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1.
Studies of the crystal chemistry of nonsuperconducting PrBa2Cu3O7 indicate that this compound is strictly isostructural with its superconducting RBa2Cu3O7 (R = Y, rare earth) analogs. Crystallographically, Pr is present in the trivalent state according to the structural trends exhibited by the RBa2Cu3O7 series as a function of R3+ ionic radius. The sole structural anomaly attributable to the presence of Pr3+ in the YBa2Cu3O7 structure is a next-next-nearest neighbor effect and consists of an unusually short axial Cu-O distance, i.e., a short bond length between the in-plane copper and the chain oxygen. The correlation of this anomaly with the nonmetallic/nonsuperconducting properties of PrBa2Cu3O7 supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa2Cu3O7 structure plays a critical role in mediating the appearance of superconductivity. The mechanism by which the f-electrons in Pr3+ (f
2) interact with the Cu-O manifold to produce the nonmetallic behavior of PrBa2Cu3O7 remains unknown; however, superconductivity is turned back on for Nd3+ (f
3), immediately next to Pr and just slightly smaller. Careful comparative studies of superconducting NdBa2Cu3O7 and nonmetallic PrBa2Cu3O7 are needed to elucidate the critical difference in the behavior of the f-electrons and may shed light on the fundamental mechanism of high-temperature superconductivity in copper oxides. 相似文献
2.
L. P. Guo G. R. Liu W. L. Zhou R. H. Yi L. Li Y. Yang J. Q. Li Y. Q. Zhou Z. X. Zhao 《Journal of Superconductivity》1995,8(1):155-161
Detailed transmission electron microscopic study has been carried out on heteroepitaxial YBa2Cu3O7/SrTiO3/YBa2Cu3O7 trilayer thin films grown on (100)SrTiO3 substrates prepared by DC and RF magnetron sputtering. The microstructural results showed the existence of somea-axis-oriented YBCO grains 20–90 nm wide in thec-axis-oriented YBCO matrix. Some of thea-axis grains in the lower YBCO thin film layer have protruded into the above SrTiO3 layer, which may cause short circuit between the two YBCO superconducting layers. This is unsuitable for the application of trilayer thin films for microelectronic devices. The defects on the surface of the substrates would also influence the growth quality of the YBCO thin films. 相似文献
3.
We report on measurements of the tunneling conductance structures above the superconducting gap energy using YBa2Cu3O7 polycrystalline junctions. The measured second derivative data are reproducible among the junctions, and the intensities of the common structures at the biases of 37–38, 47–53, 67–77, and 94–95 mV are strong enough to be assigned. These structures are in agreement with those in the neutron phonon density of states in whole energy regions when the energies are measured from the gap edge of 26±1 meV. This correspondence indicates that the electron-phonon interaction contributes to the pairing mechanism of this superconductor. 相似文献
4.
The post-annealing method of producing thin films of YBa2Cu3O7 (YBCO) has taken on a new impetus due to the recent work showing that films of the highest quality can be made by using low partial pressures of oxygen during the annealing cycle. Here it is shown that for films produced by using BaF2 as a source material, the post-annealing procedure can be closely controlled by monitoring the F that evolves due to the water vapor reaction with BaF2. The use of an ion-sensitive electrode allowed small F evolution rates (about 1 ng s–1) to be detected above background, sufficient to measure the F evolution rate from even the smallest samples used. The time interval during which F evolves was found to increase with increasing YBCO film area being annealed. 相似文献
5.
U. Dähne Y. Goncharov N. Klein N. Tellmann G. Kozlov K. Urban 《Journal of Superconductivity》1995,8(1):129-134
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 YBa2Cu3O7 thin films on NdGaO3 substrates. From the measured spectra we derived the real and imaginary part of the dynamic conductivity=
1+i
2 in the superconducting state as a function of temperature. The
1(T) and
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. Our observed frequency dependence of both
1(T) and
2(T) is consistent with a strong reduction of the quasiparticle scattering rate
–1(T) with decreasing temperature belowT
c
. 相似文献
6.
The paraconductivity of Li-doped YBa2Cu3O7–x was measured. We have found that the character of the fluctuation changes as lithium content increases: instead of a crossover from 2D to 3D behavior (D is the dimensionality), as we have observed for a low doping level, a double crossover to the 2D percolative and 3D percolative regimes becomes manifest by increasing the lithium content. 相似文献
7.
P. Burlet P. Bourges J. Bossy E. Elkaim J. Y. Henry J. P. Lauriat V. P. Plakhty L. P. Regnault J. Schweizer Y. Sidis C. Vettier F. Yakhou 《Journal of Superconductivity》1996,9(4):357-364
In the first part of this paper the results of X-ray and neutron diffraction studies of the structural aspects related to oxygen insertion in the CuO
x
planes of YBa2Cu3O6+x
compounds are presented. The second part is devoted to the inelastic neutron scattering study of the spin dynamics in the three regimes (weakly doped, heavily doped, and overdoped) of the metallic phases of YBa2Cu3O6+x
. 相似文献
8.
Post-annealing of YBa2Cu3O7 (YBCO) thin films is usually performed at 850–900°C in atmospheric-pressure oxygen. In this study, coevaporated YBCO films on LaAlO3 were post-annealed in an oxygen partial pressure of 29 Pa at temperatures in the range 700–825°C. Zero resistance transition temperatures were 89–90 K. Both d.c. (room-temperature resistance and critical-current density) and a.c. parameters (extracted from eddy-current response measurements at 25 MHz) were monitored. The optimum temperature is close to 750°C, which is on the YBCO thermodynamic stability line at this low oxygen partial pressure. 相似文献
9.
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. 相似文献
10.
The charge-transfer hypothesis is shown to be inconsistent with data for YBa2Cu3Ox: (i) The two-step behavior ofT
c(x) (with jumps from zero to 60 K and then to 90 K) is not reflected as a similar, statistically significant two-step behavior in the bond-valence-sum charge of cuprate-plane Cu ions (as once believed), (ii) as a consequence of the law of conservation of charge, the derivatives of the layer charges with respect to oxygen contentx for both the Ba-O layers and the charge-reservoir Cu-O chains have the opposite signs to those predicted, and (iii) the charge-transfer observed for superconducting YBa2Cu3Ox is not sufficient to produce superconductivity, as demonstrated by insulating PrBa2Cu3Ox, which has virtually the same layer charges. 相似文献
11.
The variation of critical current density at 77 K as a function of film thickness was studied for YBa2Cu3O7 films on (100) LaAlO3 substrates. Film thicknesses were in the range 0.2–1.6m. The films were deposited by co-evaporation and post-annealed under conditions which have previously resulted in high-quality films (750°C and an oxygen partial pressure of 29 Pa). The critical current density at 77 K exceeds 1 MA cm–2 for the thinner films, and decreases with increasing film thickness in excess of about 0.4m. The decrease is in rough agreement with a switch fromc-axis toa-axis growth at about this critical thickness. A good anticorrelation was found between room temperature resistivity and critical current density at 77 K. The results are compared to those obtained before by post-annealing at 850°C in 1 atm of oxygen. 相似文献
12.
V. Sandu Elena Cimpoiasu Carmen C. Almasan A. P. Paulikas B. W. Veal 《Journal of Superconductivity》2004,17(3):455-458
The electric transport of the charged particles in a spin texture was investigated in a strongly underdoped YBa2Cu3O6.25 single crystal in order to identify the characteristic electrical transport mechanism. The in-plane resistivity revealed three different regimes of charge transport: a chiral 2D VRH regime up to 55 K with a characteristic temperature T
d 12,400 K, an impurity band conduction regime above 55 K, and a metallic-like regime beyond 170 K. The out-of-plane resistivity has only one crossover at 115 K, but the conduction mechanisms controlling the two regimes are not clear. 相似文献
13.
Submicrometer epitaxial films of YBa2Cu3O7(YBCO) on (100) LaAlO3 were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (T
c) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (J
c) at 77 K has an average value of 2×105 A cm–2, with a tendency for decreasingJ
c with increasing film thickness (0.2 to 0.8m). Variations inJ
c are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJ
c. If the film composition is enriched in Y relative to Ba and Cu,T
c decreases by several degrees. 相似文献
14.
S. F. Xu Y. J. Tian H. B. Lu Z. H. Chen D. F. Cui Y. L. Zhou J. L. Zhang L. Li G. Z. Yang 《Journal of Superconductivity》1994,7(4):709-713
We have investigated the superconducting behavior of high-T
c YBa2Cu3O7 (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT
s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ
c increased slightly with increase in magnetic fieldB within the range ofB500 G, in the case ofB perpendicular to thec-axis of the film. 相似文献
15.
Our technique of reactive thermal co-evaporation has been extended to fabricate large films (up to 4 in.) of YBa2Cu3O7 with high quality. A rotating substrate holder is used to separate the deposition and oxidation processes. This allows free access of the metal vapors. As large substrate wafers we use Al2O3, Si, and GaAs with buffer layers of CeO2, YSZ, and MgO, respectively. On all substrates, the uniformity of thickness and composition was better than 2%. Inductively measuredT
c
andj
c
(77 K) were 87.5±0.2 K and >1×106 A/cm2, respectively, across the full wafer area. This holds also for GaAs substrates due to a new procedure of capping by Si3N4.This work was supported by the German Bundesminister für Forschung und Technologie. 相似文献
16.
Post-annealing of thin films of YBa2Cu3O7 (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 m thick, a critical current density >1 MA cm–2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO3 substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen. 相似文献
17.
The effects of electrode on the resistive switching in Nd0.7Ca0.3MnO3(NCMO)/YBa2Cu3O7(YBCO) heterostructure are investigated at room temperature. For Cu/NCMO/YBCO, resistance can be switched on-and-off from a high- to low-resistance state at a steady ratio of 25% with a pulsed-voltage of ± 3 V. On the other hand, a giant resistance-change as large as 1350% is observed with ± 5 V for Ag/NCMO/YBCO with a fast decay down to 550%. Our experimental results show clear evidences that the nature of interfaces can be modified by the electric field and it dictates the resistive switching behavior of these heterostructure devices, which are the potential candidates for the random access memory. 相似文献
18.
Analysis of electronic structure and charge density of the high-temperature superconductor YBa2Cu3O7
Self-consistent linearized augmented plane wave (LAPW) method calculations of the band structure, density of states, Fermi surface, Coulomb potential, charge density, core-level shifts, and electron-phonon interaction are presented for Y1Ba2Cu3O7. The calculated Sommerfield parameter is 4.35 mJ(mole Cu)–1 K–2, roughly about a factor of 2 smaller than experimentally deduced values of the enhanced value=(1 + )0, suggesting that the Fermi surface mass enhancement is of the order of unity. The crystal charge density is best represented by overlapping spherical ionic densities when the Cu and O ions are assigned charges of +1.62 and –1.69, respectively, corresponding to about 0.3 holes per oxygen atom. Core-level energies for the inequivalent atoms differ by as much as 0.45 eV for Cu and 0.7 eV for O, amounts which may be detectable by core-level spectroscopies. These results provide important information on the character and magnitude of ionic contributions to bonding in these materials. Within the rigid muffin-tin approximation, calculated McMillan-Hopfield parameters yield estimates for the electron-phonon strength that appear to be too small to account for the observedT
c. We point out an unusual band of oxygen-derived chain states below, but within 0.1 eV of, the Fermi level. 相似文献
19.
We studied the YBa2Cu3O7 – x
bulk superconductor doped with BaZrO3 up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO3 doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO3 does not lead to degradation of the superconducting properties of bulk YBa2Cu3O7 – x
. 相似文献
20.
The temperature dependence observed in the mid- and near-infrared optical properties of YBa2Cu3O7 is explained in terms of the Drude model for free charge carriers. In the Drude model, the linear temperature dependence of the dc resistivity arises from the free charge carriers having a temperature-dependent mean free path. This temperature dependence results in the plasmon contribution to the dielectric constant having a damping coefficient which also varies linearly with temperature. We find that the temperature dependence which is observed in the absorption and reflection spectra of YBa2Cu3O7 is consistent with this simple model. 相似文献