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1.
Surface structures of the high Tc oxide superconductors Tl 2Ba 2Ca 2Cu 3O 10 and La 2CuO 4 have been studied by high resolution electron microscopy. In addition to the single atomic layer of La on the (113) surface observed previously, several atomic layers of the C-form of La 2O 3 were found to coat the (001) surfaces of La 2CuO 4. Surface reconstruction with the incorporation of excess Ba on the (110) surface of Tl 2Ba 2Ca 2Cu 3O 10 has been observed together with a stepped (11 l) surface produced by a combination of the (110) and (001) planes. The (001) surface of Tl 2Ba 2Ca 2Cu 3O 10 was also found to terminate with a Ba-O layer. Possible mechanisms for the formation of these surface structures are discussed. 相似文献
2.
Analysis of the copper K-edge X-ray absorption finestructure on Tl 2Ba 2CuO 6, Tl 2Ba 2CaCu 2O 8, and Tl 2Ba 1.2Sr 0.8 CuO 6 indicates that the copper-equatorial oxygen local structure changes in the vicinity of the superconducting transition. These data also show that the radial distribution function for these bonds is complex with several oxygen positions relative to the copper, indicating that the local structure of the CuO 2 planes differs significantly from the average crystal structure. These results suggest the presence of polarons in these planes, whose structure changes across the superconducting transition, and are thus coupled to the hole pairing states. 相似文献
3.
The superconductivity of Tl 0.5Pb 0.5Sr 2−xNd xCuO 5−δ was observed for x ≥ 0.3, although the calculated Cu valence assuming Tl 3+ and Pb 4+ is slightly above or below 2.0. The binding energy of the Tl and Pb 4f core levels measured by XPS shifted to higher energy than those of Tl 3+ and Pb 4+ in the reference materials, showing that the Tl and Pb valences are lower than 3 + and 4 +, respectively. These observations strongly suggest that holes in the CuO 2 sheet are created by charge transfer from the (Tl,Pb)O layer, similar to the double-layered Tl---Ba cuprates. In contrast, the temperature variation of electrical conductivity of Tl0.5Pb0.5Sr2CuO5−δ changed from metallic to semiconducting with increasing oxygen deficiency, δ, and no superconductivity was observed through any control of δ. Although XPS measurement also suggested that Tl and Pb valences lowered with increasing oxygen deficiency, δ, the reason why the system did not show superconductivity can be understood by the depletion of oxygen from the CuO2 plane during deoxygenation. 相似文献
4.
The ambient-pressure synthesis of a new tetragonal alkaline-earth superconducting cuprate, Sr 0.8Ba 1.2CuO 3+δ, from a cupro-oxycarbonate is reported. Magnetic-susceptibility measurements show the presence of a superconducting transition 50 K in a post-annealed sample. The crystal structure, refined from time-of-flight powder neutron-diffraction data was found to have an oxygen-deficient La 2CuO 4-type tetragonal T structure ( a = 3.8988(3) Å and C = 12.815(3) Å) with oxygen vacancies located within the CuO 2 planes. Ordering of these oxygen vacancies is responsible for the observation of a superlattice in both neutron- and electron-diffraction measurements. An interpretation of the electron-diffraction patterns suggests that the superlattice in Sr 0.8Ba 1.2CuO 3+δ and also in the isostructural superconductor Sr 2CuO 3+δ are of an identical nature. 相似文献
5.
Cu K-edge and La L 3-edge X-ray absorption spectra have been measured on a series of nominal La xxCuO 4 samples with x≤2.00. The extended X-ray absorption fine structure (EXAFS) results show that the x<2.0 samples always consist of stoichiometric ( La/ Cu=2/1) La 2CuO 4 and CuO. The number of La vacancies in the La 2CuO 4 structure, if any, should be very small. The sensitivity of the EXAFS technique to impurity phases under favorable conditions is demonstrated. 相似文献
6.
The thermoelectric power (TEP) S versus temperature has been systematically investigated for several series of the superconducting cuprates Tl(Ba,Sr) 2Ca m−1Cu mO 2m+3−δ ( m = 2, 3) and Tl 2Ba 2Ca m−1Cu mO 2m+4+δ ( m = 1, 2, 3). The consideration of the S( Tc) curves allows two important points to be found evidence for. The first one deals with the fact that all these superconducting thallium cuprates are systematically overdoped whatever Tc, and whatever the number of Cu or Tl layers; no underdoped superconducting cuprate could be obtained. The second point shows that there exist two classes of Tl cuprates: the weakly overdoped cuprates that exhibit a Tc max ≥ 100 K (all the triple copper layer cuprates and the 2212 cuprates) and those which can be heavily doped that exhibit a Tc max ≤ 90 K (the 2201 and the 1212 cuprates). The different behavior of thallium cuprates compared to YBa 2Cu 3O 7−δ and to bismuth cuprates is discussed. 相似文献
7.
Temperature and angular dependences of 205Tl line width, Δ
, and transverse relaxation time, τ, have been measured in two crystals of Tl 2Ba 2CuO x with Tc = 32 and 115 K, respectively. In the normal state both Δ
and τ have been found to be temperature-independent and not to vary with varying Tc, implying that the size of inhomogeneous broadening is also invariable. Analysis of the line width angular dependence shows that the inhomogeneous broadening originates, most probably, from local distortions associated with Cu substitution for Tl. A conclusion is that variations in Tc are not related to this substitution. 相似文献
8.
The annealing characteristics and the superconducting properties of Tl 2Ca 2Ba 2Cu 3O 10 thin films sputter-deposited onto yttrium- stabilized ZrO 2 substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest Tc superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest Tc achieved tends to degrade with increasing substrate temperatures, a zero resistance Tc of 121 and ≈90 K, respectively, being observed for the films deposited at -ambient temperature and at 500°C. The formation of the highest Tc phase (Tl 2Ca 2Ba 2Cu 3O 10) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower Tc phases such as Tl 2Ca 1Ba 2Cu 2O 8. 相似文献
10.
The heteroepitaxy in DyMnO 3/Er 1Ba 2Cu 3O 7-δ bilayer thin films on LaAlO 3 (100) substates was characterized by four-circle X-ray diffractometry. The Er 1Ba 2Cu 3O 7-δ thin films on LaAlO 3 (100) substrates were prepared by molecular-beam deposition (MBD) and post-growth annealing in wet and dry O 2 at 880°C, whereas the DyMnO 3 thin films on the Er 1Ba 2Cu 3O 7-δ/LaAlO 3 (100) heterostructure were deposited by MBD and post-growth annealing in dry O 2 at 750°C. The conventional X-ray diffraction (XRD) patterns as well as pole figures (φ-scans) for specific ( hkl) reflections were acquired. The Er 1Ba 2Cu 3O 7-δ thin film in the DyMnO 3/Er 1Ba 2Cu 3O 7-δ/LaAlO 3 (100) heterostructure showed [001] oriented epitaxial growth, as expected. The DyMnO 3 thin film on the Er 1Ba 2Cu 3O 7-δ epilayer in the heterostructure grew with (110) epitaxy in its metastable orthorhombic phase (lattice constants: ao=5.272 Å, bo=5.795 Å and co=7.38 Å). The heteroepitaxial relationships at the orthorhombic-DyMnO O 3 (110) /Er 1Ba 2Cu 3O 7-δ (001) interface was determined as the following: DyMnO 3 (110) Er 1Ba 2Cu 3O 7-δ (001), DyMnO 3 [1
0] ¶r; Er 1Ba 2Cu 3O 7-δ[100] or Er 1Ba 2Cu 3O 7-δ[010], and DyMnO 3 [001] ¶r; Er 1Ba 2Cu 3O 7-δ[010] or Er 1Ba 2Cu 3O 7-δ [100]. 相似文献
11.
Superconductivity and crystallographic properties of La 2 − xM xCuO 4 − δ (M = Na, K) are studied. In the La 2 − xM xCuO 4 − δ system, superconductivity is detected for x 0.2. Oxygen content analysis shows that the system has more oxygen vacancies than the La 2 − xSr xCuO 4 − δ system. These oxygen vacancies may reduce the hole concentration, and high Na-doping is needed to produce superconductivity. In the La 2 − xK xCuO 4 − δ system, superconductivity is observed for the first time. Resistivity and magnetic susceptibility measurements show that Tc(onset) is 40 K and the Meissner volume fraction is about 4% for x = 0.7. The system changes from orthorhombic to a tetragonal K 2NiF 4 structure at x ≈ 0.3 and only tetragonal samples show superconductivity. 相似文献
12.
An account is given of the synthesis and structural investigation of a newly discovered tetragonal (Tl, Pb, Cu)Sr 2(Tl, Pb, Cu) 2Cu 2O 8−δ compound (Tl-1222 * phase) with both (Tl, Pb, Cu)O single and oxygen depleted (Tl, Pb, Cu) 2O x double layer motifs, space group I4/mmm, lattice parameters a=3.8405(2), c=29.2536(5) Å, and Dx=7.46 g cm −3. Micaceous and highly reflecting black single crystals of 1 mm×1 mm×3–5 μm size were obtained from a Tl 2O 3- and CuO-rich partial melt. The as-grown compound is nonsuperconducting. The very thin crystals are extremely liable to bend out of shape. Deterioration could not be observed within the period of a year under ambient conditions. The complex crystal structure has been solved using single crystals. We take note of the similarity of the unit cell dimensions to superconducting (Tl, Cu) 2Ba 2(Ca, Tl)Cu 2O 8 (Tl-2212 phase) with TlO double rocksalt layers, and compare crystal-chemical details of similar compounds. Apart from the possibility of obtaining superconducting properties by suitable doping, the mechanical properties of the new material are very attractive from the point of view of its use as highly scattering composite layers of an X-ray monochromator. 相似文献
13.
Pr concentration dependence of the superconducting transition temperature Tc in the Ho 1−xPr xBa 2Cu 3O 7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y 1−xPr xBa 2Cu 3O 7−δ reference system. Both systems have the same value of the critical concentration xc=0.58, in accordance with nearly equal ionic radii of Ho 3+ and Y 3+ ions. It has been shown that the Tc( x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr IV-ions, if one takes also into account that the number of these ions changes with x. 相似文献
14.
The polarized Raman spectra of Nd 1+xBa 2−xCu 3O 7−δ (−0.023≤ x≤0.107) and Pr 1+xBa 2−xCu 3O 7−δ (0.01≤ x≤0.15) single crystals have been investigated. It was found that the Cu(2) Ag mode softens by 6 cm −1 in Nd 1:2:3 and 4 cm −1 in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x( yy)
geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains. 相似文献
15.
Photoelectron spectroscopy results on single crystals of the superconductors Bi 2Sr 2CaCu 2O 8,Bi 2Sr 2CuO 6, Ba 0.6K 0.4BiO 3 and the semiconductor Ba 0.9K 0.1BiO 3 are reported for the photon energy region around the O K absorption threshold. The development of the O- KVV Auger structure has been carefully monitored as a function of photon energy. A non-monotonic behavior displaying a feature at a constant binding energy of about 14 eV was found for Bi 2Sr 2CaCu 2O 8 and Bi 2Sr 2CuO 6 in a narrow photon energy region of 1 eV at the main edge of the O K absorption spectrum around 530 eV. The corresponding enhancement, connected with the autoionization of O 2 p states, is absent in Ba 1−xK xBiO 3 in contrast to Bi 2Sr 2CaCu 2O 8 and Bi 2Sr 2CuO 6. The resonant enhancement is more pronounced for Bi 2Sr 2CuO 6 as compared to Bi 2Sr 2CaCu 2O 8, which can be explained by a lower charge carrier concentration in the former case, leading to a more localized nature of intermediate O 2 p states. The model parameters Cu d– d and O p– p Coulomb interactions and the charge transfer energy Δ are estimated from the experiments. 相似文献
16.
Finite deformation theory is used to obtain the strain energy density of a tetragonal 2–1–4-type single crystal of the high-temperature superconductor La 2−xSr xCuO 4. The complete set of second and third-order elastic constants of the high-temperature superconductor La 2−xSr xCuO 4 ( x = 0.16) is calculated by taking into account the interactions between nine nearest-neighbour atoms in the lattice and using Mie–Grüneisen interatomic potential. For the sake of comparison we have also computed the values of these constants for x = 0.13–0.20. The values of third-order elastic constants of La 2−xSr xCuO 4 ( x = 0.13–0.20) are negative and their absolute magnitudes are one order higher than those of the second-order elastic constants. 相似文献
17.
Infinite-layer-type superconductors Sr 1−xLn xCuO 2 are synthesized under high pressure of 3 GPa for Ln=Sm, Gd as well as for Ln=La, Nd. Their chemical and superconducting properties are systematically studied as functions of doping concentration and the kind of lanthanide ion. As a result, it is demonstrated that the variation of these properties with doping concentration is similar for all the examined Ln 3+ ions. The solubility limit lies at x ≈ 0.10. CuO 2 sheets are expanded with increasing x, while their spacing decreases. The Tc onset determined by magnetic measurements remains constant for any doping concentration; only the Meissner fraction increases with increasing x. 相似文献
18.
We have synthesized Hg(Pb)-1223 with substitutions of Sr for Ba, i.e. Hg 0.7Pb 0.3Ba 2−xSr xCa 2Cu 3O z ( x = 0.0–2.0) by the ampoule method. The spray-drying method and thermal decomposition under vacuum were applied for the preparation of homogeneous precursors. Samples with Tc = 110–128 K were obtained without oxygen annealing. We found that substitution of Ba by Sr remarkably relaxed the requirements on the precursors preparation and allowed their handling under ambient conditions. 相似文献
19.
The modulation of the (221) superconducting Bi 2Sr 2−xCa xCuO 6 phase has been analyzed by X-ray diffraction, using four-dimensional symmetry theory. The pseudo-orthorhombic diffraction pattern is a superposition of two twin related components, such that the main reflections with hkl and hk
m superimpose, but the satellite hklm and hk
m do not. The latter feature allows separation of the twin intensities. The modulation analysis in superspace group P:Aa:1 shows displacements similar to those observed in the 2212 compound [Y. Gao et al., Science 241 (1988) 954] but with generally larger displacements. In particular, the c-axis displacement amplitude of the Cu atoms is increased to almost 0.5 Å. This and the obliqueness of the q vector ( q=0.214 a*+0.61 c*) indicate the absence of a restraining influence exerted by the CuO 2/Sr(Ca)/CuO 2 double layers in the multi-Cu layer phases. 相似文献
20.
Nd2CuO4±δ is the non-superconducting prototype of the Re2−xMxCuO4t− y family (Re=Pr, Nd, Sm and M=Ceor Th) of n-type oxide superconductors. Four-probe DC conductivity, EMF in P(O 2) gradient, and thermopower measurements have been used to characterise its electric transport and defect structure between 300 and 900°C and between 5×10 −4 and 1 atm oxygen partial pressure. The results show that Nd2CuO4±δ can be oxygen under-stoichiometric (with n-type conductivity), near-stoichiometric, and over-stoichiometric (with p-type conductivity) in different T, P(O2) ranges. 相似文献
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