共查询到20条相似文献，搜索用时 23 毫秒
1.
Mannhart J Gross R Hipler K Huebener RP Tsuei CC Dimos D Chaudhari P 《Science (New York, N.Y.)》1989,245(4920):839-841
Spatially resolved resistivity measurements of current transport across individual grain boundaries have been made on superconducting YBa(2)Cu(3)O(7). These experiments were done by low-temperature scanning electron microscopy with a resolution of 1 to 2 micrometers, and they show directly the limitation of the critical current density caused by grain boundaries in YBa(2)Cu(3)O(7). Furthermore, complex spatial patterns of the current transport across grain boundaries were observed. These patterns reflect self-excited resonances of the grain boundaries and are closely correlated to the unexplained "sub-gap structure" in the current-voltage characteristics of polycrystalline YBa(2)Cu(3)O(7). 相似文献
2.
Subramanian MA Torardi CC Calabrese JC Gopalakrishnan J Morrissey KJ Askew TR Flippen RB Chowdhry U Sleight AW 《Science (New York, N.Y.)》1988,239(4843):1015-1017
A new superconductor that displays onset behavior near 120 K has been identified as Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y), with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 A, b= 5.414A, and c = 30.904 A. The structure contains copper-oxygen sheets as in La(2)CuO(4) and YBa(2)Cu(3)O(7), but the copper-oxygen chains present in YBa(2)Cu(3)O(7) do not occur in Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y). The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca(2+) and Sr(2+) cations between the adjacent Cu-O sheets; Sr(2+) cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y) show a resistivity drop at about 116 K and apparent zero resistivity at 91 K. 相似文献
3.
Eom CB Marshall AF Triscone JM Wilkens B Laderman SS Geballe TH 《Science (New York, N.Y.)》1991,251(4995):780-783
A modulated structure has been fabricated from high transition temperature superconductors where the individual CuO(2) planes are composed of alternating superconducting and insulating strips. This structure is made by growing a-axis-oriented YBa(2)Cu(3)O(7)/PrBa(2)Cu(3)O(7) superlattices by 90 degrees off-axis sputtering on (100)SrTiO(3) and (100)LaAlO(3) substrates. Superlattice modulation is observed to a modulation wavelength of 24 angstroms (12 angstroms-YBa(2)Cu(3)O(7)/12 angstroms-PrBa(2)Cu(3)O(7)), both by x-ray diffraction and by cross-sectional transmission electron microscopy. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 3 percent. Quasi-one-dimensional conductivity should be obtainable in these structures. 相似文献
4.
Changes in the copper K-edge x-ray absorption spectrum of YBa(2)Cu(3)O(7) across the critical temperature indicate that, accompanying the superconducting transition, the mean square relative displacement of some fraction of the Cu2-O4 bonds becomes smaller or more harmonic (or both), that there may be a slight increase in the associated Cul-O4 distance, and that electronic states involving these atom pairs become more atomic-like. If there is an association between the superconductivity and this lattice instability, then the bridging axial oxygen is of central importnce in determining the high tranition temperature of YBa(2)Cu(3)O(7). Because this structural perturbation will affect the dynamic polrizability of the copper oxygen sublattice, it is consistent with an excitonic pairing mhanism in these materials. 相似文献
5.
Kaindl RA Woerner M Elsaesser T Smith DC Ryan JF Farnan GA McCurry MP Walmsley DG 《Science (New York, N.Y.)》2000,287(5452):470-473
Optical spectra of high-transition-temperature superconductors in the mid-infrared display a gap of in-plane conductivity whose role for superconductivity remains unresolved. Femtosecond measurements of the mid-infrared reflectivity of YBa(2)Cu(3)O(7-delta) after nonequilibrium optical excitation are used to demonstrate the ultrafast fill-in of this gap and reveal two gap constituents: a picosecond recovery of the superconducting condensate in underdoped and optimally doped material and, in underdoped YBa(2)Cu(3)O(7-delta), an additional subpicosecond component related to pseudogap correlations. The temperature-dependent amplitudes of both contributions correlate with the antiferromagnetic 41-millielectronvolt peak in neutron scattering, supporting the coupling between charges and spin excitations. 相似文献
6.
Copper K-edge x-ray absorption data indicate that an axial oxygen-centered lattice instability accompanying the 93 K superconducting transition in YBa(2)Cu(3)O(7) is of a pseudo-(anti)ferroelectric type, in that it appears to involve the softening of a double potential well into a structure in which the difference between the two copper-oxygen distances and the barrier height have both decreased. This softer structure is present only at temperatures within a fluctuation region around the transition. A similar process involving the analogous axial oxygen atom also accompanies the superconducting transition in T1Ba(2)Ca(3)Cu(4)O(11), where the superconducting transition temperature T(c) is ~120 K. The mean square relative displacement of this oxygen atom in YBa(2)Cu(3)O(7) is also specifically affected by a reduction in the oxygen content and by the substitution of cobalt for copper, providing further evidence for the sensitivity of the displacement to additional factors that also influence the superconductivity. On the basis of the implied coupling of this ionic motion to the superconductivity, a scenario for high-temperature superconductivity is presented in which both phonon and electronic (charge transfer) channels are synergistically involved. 相似文献
7.
The surface microstructures of c-axis-oriented films of YBa(2)Cu(3)O(7), deposited by off-axis magnetron sputtering on MgO and SrTiO(3) single crystal (100) substrates, have been investigated with scanning tunneling microscopy and atomic force microscopy. There is strong evidence that the films nucleate as islands and grow by adding material to the edge of a spirally rising step. This results in columnar grains, each of which contains a screw dislocation at its center. This microstructure may be of significance in determining superconducting properties such as critical current, and represents a significant difference between thin films (especially those grown in situ) and bulk materials. 相似文献
8.
Skelton EF Drews AR Osofsky MS Qadri SB Hu JZ Vanderah TA Peng JL Greene RL 《Science (New York, N.Y.)》1994,263(5152):1416-1418
Evidence of structural inhomogeneities in two high-transition-temperature superconductors, YBa(2)Cu(3)O(7-delta) and Nd2-xCexCuO4-y, is presented. When samples were illuminated by highly collimated x-rays produced on a synchrotron wiggler, small changes in the lattice were detected over a spatial scale of 10 micrometers. These changes are interpreted as evidence of variations in the oxygen content in one case and in the cerium content in the other; both affect the superconducting properties. The existence of such structural inhomogeneities brings into question whether exotic experimental results obtained from superconducting materials with high transition temperatures actually reflect intrinsic properties. 相似文献
9.
Keimer B Dogbrevean F Aksay IA Erwin RW Lynn JW Sarikaya M 《Science (New York, N.Y.)》1993,262(5130):83-86
A detailed small-angle neutron scattering study of the vortex lattice in a single crystal of YBa(2)Cu(3)O(7) was made for a field of 0.5 tesla inclined at angles between 0 and 80 degrees to the crystalline c axis. The vortex lattice is triangular for all angles, and for angles less than or equal to 70 degrees its orientation adjusts itself to maximize the pinning energy to densely and highly regularly spaced twin planes. These observations have important implications for the microscopic flux-pinning mechanism, and hence for the critical current achievable in YBa(2)Cu(3)O(7). For large angles (about 80 degrees) the vortex lattice consists of independent chains in the orientation predicted by anisotropic London theory. 相似文献
10.
Bourges P Sidis Y Fong HF Regnault LP Bossy J Ivanov A Keimer B 《Science (New York, N.Y.)》2000,288(5469):1234-1237
A comprehensive inelastic neutron scattering study of magnetic excitations in the near optimally doped high-temperature superconductor YBa(2)Cu(3)O(6.85) is presented. The spin correlations in the normal state are commensurate with the crystal lattice, and the intensity is peaked around the wave vector characterizing the antiferromagnetic state of the insulating precursor, YBa(2)Cu(3)O(6). Profound modifications of the spin excitation spectrum appear abruptly below the superconducting transition temperature T(c), where a commensurate resonant mode and a set of weaker incommensurate peaks develop. The data are consistent with models that are based on an underlying two-dimensional Fermi surface, predicting a continuous, downward dispersion relation connecting the resonant mode and the incommensurate excitations. The magnetic incommensurability in the YBa(2)Cu(3)O(6+)(x) system is thus not simply related to that of another high-temperature superconductor, La(2-)(x)Sr(x)CuO(4), where incommensurate peaks persist well above T(c). The temperature-dependent incommensurability is difficult to reconcile with interpretations based on charge stripe formation in YBa(2)Cu(3)O(6+x) near optimum doping. 相似文献
11.
Ramesh R Inam A Chan WK Wilkens B Myers K Remschnig K Hart DL Tarascon JM 《Science (New York, N.Y.)》1991,252(5008):944-946
Thin-film heterostructures of Bi(4)Ti(3)O(12)Bi(2)Sr(2)CuO(6+x), have been grown on single crystals of SrTiO(3), LaAlO(3), and MgAl(2)O(4) by pulsed laser deposition. X-ray diffraction studies show the presence of c-axis orientation only; Rutherford backscattering experiments show the composition to be close to the nominal stoichiometry. The films are ferroelectric and exhibit a symmetric hysteresis loop. The remanent polarization was 1.0 microcoulomb per square centimeter, and the coercive field was 2.0 x 10(5) volts per centimeter. Similar results were obtained with YBa(2)Cu(3)O(7-x) and Bi(2)Sr(2)CaCu(2)O(8+x), and single-crystal Bi(2)Sr(2)CuO(6+x)as the bottom electrodes. These films look promising for use as novel, lattice-matched, epitaxial ferroelectric film/electrode heterostructures in nonvolatile memory applications. 相似文献
12.
Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) results for copper-63, oxygen-17, and yttrium-89 nuclei in the superconducting composition range of YBa(2)Cu(3)O(6+x) (0.4 相似文献
13.
Kang S Goyal A Li J Gapud AA Martin PM Heatherly L Thompson JR Christen DK List FA Paranthaman M Lee DF 《Science (New York, N.Y.)》2006,311(5769):1911-1914
We demonstrated short segments of a superconducting wire that meets or exceeds performance requirements for many large-scale applications of high-temperature superconducting materials, especially those requiring a high supercurrent and/or a high engineering critical current density in applied magnetic fields. The performance requirements for these varied applications were met in 3-micrometer-thick YBa2Cu3O(7-delta) films epitaxially grown via pulsed laser ablation on rolling assisted biaxially textured substrates. Enhancements of the critical current in self-field as well as excellent retention of this current in high applied magnetic fields were achieved in the thick films via incorporation of a periodic array of extended columnar defects, composed of self-aligned nanodots of nonsuperconducting material extending through the entire thickness of the film. These columnar defects are highly effective in pinning the superconducting vortices or flux lines, thereby resulting in the substantially enhanced performance of this wire. 相似文献
14.
The structural and electronic effects of lead substitution in the high-temperature superconducting materials Pb(x)Bi(2-x)Sr(2)CaCu(2)O(8) have been characterized by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Large-area STM images of the Bi(Pb)-O layers show that lead substitution distorts and disorders the one-dimensional superlattice found in these materials. Atomic-resolution images indicate that extra oxygen atoms are present in the Bi(Pb)-O layers. STS data show that the electronic structure of the Bi(Pb)-O layers is insensitive to lead substitution within +/-0.5 electron volt of the Fermi level; however, a systematic decrease in the density of states is observed at approximately 1 electron volt above the Fermi level. Because the superconducting transition temperatures are independent of x(Pb) (x 相似文献
15.
Fischer GM Mayer B Gross R Nissel T Husemann KD Huebener RP Freltoft T Shen Y Vase P 《Science (New York, N.Y.)》1994,263(5150):1112-1114
With low-temperature scanning electron microscopy, the magnetic flux states in high critical temperature Josephson junctions have been imaged. The experiments were performed with YBa(2)Cu(3)O(7-delta) thin-film grain boundary Josephson junctions fabricated on [001] tilt SrTiO(3) bicrystals. For applied magnetic fields parallel to the grain boundary plane, which correspond to local maxima of the magnetic field dependence of the critical current, the images clearly show the corresponding magnetic flux states in the grain boundary junction. The spatial modulation of the Josephson current density by the external magnetic field is imaged directly with a spatial resolution of about 1 micrometer. 相似文献
16.
Eom CB Marshall AF Laderman SS Jacowitz RD Geballe TH 《Science (New York, N.Y.)》1990,249(4976):1549-1552
YBa(2)Cu(3)O(7) films have been grown epitaxially on SrTiO(3) (100) and LaAlO(3) (100) substrates with nearly pure a-axis orientation and with transition temperature T(c) (R = 0) of 85 K. A unique feature of these films is their smooth surface. These smooth surfaces enable the growth of short-period superlattices with well-defined modulations. The films are untwinned and the grains grow with their c-axis along one of two perpendicular directions on the substrate ([100] or [010]). The fabrication of sandwich-type Josephson junctions with good characteristics may now be possible because unlike c-axis-oriented films, the superconducting coherence length of these smooth films is appreciably large perpendicular to their surfaces. 相似文献
17.
Loye HC Leary KJ Keller SW Ham WK Faltens TA Michaels JN Stacy AM 《Science (New York, N.Y.)》1987,238(4833):1558-1560
The effect of oxygen isotope substitution on the superconducting transition temperature, T(c), has been measured for BaBi(0.25)Pb(0.75)O(3) (T(c), approximately 11 K) and Lal(1.85) Ca(0.15)CuO(4) (T(c) approximately 20 K), and is compared to the shifts observed for La(1.85)Sr(0.15)CuO(4) (T(c) approximately 37 K) and YBa(2)Cu(3)O(7) (T(c) approximately 92 K). For all four materials, the transition temperature is shifted to lower temperature upon substitution of oxygen-18 for oxygen-16. The observed shifts demonstrate that phonons are involved in the electron-pairing mechanism in these oxide superconductors. 相似文献
18.
Boris AV Kovaleva NN Dolgov OV Holden T Lin CT Keimer B Bernhard C 《Science (New York, N.Y.)》2004,304(5671):708-710
The temperature-dependent redistribution of the spectral weight of the CuO2 plane-derived conduction band of the YBa2Cu3O6.9 high-temperature superconductor (superconducting transition temperature = 92.7 kelvin) was studied with wide-band (0.01- to 5.6-electron volt) spectroscopic ellipsometry. A superconductivity-induced transfer of the spectral weight involving a high-energy scale in excess of 1 electron volt was observed. Correspondingly, the charge carrier spectral weight was shown to decrease in the superconducting state. The ellipsometric data also provide detailed information about the evolution of the optical self-energy in the normal and superconducting states. 相似文献
19.
Kawasaki M Takahashi K Maeda T Tsuchiya R Shinohara M Ishiyama O Yonezawa T Yoshimoto M Koinuma H 《Science (New York, N.Y.)》1994,266(5190):1540-1542
The atomically smooth SrTiO(3) (100) with steps one unit cell in height was obtained by treating the crystal surface with a pH-controlled NH(4)F-HF solution. The homoepitaxy of SrTiO(3) film on the crystal surface proceeds in a perfect layer-by-layer mode as verified by reflection high-energy electron diffraction and atomic force microscopy. Ion scattering spectroscopy revealed that the TiO(2) atomic plane terminated the as-treated clean surface and that the terminating atomic layer could be tuned to the SrO atomic plane by homooepitaxial growth. This technology provides a well-defined substrate surface for atomically regulated epitaxial growth of such perovskite oxide films as YBa(2)Cu(3)O(7-delta). 相似文献
20.
Precision isothermal solution calorimetry was used to determine the standard-state enthalpy of formation of a number of phases in the Y-Ba-Cu-O system. An analysis of the data indicates that YBa(2)Cu(4)O(8) is thermodynamically metastable under ambient conditions. Taken together with the results from previous investigations, these data show that all of the superconducting compounds in the Y-Ba-Cu-O system are thermodynamically metastable at low temperatures. 相似文献