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Il Nuovo Cimento D - The results of low-temperature magnetic-relaxation experiments in TlBaCaCuO superconductors (2212 and 2223 phase) are reported. They give evidence of a crossover from a... 相似文献
3.
We have tried to understand the field dependence of magnetization of high temperature superconductors in the light of phenomenological
theory. Especially, the field dependence of dM/d lnB of polycrystalline Bi(2212) is understood by incorporating the overlap of vortices in the London theory. 相似文献
4.
It is conceivable that the high-T
c
superconducting perovskites are conventional electronphonon superconductors. In this case one expects significant strong-coupling effects because of the unusually high ratiok
B
T
c
/ of the order 0.1 and greater. We use a set of reasonable models for the Eliashberg function 2
F() (which takes into account available information on the phonon spectra and which fit the measuredT
c
's) and calculate strong-coupling effects in the specific heatc
s
(T)/T
c
, the ratio 0/k
B
T
c
, the critical fieldsH
c
(T) andH
c2
(T) including Pauli limiting, and other measurable quantities. Strongcoupling corrections turn out to be in the range of 0 to about 100%, depending on the quantity of interest. We discuss the perspectives of using strong-coupling effects as indicators for conventional electron-phonon superconductivity in the new materials. 相似文献
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Based on a reexamination of experimental facts a simple model called Double-Pairing Superconductivity is set up to describe the high temperature layered superconductors (HTLS). A mean-field scenario is carried out to investigate the model Hamiltonian. Within our model some of the exotic properties of HTLS may be understood, at least qualitatively. 相似文献
7.
M. Braden P. Böhm F. Seidler H. Kalenborn D. Wohlleben 《Zeitschrift für Physik B Condensed Matter》1990,79(2):173-176
We report on magnetostriction measurements of superconducting and nonsuperconducting YBa2Cu3O7– and of two Bi samples with the nominal compositions Bi2CaSr2Cu2O
x
and Bi0.8Pb0.2Sr0.8CaCu1.5O
x
. Both types of high temperature superconductors show a nearly identical strongly hysteretic field dependence of the magnetostriction (MS) (l/l+(2–6)*10–6 at 5 T and 1.5 K). In both cases the MS is a linear function of field in the region 1–5 T, which we explain by striction due to surface currents. Between 1.5 and 35 K the hysteresis of the MS decreases strongly with increasing temperature, which implies a decrease of the pinning force. The MS of quenched nonsuperconducting YBa2Cu3O7– is at least two orders of magnitude smaller than that of the superconductor. 相似文献
8.
We generalized the Coherent Potential Approximation (CPA) method for the calculation of the critical temperature of superconductors of lower dimensionality. Two dimensional (2d) and one-dimensional (1d) models have been considered. The critical temperature is calculated as function of the electronic concentration and of the position of the Fermi level. The critical temperature can be expressed with either of these two parameters. We also calculate the dependence ofT
c on an additional parameter which is the measure of the interplane coupling. 相似文献
9.
The superconductivity-induced self-energy of phonons has been calculated in the lowest-order conserving approximation using conventional strong-coupling theory and also including scattering at nonmagnetic impurities. Results for the dependence of the shift and the width of phonons on temperature and scattering rates are presented and their sensitivity on the strength of the electron-phonon coupling and on impurities is pointed out. The theory is applied to optical data in YBa2Cu3O7 using the experimental one-phonon density, the value ofT
c
and [*=0.25 as inputs. Neglecting anisotropies and the momentum dependence of the electron-phonon coupling the resulting strong-coupling model with 2.9 yields results which are in good agreement with the data, in particular, if impurity scattering is taken into account. 相似文献
10.
A. H. Morrish X. Z. Zhou L. Y. Luo Z. W. Li I. Maartense 《Hyperfine Interactions》1990,55(1-4):1343-1353
The high-temperature superconductor, Tl2CaBa2(Cu1?x Fe x )2O8+δ (the 2122 compound), has been investigated by a number of techniques, including X-ray diffraction, resistance and ac susceptibility measurements, and Mössbauer spectroscopy. The procedures followed to make close to single-phase samples are described. The decrease in the critical temperature for superconductivity,T c , is less than for the iron-doped 123 compounds. The Mössbauer spectra at 77 K and above consist of an asymmetric doublet. Below about 10 K magnetic hyperfine splitting occurs; relaxation effects are still present at 2.3 K. The spectra can be fitted with two overlapping patterns. Their origin is discussed: comparisons are made with other high-T c superconductors. 相似文献
11.
The effect of a quantum phase transition associated with the appearance of fermionic condensation in an electron liquid on the properties of superconductors is considered. It is shown that the electron system in both superconducting and normal states exhibits characteristic features of a quantum protectorate after the point of this Fermi-condensate quantum phase transition. The single-particle spectrum of a superconductor can be represented by two straight lines corresponding to two effective masses M FC * and M L *. The M FC * mass characterizes the spectrum up to the binding energy E 0 , which is of the order of the superconducting gap in magnitude, and determines the spectrum at higher binding energies. Both effective masses are retained in the normal state; however, E 0 ?4 T. These results are used to explain some remarkable properties of high-T c superconductors and are in good agreement with recent experimental data. 相似文献
12.
Tunneling spectroscopic studies of high-temperature superconductors (HTSC) are overviewed. It is demonstrated, in particular, that the superconducting gaps determined from Andreev reflections and tunneling current-voltage characteristics coincide at all doping levels. Facts are presented evidencing the strong electron-phonon interaction in the HTSC systems. The nature of the so-called pseudogap, which is observed in some tunneling experiments, is briefly discussed. 相似文献
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David R. Nelson 《Journal of statistical physics》1989,57(3-4):511-530
A theory of the entangled flux liquids which arise in the new high-T
c
superconductors is reviewed. New physics appears because of the weak interplanar couplings and high critical temperatures in these materials. Flux line wandering melts the conventional Abrikosov flux lattice, and leads to an entangled vortex state whose statistical mechanics is closely related to the physics of interacting bosons in two dimensions. The phase diagram as a function of magnetic field and temperature is discussed, and it is argued that an entangled vortex liquid appears just aboveH
c1 at all nonzero temperatures. The decay of vortex line correlations in the entangled liquid state is controlled by the superfluid excitation spectrum of the bosons. Line wandering produces drastic changes in theB(H) constitutive relation nearH
c1. 相似文献
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Roland Zeyher 《Zeitschrift für Physik B Condensed Matter》1990,80(2):187-192
It is shown that the long-range part of the electron-phonon coupling (rigidly displaced ionic potentials screened by a background dielectric constant and by holes moving along the CuO2 planes) is large in YBa2Cu3O7 due to imperfect screening. Using a simple phenomenological model for the phonon part the long-range contribution to the Eliashberg function 2
F() is evaluated yielding a value of about 2.4 for . The calculation suggests that this long-range part which is neglegible in usual metals plays an important role in high-T
c
oxides. 相似文献
17.
Eggenhffner R. Masini R. Bellingeri E. Cimberle M. R. Ferdeghini C. Costa G. A. Kaiser E. Signorelli E. 《Il Nuovo Cimento D》1994,16(10):1659-1664
Il Nuovo Cimento D - The formation of Hg-based HTSC materials was investigated through the development of non-conventional techniques. The former was based on a singlestep synthesis at temperatures... 相似文献
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We have solved the self-consistent equation for self-energy of a hole in a quantum antiferromagnet. The optical conductivity is estimated. The results are in good agreement with experiments and numerical simulations. 相似文献
20.
M. Crişan 《Zeitschrift für Physik B Condensed Matter》1989,74(2):151-153
We calculated the ratio 2(0)/k
BTc within the Eliashberg theory for a two dimensional (2D) high temperature superconductor. The deviations from the value 3.53 as well as from (0)/k
BTc2 are discussed, suggesting the possibility of the origin of great values of this ratio.Work performed under the contract with IPTM 相似文献