共查询到18条相似文献,搜索用时 125 毫秒
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本文中分析了非过渡金属非晶态超导体的超导参量、声子谱参量与霍耳系数之间的经验关系。研究了非晶态超导体的Tc,并得出,声子谱的软化所导致的Tc的提高幅度与电-声子耦合常数λ的提高幅度成线性关系;声子谱的高频截止频率愈高,其Tc也愈高。讨论了利用声子谱的软化虽然能大幅度地提高Tc值,但要获得包括金属Be在内的非过渡金属的高Tc非晶态超导体的希望是渺茫的。还讨论了非晶态超导体的上临界场Hc2和能隙2Δ0所表现出的强耦合效应等问题。
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研究了Gd1-xCaxBa2Cu3O7-y(0.0≤X≤0.20)高温超导体在常压和高压下的超导电性在1-300K温度范围内,利用Bridgman对顶砧获得压力达9.0GPa,测量了(X=0.10,0.15,0.20)样品的dTc/dp分别为7.68,7.8和4.46K/GPa。发现Tc的压力导数随着ca2+含量的增加而下降,分析了氧含量对Tc和dTc/dP的影响.利用常压下晶格参数精修值和阳离子与氧离子间距随压力的改变,说明CuO2面在超导电性上的作用,用CuO2面之间耦合解释Tc(P)曲线的非线性关系。
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提出了能够很好地描述非过渡金属无序和非晶态超导体的2Δ0/(kBTc)与声子谱参量之间关系的一个公式:2Δ0(kBTc=4.95[1-(T0<ω>1/2)/A(1/(λω0)+1/(20λ<ω>)+1/(20<ω>))]。计算了大量已知声子谱的非晶和无序超导体的能隙2Δ0对Tc的比,结果表明在百分之几的范围内与实验值符合。指出了非过渡金属和合金的非晶态超导体,既可以是一个2Δ0/(kBTc)值远大于BCS理论值(3.53)的强耦合超导体,也可以是一个2Δ0/(kBTc)值比BCS理论值还要小得多的弱耦合超导体。
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对于自旋I>1/2的核,其运动的相关时间τc以及核四极耦合常数NQCC是表征分子局部和微观动力学结构的重要参数. 然而,在溶液中,这些参数均难以测定. 该工作以聚氨大环-氰基合钴超分子体系为例,利用在高分子体系中广泛使用的激光光散射技术成功地测量了溶液中超分子络合物[12]aneN4[Co(CN)6], [18]aneN6[Co(CN)6], [24]aneN8[Co(CN)6],[16]aneN4[Co(CN)6], [24]aneN6[Co(CN)6] 和 [32]aneN8[Co(CN)6] 的水合半径,从而根
据Stokes-Einstein-Debye理论计算出了分子转动的相关时间τc. 这些超分子的水合半径在0.4~0.6 nm之间. 对应的转动相关时间τc在3.8~9.5×10-11 s之间. 这一时间范围对于分子量在300~500的分子,有相当的合理性. 进而,通过测量超分子在稀溶液中的纵向弛豫时间T1,结合计算出的相关时间τc,我们得到了该超分子体系中59Co的四极耦合常数,其范围在0.5~2.2 MHz之间. 我们发现,这些超分子在溶液中的四极耦合常数明显小于在固体状态时的值(5~7 MHz). 导致这一现象很可能的原因是在固体状态下59Co核周围的电场梯度是各向异性的,从而有较大的四极耦合常数值. 而在溶液中,由于分子的快速运动,各向异性被平均,因而有较小的四极耦合常数. 相似文献
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We investigate the direct effects of phonon softening on Tc in Nb3Sn. It is inadequate to explain the high Tc, which arises instead from an unusually large electron- phonon coupling to the two Γ'12 optical phonon modes. 相似文献
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High-temperature dispersion relations of the phonon modes in a cubic PbTiO3 single crystal have been investigated along the [ξ 0 0] and [ξ ξ 0] directions by inelastic neutron scattering. Above T c, the phonon dispersions are only temperature-dependent close to the Brillouin zone centre where the mode softening comes through. The measurements indicate large cubic anisotropy of the elastic tensor and relatively low anisotropy of the soft mode dispersion. The differences from an earlier inelastic neutron scattering study are discussed. 相似文献
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The functional derivative δTc/δα2(ω)F(ω) of the critical temperature (Tc) with the electron-phonon spectral density (α2(ω)F(ω)) gives information on the effectiveness of various phonon modes in enhancing Tc. For an anisotropic superconductor, it is found that δTc/δα2(ω)F(ω) goes negative at some small but finite phonon energy. This contrasts with the isotropic case for which it is well known that the functional derivative is positive everywhere. Thus, very low energy phonons reduce Tc in an anisotropic superconductor which is similar to the known effects of static impurities that wash out anisotropy and hence reduce Tc. 相似文献
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A weighted phonon frequency distribution has been measured in PbF2 at temperatures 10, 302, 660 and 910 K, using a neutron scattering technique. At 10 K good agreement is found between the measured distribution and the phonon density-of-states calculated from the low temperature dispersion relation of PbF2. At the higher temperatures, near the ionic conductivity transition temperature, Tc ~ 700 K, the optic modes are observed to broaden into a high energy tail consistent with strong anharmonicity or extensive disorder. A low energy peak arising from transverse acoustic modes remains well defined even at temperatures above Tc. 相似文献
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L. Martín-Carrón A. de Andrés 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,22(1):11-16
We have studied the behavior of the Raman phonons of a stoichiometric LaMnO3 single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks
of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra.
The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three
phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O
bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three
phonon scattering processes well bellow T
c
. The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude
of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature.
Phonon width and intensity behavior around T
c
can be explained by local melting of the orbital order that begins quite below T
c
and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions.
Received 25 January 2001 and Received in final form 14 March 2001 相似文献
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A mechanism involving interaction of conduction electrons with the distortion field and the phonon modes is considered for
the newly discovered highT
c superconducting materials. This is capable of explaining the observed range ofT
c. 相似文献
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B. Stritzker 《Zeitschrift für Physik A Hadrons and Nuclei》1974,268(2):261-264
High superconducting transition temperatures, Tc, of 16.6, 15.6 and 13.6 K have been observed in Pd-(Cu, Ag and Au) alloys, charged with large amounts of H by means of ion implantation at liquid Helium-temperatures. A peculiar phase-transition indicates that weak phonon modes might be responsible for the high Tc-values. The difference between the maximum Tc-values can be described as a type of isotope effect Tc∞M ?1/2. 相似文献
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Wei Wang Jiafa SunSuwen Li Hongyan Lu 《Physica C: Superconductivity and its Applications》2012,472(1):29-33
Using first-principles calculations, we investigate electronic structure and phonon spectrum of binary iron-based superconductor FeSe in both tetragonal nonmagnetic (NM) phase and orthorhombic striped antiferromagnetic (SAF) phase. It is found that the softening of atomic vibration modes and main electron-phonon coupling contribution from low-frequency Eliashberg spectral function α2F(ω) in SAF phase of FeSe lead to the enhancement of electron-phonon coupling strength λep and logarithmically average frequency ωln. However, the obtained superconducting Tc in SAF phase just increases up to 0.34 K, even though Coulomb pseudopotential μ∗ is limited to zero. As a result, our magnetic phonons calculation still rules out phonon mediated superconductivity, although the electron-phonon coupling through the spin channel play an important role in FeSe. 相似文献