We report on ab-plane resistivity ((rho) ) and angle-resolved photoemission (ARPES) spectra for Bi₂Sr₂CaCu₂O_8+x single crystals irradiated with neutrons or electron-beam irradiation. Both the normal and superconducting states were measured with angle-resolved photoemission. Electron-beam irradiation leads to an increase in the residual resistivity, and a decrease in the superconducting transition temperature (T_c). The resistivity data does not indicate any pseudogap; the resistivity is linear from T_c to 300 K for all levels of disorder, and the slope (d(rho) /dT) is the same for all levels of disorder. The superconducting state ARPES data exhibits no change in the binding energy of the 'peak' for Brillouin zone locations near the (O,(pi) ) point. The peak spectral intensity decreases with increasing disorder, the gap fills in, but the peak neither shifts nor broadens. The normal state exhibits a pseudogap developing with disorder; the size of the pseudogap increases as the residual resistivity increases. The pseudogap is anisotropic, largest near the (O,(pi) ) point and zero in the <(pi) ,(pi) > direction. Neutron-beam irradiation causes an increase in the residual resistivity. The resistivity data exhibit a change of slope and indications of a pseudogap for neutron irradiation. For normal state ARPES data of neutron-beam irradiated samples, there is also an anisotropic pseudogap; it is also zero in the <(pi) ,(pi) > direction and large near the (O,(pi) ) point. We discuss implications of these data.

Planar tunneling spectroscopy is used to investigate the quasi- particle density of states of YBCO. The tunneling conductance, taken as a function of temperature, magnetic field, crystallographic orientation, Pr, Zn and Ni doping and ion-induced damage confirms that the observed zero-bias conductance peak (ZBCP) is an Andreev Bound State (ABS). This ABS occurs at the interface of an unconventional superconductor, that breaks the order parameter reflectional symmetry: in this case, the (110) surface of YBCO. An applied magnetic filed causes a splitting of the ZBCP, which is due to the Doppler shift arising from the scalar product of the quasiparticle velocity with the superfluid momentum, V_F(DOT)P_S. A dramatic dependence of the ZBCP splitting with the direction of the applied field demonstrates that the transport properties of the ABS are highly anisotropic with respect to the crystal axes. In zero field, the ZBCP splits below T_s approximately 8K, which is a manifestation of a phase transition into a state that breaks time-reversal symmetry.

The pairing symmetry and the superconducting gap in high- temperature superconducting cuprates are investigated as a function of the hole doping level (x) and temperature (T), using directional scanning tunneling spectroscopy (STS). It is found that the predominant pairing symmetry is (Formula available in paper), which is insensitive to the variations in T and x. In contrast, the maximum superconducting gap ((Delta) _d) in YBa₂Cu₃O_7-(delta ) and La_2-xSr_xCuO_4-(delta ) scales with the superconducting transition temperature (T_c), and the ratio of (2 (Delta) _d/k_BT_c) increases with decreasing doping level. The dominance of d_x2_-y2 pairing is consistent with strong spatial variations in the local quasiparticle spectra near non-magnetic impurities such as Zn and Mg in a (Zn,Mg)-doped YBa₂Cu₃O_7-(delta ) single crystal. To further elucidate the nature of the pairing state, the c-axis spin- polarized quasiparticle transport in the superconducting state of YBa₂Cu₃O_7-(delta ) is investigated by studying the critical currents and STS under the injection of electrical currents from the underlying ferromagnetic La_0.7Sr_0.3MnO₃ layer in various ferromagnet-insulator-superconductor (F-I-S) heterostructures. The temperature dependent spin diffusion length ((delta) _s) and signatures of nonequilibrium quasiparticle distribution under spin injection in d-wave superconductors are determined for the first time.

We study theoretically the Josephson coupling in a ferromagnetic weak link between both isotropic s-wave, and layered d-wave superconductors. For strong ferromagnetic barrier influence, the conventional coupling is obtained, with ground state phase difference across the link 0 < (phi) _gs <EQ (pi) . When the weak link is a part of a superconducting ring, this is accompanied by the flow of spontaneous supercurrent, of intensity which depends on the reduced inductance l equals 2(pi) LI_c(T)/(Phi) ₀, and is non-zero only for l greater than a critical value. For l >> 1, another consequence of the unconventional coupling is the anomalous quantization of the magnetic flux.

Assuming tunneling perpendicular to the layers, Andreev bound states are studied theoretically in atomic-scale S/F superlattices, where S is a conventional superconductor, and F is a ferromagnet. The nonmonotonic dependence of the critical current on the exchange field h in F layers is a consequence of (pi) -phase formation. The shape of the spectra of spin-splitted densities of states (DOS) depends strongly on h and on the transfer integral t between the layers; for an appropriate choice of these parameters zero-energy bound states (ZES) may appear. Considering in-plane tunneling in high-T_c S/F/S Josephson weak links with thin, ferromagnetic metal barrier, DOS is calculated assuming that the a-axes of the crystals in two S electrodes are misoriented for an angle (theta) . In the anisotropic d-wave case the number of peaks in DOS, corresponding to the Andreev bound states, can be greater than in the isotropic s-wave case. DOS strongly depends on h, (theta) , and on the macroscopic phase difference (phi) across the link.

A novel experiment for determination of the charge related to vortices in thin superconducting film is proposed and a number of related experimental set-ups are also theoretically considered. The methods are based on the Torricelli-Bernoulli effect in superconductors and the phenomenology of the effect is briefly discussed. The vortex charge is expressed via the effective mass of the Cooper pairs, thus both parameters, inaccessible by now, could be simultaneously determined. The experiment would require layered metal-insulator-superconductor structures and standard electronics employed in kinetic measurements. The quality of the insulator- superconductor interface should be high enough as to allow for observation of electric field effects similar to those investigated in superconducting field-effect transistors. The development of layer-by-layer growth technology of oxide superconductors provides unique possibility for investigation of new fundamental effects in these materials. In particular, the structures necessary for determination of the vortex charge could be used to study the superconducting surface Hall effect, Bernoulli effect, the superfluid density, etc. In conclusion, the systematic investigation of new effects in oxide superconductors is envisaged as an important part of the material science underlying the oxide electronics.

We investigated the current-voltage (I-V) curves of high-T_c superconductors at very low and very high dissipation levels. In the limit of low driving currents the barriers for vortex movement become infinite in the vortex-glass state. Using long measurement bridges up to 0.5 m we were able to sample an electric field range from 1 V/m down to 10^-8 V/m in one experimental setup. The resulting I-V curves allowed us to verify an excellent glass scaling of the I-V curves, which revealed an increased dynamical exponent of the glass transition. We also found a considerable dependence of the vortex-glass scaling on the probed electric-field range. At very high dissipation levels the I-V curves of type II superconductors in magnetic fields can show voltage jumps due to flux-flow instabilities at high vortex-velocities. They have been investigated in Bi₂Sr₂CaCu₂O₈ and YBa₂Cu₃O₇. The results have been interpreted in the framework of the theory of Larkin and Ovchinnikov. The extensions introduced by Bezugly and Shklovskij account for unavoidable quasiparticle heating during the measurement. The influence of avoidable heating effects on the I-V curves was studied experimentally by pulsed measurements with a time resolution in the microsecond range. In Bi₂Sr₂CaCu₂O₈ we found the instability in the vortex-liquid phase. However, in YBa₂Cu₃O₇ a remarkable coincidence between the vortex-glass phase, which manifests itself at low dissipation, and the existence of the high dissipative flux- flow instability was observed.

In this article we describe the synthesis of a new superconducting lead cuprate PbSr₂CuO_5+(delta )(T_c approximately 40 K) by molecular beam epitaxy (MBE). The new superconductor is the first member of the Pb-12(n-1)n homologous series, which contain a single PbO layer as the charge reservoir block. This material is a thermodynamically quasi-stable phase, and has not been synthesized in bulk. Low-temperature synthesis by MBE and an appropriate choice of substrates were essential to our discovery of the PbSr₂CuO_5+(delta ) superconductor. This work demonstrates that MBE offers a novel synthetic route in the material search for new superconducting cuprates. Furthermore, we are attempting to synthesize higher n members of the Pb-12(n-1)n homologous series, but have encountered limited success partly because the volatility of Pb and PbO_x limits the growth temperature.

In YBa₂Cu₃O_6+x compound the tetragonal to orthorhombic transition occurs around x equals 0.3, followed by a continuum variation of lattice parameters. Hence both, the structural and superconducting properties, depend upon the oxygen content in CuO chains. Conversely, the epitaxial stress, exerted by the substrate on YBCO films, modified the lattice parameters influencing the oxygen stability in the chains. The understanding of this mechanism is essential when growing epitaxial films for in- situ photoemission studies as well as for tunneling experiments, since the oxygen stability up to the top surface unit-cell is a central issue. We have studied this effect on c-axis oriented YBCO films grown by laser ablation on (001) STO single crystals. Accurate x-ray diffraction analysis of thick films (t GRT 500 angstrom) indicates the presence of two distinct layers, one strained and the other relaxed. Detailed analysis shows that the relaxed layer is as well oxidized as bulk samples, while the strained one is oxygen deficient. Furthermore, despite an oxygen content of about x equals 0.65, the strained layer is in the tetragonal phase (in bulk, the tetragonal phase exists for x < 0.3). We discuss these results in terms of competition between the chemical pressure induced by oxygen inclusion in the chains, and the uniaxial stress within the film.

We report laser-ablation studies of highly-oriented thin films of the electron-doped infinite-layer copper-oxide compounds Sr_{1- x}La_xCuO₂. The primary synthesis variables were substrate or buffer layer material, temperature, laser fluence, target- substrate distance, and oxygen pressure. The films were characterized by x-ray diffraction, atomic force microscopy (AFM), Rutherford back-scattering (RBS), and electrical resistivity. Films were deposited on strontium titanate (001) or on buffer layers of T'-phase copper oxides, Ln₂CuO₄ (Ln equals Pr, Sm) on SrTiO₃ (001). The in-plane lattice constants of such T'-phase materials (a equals 0.391 - 0.396 nm) could provide a structure more amenable to electron doping than strontium titanate (a equals 0.390 nm). Extremely flat buffer layers were obtained from stoichiometric targets of Sm₂CuO₄ and Pr₂CuO₄. However, ablation from stoichiometric infinite-layer targets onto buffer layers resulted in mixtures of infinite-layer and chain/ladder phases. Non-stoichiometric deposition was confirmed by RBS analysis. We thus utilized non-stoichiometric targets to obtain single-phased infinite-layer films. The x-ray rocking curves of highly-oriented epitaxial infinite-layer films exhibited full- widths at half maximum as narrow as 0.05 degrees. Infinite-layer films grown on T'-phase buffer layers exhibited lattice constants closer to those of the bulk superconductor than films grown directly on SrTiO₃.

We have measured the reversible magnetization of grain-aligned HgBa₂Ca₃Cu₄O_10+(delta ) with the external magnetic fields of 1 T c, and thus the system is a highly anisotropic. This is supported by the two-dimensional scaling behavior of the magnetization at the critical region. From the model of Hao et al. we obtained various superconducting parameters such as the critical fields, the coherence length, and the penetration depth. Especially, the zero- temperature penetration depth (lambda) _ab(0) is estimated to be 157 nm which is the smallest among Hg-based superconductors. We infer that this is due to the large hole concentration within the CuO₂ plane. Finally, we analyzed the fluctuation-induced susceptibility in the low-field regime using the modified Lawrence- Doniach model.

We have estimated velocities of vortices introduced between the superconducting layers of high-temperature superconductors under a driving current flowing along the c-axis. The measured vortex velocity is approximately 4 m/s in the micro-bridges with typical sizes of 60 micrometer long and 40 X 50 micrometer² area fabricated from La_2-xSr_xCuO₄ (LSCO) single crystals by the electro-discharge machining (EDM) technique. In the framework of flux creep model can explain that the creep occurs in the bridges with large size causing a low bias current density. On the other hand, we observed a sharp up-turn in flux-flow branches on current-voltage characteristics of the micro-bridges fabricated by the FIB technique, which are smaller than the bridges fabricated by the EDM. This indicates that the motion of Josephson vortices in LSCO matches the propagation of an electromagnetic wave with one of the modes, which corresponds to the mode velocity of 1.6 X 10⁵ m/s, in the multi-layered structure. The high-speed flux- flow of Josephson vortices can be utilized for high-speed switching gates or electromagnetic wave oscillators in millimeter and sub- millimeter wave bands.

Advances in thin film deposition techniques, in particular molecular beam epitaxy, have made it possible to produce heterostructures of high temperature superconductors and manganite perovskites. The latter exhibit the phenomenon of colossal magnetoresistance (CMR). The half-metallic character of the CMR compounds results in their carriers being spin polarized. Experiments with these heterostructures have demonstrated that the injection of spin polarized carriers into high temperature superconductors results in a reduction of parameters such as the critical current and critical temperature.

Double-barrier Nb/Al-AlO_x-Al-AlO_x-(Al/)Nb devices with various thickness and purity of the middle Al layer were fabricated and investigated. It is found that the subgap structure that appears in the current voltage characteristics is very sensitive to the parameters of the middle film. In addition to the formerly reported gap-difference structure, we observed its half-voltage 'subharmonic' and a novel magnetic-field-sensitive structure that develops at a voltage V approximately equals (Delta) _Nb/e ((Delta) _Nb is the superconducting energy gap of Nb). In general, the devices with a cleaner Al electrode reveal better reproducibility of their characteristics, whereas the devices with 'impure' Al (deposited in presence of a small amount of oxygen added to the Ar) tend to have more complicated and difficult to reproduce subgap structure. In addition, an anomalously large Josephson critical current was observed for devices with a small thickness of the middle Al layer.

In this paper, we theoretically predict the best efficient way for electromagnetic wave emission by Josephson plasma excitation in intrinsic Josephson junctions. First, we briefly derive basic equations describing dynamics of phase differences inside junction sites in intrinsic Josephson junctions, and review the nature of Josephson plasma excitation modes based on the equations. Especially, we make an attention to that Josephson plasma modes have much different dispersion relations depending on the propagating directions and their different modes can be recognized as N standing waves propagating along ab-plane in cases of finite stacked systems composed of N junctions. Second, we consider how to excite their modes and point out that excitations of in-phase mode with the highest propagation velocity among their N modes are the most efficient way for electromagnetic wave emissions. Finally, we clarify that in-phase excitations over all junctions are possible by using Josephson vortex flow states. We show simulation results of Josephson vortex flow states resonating with some Josephson plasma modes and predict that super-radiance of electromagnetic field may occur in rectangular vortex flow state in which spatio- temporal oscillations of electromagnetic fields are perfectly in- phase.

Irradiating a microwave of several tens GHz on Bi₂Sr₂CaCu₂O_8+(delta ) (Bi-2212) single crystals, we observed genuine zero-crossing current steps from intrinsic Josephson junctions forming on the surface of the crystals in contact with normal- metallic (AU) electrodes. The critical current of the surface intrinsic Josephson junction was significantly suppressed due to the proximity contact to the normal-metal electrode, which allowed us to isolate the microwave response of the 'surface junction' from that of rest of the 'inner junctions' in a stack. This indicates that sufficiently small tunneling critical current density is essential to observing the zero-crossing current steps. An attempt to reduce the interlayer coupling using HgI₂-intercalated Bi₂Sr₂CaCu₂O_8+(delta ) single crystals, however, gave rise to coherent fluxon motions.

The high-T_c superconductor YBa₂Cu₃O₇/ferroelectric Pb(Zr,Ti)O₃ (YBCO/PZT) heterostructures, La_1-xCa_xMnO₃ thin film and La_1-yCa_yMnO₃/La_{1- x}Ca_xMnO₃ (x does not equal y) heterostructures have been prepared and investigated. For YBCO/PZT heterostructures, it was found by conducting atomic force microscope (C-AFM) that the leakage current distributes in the local defect region with plane size of 10 nm order. This is a new discovery of the investigation of leakage current mechanism from the surface morphology of PZT/YBCO heterostructures. One kind of ordered surface structure was observed for the first time in the films of (La_1-xCa_x)MnO₃ with x >= 0.5, which has shown an important role for the formation of interfacial structure and CMR effect of the La_{1- y}Ca_yMnO₃/La_{1- x}Ca_xMnO₃ (x >= 0.5, x does not equal y) heterostructures.

Midgap states (MGS) exist at the Fermi level at surfaces and interfaces of d-wave superconductors for order parameter orientations such that a sign change of Andreev reflection can add a (pi) -shift in the Bohr-Sommerfeld quantization condition. MGS are basically robust features, but the detailed density of states, manifested e.g. in zero bias conductance peaks (ZBCP), is in principle a sensitive probe of junctions and interfaces, providing information about normal regions with suppressed order parameter, non-specular scattering from rough or disordered interfaces, impurity states, lifetimes, etc. Resonance coupling of MGS in d₄₅/d_{+/- 45} SNINS junctions gives rise to large equilibrium Josephson critical current varies direct as (root)D, where D is the normal transparency of the junction. Spontaneous time-reversal symmetry breaking (TRSB) in d₀/d₄₅ junctions at low temperatures k_BT << D(Delta) ₀ is driven by a gain in Josephson energy for transparencies D >> (epsilon) ₀/(lambda) .

High-T_c Superconducting QUantum Interference Devices (SQUIDs) have been used to detect magnetic fields produced by a variety of chemical reactions. We call this phenomenon chemomagnetism. Reactions studied thus far include solid-gas reactions (metal oxidation during solid combustion), solid-liquid reactions (between metals and acids or bases), liquid-liquid (acid-base reactions), and replacement reactions (liquid-solid and liquid-liquid). The observed chemomagnetic fields are most likely generated by the flow of ions and electrons during the intermediate processes of a typical reaction, and often exhibit complex temporal behavior. The time-dependence of these fields, and their magnitudes, depend on the specific chemical reactions and reactant concentrations. For example, chemical reactions involving the oxidation of metals from the same row of the periodic table generate similar time-dependent magnetic signals. We also observe the formation of permanent magnetic fields during the synthesis of ferromagnetic materials. SQUIDs may prove to be important tools for noninvasively studying the dynamics of chemical reactions.

We present comprehensive experimental and numerical studies of the subpicosecond switching dynamics of YBa₂Cu₃O_7-x (YBCO) grain-boundary Josephson junctions, excited by single-picosecond electrical pulses. The test structures were patterned in 100-nm- thick YBCO films grown by pulsed laser deposition on (100) MgO bicrystal substrates. Each sample consisted of a coplanar strip (CPS) transmission line, a microbridge acting as the electrical pulse generator, and a single Josephson junction positioned between the CPS lines about 100 micrometer away from the bridge. The junctions were characterized by the nonhysteretic current-voltage characteristics with the characteristic voltage approximately equal to 2.0 mV at 20 K (temperature of our experiments). A train of 100- fs-wide optical pulses from a Ti:sapphire laser photo-excited the microbridge and generated 2-ps-wide electrical pulses, which were then applied to switch the junction. In addition to the input pulse, the junction was dc-biased at +0.7 I_c, -0.7 I_c, +1.5 I_c, -1.5 I_c, and zero-I_c, where I_c is junction critical current. Time-resolved dynamics of the junction response was studied with the help of our cryogenic electro-optic sampling system, which can be regarded as a sampling oscilloscope featuring < 200-fs time resolution and < 150-(mu) V voltage sensitivity. We obtained 0.7-ps-wide single-flux-quantum (SFQ) pulses generated as a result of the junction switching process. The measurements were compared to numerical computations based on the equivalent circuit containing a resistively shunted Josephson junction, and we have found a satisfactory agreement between our simulations and experimental data. We believe our findings provide experimental confirmation of the potential of YBCO Josephson gattes as building blocks of ultrafast (sub-THz) digital electronics.

Thin films of YBa₂Cu₃O_7-x (YBCO) were prepared on different kinds of substrates (SrTiO₃, LaAlO₃, sapphire or silicon) with critical current densities j_c (77K) > 2 (DOT) 10⁶ A/cm² and critical temperatures (zero resistance) of up to 90 K. We used laser deposition techniques for film preparation on SrTiO₃-substrates with a maximum substrate size of 10 X 10 mm². Applying bicrystallin substrates made from this material a pronounced RSJ-behavior of the grain boundary Josephson junctions was observed with I_CR_N-products of 410 (mu) V and critical current density of 1.2(DOT)10⁵ A/cm² in maximum on 24 degree grain boundaries. On that basis planar galvanically coupled dc-SQUID-gradiometers were patterned on this limited substrate size with a field gradient resolution of 308 fT/(cm(root)Hz) in the white noise level and 2 pT/(cm(root)Hz) at 1 Hz in electrically and magnetically unshielded environment. This extraordinary field gradient resolution even in unshielded environment enables this kind of dc-SQUID sensor for measurements of the magneto-cardiogram (MCG) of the human heart as well as for investigations in non-destructive testing (NDT). The layout of the dc-SQUID as well as the antenna layout of the gradiometer antennas were varied in order to determine their influence on the whole sensor performance. For the dc-SQUID layout itself we present a gradiometric scheme with decreased parasitic area of the dc-SQUID in the gradiometer. Furthermore the additional use of buffer layers to prevent interdiffusion, lattice mismatch and internal stress by different thermal expansion coefficients enables the use of silicon substrates for YBCO thin film deposition. Gradiometric flip-chip- antennas were patterned on 2'-silicon-substrates and combined with dc-SQUID-gradiometers explained above. With this sensor concept the resolution was increased by a factor of 6 in shielded environment thus also the human MCG was measured. By the integration of superconducting antennas with Hall-effect sensors on the same substrate a hybrid sensor concept is introduced having less sensitivity compared to SQUID-based sensors but improved dynamic range enabling their application in a NDT-measurement system in highly disturbed environment.

We report on the novel nonequilibrium microwave emissions from quasiparticle-injected high-Tc superconductors. The phenomena have been observable for the current-injected YBa₂Cu₃O_{7- y}(YBCO)/I/Au or Bi₂Sr₂CaCu₂O_y(BSCCO)/I/Au thin- film tunnel junctions and BSCCO single-crystal mesa samples. For the thin-film tunnel junctions, the emitted radiation appears as broadband. The different emission characteristics between the YBCO and BSCCO tunnel junctions strongly suggest the possibility of Josephson plasma emission. On the other hand, for the mesa samples, the radiation appears as three different modes depending on the bias point in the hysteretic current-voltage characteristics: Josephson self-emission, nonequilibrium broad emission and sharp emission. The latter two emission are identified as Josephson plasma emission.

We have grown heteroepitaxial thin film structures composed of various transition metal oxides such as the colossal magnetoresistance manganites, superconducting cuprates, ruthenates as well as insulating titanates on SrTiO₃, NdGaO₃ and LaAlO₃ substrates using a UHV laser molecular beam epitaxy (laser-MBE) system. The film growth was controlled in-situ using high pressure RHEED as well as scanning probe techniques (AFM/STM). The fabricated films were analyzed by x-ray diffraction, transmission electron microscopy and the measurement of the transport properties. The manganite, ruthenate and titanate thin film structures show good epitaxy with small mosaic spread. The observation of RHEED oscillations during the film deposition indicates a layer by layer growth mode. This is further supported by the observed small surface roughness of typically less than 3 nm rms for a 100 nm thick film. We also could find a clear correlation between the observed RHEED pattern and the surface morphology measured by AFM/STM. Our analysis shows that UHV laser MBE is well suited for the fabrication of complicated heteroepitaxial thin film structures required for oxide electronics.

Continuous single c-domain/single crystal thin films of PbTiO₃ (PT), 10 to 300 nm in film thickness, were epitaxially grown on the miscut (001)SrTiO₃ substrates. The PT thin films were grown under a step-flow growth and their surface was atomically flat. The interface between thin films and the substrates were coherent. These sputtered PT thin films were tetragonally deformed by the thin films were tightly bonded to the substrates. The lattice parameters did not change at the Curie temperature. The PT thin films exhibited modified ferroelectric properties and/or second order like ferro-paraelectric phase transition. The PT thin films exhibited a diffused peak at the Curie temperature of 520 degrees Celsius. The P/E hysteresis curve showed a sharp switching property with a rectangular pattern and a high coercive field of 400 to 500 kV/cm.

We have used pulsed laser ablation to grow a series of PbTiO₃/BaTiO₃ (PTO/BTO) multilayers with a modulation wavelength (Lambda) that varies between 48 angstrom < (Lambda) < 360 angstrom. These samples were grown on MgO substrates buffered with 50 angstrom of SrTiO₃(STO). By modeling the x-ray diffraction patterns we have determined that the PTO layers are a-axis oriented and the BTO layers are c-axis oriented. This result is in contrast to individual thin films of PTO in which we find the c-axis to be perpendicular to the plane of the film, as well as with PTO/STO superlattices we have grown on the same buffered substrates. Raman spectroscopy reveals only PTO modes in the spectra of seven different PTO/BTO multilayers which, from symmetry arguments, is consistent with the x-ray model determination of a-PTO/c-BTO oriented superlattices. We also observe that the soft mode and the A₁(2TO) Raman line shift abruptly in frequency above (Lambda) equals 240 angstrom. We ascribe this to the possible strain relaxation in the multilayer, in which misfit dislocations appear at a critical wavelength (Lambda) _c. The E(2TO) line in the vicinity of 200 cm^-1 is (Lambda) dependent as well as symmetric and we have modeled it as a confined mode: a signature of the modulated structure.

We have used the reversible ferroelectric polarization of the perovskite oxide Pb(Zr_0.2Ti_0.8)O₃ in Pb(Zr_0.2Ti_0.8)O₃/Nd_1.13Ba_1.87Cu₃O_7-δ epitaxial heterostructures in order to electrostatically modulate the carrier concentration in thin Nd_1.13Ba_1.87Cu₃O₇ films via the ferroelectric field effect. For 100 angstrom thick Nd_1.13Ba_1.87Cu₃O₇ layers, we observe a correlated change in the normal state resistance and in the inverse Hall constant.

We report the magnetic field-induced superconductivity in Sr_{1- x}K_xBiO₃ superconductors. A reentrant superconducting- normal resistive transition is observed in certain samples and, by applying the external magnetic field (H) or increasing the current (I), the reentrant normal state goes back to the zero resistivity: the recovery of superconductivity by applying H or increasing I. The magnetotransport of investigation on different samples reveals an interesting relation between the observed field-induced superconductivity and the normal state transport properties. Possible origins of this unusual phenomenon in Sr_1-xK_xBiO₃ are discussed.

We report the analogies between the electrical properties at low temperatures of oxygen deficient cuprates and manganites thin films under illumination by UV or visible light. For the cuprates, a decrease of the oxygen content decreases the critical temperature of the transition from the normal to the superconducting state while for the manganites it decreases the transition temperature at which the insulator-metal transition occurs. For full oxygenated cuprates and manganites thin films there is no effect of the illumination on the electrical properties of the films. For small oxygen deficient cuprates (in the normal state) and manganites thin films, light increases substantially the conductivity leading to a persistent photoinduced conductivity (PPC) and the effect of persistent photoconductivity increases with oxygen deficiency. For high oxygen deficiency, the cuprates and the manganites are insulating. In that case, a transient photoconductivity is observed in the cuprates and manganites. Moreover, a photoinduced insulator- metal (I-M) transition appears in these insulating thin films.

In order to unveil the mechanism responsible for the large decrease of T_c ((Delta) T_c approximately equals -30 K) found in YSr₂Cu₃O_6+x with respect to YBa₂Cu₃O_6+x, we compare structural and electronic properties of these two cuprates. We report X-ray, neutron diffraction, resistivity and ac susceptibility data and results of ab initio electronic structure calculations carried out within the local density approximation. Main structural differences between the two phases are: (1) the absence of long CuO chains in YSr₂Cu₃O_6+x possibly due to a large orthorhombic distortion predicted by calculations; (2) the strong compression of the CuO₅ pyramids along the z- direction, which reduces the metallic character of the bond between the chain copper and the apical oxygen. This is expected to hinder the hole transfer from the CuO chains to the superconducting CuO₂ planes, thus reducing the effective doping of the planes with respect to the optimum doping level. Both the disorder associated with short CuO chains and the reduced hole transfer would qualitatively account for the observed reduction of T_c.

Correlation between the nanoscopic disorder in the nanostructures in the a-b planes of high T_c superconductors and the macroscopic properties has been studied. It has been found that nanostructures govern the temperature dependence of the transport and magnetic properties such as: the critical current density J_c(T), the superfluid density n_s(T)varies direct as1/(lambda) _ab²(T) and the normalized magnetic relaxation rate S(T). They also determine magnetic flux pinning: the dependence of the energy barrier on the current density U_eff(J). Irradiation with heavy ions affects the above properties through the change of the disorder in the nanostructure array. The studies have been performed on YBCO and TlBCCO thin films.

In La_2-xSr_xCuO₄(La214) and Bi₂Sr₂CaCu₂O_8+(delta ) (Bi2212), scanning tunneling spectroscopy (STS), break-junction tunneling spectroscopy (BJTS) and Raman scattering spectroscopy demonstrate that the superconducting-state energy gap (Delta) _o at T << T_c increases monotonically with the lowering of hole-doping level p even below the optimal doping p_o, where T_c begins to decrease. On the basis of electronic specific heat, magnetic susceptibility, resistivity and STS data in La214, it is suggested that a pseudogap of almost the same energy scale as (Delta) _o, a small energy scale pseudogap (SPG), will develop progressively below a certain temperature T* in La214 as in Bi2212. In the two systems, the size of SPG ((Delta) _PG^S), comparable to (Delta) _o, essentially scales with SPG temperature T*; (Delta) _PG^S approximately (Delta) _o approximately (2.3 +/- 0.3)k_BT*. We also report the relation among (Delta) _o, higher crossover temperature T_max (> T*) and the corresponding large energy scale pseudogap (LPG) (Delta) _PG^L; (Delta) _PG^L approximately 3k_BT_max approximately (alpha) (Delta) _o ((alpha) equals 3 to 4 for Bi2212 and (alpha) equals 6 - 8 for La214). Furthermore, we report that T_c almost scales with p(Delta) _o over a wide p range, except for high doping levels (p/p_{o approximately}> 1.3), where T_c is very close to the BCS expectation (T_c equals 2(Delta) _o/4.3k_B). We discuss some scenarios for the superconducting transition, where the energy scale in determining T_c is expected to be of order p(Delta) _o.

The structural and superconducting properties of HgBa₂CuO_4+(delta ) have been investigated over a large doping range (0 < (delta) < 0.25). Two series of samples were measured in which oxygen contents were varied through different annealing conditions. Both series are found to have non-parabolic doping behavior as well as unusual structural phenomena in the underdoped region and at maximum T_c. Samples of identical oxygen content are found to have variations in T_c large as 30 K. These differences in superconducting and structural properties are evidence of oxygen/charge ordering present in HgBa₂CuO_4+(delta ). Results are discussed in terms of structural distortions present and possible charge ordering and the formation of filamentary condensates.

The recently discovered superconductivity in the titled materials, at 45 K and 90 K, respectively, is almost as surprising phenomenologically as high-temperature superconductivity (HTSC) itself. Nevertheless, the microscopic theory of filamentary intermediate phases, recently developed for semiconductor impurity bands and layered HTSC cuprates, and even for network glasses, explains the existence of these phenomena with no new assumptions. The ultimate mechanism responsible for filamentary formation is self-organization, which minimizes the dopant-related free energy at the formation temperature.

We discuss a new class of phenomena based on strong interaction between magnetic flux from magnetic textures and supercurrents in emerging mesoscopic, heterogeneous magneto-superconducting systems. These systems include coupled smooth and textured magnetic/superconducting films, magnetic dots, columns, stripes embedded in super-conducting material, etc. The frozen magnetic flux from magnetic textures or topological defects can pin vortices or create them, changing drastically the properties of the superconductor. On the other hand, the magnetic field from supercurrents (vortices) strongly interacts with the magnetic subsystem. This results in spontaneous ground state supercurrents and in formation of coupled magnetic-superconducting topological defects. We discuss possible experimental realization of mesoscopic magneto-superconducting systems.

Many epitaxial heterostructures of transition metal oxides with perovskite structure can be found in literature, accounting for the great interest on electronic devices based on correlated materials. Within this context, the role of traditional semiconductors (Si, Ge) can be played by SrTiO₃, which can become metallic with a carrier concentration as low as 10¹⁸ cm^-3 and with an electron mobility as high as 10⁴ cm²/(V(DOT)s), comparable to the one commonly found in silicon. First, we studied the dramatic effect of oxygen deficiency on transport properties of SrTiO_3-(delta ) homoepitaxial thin films deposited by Pulsed Laser Deposition (PLD) in Ultra High Vacuum (UHV) conditions. Then, we explored the feasibility of employing e-doped strontium titanate as semiconducting layer in field effect Metal-Insulator- Semiconductor (MIS) heterostructures. We deposited MIS epitaxial heterostructures, where the wide band gap insulating layer was made of MgO (E_gap approximately equals 8 eV). Field effect measurements performed by an a.c. technique showed an increase in conductance up to 90% at 6 Volts of gate voltage. This promising result could open new perspectives in crystalline oxides electronics.

Superconducting multilayers, whose growth and structure are determined by the kinetic deposition and not by thermodynamics, are a powerful tool for investigating High T_c superconductors (HTSC) properties and for possible future applications. By means of Pulsed Laser Deposition (PLD), we grew such materials alternating infinite layer phases both pure and doped, to supply CuO₂ planes and charge reservoir (CR) blocks respectively. We found out that using CR chemically doped with high concentrations of Ag and Sc substituted on the copper site in infinite layer compounds leads to a semiconducting behavior, whereas oxygen doping seems to be much more effective, since the BaCuO₂ infinite phase, grown in high oxygen partial pressure, does actually bring about a superconducting behavior. We deposited BaCuO₂/CaCuO₂ superlattices with various periodicities and studied their transport and structural properties. By varying the deposition parameters, we reached both kinetic and thermodynamic growth and we observed semiconducting and superconducting behaviors.

Temperature variations of dielectric and/or ferroelectric properties were evaluated for the continuous single c-domain/single crystal thin films of Pb-Ti-O families, PbTiO₃ (PT) and Pb_0.8La_0.2TiO_0.95 [PLT(20)]. The thin films of Pb-Ti- O families, 100 to 300 nm in film thickness, were epitaxially grown on the miscut (001)SrTiO₃) (ST)substrates by using rf-magnetron sputtering. The sputtered PT and PLT(20) thin films were tetragonally deformed, but the thin films were tightly bonded to the substrates. These thin films did not show the temperature anomaly in the lattice parameter unlikely to the bulk ferroelectric materials. The dielectric properties of the Pb-Ti-O thin films were evaluated in a Au/Pb-Ti-O/SRO/ST heterostructure with evaporated Au top electrodes and sputtered SRO (SrRuO₃) base electrodes. The dielectric measurements showed the hysteresis feature at the heating up and cooling down stage probably due to the temperature change of their interfacial structure. The temperature-dielectric properties of the PT thin films showed the diffused peak at about 520 degree(s)C - 525 degree(s)C, which would corresponded the Curie temperature. The P/E hysteresis curves of the PT thin films showed zero polarization at 525 degree(s)C. The similar diffused temperature anomaly was also observed for the PLT(20) thin films.

Thin films of Pb-Ti-O families, PbTiO₃ (PT) and (Pb,La)TiO₃ (PLT), were grown at 600 degree(s)C on (100)MgO substrates by rf- magnetron sputtering at various cooling rates (3 degree(s)C/min to 33 degree(s)C/min). It is found that c-axis orientation of the PT thin films was enhanced by the increase of the cooling rates and the a- axis orientation of PT thin films was enhanced by the decrease of the cooling rates. While, c-axis orientation of the PLT thin films was achieved independent of the cooling rates. The growth mechanism of the a-axis orientation of PT thin films will be governed by an epitaxial growth and the c-axis orientation will be caused by the preferred orientation. The cooling rates are essential for the control of the orientation of PT thin films on (100)MgO substrates.

A model is presented to explain the recently observed voltage jumps in BISCCO superconducting thin films. The current approach provides an additional evidence to the validity of the flux-flow instability model. The field and temperature dependencies of the critical vortex velocity were attributed to the physics of the vortex core in clean superconductors (high-Tc superconductors) which is very different from the physics of the vortex core in dirty superconductors. The quasiparticles diffusion coefficient D_qp was calculated based on the assumption that the quasiparticle motion is considered as a transport phenomena on random walks. In addition, the effects of scattering of normal excitations by quasiparticles localized in the vortex core moving together with the vortex on the inelastic scattering time of quasiparticles (Tau) _in was also calculated. The predictions of the model and the experimental observations are in excellent agreements.

In light of recent experiments study of the collective excitations in unconventional superconductors (USC) becomes very important. We build by path integration technique 2D and 3D models of p- and d- pairing for superfluids and superconductors (SC). Within these models we calculate the collective excitations in different USC [high temperature superconductors (HTSC), heavy fermion superconductors (HFSC) etc.] under p- and d-pairing. We consider both bulk and 2D systems. Some recent ideas concerning realization in HTSC of the mixtures of different states are investigated. In particular, we consider the mixture of d _x ² _-y ² and d _xy states in HTSC. Obtained results could be used for interpretation of the sound and microwave absorption data as well as for identification of the type of pairing and order parameter in unconventional superconductors.

A novel cation-exchange process has been developed for synthesis of bulks and epitaxy of thin films of highly volatile Hg-based high- temperature superconductors (Hg-HTS's). Unlike in a conventional thermal reaction process, precursor matrices of Tl-based high- temperature superconductors were employed in the cation exchange process and annealed in Hg vapors, in which Tl cations were perturbed thermally and replaced subsequently by the Hg cations. A large processing window has been observed suitable for such an atomic perturbation process. This allows Hg-HTS's to inherit high- quality crystalline structures, surface and film/substrate interface morphologies from their Tl-HTS's precursor matrices, eliminating difficulties in synthesis and epitaxy of Hg-HTS's associated with the high volatility of Hg-related compounds. Nearly pure phase HgBa₂CaCu₂O_6+(delta ) and HgBa₂Ca₂Cu₃O_8+(delta )bulks and epitaxial thin films were obtained reproducibly with exciting physical properties including high T_c's, high J_c's, and low microwave surface resistance. This cation-exchange technique may also provide a general scheme for synthesis of other volatile compounds with pre-designed structure and composition.

We have performed several experiments to explore the role of local lattice in the cuprate superconductors. The Cu K-edge x-ray absorption fine structure (EXAFS) results provide a direct evidence for the Jahn-Teller polaronic distortions of the CuO₂ lattice in these materials. The Jahn-Teller polarons in the cuprates are found to be associated with Q₂ mode which is degenerate with the Q₃ Jahn-Teller mode. It is discussed that the Jahn-Teller polarons get ordered in the stripes. The temperature dependent Cu K-edge X-ray absorption near edge structure (XANES) measurements reveal a particular change in the local lattice at the charge stripe ordering temperature in different superconducting cuprate systems. This response of the local lattice to the stripe charge ordering has been used to study oxygen isotope effect (¹⁶O yields ¹⁸O) in the La_1.94Sr_0.06CuO₄ superconducting system. The results show that the isotope substitution introduces a large increase to the onset temperature of the charge-stripe ordering by approximately 60 K, providing a compelling evidence for the vital role of the electron-lattice interactions to be an important ingredient for the charge-stripe ordering in the high T_c cuprate superconductors.

Emphasis on the analysis of high T_c phase diagrams is made by showing coupling between the charge and spin degrees of freedom. Based on an improved SU(2) slave-boson approach which shows the coupling, we derive a satisfactory phase diagram of high T_c cuprates which displays both the superconducting and pseudogap phases in the plane of temperature vs. hole doping rate.