非晶态超导体的转变温度Tc与原子质量的关系

本文探讨非晶态超导体的T_c与原子质量M之间的关系,发现在具有相同价电子数的同族元素中,超导T_c与原子质量的立方根成反比,即T_c∝l/M^1/3。讨论了晶态超导体的T_c问题,其中包括高T_c氧化物超导体。 关键词：     

Relationship between T c and electronegativity differences in compound superconductors

The electronegativity differences, N, between the anions and the cations in suggested resonating elements of some representative high-temperature superconductors with T _c - 10 K are evaluated adopting Pauling's scale. The relationship between N and T _c was found to separate all the examined high-T _csuperconductors onto two curves: One for the cuprate superconductors having two-dimensional layered structures was a straight line, T _c=29.8+4.1N; the other correlation curve representing the remaining compound superconductors including the doped 113 perovskite and the perovskite-related 214 structures was at lower T _c values but also suggested that increasing electronegativity differences was related to increasing T _c.This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098     

低通量慢中子对Bi系超导体的辐照效应

本文研究低通量慢中子对Bi系超导体的辐照效应。实验结果表明:(1)Bi系超导体在适量慢中子辐照后,临界电流密度J_c和零电阻温度T_c0都有不同程度提高;(2)低通量慢中子和高通量快中子对Bi系超导体具有相似的辐照效应。 关键词：     

Ll2超导体的Tc表达式

通过对L1₂超导体的研究,我们得出了这类超导体临界温度的表达式T_c=(15.9T_BV(B)G_A)/(M^1/2V(L1₂)G_B)。其中T_B是B组元的超导转变温度,V(B)是B组元的原子体积,V(L1₂)为L1₂化合物的平均原子体积,M是L1₂化合物的平均原子量,G_A,G_B分别为A组元、B组元的Gordy电负性值。 关键词：     

Temperature dependence of κc for type-II superconductors

The temperature dependence of the critical value κ_c of the Ginzburg-Landau parameter which separates type-II/1 from type-II/2 superconductors is theoretically computed by means of a microscopic calculation of the boson characteristic function at nonzero temperature, assuming that the temperature is not too close to T_c. Agreement with experimental data is satisfactory in the region $\text{0?T}\text{T}{}_{\mathrm{c}}\text{?0.6}$.     

Anomalous temperature dependence of the order parameter of a superconductor with weakly correlated impurities

It is shown that weak correlations between pair-breaking impurities in superconductors influence the temperature dependence of the order parameter within the Ginzburg-Landau region if correlation radius of impurities R is greater than the coherence length of the superconductor ξ₀. The dependence of the square of the average order parameter on a temperature difference T _c − T changes its slope in a region $ \xi _0 \sqrt {{{T_c } \mathord{\left/ {\vphantom {{T_c } {\left( {T_c - T} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {T_c - T} \right)}}} \sim R $ \xi _0 \sqrt {{{T_c } \mathord{\left/ {\vphantom {{T_c } {\left( {T_c - T} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {T_c - T} \right)}}} \sim R . The influence of correlations of impurities on other thermodynamic properties of superconductors is discussed.     

非晶态超导体的2△0/(kBTc)和声子谱参量

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/(k_BT_c)与声子谱参量之间关系的一个公式:2Δ₀(k_BT_c=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T_c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/(k_BT_c)值远大于BCS理论值(3.53)的强耦合超导体,也可以是一个2Δ₀/(k_BT_c)值比BCS理论值还要小得多的弱耦合超导体。 关键词：     

λ小的超导体临界温度理论(Ⅲ)

在本文中,作者将前文得到的T_c公式与Eliashberg方程数值解和实验值进行了比较,在小λ区域,它们符合得很好。 关键词：     

A specific heat and magnetization study of highT c ceramic superconductors

From the temperature dependence of the specific heat of the semiconductor La₂CuO₄ and the high temperature superconductors La_1.8Sr_0.2CuO₄ (T _c =37.2 K) and YBa_1.9K_0.1Cu₃O_6.9 (T _c =91.5 K) in the range 1.5–30 K, a strong similarity of the lowfrequency part of their phonon density of states with a peak around 10 meV could be inferred. In the case of La_1.8Sr_0.2CuO₄ the thermodynamical critical field belowT _c has been determined and using the Rutger's formula and the BCS model, a Sommerfeld coefficient γ=9 mJ·mol^?1 K^?1 was obtained, which, taking into account recent results of band structure calculations leads to an electron-phonon enhancement factor γ=1.3, value compatible withT _c =36 K when using McMillan's formula forT _c . A systematic study of the magnetization offered evidence for strong flux trapping effects at higher fields and for Meissner shielding by superconducting Josephson currents in fields below 6 mT at 4.2 K.     

非晶态超导体转变温度Tc的经验公式

本文提出一个非晶态非过渡金属超导体的T_c经验公式,T_c=Aλ<ω>^1/2/(<ω>/ω₀+(1+λ)/20),式中A=(1/5)^(K1/2)。计算值和实验值,以及和Garland理论值的比较表明,T_c经验公式能很好地描述非晶态超导体的T_c值。 关键词：     

A quantum of history

The critical temperature, T _c, for all presently known superconductors does not exceed 20°K. This fact obviously limits the range of applications of superconductivity in technology in a very fundamental way. On the whole, the reason why the value of T _c for ‘ordinary’ superconductors should not exceed 20–40 °K is fairly well understood on the basis of the existing theory of superconductivity. At the same time, there apparently could exist high temperature superconductors for which the temperature T _c would reach hundreds of degrees, or at least liquid air temperature. Possible means of producing high temperature superconductors are considered in this article. Special attention is paid to what can be called the exciton mechanism of superconductivity.     

超导临界温度理论(Ⅱ)

在文献[1]中,我们导出了超导临界温度T_c的一个严格级数表式。本文讨论这个级数的收敛范围,以及通过解析延拓来扩展收敛范围的可能性。结论是:我们的T_c级数(指文献[1]原来的级数,或者经过延拓后的级数)在∞>λ>λ₀的整个范围内,都是收敛的。这里λ₀是Matsubara表象中使决定T_c的方程具有正实数解的最小的λ值。实际上,就是库仑赝势。因此,这就是说,也许除了少数非常弱耦合的超导体以外,我们的T<     

Are the high Tc superconductors d-wave?

Although it has been widely accepted for several years that the normal state of high T_c superconductors is anomalous, only recently has there been growing evidence for the anomalous nature of the superconducting state. A number of recent experiments show clear evidence for the existence of low-lying excitations in the superconducting state. Moreover, the observation by Bonn, Hardy and coworkers of a linear temperature dependence in the microwave surface resistance and penetration depth of YBCO at low temperatures suggests d-wave pairing with line nodes. The evidence for and against unconventional superconductivity in the high T_c oxides is reviewed. Specific topics discussed include the different behaviours observed for the penetration depth in thin films and in single crystals, what is known about the absolute value of the penetration depth, the role of disorder and inelastic scattering, the relative merits of strongvs. weak correlation theories of d-wave superconductivity in the high T_c oxides, and most important, what have we actually learned about the penetration depth of high T_c superconductors fromμSR?     

INFLUENCE OF SEED CRYSTAL DIMENSION ON CRYSTAL GROWTH FROM SOLUTION UNDER MICROGRAVITY

The influence of the dimension of seed crystal on the characteristics of crystal growth from solution under microgravity is studied. The dimensionless average velocity of fluid V_av, the dimensionless maximum velocity of fluid V_max, the temperature distribution index S_θ, the concentration distribution index S_φ and the dimensionless average growth rate of crystal \overline{V}_cg are calculated by only taking into account the variation of the solution density caused by the temperature change, that caused by the concentration change being neglected. In certain regions of the parameter (Ra, Pr,Sc and μ) space, some scaling laws are generated: the scales of S_φ and \overline{V}_cg are given by power functions of μ with negative exponents. It is shown that the characteristics of crystal growth for small seed crystal are different from those for large seed crystal.     

复合颗粒超导体的壳层模型

本文提出了描述复合颗粒超导体的壳层模型.此模型把复合颗粒分为壳层和内核两部分来处理。根据解非均匀体系Eliashberg方程的结论,对两部分谱函数进行权重相加,用对弱强耦合超导体都适用的Louie和Cohen临界温度T_c的公式,计算了Pb,Sn,In等不同尺寸复合颗粒的T_c。计算结果与实验基本符合。与已有的理论计算不同,我们的计算给出了Pb的T_c随尺寸减小而下降的结果。 关键词：     

The vortex-like excitations detected by Nernst signals in high-Tc superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

Model of superconducting domains––a unified theory of high- and low-Tc superconductors

A unified model of the superconducting mechanism has been put forward. The model suits not only to high-T_c but also to low-T_c superconductors. It is found that there are superconducting domains (SD) in crystal. When T⩽T_c, all the SD’s in the whole crystal are connected with one another. We have obtained the formula of T_c. On the basis of the formula and theory of quantum mechanics, the different behaviours of isotopic effects in low- and high-T_c superconductors as well as C₆₀, the triangular peak of T_c of transition metals, Matthias rules, and other effects are explained. New superconductors with higher T_c are predicted.     

A型超导体的近似临界温度公式(Ⅰ)——μ*=0情形

本文用数值解方法从Eliashberg方程计算出超导临界温度T_c,并考察T_c对有效声子谱的依赖关系。在这个研究中,a²F(ω)被取为双δ函数谱,并允许其中的谱参数可以在很宽范围内改变。作者发现在λ<∧区域(即在T_c级数解的收敛圆外),T_c除了依赖λ和矩比外,还依赖T_c级数解的收敛半径倒数Λ;它们之间的关系是有规律的。在这些结果的启示下,本文在μ^*=0情形,用弥合数值解的方法得到一个适用于λ<Λ区域的T_c近似公式。接着,本文作者对吉光达和吴杭生的一篇文章进行了研究,指出:该文提出的超导体分类建议及其工作的主要结论是对的。但其中对决定A型超导体临界温度主要参量问题进行的分析,只适用于这样一些A型超导体,它们的收敛半径倒数Λ或者比λ₀小,或者虽比λ₀大、但λ又小于λ₀,其中λ₀是个依赖谱形状的参量,它的定义在正文中给出。对另一些A型超导体(λ₀<λ<Λ),决定T_c的主要参量不再是λ,而是δ=1/∧^0.5(ω_1/2/ω_log)^5.5λ^1.55。 关键词：     

The vortex-like excitations detected by Nernst signals in high-T c superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

The relation betweenT c and anharmonicity in Sn-base A15 superconductors

The absolute value of the recoil-free fraction was measured accurately at three reference temperatures in the range 300 K down to 4 K, in two Sn-baseA 15 superconductors with disparate superconducting properties. The higher-T _c compound Nb₃Sn (T _c≈18K) exhibits low-temperature anharmonicity; this is in contrast to the lattice-dynamics of the low-T _c isomorph V₃Sn (T _c≈4K) in which it is observed that harmonic binding of the Sn atoms is prevalent down to low temperatures. The difference in the superconducting properties of the two compounds is shown to correlate with the considerable difference in their lattice-dynamics.