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：     

Ta对C—15相V2Hf和V2(Zr0.5Hf0.5)系列声子性能的影响

本文用中子飞行时间方法对C-15相的超导材料V₂Hf,V₂Ta和V₂Hf_0.8Ta_0.2以及V₂Zr_0.5。Hf_0.5和V₂Zr_0.5Hf_0.33Ta_0.17的热中子非弹性散射谱作了测量,并计算出相对的广义声子态密度。结果与早先发表的Nb对C-15相V₂Zr和V₂(Hf_0.5Zr_0.5)系列声子性能的影响一致:声子频率随超导转变温度T_c增加而软化,随T_c减小而硬化。这表明,对于此类材料弹性软化在一定程度上对提高T_c起了作用。结果还进一步表明V₂Zr或V₂Hf与V₂(Zr_0.5Hf_0.5)之间有着质的差别,V₂Hf加Ta后,T_c增加,声子频率软化,而V₂(Zr_0.5Hf_0.5)加Ta后,T_c减小,声子频率则略有硬化。这与V₂Zr和V₂(Hf_0.5Zr_0.5)加Nb的结果是一致的。此结果可以用角动量分波表象的能带论方法分析电-声耦合相互作用得出的杂化理论来定性解释。 关键词：     

归一有效声子谱谱形对超导临界温度Tc的影响

作者设计了一个图分析法,用来分析Eliashberg方程的数值解。从这个分析,得到如下结论:硬化归一有效声子谱g(ω),可使超导材料的T_c提高。而g(ω)软化,只能提高小λ超导体的T_c。 关键词：     

金属氢高温超导电性

本文利用晶体的平均电子半径r_s、离子质量M和声速v_j等较少参量表示Z价金属的有效声子谱α²F(ω)、电声子耦合常数λ和谱面积λ/2〈ω〉,并用Dynes的T_c公式,求得了金属氢、铅和铌的超导临界温度。计算表明,在零压下金属氢的λ=2.55,T_c=162K。 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：     

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：     

超导临界温度理论(Ⅲ)

本文定出超导临界温度T_c级数公式(1)的前几项系数。对于形式为α²F(ω)=(λω)/2[a₁δ(ω-ω₁)+(1-a₁)δ(ω-ω₂)]的双δ型有效声子谱及若干具体材料的谱,将级数公式计算的T_c与Allen-Dynes公式(以下简称A-D公式)及Eliashberg方程的数值解作了比较。计算表明,当级数(1)收敛时,级数公式计算的结果较A-D公式更接近于数值解。此外,本文还给出了一个近似的T_c级数公式,得到了估计该T_c级数收敛半径的方法,并计算了若干材料的收敛半径值。因此,可估计级数公式(1)的适用范围。 关键词：     

N-S多层薄膜结构的超导临界温度讨论

本文推广了McMillan隧道模型,并应用于厚度小于超导相干长度的超导和正常膜组成的N-S多层薄膜结构。计算了它的序参量和超导临界温度。由于界面区域电声子作用的修正以及不同原子的掺杂所引起的T_c增强的可能性也作了讨论。 关键词：     

隧道电子谱的温度修正

由隧道电子谱通过强耦合超导能隙方程反演面求得有效声子谱的过程,要求电子态密度在T→OK条件下测量,否则会存在相当大的误差,所求得的有效声子谱也与真正的谱形偏离很远,本文给出一种校正这个温度效应的方法,可克服这个困难,采用此方法后,即使在T/T_c=0.22的条件下测量隧道谱,也可以得到正确的有效声子谱、λ与μ^*值。 关键词：     

非晶态合金La72Si28晶化过程中生成的新亚稳超导相

本文报道了在非晶态合金La₇₂Si₂₈晶化过程中生成新亚稳超导相的实验结果。非晶态合金La₇₂Si₂₈是用液态急冷法制备的,该合金经250℃,30min热处理后,得到了一个新的亚稳超导相,该相具有正交结构,其晶格参数为a=9.121?,b=6.821?,c=3.833?,超导临界温度T_c为3.02K,高于目前已知的La-Si系超导相的T_c值。 关键词：     

On the theory of localized superconductivity: Fluctuational diamagnetism

The diamagnetic susceptibility of a twinning plane at temperatures slightly higher than the localized superconductivity temperature T_c as well as the heat capacity jump at T = T_c are calculated. The possibility of an appreciable increase of the superconductivity temperature in small particles containing twinning planes is studied.     

Compatibility of BCS pairing and the peierls distortion in two-band systems

The effect of the Peierls transition on superconductivity in a two-band system, where one of the bands is flat, is examined theoretically. We find that superconductivity appears at the background of the insulating phase (T_P >; T_c). At T_c > T_P only the superconducting transition is possible.     

Studies of chemical diffusion in La1-X SrXCoO3-δ

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zig-zag nanotube can become superconducting at around 15?K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature T _c less than 1?K. To explain the observed higher critical temperature we explore the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature T _c in a single-wall nanotube. Our theoretical results can explain the observed T _c in a single-wall nanotube. In particular, we find that T _c is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons and the radius of the nanotube. We then consider superconductivity in a bundle of single-wall nanotubes and find that bundling of nanotubes does not change the critical temperature significantly. Going beyond carbon nanotubes we show that in a metallic hollow nanowire T _c has some sort of oscillatory behaviour as a function of the surface number density of electrons.     

On the critical temperature of a superconducting system

Based on the assumption that in the groundbcs state the net gain in energy is equivalent to the repulsive electron-ion and electron-electron Darwin interactions, an expression forT _c has been obtained which depends on only a few atomic parameters. The theory provides a criterion for the occurrence of superconductivity and yields satisfactory values ofT _c for metals and alloys, and ternary chalcogenides and borides. It explains the difference inT _c in the crystalline and amorphous states as well as the pressure dependence ofT _c . The possibility of occurrence of high temperature superconductivity has been explored.     

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.     

Re-entry in ferromagnetic superconductors

The re-entry phenomenon in magnetic superconductors is studied using the generalized Ginzburg-Landau free energy introduced by Blount and Varma. The re-entry temperature T_c2 is simply that temperature at which the magnetization acts as a source of induction strong enough to destroy superconductivity. Above T_c2 ferromagnetism and superconductivity coexist. The structure is an Abrikosov vortex lattice, with ferromagnetic magnetization spreading widely around the vortex cores. Within our approximations, the phase transition at T_c2 is of second order.     

Scanning tunneling microscopy and spectroscopy on iron-pnictides

Tremendous excitement has followed the recent discovery of superconductivity up to T_c = 56 K in iron–arsenic based materials (pnictides). This discovery breaks the monopoly on high-T_c superconductivity held by copper-oxides (cuprates) for over two decades and renews hope that high-T_c superconductivity may finally be theoretically understood and widely applied.Since scanning tunneling microscopy (STM) and spectroscopy (STS) have been key tools in the investigation and understanding of both conventional and unconventional superconductivity, these techniques are also applied to the pnictides. While the field is still in its early stages, several important achievements by STM and STS have been reported on the pnictides. In this paper, we will review their contribution towards an understanding of superconductivity in this new class of materials.     

Workshop on fast structural changes

Copper-oxide (cuprate) high-temperature superconductors are doped Mott insulators. The undoped parent compounds are antiferromagnetic insulators, and superconductivity occurs only when an appropriate number of charge carriers (electrons or holes) are introduced by doping. All cuprate materials contain CuO₂ planes (Figure 1a) in their crystal structure; the doped carriers are believed to go into these CuO₂ planes, which are responsible for high-temperature superconductivity. High-temperature superconductors are characterized by their unusual physical properties, both in the superconducting state (below the superconducting transition temperature T_c) and in the normal state (above T_c). Since the discovery of high-temperature superconductivity in 1986 [1], these unusual physical properties and the mechanism of superconductivity have been prominent issues in condensed matter physics [2].     

Possible universal cause of high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> superconductivity in different metals

Using the theory of high-temperature superconductivity based on the idea of the fermion-condensation quantum phase transition (FCQPT), we show that neither the d-wave pairing symmetry, the pseudogap phenomenon, nor the presence of the Cu-O₂ planes is of decisive importance for the existence of high-T _c superconductivity. We analyze recent experimental data on this type of superconductivity in different materials and show that these facts can be understood within the theory of superconductivity based on the FCQPT. The latter can be considered as a universal cause of high-T _c superconductivity. The main features of a room-temperature superconductor are discussed.     

Studies on the superconductivity for the “111” type iron arsenide superconductor

The “111” type Li _x FeAs (x ranges from 0.8 to 1.2) with Cu₂Sb type tetragonal structure was synthesized with T _c 18 K. The isostructure NaFeAs was further studied, which shows superconductivity with T _c up to 26 K. The effect of pressure on superconductivity was investigated.