共查询到20条相似文献,搜索用时 8 毫秒
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铜氧化物高温超导现象自30年前被发现以来,对现代凝聚态物理的发展产生了极其重要的影响,然而其微观机制至今依然是一个谜。近年来,多种实验手段的研究结果发现,在铜氧化物高温超导体中电子除了形成库珀对,还可能形成多种其他新奇的有序态,例如自旋有序态、电荷有序态以及库珀对密度波等。这些有序态的起源及其与赝能隙态和超导态的关联对于理解高温超导机理可能具有重要的意义。文章将主要从实验的角度对铜氧化物超导体中的电子有序态做一个概述。 相似文献
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David Emin 《哲学杂志》2013,93(31):2931-2945
AbstractPlanar-large-bipolarons can form if the ratio of the surrounding mediums’ static to high-frequency dielectric constants is especially large, ε0/ε∞ >> 2. A large-bipolaron in p-doped La2CuO4 is modelled as two electrons being removed from the out-of-plane orbitals of four oxygen ions circumscribed by four copper ions of a CuO2 layer. These oxygen dianions relax inwardly as they donate electrons to the surrounding outwardly relaxing copper cations. This charge transfer generates the strong in-plane electron–lattice interaction needed to stabilise a large-bipolaron with respect to decomposing into polarons. The lowest-energy radial in-plane optic vibration of a large-bipolaron’s four core oxygen ions with their associated electronic charges has d-symmetry. Electronic relaxation in response to multiple large-bipolarons’ atomic vibrations lowers their frequencies to generate a phonon-mediated attraction among them which fosters their condensation into a liquid. This liquid features distinctive transport and optical properties. A large-bipolaron liquid’s superconductivity can result when it undergoes a Bose condensation yielding macroscopic occupation of its ground state. The synchronised vibrations of large-bipolarons’ core-oxygen ions with their electronic charges generate this Bose condensate’s dynamic global d-symmetry. 相似文献
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Expressions for the attenuation coefficients of longitudinal and transverse ultrasonic waves are developed for steel with pearlitic microstructure. This type of lamellar duplex microstructure influences attenuation because of the lamellar spacing. In addition, longitudinal attenuation measurements were conducted using an unfocused transducer with 10 MHz central frequency on the cross section of a quenched railroad wheel sample. The dependence of longitudinal attenuation on the pearlite microstructure is observed from the changes of longitudinal attenuation from the quenched tread surface to deeper locations. The results show that the attenuation value is lowest and relatively constant within the quench depth, then increases linearly. The experimental results demonstrate a reasonable agreement with results from the theoretical model. Ultrasonic attenuation provides an important non-destructive method to evaluate duplex microstructure within grains which can be implemented for quality control in conjunction with other manufacturing processes. 相似文献
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Cu K-absorption edges of YBa2Cu3O6.9, Tl2CaBa2Cu2O8 and Bi2CaSr2Cu2O8 show similar features. Copper is mainly in the 2+ state in these cuprates suggesting the likely presence of oxygen holes.
Contribution No 616 from the Solid State and Structural Chemistry Unit. 相似文献
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Two aspects of ultrasonic attenuation in superconductors are examined: (i) electron drag on dislocations and (ii) anomalous
results in energy gap measurements. None of these features is physically understandable at present. 相似文献
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Kazuhisa Nishi 《Physics letters. A》2018,382(45):3293-3297
We design a new structure for a cuprate superconductor indicating the possibility of higher temperature superconductivity using our recently proposed composite fermions theory. It is constructed with modulation-doped superlattice structures, which are often used in the design of semiconductor superlattice devices. The superconductive critical temperature () was calculated in the superlattice structures of the superconductor in which the optimal doped CuO2 layer was sandwiched between two less-doped CuO2 layers. We find that if these structures could be realized in a cuprate superconductor such as Bi2Sr2Ca2Cu3O10 or HgBa2Ca2Cu3O9, the highest could attain the level of 300 K at atmospheric pressure. 相似文献
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《Physics letters. A》2014,378(18-19):1309-1312
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An analytical solution for a three dimensional integral representation of the backscattering (BS) coefficient in polycrystals with elongated (generally ellipsoidal) grains is obtained; it is a natural generalization of the known explicit result for the BS coefficient in polycrystals with spherical grains. New insights into the dependence of the BS signal on frequency and averaged ellipsoidal grain radii are obtained. In particular it has been shown that the dominant factor for the backscattering is the averaged interaction length of the ellipsoidal grain in the direction of wave propagation, instead of the ellipsoidal cross-section. The theory was applied to a simplified model of Ti alloy duplex microstructure and was compared with experiment. For the experimental data analysis directional backscattering ratios are introduced and shown to be advantageous for characterization of duplex elongated microstructures/microtextures. In addition to the geometrical parameters of the elongated microtextures, the BS directional ratios depend on the newly introduced nondimensional material parameter q. The parameter q exhibits the relative contribution of the second phase (crystallites) to the backscattering signal, the effect of which is measurable and important. Comparison of the model with experiment shows there is a significant advantage in using the directional ratios of backscattering coefficients for data analysis. 相似文献
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The ultrasonic attenuation in two-band superconductors with non-magnetic and paramagnetic impurities in the presence of persistent currents is studied. The case of high impurity concentrations is considered. Assuming space-independent sound waves, the absorption energy in the case of relatively low frequencies at low and zero temperatures is calculated. 相似文献
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T. Schneider 《Zeitschrift für Physik B Condensed Matter》1991,85(2):187-195
A central concern in understanding the mechanism for the occurrence of superconductivity in cuprates is the interaction driving the phase transition and their dimensionality. As physical systems near a phase transition have a marked dependence on dimensionality, this can be explored with symples where one of the physical dimensions is reduced and becomes comparable to the correlation length. Recently, it became possible to fabricate sufficiently thin cuprate slabs, revealing a fall ofT
c with reduced thickness, becoming pronounced for slabs a few unit cells thick. Related effects have been observed in the YBCO bulk compounds 123, 124 and 247. We analyze the experimental data by invoking finite size scaling and a Ginzburg-Landau treatment. The main conclusions include the following: the fall ofT
c with decreasing thickness corresponds to a dimensional crossover, revealing the three-dimensional nature of the interaction mediating superconductivity; there is a predominance of two-dimensional fluctuations and boundaries with reduced thickness; there are crossover phenomena reminiscent of4He films and thin slabs of conventional super-conductors. 相似文献
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We have measured the in-plane resistivity of Bi2Sr2CaCu2O8+δ and Tl2Ba2 CaCu2O8+δ single crystals in the temperature range 70–300 K. The thermodynamic fluctuations in the conductivity of both the samples
start around ∼ 125 K. We find the Lawrence and Doniach [1] model to be inadequate to describe the fluctuation conductivity
in these materials. The modification suggested by Ramallo et al [4] where by the conductivity is enhanced due to the presence of two superconducting layers in each unit cell is also not
adequate. We suggest the fluctuation conductivity to be reduced due to the reduction in the density of states (DOS) of the
quasiparticles which results due to the formation of Cooper pairs at the onset of the fluctuations. The data agrees with the
theory proposed by Dorin et al [5] which takes into account this reduction in DOS. 相似文献
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Electron-phonon (e-ph) renormalization effects in a model cuprate system CaCuO2 are studied by employing density functional theory based methods. Whereas calculations based on the local spin-density approximation (LSDA) predicts negligible e-ph coupling effects of the half-breathing Cu-O bond stretching mode, the inclusion of a screened on-site Coulomb interaction (U) in the LSDA+U calculations greatly enhances the e-ph coupling strength of this mode. The full-breathing mode, on the other hand, shows a much weaker e-ph renormalization effect. 相似文献
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D. Brinkmann 《Applied magnetic resonance》2004,27(1-2):207-213
This presentation gives a personal review of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spin-lattice relaxation studies in cuprate superconductors mainly dealing with the YBa2Cu4O8 compound with many examples from the Zürich laboratory. The studies were performed in both the normal and the superconducting state with various NMR isotopes (e.g.,17O,63,65Cu,135,137Ba). The relatively broad signals were mostly obtained by a phase-alternating add-subtract spin-echo technique. We will discuss the general behavior of spin-lattice relaxation in the normal state and the calculation of the dynamic spin including an approach (on the basis of thet-J model) to calculate the relaxation for plane copper, oxygen, and yttrium. An application of the Luttingerliquid model to the relaxation of chain copper in YBa2Cu3O7 and YBa2Cu4O8 is also given. We then will deal with characteristic features of the YBa2Cu4O8 structure: the spin gap, an electronic crossover in the normal state, the single-spin fluid model, and the d-wave pairing. 相似文献
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K. Sadhana K. PraveenaS.R. Murthy 《Journal of magnetism and magnetic materials》2011,323(23):2977-2981
The nanocrystalline MgCuZn ferrites with particle size (∼30 nm) have been synthesized by microwave-hydrothermal (M-H) method at 160 °C/45 min. The powders were densified at 750-900 °C/30 min using microwave sintering method. The sintered samples were characterized using X-ray diffraction and scanning electron microscope. The grain sizes of the sintered samples are in the range of 60-80 nm. The ultrasonic velocities have been measured on MgCuZn ferrites using the pulse transmission method at 1 MHz. The ultrasonic velocity is found to decrease with an increase of temperature. A small anomaly is observed around the Curie temperature, 520 K. The anomaly observed in the thermal variation of longitudinal velocity and attenuation is explained with the help of magneto-crystalline anisotropy constant. 相似文献