关于高Tc问题的一个建议

我们建议:把超导体分成A型和B型两种。前者,λ<Λ,后者,λ>Λ。Λ的物理意义在正文中给出。本文分析表明了,有效声子谱对这两种超导体T_c的影响是不同的,因而提高它们T_c方法也是不一样的。提高A型超导体T_c的最有效方法是增大λ。提高B型超导体T_c的最有效方法是增大又λ〈ω²〉。 关键词：     

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：     

非过渡金属非晶态超导电性的研究

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T_c,并得出,声子谱的软化所导致的T_c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T_c也愈高。讨论了利用声子谱的软化虽然能大幅度地提高T_c值,但要获得包括金属Be在内的非过渡金属的高T_c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H_c2和能隙2Δ₀所表现出的强耦合效应等问题。 关键词：     

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：     

高Tc超导体的表面微波吸收

测量不同粒度及低真空条件下高T_c超导样品Bi-Pb-Sb-Sr-Ca-Cu-O的非共振微波吸收信号,实验表明,这个对温度和磁场十分敏感的信号还与样品颗粒的大小及氧气氛有关;并经计算证明,信号强度与近表面趋肤深度壳层Meissner态的体积成正比。 关键词：     

非磁杂质对高Tc超导电性的影响

本文证明了在氧化物高T_c超导体的Anderson晶格模型中,由于序参量是能量的函数,非磁掺杂也具有拆对效应,在稀掺杂情况下,求出了T_c随杂质浓度增加而线性下降的规律,与实验结果相符,文中还计算了掺杂对超导态密度中能隙的影响,结果表明,当非磁掺杂浓度增加时,零温能隙的减小比T_c的下降慢得多,从而在转变为正常态之前可能不出现类似通常BCS超导体磁性掺杂的无能隙区。 关键词：     

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：     

超导临界温度级数公式的应用

从超导临界温度级数公式(1)出发,得到了B类超导体同位素效应的表达。在μ^*=0及μ^*≠0时,我们得到了T_c上限的级数表达式,其结果与数值解符合甚好。最后,在μ^*=0时,讨论了T_c与各种谱参数的关系。由级数表达式可以清楚地看出,当固定不同参数时,T_c的行为完全不同,从而统一解释了不同作者关于声子谱对T_c影响的不同结论。我们着重指出,在大多数情况下,级数公式(1)仅取前两项,可作为数值解的很好近似,从而大大简化了级数公式。预计可以由此简化公式出发研究其它问题。 关键词：     

低温/高温复合超导体分流温度的定性计算

本文基于超导体幂指数模型对低温超导体和高温超导体的分流温度分别作了数值计算,计算结果与现有的分流温度估算公式结果符合的很好.本文还讨论了幂指数模型中不同n值对低温/高温复合超导体分流温度的影响,并对复合超导体分流温度做了数值计算,结果显示在相同传输电流下,低温/高温复合超导体的分流温度比传统低温超导体的分流温度高3.2K以上,这表明这种复合超导体较传统低温超导体具有更好的稳定性.     

CeCu2Si2和UBe13的超导理论(Ⅱ)——热力学量的计算

根据文献[1]中给出的模型和能隙方程,计算了超导态的热力学临界磁场,T_c处的比热跃变以及T_c以下的比热行为。结果表明:有关热力学量的临界值与实验结果定性相符;T_c以下的比热行为在整个温区与CeCu₂Si₂和UBe₁₃的实验结果符合。 关键词：     

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.     

Upper critical field in the heavy fermion superconductors

The upper critical field H_c2 (T) and its temperature derivative at T_c in the heavy fermion superconductors have been calculated by using the simplified Kondo lattice model.     

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：     

Proposal for a particle accelerator driven by a far-infrared free-electron laser

We propose a scheme of particle accelerators driven by far-infrared free-electron lasers on the basis of oversized periodically or helically corrugated hollow waveguides with cooled walls of high-T_c superconductors. In this context, we discuss the basic principles of particle acceleration in periodic and helical hollow waveguides, the relevant modes and fields of these waveguides, attenuation and damage threshold in the waveguides assuming walls made of standard metals as well as high-T_c superconductors, and the energy gain of particles for two modes of coupling laser radiation into the waveguides.     

The flux distribution and hysteresis losses in highT c superconductors including viscous forces on flux lines

By comparing the contributions of pinning and viscous forces to the restoring force on flux ines in type II and highT _c superconductors, it is shown that the flux flow in highT _c superconducto rs should play an important role in determining the magnetic flux distribution and hence the hyste resis losses in a.c. fields. Both quantities are calculated in the extreme case of very large viscous forces with respect to the pinning force. The magnetic field and frequency dependence of the losses are changed with respect to the results obtained from the critical state model. The theoretical results are qualitatively confirmed by a.c. susceptibility measurements at different magnetic field amplitudes and frequencies. The quantitative differences indicate that the flux flow effects in highT _c superconductors are by far not so strong as expected and supposed by some theories. The reasons for these discrepancies are discussed.     

Upper critical field of high Tc superconductors

The upper critical field H_c2 of the high T_c superconductors such as La-Sr-Cu-O and La-Ba-Cu-O systems has been calculated by using the modified Suhl's et al. two-band model. A modification to Gor'kov theory is found for the ratio of K₁(T) at T=0 and T=T_c. The results may explain the discrepancy between the BCS theory and experimental data.     

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.