation effects and spin density fluctuations, have been considered by means of the Fermi-liquid approach. It is shown that the electron-electron interaction mediated by the virtual charge density excitations in a layered 2D correlated Fermi liquid has the short-range attractive character at the intermediate electron density. It is found that such interaction leads to superconductivity at the intermediate density of carriers and the phase transition to the insulating state with the decrease of the density. The dependence of transition temperature T_c on the doping density, interlayer spacing and dielectric constant of the cuprate under investigation is also shown. A optimum T_c for a single-layer cuprate and the possibility to reach room temperature superconductivity are predicted as well. This indicates a direction to synthesize the compounds with superconductivity at higher temperature.

Using a molecular beam epitaxy deposition technique, c-axis La_2-xSr_xCuO_{4+/- (delta} ) ultrathin films have been prepared on (001) SrTiO₃ substrates. Several superconductive properties such as the critical temperature T_c, the penetration depth (lambda) _ab(0), the activation energy for flux flow (Delta) U and the Hall coefficient R_H are reported for the same set of films. As the dopant content is increased, maximum values for T_c and (Delta) U are observed near the optimum doping while R_H decreases continuously.

Interaction between magnetic vortices may result in strong thermo-galvano-magnetic effects in thin superconducting films, whose thickness is much smaller than London penetration length. In particular, the Nernst coefficient acquires a contribution compared to that observed in semiconductors. The non-equilibrium vortex-antivortex pair production by some external currents anticipated to occur easily in granular films may lead to the same order of magnitude for the transverse voltage as provided by the Nernst effect. External microwave radiation absorbed inhomogeneously and resulting in a gradient of normal excitations along the film should cause the vortices motion and, consequently, the dc-voltage across the film. These strong effects are to be observed within the range of parameters where the Charged 2D Coulomb Gas Model is still valid. The connection of our results with the experiments of Gerber and Deutcher is discussed. We suggest to employ the effect of photoinducing the vortices motion in thin superconducting films to detect weak microwave radiation.

We present a systematic study of compositional and doping effects in Bi₂Sr₂CaCu₂O_y high-T_c superconductors performed with photoemission spectroscopy. The study has been extended to Y-doping and I-intercalation of Bi-2212 high quality single crystals. The main results is that each type of dopant affects the crystal composition in its own way. Yttrium affects the Ca and Sr planes, producing a charge transfer into the CuO planes. For I-doping, we find that the main effect is a change in the interplanar distance, but X-ray Photoemission Spectroscopy (XPS) allows to see that the decrease of the over-doping of copper planes (hole doping). We performed also a comparative study by Angle Resolved Ultraviolet Photoemission Spectroscopy (ARUPS) between this sample and an oxygen annealed specimen. XPS Cu2p core level data establish that the hole concentration in the CuO₂ planes is essentially the same for these two kinds of samples. ARUPS measurements show that electronic structure of the normal states near the Fermi level has been strongly affected by iodine intercalation.

The influence of temperature and pump power on phonon modes in YBa₂Cu₃O_x (x equals 6.9, 6.4) was investigated. In YBa₂Cu₃O_6.4 a laser-heating up to 550 K results in decrease of linewidth and frequency of apex oxygen mode. This results is interpreted as an ordering of the chain oxygens.

There exists several techniques to synthesize TlBaCaCuO from precursor films. Recently, a two zone furnace technique was developed by DeLuca. Based on our experience on the two zone furnace technique, we systematically conducted a series of experiments in a tube furnace with a temperature gradient distribution. The BaCaCuO precursor films were fabricated by an ink method, and the Tl diffusion source was Tl₂O₃. Superconductive films were characterized by SQUID magnetization measurement, X-ray diffraction, SEM, and EDS. Experimental results will be summarized.

In this paper the possibility that high critical temperature superconductor (HCTS) electrodes are non homogeneous is considered. The effect of such a configuration on current vs. voltage characteristics in HCTS junctions is analyzed and modelled in significative limits. Typical features, such as the depression of gap structures and finite zero bias conductances, are explained according to the proposed model. This last is based on arguments of proximity effect between weakly coupled bilayers (superconductor--normal metal). It allows to study tunnel and point contact (PCJ) junctions, providing results qualitatively different from classical theories on PCJ. The low transmissive nature of such an interface seems to play a fundamental role in determining current vs. voltage characteristics in HCTS junctions. The results of the presented model suggest also PCJ as a sensitive tool to evaluate the transmission probability of interfaces between a superconductor and a metal.

We have found that the structure of the CuO₂ plane of Bi₂Sr₂CaCu₂O_8+y (Bi2212) is anisotropic. Experimental investigation of the Cu site configurations in the CuO₂ plane of Bi₂Sr₂CaCu₂O_8+y (Bi2212) by XANES and EXAFS shows that the quasi 2D Fermi liquid is confined in a superlattice of quantum stripes of width L running in the x direction. We have found that the recently proposed mechanism to rise the critical temperature by the confinement of an electron gas in superlattices and tuning the Fermi level to a shape resonance is verified. In fact the component of the Fermi wavevector k_Fy in the y direction perpendicular satisfies the condition k_Fy equals 2(pi) /L in Bi2212. This resonance gives an amplification of the critical temperature T_c by a factor of order of 5 in comparison with the critical temperature of the homogeneous CuO₂ plane estimated to be about 15 - 20 K.