high-Tc Superconductor YBa2Cu3O7 Research Articles

Electron and Phonon Thermal Conduction in Epitaxial High-Tc Superconducting Films

Electrons and phonons are the carriers of heat in the a-b plane of the high-Tc superconductor YBa2Cu3O7. In the absence of boundary scattering, the a-b plane thermal conductivity and the mean free path of each carrier type are calculated as functions of temperature using kinetic theory, the two-fluid model of the superconducting state, and experimental data for the thermal conductivity and electrical resistivity of a single crystal. The reduction by boundary scattering of the effective a-b plane thermal conductivity along an epitaxial YBa2Cu3O7 film is predicted as a function of temperature and film thickness. The size effect on the phonon conductivity dominates over the size effect on the electron conductivity. The predicted electron mean free path is limited by scattering on defects and is in very good agreement with experimental data from infrared spectroscopy.

Origin of tweed texture in the simulation of a cuprate superconductor

The origin of metastable tweed texture (microstructure) is studied by computer simulation. A two-dimensional model of 99*99 unit cells represents a layer with an oxygen deficit of the high-Tc superconductor YBa2Cu3O7- delta and exhibits the ferroelastic tetragonal-orthorhombic phase transition. The tweed texture is known to be important for flux pinning. In the model, the local ordering of the oxygen atoms produces long-range strain fields, which have been studied computationally by a molecular dynamics technique. The system has a strong tendency to form the tweed texture, as observed experimentally. Well above the structural phase transition temperature, the strain fluctuations show well developed embryos of the tweed texture, whose temperature dependence agrees with theoretical estimates obtained using the independent-site approximation. On quenching to below the transition temperature, the texture first becomes more regular in spacing and coarsens before the system orders macroscopically: the kinetics behaviour is quite different from the traditional model of nucleation and growth.

Evidence of Distortion in the Oxygen Environment around Praseodymium in Y1−xPrxBa2Cu3O7 from Praseodymium K-Edge XAFS

ABSTRACTIn constrast to other rare earths, replacement of Y with Pr in the high Tc superconductor YBa2Cu3O7 has a pronounced effect on Tc, with Tc going to zero at ≈ 55% Pr. We present Pr XAFS K-edge data for various concentrations of Pr in Y1−xPrxBa2Cu3O7. The Pr K-edge data show that the amplitude of the first Pr-O peak is greatly reduced when compared to the first Y-O peak in pure YBa2Cu3O7. In constrast, for the Y K-edge data in these alloys, there is little if any change in the oxygen environment, while the Cu K-edge data show at most a 10% reduction in the first Cu-0 peak. Fits to the Pr data suggest either that oxygen atoms in the CuO2 planes are missing or that some oxygen atoms about the Pr become highly disordered. Either result could help explain the reduction of Tc with increasing concentration of Pr.

酸化物高温超伝導体YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7−<I>y</I></SUB>に対するSnドーピングと熱処理条件

This study was performed in order to establish the heat treatment for substitution of Sn for Cu in high-Tc superconductor YBa2Cu3O7−y. Two methods of YBa2(Cu, Sn)3O7−y preparation were tired. The crystal structure and the impurity phase in the fired samples were examined by X-ray diffraction, and the superconductivity was estimated by the ac-susceptibility measurement. First the substitution of Sn for Cu in CuO prior to the synthesis of YBa2(Cu, Sn)3O7−y was tried and solubility of Sn in CuO was obtained as a function of firing temperature. Since it was found that Sn is substituted for 0.5 at%Cu at most, this preparation method was abandoned. Second, the ordinary firing of the mixture of 2BaCO3, 1/2Y2O3, 3(1−x)CuO and 3xSnO2 was performed. The Sn solubility was obtained as a function of firing temperature from 900°C to 970°C. Sn of x=0.011 dissolves at 960°C. From the lattice constant, it was shown that Sn is substituted not only for Cu(1) site but also for Ba or Y sites in the case of large x and high firing temperature. In the samples fired at high temperatures, Sn doping did not change Tconset but it decreased the superconductive volume mainly by the orthorhombic-tetragonal phase transition. In the samples fired at 900°C, the Meissner signal decreased by 40%, though Sn of only x=0.001 is soluted, which means that Sn may occupy the Cu(2) site in the samples.

溶融法で作成したＹＢａＣｕＯ系超伝導体の磁化緩和

The magnetic relaxation in high-Tc superconductors YBa2Cu3O7 system prepared by modified melt-textured growth method was investigated. The relaxation rate, S, given by M-1dM/d log (t), is independent of magnetic field and temperature below a given temperature. T0, which is decreased with magnetic field. The pinning potential evaluated from the temperature-independent S is increased with temperature, but the potential is apparent. On the other hand, S in the range higher than T0 is increased with temperature. This gives the potential dependent on log (Jc).

Residual losses in epitaxial thin films of YBa2Cu3O7 from microwave to submillimeter wave frequencies

We have measured the residual loss in five epitaxial a-b plane films of the high-Tc superconductor YBa2Cu3O7. Microwave measurements near 10 GHz were made by resonance techniques at 4 K. Submillimeter measurements from ∼1.5 to 21 THz were made at 2 K by a direct absorption technique. We use a model of weakly coupled superconducting grains and a homogeneous two-fluid model to fit the data for each film below the well-known absorption edge at 13.5 THz. When the penetration depth determined from muon spin rotation measurements is used to constrain each model, the weakly coupled grain model is able to fit the measured absorptivities for all films, but the two-fluid model is less successful.

Screening of low frequency electromagnetic fields by the high-Tc superconductor YBa2Cu3O7

The temperature and magnetic field dependences of the magnetization and the electromagnetic shielding efficiency in polycrystalline samples of the high-Tc superconductor YBa2Cu3O7-δ have been studied. At all temperatures investigated, two field regimes can be identified: A low field regime with an ideal diamagnetic behaviour and a large shielding factor, and an intermediate field regime, typically above a few mT, with a reduced diamagnetic susceptibility and a poor shielding efficiency. Using arguments from a modified critical state model, with a strongly field dependent critical current density, the transition to the intermediate field regime is attributed to the decoupling of the superconducting grains. It is argued that this effect is caused by the weakening of the intergrain Josephson junctions in the presence of a magnetic field.

Partial densities of states with polarized oxygen Kα X-ray emission of YBa2Cu3O7−δ

O Kα X-ray emission spectra of the high-Tc superconductor YBa2Cu3O7−δ were evaluated in terms of the linear pola rization along the x-,y- and z-direction. These partial spectra represent the electronic densities of states of the valence band with 2px-, 2py- and 2pz-like symmetry. The measurements which are representative for the bulk material were done by using an electron microprobe. The polarization factor of the spectrometer crystal was determined. The O 2px- and O 2py-spectra show a main peak at a binding energy of 1.80 eV and a shoulder at about 4.3 eV as well as a band width of about 8 eV. They differ for binding energies around the shoulder. The O 2pz spectrum which represents the density of states resulting from oxygen p-orbitals perpendicular to the CuO2-planes shows a different shape, a lower integrated intensity, and a smaller energetic breadth compared with the O 2px- and O 2py-spectra. The agreement between recently calculated partial densities of states and the measured X-ray emission spectra is satisfactory.

Structural origin of reduced critical currents at YBa2Cu3O7–δ grain boundaries

THE critical current density across individual grain boundaries in thin films of the high-Tc superconductor YBa2Cu3O7–δ (YBCO) has been found1–4 to be inversely proportional to lattice misorientation for tilts up to ∼10°. Reports of impurity segregation5,6 at grain boundaries, and variations in the chemical stoichiometry7,8, have led to the view that deviations from the ideal composition are responsible for the depressed superconducting order parameter at the boundary. Here we present images of YBCO grain boundaries obtained by a scanning transmission electron microscope in Z-contrast mode9,10, which show that chemical segregation does not necessarily occur at these boundaries. A simple model of the strain associated with the grain-boundary dislocations provides a reasonable physical explanation of the suppressed superconductivity. The surprisingly large effect of strain implied by our model has implications beyond critical currents, for the physics and applications of any thin-film YBCO structures involving strained epitaxial layers.

Microwave hall mobility and conductivity in crystals of various (BEDT-TTF) radical salts

We report investigation of tthe microwave transport properties carried out on the organic conductors and superconductors of (BEDT-TTF)-salts: α-, αt- and ß-(BEDT-TTF)2I3, α-(BEDT-TTF)3(NO)3)2) and ξ-(BEDT-TTF)2Cu(NCS)2. The anisotropy and temperature dependence (300 K to 4 K) of the high-frequency conductivity was measured in a microwave cavity at 10.3 GHz. There is low anisotropy in the high conducting plane of the quasi two-dimensional crystals of the (BEDT-TTF)-family, e.g. the ab-plane in ß-(BEDT-TTF)2I3; perpendicular to this the microwave conductivity is one order of magnitude lower. At room temperature the microwave Hall mobility at 9.5 GHz of the different phases of (BEDT-TTF)2I3 and (BEDT-TTF3(NO3)2 is 100 to 200 cm2/Vs in the high conducting planes of the crystals. With about 1200 cm2/Vs the high-frequency mobility of the superconductor (BEDT-TTF)2Cu(NCS)2 is similar to the one measured in the (a,b-plane of the high-Tc superconductor YBa2Cu3O7.

Muonic x-ray intensity measurements on a highT c superconductor

Muonic x-ray spectra from the high-Tc superconductor YBa2Cu3O6.95 have been measured in the superconducting and normal states. No significant differences were found between the two spectra when comparing the intensities of 27 lines.

Correlations betweenT c andn s/m * (carrier density/ effective mass) in high-T c and organic superconductors

We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenT c and the relaxation rate σ αn s/m * (carrier density/effective mass) of hole-doped high-T c cuprate superconductors (La2, Sr3)CuO4, YBa2Cu3O7 (Y1−xPrx)Ba2Cu3O7, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)2Cu(NCS)2.

Thermal neutron effect on YBa2Cu3O7

High-Tc superconductor YBa2Cu3O7 was irradiated by thermal neutrons. Significant effects on the critical temperature were found. The changes were also investigated by X-ray diffraction technique.65Cu (n, γ)66Cu was the only reason for the degradation in superconducting properties. The existence of an isotope effect is argued.

Compositional and Structural Properties of Sputtered Plzt Thin Films

AbstractThin films of lanthanum-modified lead zirconate-titanate (PLZT) were prepared by rf magnetron sputtering from a single oxide target onto a heated substrate. The target consisted of Pbl-xLaxZryTi1-yO3 with composition close to x=8% and y= 65%, either as a loose powder or a solid sintered disk. Under appropriate conditions, the desired perovskite phase formed in situ without any subsequent post-anneal. Film composition and structure were correlated with deposition parameters, including substrate temperature, target composition, gas pressures, and target aging. For deposition onto MgO or A12O3 crystalline substrates, perovskite PLZT films formed if there was sufficient Pb at the target surface, sufficient oxygen in the sputter gas (≈ 50%), and a substrate temperature >≈600°C. Target heating led to excessive Pb loss from the loose powder target; this was much less significant for the solid target. In addition, it was found that deposition onto an epitaxial perovskite substrate promoted formation of the perovskite phase, leading to an epitaxial film. A prototype ferroelectric capacitor was fabricated by depositing a conducting perovskite film (the high-Tc superconductor YBa2 Cu3O7 ) on a perovskite substrate, sputtering PLZT on top, with Ag for a top electrode. Measurements indicate a remanent polarization of 5 μC/cm2 and a coercive field of 900 V/cm.

Patterned YBa2Cu3O7−x thin films from photopolymerizable precursors

A technique which combines the fabrication and patterning of thin films of the high Tc superconductor YBa2Cu3O7−x has been developed. The technique possesses the essential features of the metalorganic decomposition method with the additional attribute that the metalorganic precursor is photopolymerizable. Patterns are generated directly in the precursor film using optical exposure through a mask followed by development in a solvent. A subsequent thermal treatment transforms the patterned precursor film to the oriented superconducting phase with c axis perpendicular to the substrate surface. Resistivity measurements for such a patterned film on a single crystal (100)MgO substrate show an onset to the superconducting state occurring at 85 K with zero resistivity below 67 K.

H-modulated microwave absorption and resistive transition in the high-Tc superconductor YBa2Cu3O7

Abstract The derivative microwave absorption near T c in presence of a sufficiently high field ( H ⩾1 kOe) is related to the temperature variation of resistivity. This idea, originally proposed by Kim et al. (1988), is extended to take into account the effects of the anomalous resistive tail by using the thermoactivated flux-creep model proposed by Tinkham (1988). This gives a simple explanation for some relevant features observed in the temperature behaviour of the ESR absorption, as the asymmetry of the peak at T c , with the long tail extending toward low temperatures, and the field dependence of height and linewidth, for which the model provides H −1 and H 2/3 behaviour, respectively, ESR data on YBa 2 Cu 3 O 7 powder are in satisfactory agreement with this picture, when the role of the intrinsic 2D-character of this compound is properly taken into account. This allows us to deduce consistent values for the parameters a ‖ and a ⊥ of the anisotropic resistivity.

Microwave Properties of Inhomogeneous YBa2Cu3O7

Microwave properties of the powder specimen of high-Tc superconductor YBa2Cu3O7 were examined with the standing-wave method at 9.12 GHz. From the analysis based on the coexistence of superconductivity and dielectricity, the phenomenological penetration depth is found to be as large as 0.3 cm in the powder specimen. The temperature dependence of the penetration depth in the powder specimen resembles that of London penetration depth in the compact material. The imaginary part of the dielectric permittivity diverges at the superconducting phase transition, the origin of which can be attributed to either fluctuations or the Josephson effect.

Measurements of the electric field gradient at cadmium in the high Tc superconductor YBa2Cu3O6+x

The electric field gradient (EFG) at111Cd probe atoms in YBa2Cu3O6+x has been studied by the perturbed angular correlation (PAC) spectroscopy. After annealing at temperatures below 1000 K the probe atoms are located at the Cu(1) site; above 1100 K they preferentially occupy the Y site, whereas in the intermediate temperature range they also reside on the Ba site. A reduction of the oxygen content from x=1.0 to 0.2 resulted in a decrease of the asymmetry parameter measured at the Ba site, but in an increase of it at the Y site, indicating there a local distortion of the YBa2Cu3O6+x lattice by the Cd impurity. Two different surroundings for the Cd atoms at the Cu(1) site were observed.

Microstructures of Cu- and Y-rich YBa2Cu3O7−x thin films

The defect microstructure of Cu- and Y-rich thin films of the high Tc superconductor YBa2Cu3O7−x (123) is of interest for several reasons. The highest Tc' reported for well-ordered stoichiometric films are typically 90-91 K. Post-annealed Cu- and Y-rich films may exhibit Tc's of 95-96 K. X-ray diffraction showed these non-stoichiometric films to be a highly faulted mixture of the 123 and 248 (Y2Ba4Cu8O16,Tc=80 K) phases. In situ films made by sputtering or electron beam evaporation typically show sharp but depressed Tc's (75-85 K); for our films these values approach 90 K when the films are Cu- and Y-rich. Both post annealed and in situ films exhibit high critical currents on and off stoichiometry; the pinning characteristics, which may be highly influenced by the defect microstructure, are superior for the in situ techniques.Transmission electron microscopy analysis shows that excess Cu and Y can be incorporated both as stacking fault (SF) defects and as second phase precipitates.

Mössbauer effect of tin-119 doped in highT c Y−Ba−Cu−O superconductor

Mossbauer measurements at temperatures ranging from 12 K to 300 K have been carried out on highTc superconductor YBa2Cu3O6+δ samples doped with Sn (5,2 and 0.5 wt%). The temperature dependent recoilless fractionf(T) has a narrow and small drop nearby transition temperatureTc which is dependent on the Sn-concentration and has a drop between 200–220 K. This indicates the occurrence of phonon softening at these temperatures. The Debye temperature θD of the samples containing 5, 2 wt% Sn atoms was determined as (290±25) K and (320±13) K respectively. These results show that phonons may play an important role for electron pairing in novel oxide superconductor.