The effects of thermal pairing fluctuations on the phase transition in extreme type II superconductors with pronounced uniaxial anisotropy are examined in zero magnetic field. Approaching T_c from below, there is the crossover from quasi-2D to 3D xy-behavior, which belongs to the universality class of an uncharged superfluid. Because phase fluctuations destroy long-range order in strictly 2D systems the fluctuation dominated regime is rather large. Approaching from above, the same crossover occurs, but in this regime the fluctuations in the magnitude of the pairing field dominate. Using the Gaussian approximation we obtain the relevant correlation lengths and show the importance of fluctuations and anisotropy.

Recent neutron data on the disappearance of long-range order in the vortex lattice of BSCCO at fields of order 1 kGauss is interpreted in terms of the results of a monte carlo simulation of the thermodynamics of the vortex lattice in a pure system. We show that the neutron data is consistent with a disorder-driven phase transition from a vortex crystal at low fields to a vortex glass at higher fields.

The frequency of the plasma with the c axis polarization in the cuprate superconductors is extremely low and it is lower than the superconducting energy gap in some cuprate. Electromagnetic phenomena caused by the excitation of the plasma are theoretically studied. The light passing through a film of the cuprate superconductor strongly interacts with the transverse component of the plasma. The interference effect of the plasma waves reflected at both sides of the film causes the oscillatory frequency dependence of the reflectivity and transmissivity. When vortices are introduced by an external magnetic field, the vortex motion and the gapless excitation inside the vortex normal cores make the reflectivity and transmissivity strongly field dependent. When a static voltage is applied to a Josephson junction of the cuprate superconductor, the plasma is excited by the ac Josephson effect and the excited plasma decays by emitting light. The excited plasma brings about an anomalous current-voltage characteristics in a weak magnetic field.

We report the energy gap structure and density-of-states (DOS) of a model layered superconductor with one superconducting layer and one normal layer in a unit cell along the c- axis. In the physically interesting parameter range where the interlayer hopping strengths of the quasiparticles are comparable to the critical temperature, the peaks in the DOS curve do not correspond to the order parameter (OP) of the superconducting layer, but depend on the OP and the band dispersion in the c-direction in a complex manner. In contrast to a BCS superconductor, the DOS of layered systems have logarithmic singularities. Our simulated tunneling characteristics bear close resemblance to experimental results.

The Takahashi-Tachiki equations, describing the critical properties of proximity effect systems in the dirty limit, are solved exactly using the full eigenfunction expansion. Both parallel and perpendicular critical fields are calculated. The theory is applied to experimental data of Kanoda et al. for V/Ag and of Chun et al. for Nb/Cu, using the T_c of the superconducting material and the diffusion coefficients of both materials as fit parameters. For the 3D systems the fits compare nicely with the experimental results, but for 2D systems this is not always the case. It is found that the V and Nb critical temperatures necessary to fit the data can be larger than the corresponding bulk critical temperatures. This contrasts with what has been observed for single V and Nb films. Additionally, the diffusion coefficients resulting from the fit procedure are found to be much smaller than one would expect from resistivity measurements. In the light of these anomalies earlier less complete calculations are reconsidered. It turns out that several assertions are superficial and cannot be affirmed by the results of the present work.

We present a theoretical study of frequency and line shape for collective excitation of charge carriers below T for cuprate superconductors ..IP La SrCuC and like YBa2Cu3()7. using layered electron ga tide1. The collective excitations are studied in the frequency range h 2t QVF< T < T, where A, q. v. and T are respectively the component of wave vector in a—b plane, binding energy of Cooper pair, Fermi velocity and the temperature. The polarizability for a two dimensional fluid comprising of paired as well as unpaired electrons is calculated and used to calculate the imaginary part of inverse dielectric response function, which gives rise to the frequency and line shapes of collective excitations.The plasrnon line shapes of a layered electron gas are found very similar in nature to that of three dimensional electron gas below T.

We report on the behavior of high T_c superconductor coupled multilayer structures in magnetic field perpendicular to the ab plane. In these structures the number N of superconducting layers in the sample has been systematically varied. We extract the activation energy for flux motion from resistive measurements of the superconducting transitions. For samples containing N 24 angstroms thick DyBa₂Cu₃O₇ layers, each separated from the next by 96 angstroms of an (formula available in paper) alloy, we find that the activation energy U increases linearly with the number N of layers for N < 3 - 4, and then saturates. The linear increase of U is taken as an evidence that coupling through (formula available in paper) increases the correlated volume involved in the flux motion and thus U. For N equals 3 stacked pancake vortices move rigidly and the vortex lattice is 2D. For N > 3 - 4 shear of the vortex lattice becomes important, the vortex lattice dimensionally crosses-over to 3D, and the activation energy saturates.

Epitaxial multilayer thin films of SrCuO₂/(Sr,Ca)CuO₂ and (Sr,Ca)CuO₂/(Sr,Ca)RuO₃ have been prepared on (100) SrTiO₃ single crystal by a multitarget RF magnetron sputtering. The XRD measurements revealed that the heteroepitaxial growth of c-plane of the infinite-layer SrCuO₂ or a-plane of perovskite (Sr,Ca)RuO₃ was confirmed with c-plane of infinite-layer (Sr,Ca)CuO₂ on (100) SrTiO₃ substrate surface. Both multilayer films were successfully constructed as we designed with a minimum layer thickness of 10 angstroms. In the TEM images of the SrCuO₂/(Sr,Ca)CuO₂ multilayer, there existed planar dislocations being parallel to the ac- and bc-planes, crossing the boundaries of SrCuO₂ and (Sr,Ca)CuO₂ layers. On the other hand, TEM measurements of the (Sr,Ca)CuO₂/(Sr,Ca)RuO₃ multilayers indicated that there was no dislocation which exists commonly in the infinite-layer films. Resistivities of multilayer films at room temperature were ranging from 1 to 100 m(Omega) cm and showed semiconductor-like dependence against the temperature.

Heterostructures made of insulating PrBa₂Cu₃O₇ (PBCO) and superconducting YBa₂Cu₃O₇ (YBCO) compounds, such as PBCO/YBCO/PBCO trilayers and PBCO/YBCO superlattices, were produced by an in-situ laser ablation technique. Deposition rates were carefully adjusted, for each sample, to ensure nominal integer coverages. This minimizes interface alloying and avoids chemical overmodulation when using periodic deposition sequences. Epitaxy, chemical order, c-axis and off axis crystalline coherence were controlled by exhaustive X-ray diffraction analysis on each sample. Inductive measurements were performed on these samples in order to determine their kinetic inductance. The temperature dependence of the magnetic penetration depth was obtained from the kinetic inductance data. The results are discussed in terms of Berezinskii-Kosterlitz-Thouless transition and zero point charge fluctuations.

Epitaxially grown multi-layers of Bi₂Sr₂CaCu₂O_y (Bi-2212) and Bi₂Sr₂CuO_x (Bi-2201) have been investigated referring to those microstructure and transport properties. Thin films have been prepared by a RF magnetron sputtering technique and laminated as the superconducting and non-superconducting layers. The samples have a variety of film thickness of each layer. The surface morphology, crystallization, diffusion and layered structures of these multi-layers were evaluated by TEM, AES, SIMS, STM-AFM and X-ray diffraction measurements. BSCO films exhibit normal conducting transport property even at 4.2 K. The superconducting properties such as change of Tc, a broadening of the transition and anisotropy in those properties under magnetic field were discussed. Those results suggested that the structural and electronic condition in N-layer seriously affect the superconducting interlayer coupling between superconducting layers through non- superconducting layers.

Conductance measurements on YBa₂Cu₃O_7-(delta ) (YBCO) between layers of Y_1-xPr_xBa₂Cu₃O_{7- (delta} ) [(Y-Pr)BCO] show a transition from a bulk regime in the interior of the YBCO to a surface regime near the interfaces. The depression of the zero-resistance transition temperature in ultrathin YBCO is correlated with the depressed conductance in the surface layer. The results indicate that the changes are related to the presence of the interfaces, primarily to charge transfer between the layers, with little, it any, indication of a change in the intrinsic properties of the YBCO from bulk down to the thickness of a single unit cell.

We have studied microstructures and superconductivity in ultrathin YBa₂Cu₃O_7-(delta ) (YBCO) films. We focus on the issues of terminating layer and superconductivity in surface layer of the film.

Multilayered YBa₂Cu₃O_7-(delta ) (YBCO)/LaAlO₃/YBCO films deposited on (100)-oriented LaAlO₃ substrates have been studied for potential use in microwave circuits.

We have investigated the response of layered superconductors to an external magnetic field using the semiclassical phase approximation.

We show experimental evidence that the associated flux motion occurs as a result of a crossover from 3D vortex lines to 2D independent pancake-like vortices, residing in the Cu-O layers. This 3D to 2D crossover occurs after k_BT exceeds the Josephson coupling energy.

The effect of quantum fluctuations of vortices on the low temperature specific heat and reversible magnetization in the mixed state in highly anisotropic layered superconductors is discussed. For reversible magnetization, M, the change of slope in the dependence of M vs InB, observed in Bi(2:2:1:2) single crystals, is explained. In the mean field approach this slope should be almost B independent. We show that for magnetization quantum fluctuations are important at all temperatures except in a narrow region near T_c. The specific heat due to the vortex fluctuation contribution is predicted to be linear in T at low T and to increase logarithmically with B.

Recent progress in the proximity effect theory of superconductor-ferromagnet superlattices is reviewed. The phase diagram calculations, transition temperature Tc and upper critical fields (formula available in paper), are presented. Characteristic features in Tc and Hc2(T) dependence on layers thicknesses, including the predicted unusual oscillatory variations and new inhomogeneous superconducting state with nontrivial phase difference between neighboring superconducting layers, are discussed and compared with experimental data for V/Fe and Nb/Gd superlattices.

We theoretically study the vortex states in superconducting multilayers whose components are superconductors with nearly same transition temperatures but very different upper critical fields. We solve the nonlinear Ginzburg-Landau equations numerically for the vortex lattice state of the multilayers. Two kinds of different vortex lattice states appear between the lower and upper critical fields. From the free energy calculations, we determine the phase boundary between the two vortex lattice states and map a phase diagram in the H-T plane. The peak effect of the critical current observed in Nb/NbZr multilayers is discussed in connection with the vortex states in these multilayers.

This paper presents a study of the multilayered high-T_c cuprate semiconductors based on a model where the plasmon mediated effective interaction provides an attraction between the charge carriers.

Ultrathin ReBa₂Cu₃O_7-(delta ) (ReBCO) (Re equals Y, Nd) films in the form of trilayer sandwiches between (Pr_xY_1-x)Ba₂Cu₃O_{7- (delta} )((Pr_xY_1-x)BCO) (1 >= x >= 0) and YBCO/(Pr_xY_1-x)BCO superlattices with nominal YBCO layer thickness as thin as 1 unit- cell were grown by pulsed laser deposition. It was found that although the 1-unit-cell thick YBCO films were superconducting, the T_c value depends strongly on the doping level x in the adjacent layers. The T_c value increases when x is reduced. A number of effects, either extrinsic or intrinsic in origin, have been examined by various experiments. The results indicate that the lattice mismatch, interdiffusion between Y and Pr atoms, and oxygen deficiency are unlikely to be the primary reason. Charge redistribution and the possibility of K-T transition were suggested by Raman and transport measurement, but the effects observed may not be sufficient to explain the change of zero resistance temperature and the broadening of the transition in these sample structures.

Very precise artificial structuring of high T_c heterostructures is possible using atomic layer-by-layer molecular beam epitaxy. Cuprates are combined with other oxides, such as titanates, to make atomically precise heterostructures for studying transport and interfacial effects. Titanate slabs as thin as one unit cell thick can be grown without pinholes and provide tunneling barriers for c-axis transport. Single doped unit cells of BSCCO-2212 can also be used as barriers. These give SNS Josephson junctions at temperatures as high as 65 K. Since the crystallographic structure of the barrier is identical to the structure of the 2212 electrode material, it is easily possible to stack more than one junction in close proximity. This results in phase-locked operation of two junctions together.

We report on the behavior of high T_c superconductors coupled multilayer structures in magnetic field parallel and perpendicular to the ab plane. These structures consist of N 24 angstroms thick DyBa₂Cu₃O₇ layers, each separated from the next by 96 angstroms of an (formula available in paper) alloy. This alloy allows a coupling of the superconducting layers as demonstrated by measurements of the activation energies for flux motion U. For a N equals 15 sample one finds, in perpendicular fields, that a correlated motion of pancake vortices occurs with a characteristic length of about 480 angstroms. Additionally, U is found to be proportional to logB at low fields while at higher fields a power law behavior is observed. In parallel field we find U to be field independent up to 1 T, suggesting that the vortices are trapped at low fields in the material where weak superconductivity is induced by proximity effect.

Superconductor/insulator (SI) superlattices may be viewed as repeated (or stacked) SIS junctions connected in series. Such superlattices have a number of promising applications which we will survey. In the weakly coupled (tunnel junction) limit the possibility of fabricating high resolution X-ray detectors is being studied. For more strongly coupled junctions involving Josephson coupling, possible applications are numerous. The suggestion of Auvil and Ketterson that an SI multilayer would function as an efficient radiation source (due to coherent radiation from the layers and an increased junction phase velocity) has yet to be studied experimentally. Rippert and Ketterson have proposed that a phonon maser might be realized involving 2(Delta) recombination phonons and internal feedback via the superlattice Bragg mirror. Lomatch et al. have suggested that the added kinetic inductance of a multilayer could be used to eliminate stripline inductors from a Josephson transmission line. Finally multilayer SI tapes have been shown to have an enhanced critical current density, J_c. The mechanisms involved in the above devices and the status of efforts to fabricate them are reviewed.

The temperature and field dependence of the flux creep rate S(T,H)=M0dMJd(lnt) and critical current j(T,H) have been measured in low- Tc (Pb/Ge) and high- Tc (YBa2Cu3O7IPrBa2Cu3O7) (YBCOIPrBCO) multilayers with different coupling strength between the superconducting layers. By taking thicker insulating separator layers, the dimensional crossover from an anisotropic 3D to a quasi 2D behaviour is induced. The detailed analysis of the S(T,H) and j(T,H) dependence made it possible to identify the pinning regimes as predicted by the collective pinning theory. In YBCO/PrBCO multilayers with completely decoupled YBCO planes, a strong enhancement of the quantum flux creep rate S(T—O) has been observed. The value of S(T—O)3% in the 3D YBCO superconductor and increases up to S(T—*O) 2O% in the quasi 2D YBCOIPrBCO multilayers. We also used magnetic fields H<H3D÷2D to decouple the Cu02 planes in Bi2Sr2CaCu2O8 (BSCCO) single crystals. A great similarity between the S(T,H0) andjc(T,Ho) curves has been found in BSCCO, with the superconducting layers decoupled by the applied field, and in the YBCOIPrBCO with the YBCO layers decoupled by inserting thick insulating PrBCO layers.

We report the first direct experimental observation of the Josephson properties of Nb/Cu multilayers achieved by measurements across layers. Multilayered sandwiches with a small crossection (20 pm in diameter) consisting of 1 0 Nb/Cu/Nb junctions in series were fabricated for such measurements. Applying RF power we observed Shapiro steps in the current-voltage characteristics. Changing the temperature we have seen the sequential change in the principal Shapiro step voltage representing the change in the number of the phase-locked junctions. This phenomenon is a direct evidence for the dimensional 3D-2D crossover responsible for the subdivision of the sample in the individual layers. Simultaneously with the change of the RF step voltage, the behavior of the I-V characteristics change. For T < T2D hysteresis appears in the I-V characteristics and the slope of the tem perature dependence of the Josephson critical current, I (T), changes.

Superconducting tunnel junctions have a potential, statistically limited, energy resolution on the order of eV's. The best results to date, however, have been an order of magnitude worse than this and required operating temperatures on the order of 0.1 K. Niobium based junctions operating at approximately 1 K have shown X-ray detection capabilities, but have only achieved energy resolutions on the order of 100 eV's at best. Several mechanisms, including quasiparticle self trapping, loss of `hot' excitations, quasiparticle recombination, and loss of `cold' excitations, have been proposed to explain the degradation of energy resolution in these devices. We will present a design concept for an X-ray detector, along with recent experimental and computer modeling results, based on a 1D superlattice of superconducting tunnel junctions. This multilayered superconducting tunnel junction design has the potential for alleviating many of the potential resolution degrading mechanisms while operating in the 1 K temperature range. In addition, the possibility of engineering the device to improve the signal to noise ratio of the output and to control the transport of phonons in the structure will be discussed.

We investigated the superconducting properties of V/Cr-multilayers grown epitaxially on MgO(001) and Al₂O₂(1120) substrates. The samples with a preferred orientation of (001) and (110) were fabricated on MgO and Al₂O₂, respectively. The Cr-layer thickness was varied from d_Cr equals 2 to 100 angstroms under 300 angstroms thick V- layer. The critical temperature for (001)V/Cr abruptly drops in the range of d_Cr equals 5 to 30 angstroms, becoming a plateau above it and keeps lower than that for (110)V/Cr, in which T_c is gradually decreased up to d_Cr equals 80 angstroms. The dimensional crossover phenomenon on the parallel upper critical fields H_C2 takes place in the range of d_Cr equals 20 approximately 70 angstroms and 5 approximately 25 angstroms for (110)V/Cr and (001)V/Cr, respectively. The superconducting layers are decoupled by d_Cr equals 30 angstroms for (001) and 80 angstroms for (110). The multilayers with (001) texture exhibit a little larger anisotropy for H_C2 in such decoupling region compared with the case of (110). The strong pair breaking effect for (001) samples may be caused by the magnetic structure of Cr(001) surface.

Flux quantum logic and memory circuits using superconducting Josephson tunnel junctions have high-speed switching times (approximately 1 ps), low power dissipation (< 1 (mu) W per circuit) and low levels of thermally induced electrical noise. Current designs of such circuits employ single trilayer junctions, which impose circuit size and logic threshold limitations. A new design component, the multilayered tunnel junction, consists of a vertically stacked array (a 1D superlattice) of Josephson tunnel junctions. The introduction of multilayered junctions into superconducting electronic circuitry offers a reduction in the current device size, fault tolerances, and new device applications. We present numerical simulations of simple circuits employing multilayered Josephson junctions as design components. Comparison with conventional single flux quantum circuitry is discussed. We also present preliminary measurements of multilayered Josephson junctions fabricated for use in flux quantum devices.

The superconducting multilayer Nb/NbZr is known to show an upturn feature in the parallel upper critical field H_c2(T) at a characteristic temperature T⁺ with decreasing T. This phenomenon occurs due to the change of the position for nucleation of order parameter (Delta) (r) from Nb to NbZr layers. In order to understand the behavior of the localized (Delta) (r) in the mixed state, we have measured the critical current density J_c of Nb/NbZr as a function of H and of angle between H and film plane. In this system, the peak effect has been reported in J_c(H) above T⁺ by previous work. In this work, besides this peak, two kinds of kink structures in J_c(H) are observed at fields H_L1 and H_L2 only in the temperatures above T⁺.H_L1(T) and H_L2(T) are plotted around the extrapolated line from H_c2(T) above T⁺. From the field dependence of the angular derivative of J_c and the discussion about the free energy in the system, we will show the possibility of the phase transition at these characteristic fields.

We report on superconducting fluctuations and weak localization effects in amorphous Si/Nb multilayer films in order to investigate the coupling between different superconducting layers. We found that the coupling effect is strongly related to the transport parameters and the inelastic scattering time gives important information related to the mechanism of the coupling.

We investigate the effect of the diamagnetic fluctuating current above T_c in superconducting multilayers on the basis of the GL theory. The expression for the conductance enhanced by the diamagnetic fluctuations of the electromagnetic field is derived within the Gaussian approximation for the fluctuations of the order parameter.

We have shown, within the Ginzburg-Landau theory, that the interaction between a single vortex and a normal metal microdefect in the high T_c superconducting matrix can cause high critical current densities j_c. The interaction is primarily magnetic, except at very low temperatures T, where the core interaction is dominant. For a lattice of vortices commensurate with an array of defects in a parallel magnetic field H, strong magnetic pinning is obtained, with a non-monotonic critical current dependence on H, and with j_c of the order of 10⁷ - 10⁸ A/cm².

We consider vortex lattice interaction with transverse surface magnetoacoustic wave in high- T_c superconductor--ferrite structure. It has been found that the magnetoacoustic waves excited in the ferrite can be efficiently coupled with vortex structure in superconducting film. The nonlinear effect of vortex drift as well as the possibility of the wave amplification are discussed.

The proximity effect in S/N multilayer is studied theoreticalily for the general case of arbitrary transparency of NS boundary and arbitrary parameters of N and S materials. Small mean free paths are assumed in both metals (dirty limit). The approach based on the microscopic Usadel equations is used. The main subject of the paper is the crossover from strongly coupled layers to the case of complete decoupling. For the simplest case of thin N and S layers the well-known approximations can be derived from the Usadel equations: the Cooper limit for the case of highly transparent NS boundaries and the McMillan tunnelling model for the case of small transparency. The limits of applicability of both approximations are discussed, as well as that of the Werthamer (one-frequency) approximation. In general case the complete set of equations of the model is solved. Critical temperature of a multilayer and densities of states at different layers are calculated numerically as a function ofNS boundary transparency.

(2223)₂/(2234)₂ superlattices have been synthesized by three target sequential sputter deposition of BiO, SrCu_0.5O_1.5 and CaCuO₂ layers. The superstructure formed was analyzed in atomic layer scale by using some X-ray diffraction technique, such as interference of X-ray specular reflection, observation of the Laue's function pattern and a quantitative analysis of intergrowth. The superlattice, thus confirmed in the atomic layer scale, has a superconducting transition temperature (T_c) as high as the average of monophasic 2223 and 2234 films. The resistivity showed a similar angular dependence on the magnetic field orientation even at the temperature higher than the T_c of 2234. These results will be discussed by the hole diffusion and/or the proximity effect between the 2223 and 2234 layers.

We have fabricated and investigated three-terminal Nb/Al-AlO_x-Nb/Al-AlO_x-Nb tunnel devices with a thin (approximately equals 13 nm) Nb/Al middle layer. By measuring I-V characteristics and I_c vs. H dependences of the whole device and particular junctions separately, we have demonstrated strong interaction between the junctions. As a result, at V equals 0 the system behaves like a single junction. The pronounced feature of this state is the critical current locking of the junctions, with a `common' critical current larger than the critical currents of the individual junctions. This state can be characterized by identical phase difference distributions for both junctions, and is stable at least for the region of superconducting current and magnetic field values corresponding to the first period of the diffraction pattern of the whole structure. At V equals 0 and some value of magnetic field near to that where critical current of the system first falls to zero, the system undergoes a transition into the 2D state, in which the individual junctions have different phase distributions and behave relatively independently. In a high magnetic field, when many vortices enter the multilayer structure, along with the current locking, there is evidence of phase locking.

Novel electronic properties are observed in metals in contact with the ferromagnetic insulator europium monoxide (EuO). We have successfully prepared epitaxial bilayers and multilayers involving EuO and the metals V or Ag. With metal layers of 100 angstroms thickness or less, these structures exhibit a slope discontinuity in their resistance versus temperature at 90 K for V/EuO interfaces and at 50 K for Ag/EuO interfaces. For metal layers less than about 40 angstroms, a feature is also apparent in R(T) at 70 K, the Curie temperature of bulk EuO. The slope discontinuities are reminiscent of those observed at the Curie temperature of ferromagnetic metals. Kerr effect measurements confirm that the EuO exhibits ferromagnetic ordering at 70 K. In V/EuO structures, the V still displays superconductivity with a slightly depressed transition temperature. This suggests that such metal/ferromagnetic insulator structures may be used to prepare artificial ferromagnetic superconductors. The resistive transition is incomplete, suggesting small regions of isolated superconductivity within the sample, as has been predicted for certain special cases when a superconductor is in proximity to a ferromagnet.

A quantitative theory of the plasma resonance in layered metals is presented. It is shown that electron-impurity scaterring can suppress the plasma resonance in the normal state and sharpen it in the supercondicting state.