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Showing papers in "Journal of Superconductivity in 1995"


Journal ArticleDOI
TL;DR: In this article, a method is presented which is able to separate electron dynamics at the weak links from their strength and distribution, which explains the observed, finite rf residual lossesRres(T∼0) quantitatively and as a function of material parameters, temperatureT, fieldH, and frequencyΩ for Nb, NbN, and cuprate superconductors.
Abstract: High-frequency experiments depend sensitively on homogeneous and inhomogeneous “defects” in the normal and superconducting state. As homogeneous effects, the intrinsic scattering time is of great theoretical importance above 0.1 THz in the surface impedanceZ. Of practical importance are the planar defects, “the weak links (WL),” which interrupt the rf shielding currents and thus enhanceZeff. In the superconducting state, the Josephson currentjcJ crosses the WL in parallel with the normal, leakage currentjbl. The latter explains the observed, finite rf residual lossesRres(T∼0) quantitatively and as a function of material parameters, temperatureT, fieldH, and frequencyΩ for Nb, NbN, and cuprate superconductors. With increasing field,Z deteriorates likeH2 up toHcIJ, where Josephson fluxons (JF) penetrate into the WL yieldingδZ∫ αH. Above HcIJ≈0.1–10 mT, JF dynamics dominatesZ with hysteresis losses and reactive components. The nonlinear JF effects are enforced by thin-film edge enhancements limiting the performance of various devices by enhanced dissipation, reactance, and flux noise. A method is presented which is able to separate electron dynamics at the WL from their strength and distribution.

117 citations


Journal ArticleDOI
TL;DR: The BiO3 (BKBO) superconductor is cubic with a coherence length of 30-60 A. The basic properties of this compound are reviewed in this paper.
Abstract: Ba1−x K x BiO3(BKBO) has aT c (onset) of 34 K. It is the highest-temperature oxide superconductor which is cubic, with a coherence length of 30–60 A. The basic properties of this compound are reviewed.

51 citations


Journal ArticleDOI
TL;DR: In this paper, the Lattice dynamics of Ba.4BiO3 were investigated by inelastic neutron scattering on a superconducting single crystal (T c =26 K (midpoint)).
Abstract: The Lattice dynamics of Ba.6K.4BiO3 was investigated by inelastic neutron scattering on a superconducting single crystal (T c =26 K (midpoint)). At low frequencies the dispersion curves are very similar to those observed in BaPb.75Bi.25O3. Differences were found in the bond bending vibrations of the BiO6 octahedra which indicate that the binding in the K-doped compound is more ionic. Rather anomalous features were observed in the high frequency Bi-O bond stretching vibrations which resemble those observed in the high T c cuprates La1.85Sr.15CuO4 and YBa2Cu3O7. The observed frequency shifts are interpreted as the consequence from a strong electron phonon coupling. The data are compared to the results obtained on non superconducting Ba.98K.02BiO3.

32 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured the nonlinear surface impedanceZs in a number of YBa2Cu3O7−x thin films as a function of frequency from 1 to 18 GHz, surface magnetic fieldHrf to 1500 Oe, and temperature from 4 K to Tc. The results at lowHrf were shown to agree quantitatively with a modified coupled-grain model and at highHrf with hysteresis-loss calculations using the Bean critical-state model applied to a thin strip.
Abstract: High-Tc thin films continue to be of interest for passive device applications at microwave frequencies, but nonlinear effects may limit the performance. To understand these effects we have measured the nonlinear surface impedanceZs in a number of YBa2Cu3O7−x thin films as a function of frequency from 1 to 18 GHz, rf surface magnetic fieldHrf to 1500 Oe, and temperature from 4 K toTc. The results at lowHrf are shown to agree quantitatively with a modified coupled-grain model and at highHrf with hysteresis-loss calculations using the Bean critical-state model applied to a thin strip. The loss mechanisms are extrinsic properties resulting from defects in the films. We also report preliminary measurements of the nonlinear impedance of Josephson junctions, and the results are related to the models of nonlinearZS. The implications of nonlinearZS for devices are discussed using the example of a five-pole bandpass filter.

32 citations


Journal ArticleDOI
TL;DR: In this article, the copper isotope effect (63Cu −65Cu) was investigated for oxygen-deficient YBa2Cu3O7−δ with transition temperatures between 40 and 91 K.
Abstract: The copper isotope effect (63Cu —65Cu) was investigated for oxygen-deficient YBa2Cu3O7−δ with transition temperatures between 40 and 91 K. The isotope exponent αCu=−ΔlnTc/ΔlnmCu is negative for all transition temperatures. It is small, αCu≈−0.14, in the 60 K plateau, and unobservable for Tc=91 K. In regions away from these special conditions, αCu reaches large values of up to αCu=−0.4.

27 citations


Journal ArticleDOI
TL;DR: In this paper, the increased thermal conductivity below Tc in YBa2Cu3O7−x (YBCO) is shown to agree with the microwave electrical conductivity, affirming that increase is due to quasiparticle conduction.
Abstract: The increased thermal conductivity below Tc in YBa2Cu3O7−x (YBCO) is shown to agree with the microwave electrical conductivity, affirming that increase is due to quasiparticle conduction The field dependence, particularly in the conducting plane permits us to separate phonon and quasiparticle contributions for both YBCO and Tl2Ba2CuO6 single crystals.

26 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the high Tc superconductivity in a cuprate perovskite occurs in a superlattice of Josephson coupled quantum wires, and that this particular heterostructure provides the physical mechanism raising Tc from the low temperature range Tc <23K to the high temperature range 30K
Abstract: Here we report experimental evidence that the high Tc superconductivity in a cuprate perovskite occurs in a superlattice of Josephson coupled quantum wires. We show that this particular heterostructure provides the physical mechanism raising Tc from the low temperature range Tc<23K to the high temperature range 30K

20 citations


Journal ArticleDOI
Ping Ao1
TL;DR: In this article, the authors argue that the motion of vacancies in a pinned vortex lattice may dominate the contribution to the Hall effect in an appropriate parameter regime for a superconductor.
Abstract: We argue that the motion of vacancies in a pinned vortex lattice may dominate the contribution to the Hall effect in an appropriate parameter regime for a superconductor. Based on this consideration a model is constructed to explain the anomalous Hall effect without any modification of the basic vortex dynamic equation. Quantitative predictions are obtained. Present model can be directly tested by an observation of the vacancy motion.

19 citations


Journal ArticleDOI
TL;DR: In this article, a cylindrical copper cavity with an HTS end plate, a stripline resonator, and sapphire rod resonators are used for HTS characterization.
Abstract: Errors in measurements of surface resistanceR s of HTS materials are due to discrepancy between a mathematical model describing physical phenomena and a real measurement environment, finite accuracy of measurements of theQ-factor, and finite accuracy of constants used for calculation ofR s . In this paper we analyze errors inR s due to uncertainties in theQ-factor, geometrical coefficients, loss tangent,R scu, and other factors when a cylindrical copper cavity with an HTS end plate, a stripline resonator, and sapphire rod resonators are used for HTS characterization.

18 citations


Journal ArticleDOI
TL;DR: In this article, a first-order contraction of the mean Cu-O bond length collapses the Hubbard U and transfers spectral weight from the Hubbard bands to the gap between them within a polaron.
Abstract: Transport measurements distinguish the superconductive phase from the parent antiferromagnetic and the overdoped metallic phases in La2−xSrxCuO4. For small x, holes in the CuO2 sheets form non-adiabatic polarons containing, experimentally, five Cu atoms. Within a polaron, a first-order contraction of the mean Cu-O bond length collapses the Hubbard U and transfers spectral weight from the Hubbard bands to the gap between them. In the superconductive phase above Tc, condensation into a polaron liquid produces an ɛ(k) dispersion curve, and dynamic pseudo Jahn-Teller deformations within the polaron give a Seebeck enhancement with Tmax≈140 K. The polarons order pair-wise into 〈110〉 stripes alternating with parent-phase stripes; below Tc, the stripes are perfectly ordered. In the overdoped phase, the parent phase is squeezed out; the vibronic states become homogeneous.

18 citations


Journal ArticleDOI
TL;DR: In this paper, the authors solved the Eliashberg equations in the case of strong anisotropic electron-phonon coupling and low energy cutoff in a simplified model representing the Fermi Surface of (CuO2)n planes.
Abstract: We solve Eliashberg equations in the case of strong anisotropic electron-phonon coupling and low energy cutoff. In a simplified model representing the Fermi Surface of (CuO2)n planes, we exhibit ad-wave solution as well as a generalizeds-wave one.

Journal ArticleDOI
TL;DR: In this article, the degradation of epitaxial thin films of YBa2Cu3O7 has been studied as a function of annealing temperature in air and in vacuum; some samples had an evaporated overlayer of CaF2.
Abstract: The degradation of epitaxial thin films of YBa2Cu3O7 has been studied as a function of annealing temperature in air and in vacuum; some samples had an evaporated overlayer of CaF2. Degradation was monitored by the measurement of electrical properties after consecutive 30-min annealing treatments. The room-temperature resistance registered significant increases for all samples after annealing at temperatures above about 200°C; the critical current density at 77 K was degraded for annealing temperatures ≥400°C in air, and ≥200–250°C in vacuum. By annealing in oxygen at 550°C, electrical properties were restored in degraded bare YBCO samples annealed in vacuum, but not for those annealed in air.

Journal ArticleDOI
TL;DR: In this article, the temperature dependence of the lattice parameters of single crystal superconductors Bi2Sr1.8CaCu2Ox was investigated using a low-temperature stage X-ray diffractometer.
Abstract: Using a low-temperature stage X-ray diffractometer, we have investigated the temperature dependence of the lattice parameters of single crystal superconductors Bi2Sr1.8CaCu2Ox. The experimental results show that (i) the lattice constants increase linearly with increasing temperature above about 100 K; (ii) thec-axis lattice parameter shows a kink in the superconducting transition region, while thea, b-axis parameters do not show any anomalous behavior in this region; (iii) botha, b, andc show negative thermal expansion coefficients below 40–50 K, which may be related to the characteristics of the Cu-O bond.

Journal ArticleDOI
TL;DR: In this paper, the dependence of C60/C70 microhardness on the temperature in the range 77-570 K and on the influence of solvent residues has been examined.
Abstract: Mechanical tests of C60/C70 single crystals allow one to assign them to the category of soft and fragile materials. Their mechanical properties are comparable with those of graphite but, in contrast, they are isotropic. The dependence of C60/C70 microhardness on the temperature in the range 77–570 K and on the influence of solvent residues has been examined. Preliminary data on microhardness of pure C60 and C70 have been obtained.

Journal ArticleDOI
TL;DR: In this article, the spectra of intermediate bosons were analyzed for high temperature superconductors and it was shown that they are consistent with the standard electron-phonon model with a moderate coupling constant λ = 1−1.5.
Abstract: Calculations of the effect of intermediate bosons on the optical properties of the normal state are used to analyze the spectra of these bosons. For high temperature superconductors these spectra are in agreement with the standard electron-phonon model (the end of the spectrum is near 800 cm−1) with a moderate coupling constant λ=1–1.5. For the heavy fermion compound UPt3 the spectrum of intermediate bosons is consistent with neutron data for spin fluctuations.

Journal ArticleDOI
TL;DR: In this paper, the authors show that a class of intermetallic borocarbide superconductors (LuNi2B2C) have three-dimensional conduction with anisotropy being a minor factor.
Abstract: In spite of their layered crystal structure, the new class of intermetallic borocarbide superconductors (typified by LuNi2B2C) have three-dimensional conduction with anisotropy being a minor factor. The calculated electron-phonon coupling constants are consistent with a picture of moderate- to strong-coupling conventional superconductivity, with large contributions from boron vibrations. We report several calculated material parameters for members of this class of compounds, and compare with some of the experimental data.

Journal ArticleDOI
TL;DR: In this paper, complex dielectric response measurements have been made on a C60/C70 mixture pellet and the frequency and temperature ranges cover from 20 Hz to 1 MHz and from 30 to 300 K.
Abstract: Complex dielectric response measurements have been made on a C60/C70 mixture pellet. The frequency and temperature ranges cover from 20 Hz to 1 MHz and from 30 to 300 K. Results show that the real part of the dielectric constant is 5 with a weak thermally activated polarization contribution. This thermal polarization is believed to be related to reorientation of C60 molecules recently observed by many techniques such as NMR, sound velocity, thermal conductivity, and others. Our data yield relaxation frequencies for reorientation, indicating that the presence of an additional electric dipole moment of the orientationally ordered C60 molecules at low temperature contributes 0.2–0.3 to the static dielectric constant.

Journal ArticleDOI
TL;DR: In this article, the thermal conductivity of (Y,Pr)Ba2Cu3O7, YBa2(Cu,Zn) 3O7 and YBCO 7 single crystals from 10≥T≤280K was measured and a strong enhancement in K(T) was observed for T≈Tc/2 with a peak in K at approximately T ≥Tc 2 with a sharp change in slope of K at Tc.
Abstract: The thermal conductivity K(T) vs temperature has been measured for (Y,Pr)Ba2Cu3O7, (Y,Tb)Ba2Cu3O7 and YBa2(Cu,Zn)3O7 single crystals from 10≤T≤280K. For YBa2Cu3O7(YBCO), a strong enhancement in K(T) is observed for T≤Tcwith a peak in K(T) at approximately T≈Tc/2 with a sharp change in slope of K(T) at Tc. These results are similar to those reported previously. For the selectively doped YBCO, the peak in K(T) is shifted to lower temperatures and occurs at T≈ 20K independent of Tc and the impurity concentration. In addition, the sharp change in slope of K(T) at Tc is not apparent for the doped crystals. These results are discussed along with a theoretical model for electronic conduction including both impurity and inelastic scattering from spin fluctuations.

Journal ArticleDOI
TL;DR: A millimeter wave spectrometer for frequencies between 100 and 350 GHz consisting of continuously tunable backward wave oscillators as sources and a quasi-optical interferometer in the Mach-Zehnder configuration was used to measure the transmittivity in phase and amplitude of YBa{sub 2}Cu{sub 3}O{sub 7} thin films on NdGaO(sub 3) substrates as mentioned in this paper.
Abstract: A millimeter wave spectrometer for frequencies between 100 and 350 GHz consisting of continuously tunable backward wave oscillators as sources and a quasioptical interferometer in the Mach-Zehnder configuration was used to measure the transmittivity in phase and amplitude of YBa{sub 2}Cu{sub 3}O{sub 7} thin films on NdGaO{sub 3} substrates. From the measured spectra the authors derived the real and imaginary part of the dynamic conductivity {sigma} = {sigma}{sub 1} + {sigma}{sub 2} in the superconducting state as a function of temperature. The {sigma}{sub 1}(T) and {sigma}{sub 2}(T) values at 300 GHz were compared to corresponding values at 19 GHz determined by surface impedance measurements of the same films using a shielded dielectric resonator. The observed frequency dependence of both {sigma}{sub 1}(T) and {sigma}{sub 2}(T) is consistent with a strong reduction of the quasiparticle scattering rate {tau}{sup {minus}1}(T) with decreasing temperature below {Tc}.

Journal ArticleDOI
TL;DR: In this paper, the shift and splitting of the infrared spectra of the neutral monoanion were compared with the spectrum of a neutral molecule in different solid-state structures with each other.
Abstract: We compare infrared spectra of the C 60 − monoanion in different solid-state structures with each other as well with that of the neutral molecule We relate the shift and splitting of theT c infrared modes to the strength and anisotropy of electron-phonon coupling in those environments

Journal ArticleDOI
TL;DR: In this paper, the effect of impurities in d-wave superconductors is discussed in terms of infrared reflectance spectroscopy on high quality crystals of a number of HTSC systems.
Abstract: Recent infrared reflectance spectroscopy on high quality crystals of a number of HTSC systems shows thatall have finite conductivity in the frequency region of the superconducting gap. Results on untwinned YBCO from a number of laboratories show that this absorption is not due to experimental problems or sample-to-sample variations. Other materials also show absorption features in the gap region in the form of peaks. We discuss these results in terms of recent ideas of the effect of impurities in d-wave superconductors.

Journal ArticleDOI
TL;DR: In this article, the authors proposed a shape factor for microwave absorption in granular YBa2Cu3Ox superconductor under high-field sweep, which can reproduce the complex behavior of microwave absorption fairly well.
Abstract: The mechanism of modulated microwave absorption (MMA) is suggested from the modulation (H m ) dependence in granular YBa2Cu3Ox superconductor under high-field sweep. It is proposed that the MMA signalS be decomposed into three shape factors to interpret the experimental result. The VN factor arises from the difference in the number of vortices between the add (A) mode and the subtract (S) mode corresponding to the alternating field. The CT factor arises from the appearance of the transition region of the shielding current in the Smode where the absorption is less. The SS factor arises from the difference in the surface slope of the flux distribution between the two modes. With increasingH m , the positive VN increases linearly. The positive CT increases initially and then decreases. The slope for the Amode is very gentle, so that SS is negative, and it initially increases rapidly and then saturates. These factors can reproduce the complex behavior of theH m dependence ofS fairly well.

Journal ArticleDOI
S. Donovan1, J. Kircher1, J. Karpinski2, E. Kaldis2, Manuel Cardona1 
TL;DR: In this article, the electronic Raman scattering in YBa2Cu4O8 was investigated and it was shown that the normal state is strongly temperature-pendent in all polarizations.
Abstract: We report on the electronic Raman scattering in YBa2Cu4O8 and find that, In contrast to the other superconducting cuprates, the continuum in the normal state is strongly temperature ⪤pendent in all polarizations. This temperature dependence is found to follow a Bose-Einstein-like form. We conclude that the absence of the Bose-Einstein factor in the electronic continuum is not essential to high-Tc superconductivity, as is sometimes implied. Additionally, we report on the rearrangement of the continuum which occurs belowTc.

Journal ArticleDOI
TL;DR: In this paper, the spin-lattice relaxation of thulium and copper nuclei reveals three types of paramagnetic centers to be present in oxygen-deficient 1-2-3 superconductors.
Abstract: The169Tm “enhanced” NMR in TmBa2Cu3O6+x (x=0.5, 0.6) at temperatures below 4.2K and the63Cu(1) NQR in YBa2Cu3O6.5 at temperatures above 4.2K are used to study properties of intrinsic paramagnetic centers incorporated into superconducting materials. The spin-lattice relaxation of thulium and copper nuclei reveals three types of paramagnetic centers to be present in oxygen-deficient 1-2-3 superconductors, those are (1) two-level ones with a spin S=1/2, localized outside CuO2 bilayers, (2) singlet-ground-state paramagnetic centers with an integer spin S≥1 in CuO2 bilayers, and (3) exchange copper-oxygen clusters with a half-integer spin S≥5/2, localized in a nearest neighborhood of CuOx basal plane at boundaries of superconducting OrthoII microdomains.

Journal ArticleDOI
TL;DR: In this paper, the impurity mechanism proposed earlier is used to interpret the heat properties of hightemperature superconductors, and an expression for the heat capacity jump describing its temperature and magnetic field dependences is obtained.
Abstract: The impurity mechanism proposed earlier is used to interpret the heat properties of hightemperature superconductors. The expression for the heat-capacity jump describing its temperature and magnetic field dependences is obtained. An explanation of the dependence of the Sommerfeld constant on the stoichiometric composition is proposed.

Journal ArticleDOI
TL;DR: In this article, the electronic structure of idealized tetragonal Bi2Sr2CuO6 is compared with that of an orthorhombic √2×√2 model that includes atomic displacements due to the structural modulation.
Abstract: The electronic structure of idealized tetragonal Bi2Sr2CuO6 is compared with that of an orthorhombic √2×√2 model that includes atomic displacements due to the structural modulation. The electronic structure, particularly that related to Bi and O(3) atoms, is strongly modified near the Fermi energy. In particular the shape and topology of the Bi-O(3) related Fermi surfaces near the ¯M point are changed. With the modulation, bands occur near the Fermi energy over a significant part of the zone near this point. A method is proposed to distinguish between lattice distortions and antiferromagnetic correlations as the cause of recently observed shadow Fermi surfaces.

Journal ArticleDOI
TL;DR: In this article, angle resolved photoemission (ARPES) measurements for the insulating cuprate Sr2CuO2Cl2 have provided the first experimental data which can be directly compared to the theoretically well-studied problem of a single hole propagating in an antiferromagnet.
Abstract: Recent angle resolved photoemission (ARPES) measurements for the insulating cuprate Sr2CuO2Cl2 have provided the first experimental data which can be directly compared to the (theoretically) well-studied problem of a single hole propagating in an antiferromagnet. Some discrepancies withthe familiar 2D t— J model were observed. Here we discuss a comparison between the ARPES results and the quasiparticle dispersion of both (i) the extended t— t′— J Hamiltonian and (ii) the three-band Hubbard model.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the in-plane Hall effect and resistivity of cobalt-doped YBa2Cu3O7−δ (YBCO) single-crystal samples.
Abstract: We have measured the in-plane Hall effect and in-plane resistivity of cobalt-doped YBa2Cu3O7−δ (YBCO) single-crystal samples. The concentration of cobalt ranged from 1 to 3%. As the applied field was varied, a sign reversal of the Hall resistivity was seen for 1 and 2% Co-doped samples, but not for the 3% sample. Our results are expressed in terms of conductivities, as recently recommended by the work of Dorseyet al. and Kopninet al. In the mixed state just below the superconducting transition temperatureTc, the Hall conductivityσxy is the sum of two terms,C1/H andC2H, whereC1 andC2 are field-independent but temperature-dependent. As previously observed by Ginsberg and Manson for undoped YBCO, the coefficientC1 is approximately proportional toτ2 andC2 is approximately linear inτ, whereτ = 1−(T/T c ). The values ofC1 andC2 are presented for each cobalt concentration.

Journal ArticleDOI
TL;DR: A microscopic theory of the plasma resonance in layered metals is presented in this paper, where it is shown that electron-impurity scattering can suppress the normal state and sharpen it in the superconducting state.
Abstract: A microscopic theory of the plasma resonance in layered metals is presented. It is shown that electron-impurity scattering can suppress the plasma resonance in the normal state and sharpen it in the superconducting state.

Journal ArticleDOI
TL;DR: In this article, atomic layer deposition was used to synthesize single crystal thin films of cuprate superconductors, as well as a variety of superlattices and multilayer heterostructures with atomically abrupt interfaces.
Abstract: We have utilized atomic layer deposition to synthesize single crystal thin films of cuprate superconductors, as well as a variety of superlattices and multilayer heterostructures with atomically abrupt interfaces. For example, we made trilayer structures with the top and the bottom BiSrCaCuO electrodes separated by titanate barriers that were only 4 A thick and yet free of pinholes over macroscopic areas. This unique synthetic capability made it possible to study transport in thec-axis direction under controlled and systematically varied conditions. Taken together, these experiments provide a picture of thec-axis transport in 2201, 2212, and various heterostructures under study. In particular, they suggest presence of a large density of localized states near the Fermi level and thus support a two-component description of the electron system in these compounds.