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排序方式: 共有422条查询结果,搜索用时 93 毫秒
71.
P. Singha Deo V. A. Schweigert F. M. Peeters A. K. Geim 《Physica E: Low-dimensional Systems and Nanostructures》1998,1(1-4)
The Ginzburg–Landau equations coupled with the three-dimensional Maxwell equations are solved for the disc geometry in order to explain recent magnetization experiments. In order to explain the experimental results on a 0.5
m radius Al disc, we have to assume that the superconducting state stays in the lowest angular momentum giant-vortex state even in regions where it is not the lowest energy state. 相似文献
72.
J. Arvanitidis K. Prassides G.A. Kourouklis T. Takenobu Y. Iwasa 《Journal of Physics and Chemistry of Solids》2004,65(1):73-77
We study the effect of carbon doping on the Raman spectrum of the MgB2 superconductor. Out data show that significant changes in the Raman spectra of the MgB2−xCx compounds occur for carbon concentrations x>0.04. The E2g mode at ∼580 cm−1 hardens only moderately upon increased doping despite direct carbon substitution in the boron layers, while the dependence of its full width at half maximum (FWHM) on x reveals the competing effects of reduced electron-phonon coupling and increased disorder. The results are discussed in the framework of anisotropic lattice contraction and the position of the σ sub-bands with respect to the Fermi energy level. The relative intensity of the phonon peak at ∼770 cm−1, associated with a peak in the phonon density of states, increases considerably and dominates for x=0.08. Additionally, it hardens and broadens with increasing x in the range 0.04-0.08. These changes can be associated with carbon substitution-induced disorder in the investigated samples. 相似文献
73.
Ying-Nan Chiu 《中国化学会会志》1989,36(6):487-499
A vibrationally coupled intervalent charge transfer theory is proposed for the copper oxide-superconductors. The proposed B1g-vibrational distortion exactly interchanges the chemical environment of the neighboring copper atoms. A quadruple cell is constructed to accommodate the non-stoichiometry and a double copper-oxide-conducting chain with distorted symmetry is postulated. The distorted structure, prone to switching of the copper sites by vibration, is postulated to be the superconducting precursor. The left over-oxygen, due to oxygen deficiency, is allowed to serve as a bridge between the chains and 10 oscillate with the vibration. Symmetry of the system, before and after distortion, is used to explain the spin-angular momentum change, needed to arrive at Cooper-paired electron/holes. The O? and O= atoms are postulated to be the bridge for superexchange and charge flow between the Cu+1/Cu+3 and Cu+1/Cu+2 pairs. The same theory is used to interpret the mechanisms for the following three types of copper oxide-superconductors: YBa2Cu3OT-x; La2-xSrxCuO4 and the newly discovered electron superconductor Nd2-xCexCuO4-y. The bridge oxygen oscillation may be chosen not to come from the breathing mode but from the bending deformation mode that involves the relative motion of the Cu atoms (to the oxygen) giving rise to smaller oxygen isotope effect. 相似文献
74.
发展超导问题的局部耗散性和吸引子 总被引:2,自引:0,他引:2
本文得到了在Lorentz规范下一类发展超导问题解的整体存在性、局部耗散 性和吸引子的存在性. 相似文献
75.
H. Fjellvåg Y. Morita J.-M. Lee R.-S. Liu V.P.S. Awana Y. Matsui M. Karppinen 《Journal of solid state chemistry》2006,179(3):632-645
In this work, the first three members (s=1, 2, 3) of the Co-12s2 homologous series of multi-layered copper oxides are gradually doped with holes through high-pressure oxygenation (HPO). The phases differ from each other only by thickness of the fluorite-structured layer block, (Ce,Y,Ca)-[O2-(Ce,Y)]s−1, between two identical CuO2 planes. High-resolution transmission-electron microscopy (HRTEM) and electron diffraction (ED) analyses together with both synchrotron X-ray and neutron powder diffraction data, reveal that as a consequence of HPO the charge-reservoir CoO4-tetrahedra chains get broken and the lattice symmetry of the Co-12s2 phases changes from orthorhombic to tetragonal. Oxygen contents are analyzed for the samples with wet-chemical and thermogravimetric techniques. The valence state of copper in the CuO2 plane is determined from Cu L-edge X-ray absorption near-edge structure (XANES) spectra to be compared with the values estimated through bond-valence-sum (BVS) calculations from the crystal structure data. The positive charge induced by oxygen loading (or aliovalent CaII-for-YIII substitution in CoSr2YCu2O7+δ) is found not to be completely accommodated in the CuO2 planes but be rather effectively trapped at the charge-reservoir Co atoms. Superconductivity appears in the Co-1212 (CoSr2YCu2O7+δ) samples with the copper valence of 2.13 or higher, whereas in the Co-1222 (CoSr2(Ce0.25Y0.75)2Cu2O9+δ) and Co-1232 (CoSr2(Ce0.67Y0.33)3Cu2O11+δ) samples Cu valence does not increase high enough to induce superconductivity. 相似文献
76.
77.
Nicolas Emery Jean-François Marêché Geneviève Loupias 《Journal of solid state chemistry》2006,179(4):1289-1292
In this paper, we report the discovery of superconductivity in Li3Ca2C6. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors [T. Enoki, S. Masatsugu, E. Morinobu, Graphite Intercalation Compounds and Applications, Oxford University Press, Oxford, 2003]. It is probably not astonishing, since it is generally admitted that low dimensionality promotes high superconducting transition temperatures. Superconductivity is lacking in pristine graphite, but after charging the graphene planes by intercalation, its electronic properties change considerably and superconducting behaviour can appear. Li3Ca2C6 is a ternary GIC [S. Pruvost, C. Hérold, A. Hérold, P. Lagrange, Eur. J. Inorg. Chem. 8 (2004) 1661-1667], for which the intercalated sheets are very thick and poly layered (five lithium layers and two calcium ones). It contains a great amount of metal (five metallic atoms for six carbon ones). Its critical temperature of 11.15 K is very close to that of CaC6 GIC [T.E. Weller, M. Ellerby, S.S. Saxena, R.P. Smith, N.T. Skipper, Nat. Phys. 1 (2005) 39-41; N. Emery, C. Hérold, M. d’Astuto, V. Garcia, Ch. Bellin, J.F. Marêché, P. Lagrange, G. Loupias, Phys. Rev. Lett. 95 (2005) 087003] (11.5 K). Both CaC6 and Li3Ca2C6 GICs possess currently the highest transition temperatures among all the GICs. 相似文献
78.
The new equiatomic silicides NbOsSi and TaOsSi as well as ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) were prepared from the elements by arc-melting. These silicides crystallize with the orthorhombic TiNiSi type structure, space group Pnma. Irregularly shaped crystals of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi were separated from the annealed samples and investigated by single-crystal X-ray diffraction (a = 640.46(7), b = 404.07(5), c = 743.66(8) pm, wR2 = 0.0285, 390 F2 values, 20 variables for ZrOsSi; a = 629.78(6), b = 388.72(4), c = 727.48(7) pm, wR2 = 0.0350, 397 F2 values, 20 variables for NbOsSi, a = 626.80(6), b = 389.36(4), c = 726.22(7) pm, wR2 = 0.0501, 385 F2 values, 20 variables for TaOsSi, a = 653.48(8), b = 395.35(4), c = 739.19(8) pm, wR2 = 0.0427, 413 F2 values, 20 variables for ZrIrSi and a = 646.34(12), b = 393.57(7), c = 736.8(14) pm, wR2 = 0.0582, 371 F2 values, 20 variables for HfIrSi). The striking structural motifs in the new osmium compounds are three-dimensional [OsSi] networks (Os–Si: 240–251 pm) in which the osmium atoms have strongly distorted tetrahedral silicon coordination. High-pressure/high-temperature experiments (9.5 GPa/1520 K) on TaOsSi gave no hint for a structural phase transition. Temperature dependent measurements of the magnetic susceptibility and the electrical conductivity of NbOsSi and TaOsSi showed superconductivity below TC = 3.5 and 5.5 K, respectively. 29Si solid state MAS NMR investigations of the prepared silicides approved the structural models and showed a correlation between the observed 29Si resonance shifts and the electronegativity of the involved refractory metal. 相似文献
79.
Martin Dressel Dan Wu Boris Gorshunov 《Journal of Physics and Chemistry of Solids》2011,72(5):514-518
THz and infrared spectroscopies are widely utilized to investigate the electrodynamic properties of the novel iron-based superconductors in the normal and superconducting states. Besides electronic excitations and correlations, electron-phonon coupling and the influence of magnetism, the experiments yield important information on low-lying excitations and help to clarify the number and symmetry of superconducting gaps. While the experimental data of different groups converge, the interpretation is still under debate. Here we review the status of optical investigations on the superconducting state for the 122 and 11 family of iron pnictides. 相似文献
80.
Xinghua Guo Maozhong An Peixia Yang Caina Su Yuhong Zhou 《Journal of Sol-Gel Science and Technology》2009,51(3):335-347
Despite tremendous efforts in research and development after the discovery of high-temperature perovskite-type layered superconductors,
and high expectations expressed in market projection potential electrical commercial applications of these materials, the
popularity of these superconductors is taking off very slowly. One of the biggest hurdles facing the widespread application
of the (RE)Ba2Cu3O7 is developing a manufacturing process of flexible multifilamentary coated conductors, comprised of a buffered metallic substrate
and a superconducting layer (Glowacki, in Frontiers in Superconducting Materials. Springer, New York, 2005), that will produce
it in long lengths and at prices competitive with copper for applications such as motors, generators, transmission cables,
and other power systems (Lectures on Superconductivity, University of Cambridge, Cambridge, 2008). The low-cost powder-in-tube
or screen-printing processes, that are successful in the manufacturing of medium temperature superconductor MgB2 and high-temperature Bi-family superconductors cannot be used for (RE)Ba2Cu3O7 conductors due to their granularity and ceramic-like mechanical bulk properties. Deposition of a (RE)Ba2Cu3O7 coating using physical deposition techniques is too expensive to provide low-cost highly textured coated conductor for AC
applications (Lectures on Superconductivity, University of Cambridge, Cambridge, 2008). Therefore, the obvious choice is the
chemical route and the sol gel deposited using ink-jet printing (Glowacki, in Frontiers in Superconducting Materials. Springer,
New York, 2005; Lectures on Superconductivity, University of Cambridge, Cambridge, 2008) is currently recognised as potentially
the best way to manufacture 3D multifilamentary high temperature superconductor (Glowacki, in Frontiers in Superconducting
Materials. Springer, New York, 2005). In this paper the perspective of deposition of buffer layer and multifilamentary (RE)Ba2Cu3O7 superconducting layer by sol gel ink-jet printing is discussed with respect to best conductor AC performance, and also addresses
the future research that is required. 相似文献