s＋d混合波对称下联合模型的超导电性

提出了一个联合模型，在ｓ＋ｄ混合波对称性下，合并考虑电子间配对相互作用的各向异性及二维外Ｖａｎ Ｈｏｖｅ奇异性对超导电性的影响，理论结果表明：Ｖａｎ Ｈｏｖｅ奇异性及配对相互作用的各向异性都是使Ｔｃ提高的重要因素。     

s＋d混合波对称性下计入库仑作用和Van Hove奇异性的超导转变…

本文在ｓ＋ｄ混合波对称性下研究了Ｖａｎ Ｈｏｖｅ奇异性、准粒子配对相互作用各向异性并再计入库仑排斥势对超导转变温度（Ｔｃ）和同位素效应的联合效应。结果表明，Ｖａｎ Ｈｏｖｅ奇异性和各向异性都能显著地增高Ｔｃ，库仑作用则降低Ｔｃ。库仑作用和Ｖａｎ Ｈｏｖｅ奇异性都能减小同位素效应指数（α），各向异性和ｄ波成分则增大α。计入库仑作用后α就随ＣｕＯ２面内各向异性参量增大而增大。由此可以推测，如果实验上肯     

s+d混合波对称性下计入库仑作用和Van Hove奇异性的超导转变温度和同位素效应

本文在ｓ＋ｄ混合波对称性下研究了ＶａｎＨｏｖｅ奇异性、准粒子配对相互作用各向异性并再计入库仑排斥势对超导转变温度（Ｔｃ）和同位素效应的联合效应．结果表明，ＶａｎＨｏｖｅ奇异性和各向异性都能显著地增高Ｔｃ，库仑作用则降低Ｔｃ．库仑作用和ＶａｎＨｏｖｅ奇异性都能减小同位素效应指数（α），各向异性和ｄ波成分则增大α．计入库仑作用后α就随ＣｕＯ２面内各向异性参量增大而增大．由此可以推测，如果实验上肯定了高温超导体的序参量是以ｄ波为主，则高温超导电性为非声子机制的可能性就比较大．此结论与根据有关文献作出的判断相符．     

二能带交迭高温超导理论的Van－Hove奇异性与超导转变温度和反常同位素效应

本文利用二交迭能带理论，计算费米面附近电子态密度的Ｖａｎ－Ｈｏｖｅ奇异性对超导转变温度和同位素指数因子的影响，计算结果表明：费米面附近电子态密度的Ｖａｎ－Ｈｏｖｅ奇异性能同时提高超导转变温度和降低同位素指数因子，这是普通的ＢＣＳ理论不能解释的［３］．     

Bi2Sr2CaCu2O8+y掺Na样品的超导机制

用熔融法制备了Bi₂Sr₂CaCu₂O_8+y掺Ｎａ多晶样品，转变温度Ｔ_c提高到９０Ｋ以上，最大的零电阻温度达到９２．５Ｋ。在流动氧气退火后，未掺Ｎａ样品Ｔ_c显著降低，而掺Ｎａ样品Ｔ_c几乎不变，说明二者Ｔ_c提高具有不同的机制。据此我们认为Ｎａ替代了Ｂｉ而使Ｂｉｏ层电荷失去平衡，Ｃｕｏ面上电子转移到Ｂｉｏ层而形成空穴，空穴和电子的吸引相 关键词：     

Gd1-xCaxBa2Cu3O7-y高温超导体压力效应的研究

研究了Ｇｄ_1-xＣａ_xＢａ₂Ｃｕ₃Ｏ_7-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ｄＴ_c／ｄｐ分别为７．６８，７．８和４．４６Ｋ／ＧＰａ。发现Ｔ_c的压力导数随着ｃａ²⁺含量的增加而下降，分析了氧含量对Ｔ_c和ｄＴ_c／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Ｔ_c（Ｐ）曲线的非线性关系。 关键词：     

探测激光等离子体电子温度的一种新方法——受激Raman散射光谱的短波截断

利用受激Ｒａｍａｎ散射（ＳＲＳ）三波相互作用理论导出探测激光等离子体电子温度的一种新方法．对我国神光ＬＦ１２^#激光器上的ＳＲＳ实验光谱进行了分析，由ＳＲＳ光谱短波截断波长λ_s自洽地计算出激光等离子体晕区电子温度Ｔ_c．这些结果与由Ｘ射线谱测得的结果合理地符合． 关键词：     

掺Ba对Bi系2212相超导电性的影响

研究了掺Ｂａ对Bi₂Sr_2-xBa_xCaCu₂O_y（０≤ｘ≤０．１５，０．３）单晶和多晶样品超导电性的影响，结果表明，有少量Ｂａ^２＋离子进入了超导相，且有固溶度极限．对于２２１２相单晶，ｃ轴参数和Ｔ_c均随Ｂａ含量增加而增加；对于慢冷多晶样品，掺Ｂａ可明显提高Ｔ_c；然而对于淬大多晶样品，Ｔ_c没有明显变化，用掺Ｂａ 关键词：     

在Van Hove奇点模型下计入库仑作用的高温超导电性

本文在Ｖａｎ Ｈｏｖｅ奇点模型下系统地研究了库仑作用对高温超导电性的影响，对超导转变温度Ｔｃ、绝对零度下能隙Δ（０）、同位素效应指数α和Ｔｃ处的比热跃变ΔＣ（Ｔｃ）作了解析和数值计算。结果表明，在Ｖａｎ Ｈｏｖｅ奇点模型下考虑库仑作用后，我们仍能得到高的Ｔｃ，并且能较好地解释同位素效应指数很小的实验结果。同时还表明在Ｖａｎ Ｈｏｖｅ奇点模型下考虑载流子局域性和巡游性的竞争对于解释高温超导体的实验结     

(Ba_(1-x)K_x)BiO_3电子结构与超导电性

用第一原理的ＬＤＦ-ＬＭＴＯ-ＡＳＡ超元胞法，模拟由Ｘ射线吸收谱精细结构测定的ＢａＢｉＯ_３中，Ｂｉ有两种价态Ｂｉ^３＋和Ｂｉ^5＋及与之相应的两种不同键长的Ｂｉ—Ｏ八面体，以及Ｋ掺杂对晶体结构的影响．计算了Ｂａ₄Ｂｉ₄Ｏ₁₂，（Ｂａ_３Ｋ）Ｂｉ₄Ｏ₁₂，（ＢａＫ）Ｂｉ₂Ｏ₆，（ＢａＫ_３）Ｂｉ₄Ｏ₁₂，Ｋ₂Ｂｉ₂Ｏ₆（简记为（４０４），（３１４），（１１２），（１３４），（０２２））五种“样本”的电子结构．结果表明，（４０４）和（３１４）分别为Ｅ_g＝１.６ｅＶ及Ｅ_g＝１.５ｅＶ的半导体，其它“样本”为金属．总能的分析表明（１３４）是不稳定的，故溶解极限为ｘ＝０.５．以“取样”方式按伯努利分布确定任意组分各“样本”的概率，进而计算了（Ｂａ_1-xＫ_x）ＢｉＯ_３电子结构随组分的变化．最后用逾渗模型说明了超导转变温度Ｔ_c在ｘ＝０．２５附近的突变 关键词：     

On the enhancement of superconductivity due to Van Hove singularity

We have rigorously solved the standard BCS gap equation with the density of states of Van Hove type. The result shows that the enhancement ofT _c is not as effective as previously estimated. It seems unlikely that Van Hove singularity inN(E) can be theT _c -enhancement mechanism for the high-T _c Cu oxides.     

What is the highest Tc for phonon-mediated superconductivity?

We suggest that the high-temperature superconductivity can be attributed to the director-roles of the van Hove singularity between an electron-electron interaction and an electron-phonon interaction. The difference between the critical temperature and the pairing temperature is presented, and the Fermi arc, the d-wave symmetry and the poor conductivity, etc., are discussed. In particular, the non-s-wave symmetry is predicted to have the highest T _c for superconductors.      

What is the highest T c for phonon-mediated superconductivity?

We suggest that the high-temperature superconductivity can be attributed to the director-roles of the van Hove singularity between an electron-electron interaction and an electron-phonon interaction. The difference between the critical temperature and the pairing temperature is presented, and the Fermi arc, the d-wave symmetry and the poor conductivity, etc., are discussed. In particular, the non-s-wave symmetry is predicted to have the highest T _c for superconductors.     

Thermal and quantum fluctuations induced additional gap in single-particle spectrum of d-p model

The possibility of thermal and quantum fluctuations induced attractive interaction leading to a pairing gap Δ_tq in the single-particle spectrum of d-p model in the limit of a large N of fermion flavor is investigated analytically. This is an anomalous pairing gap in addition to the one with d-wave symmetry originating from partially screened, inter-site coulomb interaction. The motivation was to search for a hierarchy of multiple many-body interaction scales in high-T_c superconductor as suggested by recent experimental findings. The pairing gap anisotropy stems from more than one source, namely, nearest neighbor hoppings and the p-d hybridization, but not the coupling of the effective interaction. The temperature at which Δ_tq vanishes may be driven to zero by using a tuning parameter to have access to quantum criticality (QC) only when N?1. For the physical case N=2, the usual coherent quasi-particle feature surfaces in the spectral weight everywhere in the momentum below the pairing gap Δ_tq. Thus it appears that the reduction in spin degeneracy has the effect of masking quantum criticality.     

高温超导体混合态磁通涡旋结构

利用紧束缚模型,对s-d混合波的存在性作了讨论.在Ginzburg-Landan理论框架下,严格求解了高温超导体混合配对态磁通涡旋结构.结果表明,其磁通涡旋的分布与单个涡旋的结构密切相关.在低温下,由于序参量s波成分的存在及它与d波分量的耦合,引起局域磁场和d波分量的各向异性,使磁通涡旋呈斜格子分布,而在温度近于转变温度时,各向异性消失,涡旋态转化为三角格子.这与YBa₂Cu₃O₇样品的小角度中子散射实验的结果相符合. 关键词：     

Properties of electrons near a Van Hove singularity

The Fermi surface of most hole-doped cuprates is close to a Van Hove singularity at the M point. A two-dimensional electronic system, whose Fermi surface is close to a Van Hove singularity, shows a variety of weak coupling instabilities. It is a convenient model to study the interplay between antiferromagnetism and anisotropic superconductivity. The renormalization group approach is reviewed with emphasis on the underlying physical processes. General properties of the phase diagram and possible deformations of the Fermi surface due to the Van Hove proximity are described.     

Magnetic fluctuations and itinerant ferromagnetism in two-dimensional systems with Van Hove singularities

A criterion for ferromagnetism in two-dimensional systems with the Fermi level near Van Hove singularities (VHSs) is analyzed. In the quasistatic approximation applied to a spin-fermion model, it is shown that the spectrum of spin excitations (paramagnons) is positively defined when the interaction I between the electronic and spin degrees of freedom is sufficiently large (I > I _c). The critical interaction I _c is much greater than its value determined from the Stoner criterion; hence, the latter criterion is not an adequate criterion for ferromagnetism in the presence of Van Hove singularities in the electronic spectrum. By combining the quasistatic approximation with the method of equations of motion, the electronic self-energy is obtained in the first order in the inverse number of spin components.     

Van Hove singularity as a possible origin of the bandwidth renormalization in layered superconductors

We use angle-resolved photoemission spectroscopy to study bandwidth renormalization in layered superconductor 2H-NbSe₂. Renormalization of the conduction band in comparison with the very similar 2H-TaSe₂ is of the factor of two. We discuss the possibility that the Van Hove singularity could be responsible for this effect not only in dichalcogenides but also in pnictides.     

Spontaneous deformation of the fermi surface due to strong correlation in the two-dimensional t- J model

The Fermi surface of the two-dimensional t- J model is studied using the variational Monte Carlo method. We study the Gutzwiller-projected d-wave superconducting state with an additional variational parameter t(')(v) corresponding to the next-nearest-neighbor hopping term. It is found that the finite t(')(v)<0 gives the lowest variational energy in the wide range of hole-doping rates. The obtained momentum distribution function shows that the Fermi surface deforms spontaneously. It is also shown that the Van Hove singularity is always located very close to the Fermi energy. Using the Gutzwiller approximation, we show that this deformation is due to the Gutzwiller projection operator or the strong correlation.     

Competition mechanism between singlet and triplet superconductivity in the tight-binding model with anisotropic attractive potential

Based upon the tight-binding formalism a model of a high-T_c superconductor with isotropic and anisotropic attractive interactions is considered analytically. Symmetry facets of the group C_4v are included within a method of successive transformations of the reciprocal space. Complete sets of basis functions of C_4v irreducible representations are given. Plausible spin-singlet and spin-triplet superconducting states are classified with regard to the chosen basis functions. It is displayed that pairing interaction coefficients and the dispersion relation, which can be characterized by the parameter η= 2t₁/t₀, have a diverse and mutually competing influence on the value of the transition temperature. It is also shown that in the case of a nearly half-filled conduction band and an anisotropic pairing interaction the spin-singlet d-wave symmetry superconducting state is realized for small values of the parameter η, whereas in the opposite limit, for sufficiently large values, the spin-triplet p-wave symmetry superconducting state has to be formed. This result cannot be obtained within the Van Hove scenario or BCS-type approaches, where the p-wave symmetry superconducting state absolutely dominates. The specific heat jump and the isotope shift as functions of the parameter η are assessed and discussed for the d-wave symmetry singlet and the p-wave symmetry triplet states.