s+d混合波对称性下联合模型的超导电性

提出一个联合模型，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电子结构上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Ｔ_c提高的重要因素；各向异性的电子配对相互作用自然导致序参量的ｄ波成分，当此各向异性增强时，ｄ波成分也增大．高温超导体的较高２Δ（０）／ｋ_ＢＴ_c值可能预示着在这些材料中ｓ波的权重远小于ｄ波权重．联合效应模型下的Ｔ_c处比热跳跃行为与经典的ＢＣＳ理论也完全不同 关键词：     

Effective interactions and superconductivity in the t-J model in the large-N limit

The feasibility of a perturbation expansion for Green's functions of the t-J model directly in terms of X-operators is demonstrated using the Baym-Kadanoff functional method. As an application we derive explicit expressions for the kernel of the linearized equation for the superconducting order parameter in leading order of a 1/N expansion. The linearized equation is solved numerically on a square lattice taking instantaneous and retarded contributions into account. Classifying the order parameter according to irreducible representations of the point group C_4v of the square lattice and according to even or odd parity in frequency we find that a reasonably strong instability occurs only for even frequency pairing with d-wavelike symmetry. The corresponding transition temperature T_c is where t is the nearest-neighbor hopping integral. The underlying effective interaction consists of an attractive, instantaneous term and a retarded term due to charge and spin fluctuations. The latter is weakly attractive at low frequencies below ,strongly repulsive up to and attractive towards even higher energies. T_c increases with decreasing doping until a d-wavelike bond-order wave instability is encountered near optimal doping at for J=0.3. T_c is essentially linear in J and rather insensitive to an additional second-nearest neighbor hopping integral t'. A rather striking property of T_c is that it is hardly affected by the soft mode associated with the bond-order wave instability or by the Van Hove singularity in the case with second-nearest neighbor hopping. This unique feature reflects the fact that the solution of the gap equation involves momenta far away from the Fermi surface (due to the instantaneous term) and many frequencies (due to the retarded term) so that singular properties in momentum or frequency are averaged out very effectively. Received: 16 June 1998 / Accepted: 14 July 1998     

Transition temperature and isotope effect near an extended van Hove singularity

Using Eliashberg theory and a model density for ² F the transition temperatureT _c and the isotope effect are calculated near an extended van Hove singularity. We show that, at least in the one-particle and the Migdal approximation, even the considered strong van Hove singularity cannot yield large enhancements ofT _c and strong reductions of of the kind observed in experiment around optimal doping.     

Van Hove singularity in Raman scattering spectra of high-T c superconductors

Experimental data are presented which supporting observation of an extended van Hove singularity in Raman spectra of high-T _c superconductors. Excitation of large-momentum electron-hole pairs is proposed as a possible mechanism of the scattering. Fiz. Tverd. Tela (St. Petersburg) 40, 27–31 (January 1998)     

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.     

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.      

The two-dimensional t-t'-U model as a minimal model for cuprate materials

The addition to the Hubbard Hamiltonian of a t' diagonal hopping term, which is considered to be material dependent for high-T _c cuprate superconductors, is generally suggested to obtain a model capable to describe the physics of high-T _c cuprate materials. In this line of thinking, the two-dimensional t-t'-U model has been studied by means of the Composite Operator Method, which allows to determine the dynamics in a fully self-consistent way by use of symmetry requirements, as the ones coming from the Pauli principle. At first, some local quantities have been calculated to be compared with quantum Monte Carlo data. Then, the structure of the energy bands, the shape of the Fermi surface and the position of the van Hove singularity have been computed as functions of the model parameters and studied by the light of the available experimental data. The results of our study show that there exists two sets of parameters that allows the model to describe the relevant features of the 1-layer compounds Nd_2-xCe_xCuO₄ and La_2-xSr_xCuO₄. On the other hand, for the 2-layer compound YBa₂Cu₃O _{7 - δ} is not possible to find a reasonable set of parameters which could reproduce the position of the van Hove singularity as predicted by ARPES experiments. Hence, it results questionable the existence of an unique model that could properly describe the variety of cuprate superconductors, as the two-dimensional t-t'-U model was thought to be. Received 29 March 2000 and Received in final form 10 August 2000     

On the influence of a square-root van Hove singularity on the critical temperature of high-T c superconductors

It is shown that the square-root van Hove singularity appearing in the density of states ν (E _F )∼(E _F −E ₀)^−1/2 as a result of extended saddle-point singularities in the electron spectrum of high-T _c superconductors based on hole-type cuprate metal-oxide compounds gives a nonmonotonic dependence of the critical temperature T _c on the position of the Fermi level E _F relative to the bottom E ₀ of the saddle. Because the divergence of ν(E _F ) is canceled in the electron-electron interaction constant renormalized by strong-coupling effects, T _c approaches zero as E _F →E ₀, in contrast to the weak-coupling approximation, where in this limit T _c approaches a finite (close to maximum) value. The dependence obtained for T _c as a function of the doped hole density in the strong-coupling approximation agrees qualitatively with the experimental data for overdoped cuprate metal oxides. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 473–477 (10 April 1998)     

Analytic solution of the BCS gap equation inD dimensions (D=1, 2, 3), at finite temperatures

The Bardeen-Cooper-Schrieffer (BCS) gap equation is solved analytically in one, two and three dimensions, for temperatures close to zero andT _c. We work in the weak coupling limit, but allow the interaction widthν≡ħω _m/E _F to lie in the interval (0, ∞) Here,ħω _m is the maximum energy of a force-mediating boson, andE _F denotes the Fermi energy. We obtain expressions forT _c and ΔC, the jump in the electronic specific heat acrossT=T _c, in the limitsν≪1 (the usual phonon pairing) andν>1 (non-phononic pairing). This enables us to see howT _c scales with the mediating boson cut off. Our results predict a larger jump in the specific heat for the caseν>1, compared toν≪1. We also briefly touch upon the role of a van Hove singularity in the density of states.     

Muonic study of type II superconductors

A theory of μSR method is developed for uniaxial anisotropic high-T _c superconductors. In two extreme cases ofH _ext‖c andH _ext⊥c analytical formulas are obtained which makes it possible to determine from the position of van Hove singularities in the Fourier-spectrum of muonium polarization, the type and parameters of the vortex lattice, such as λ _ab (λ _c ). In caseH _ext≲H _c2 we obtained the shape of the Fourier-spectrum in analytical form and the simple method of determining the Ginsburg-Landau parameter κ. Convenient expressions for numerical calculations are obtained for the arbitrary orientation of the external field, and an algorithm is provided to compute the mean fieldB, the vortex lattice parameters and the bulk field distribution in anisotropic superconductor. The Fourier spectrum of polarization based on these calculations can be used to independently check the validity of the high-T _c parameters determination for “appropriate” orientations.     

Effect of uniaxial stress on T c and the van Hove scenario in YBa2Cu3O7

The effect of uniaxial stress on T _c in YBa₂Cu₃O₇ is studied within the van Hove scenario including the interlayer coupling (3D effects). The uniaxial pressure along the a or b axis changes the in-plane anisotropy, and that along the c axis increases the interlayer coupling. Both of them change the averaged density of states and so T _c in the framework of the BCS theory. The latter effect on T _c is found to be small compared with the former, which can account for the uniaxial pressure experiment of YBa₂Cu₃O₇.     

Magnetic flux distribution in the bulk of the pure type-II superconductor niobium measured with positive muons

The spin precession of positive muons in ultra-pure Nb single crystals of high perfection, cooled down in transverse magnetic fields 0.68B _c2≤B _appl≤0.84B _c2 (B _c2=upper critical field) from temperatures well above the superconducting transition temperatureT _c=9.25 K, has been investigated in the temperature range 2.6 K≤T≤8.0 K. The experiments confirm the periodic fieldB(r) of triangular flux-line lattices as calculated from numerical solutions of the microscopic BCS-Gor'kov theory. The observed broadening of the van Hove singularities in the field distribution is discussed in terms of the combined effects of muon diffusion and random perturbations of the flux-line lattice.     

Electron-phonon coupling constant of cuprate based high temperature superconductors

The electron-phonon coupling constant in two-dimensional cuprate high temperature superconductors has been determined by the ultrasonic method. The electron-phonon coupling constant in the Van Hove scenario was found to increase with transition temperature T_c. is in the range of 0.025-0.060 which is 10-100 times smaller than the conventional three-dimensional Bardeen-Cooper-Schrieffer coupling constant. The characteristic Debye temperature θ_D does not correlate with T_c. These findings show that the interplay between the Debye frequency and electron-phonon coupling in the two-dimensional system and their variations have a combined effect in governing the transition temperature.     

电子型掺杂高温超导体La2-xCexCuO4飞秒时间分辨动力学研究

利用飞秒时间分辨光抽运探测技术研究了电子型掺杂La_2-xCe_xCuO₄(LCCO)高温超导材料的准粒子超快动力学过程.得到低温(T<0.7T_c)、转变温度附近(0.7T_c≤T≤T_c)和高温(T>T_c)三个温区内的动力学行为.研究发 关键词： 电子型掺杂高温超导体 飞秒时间分辨 准粒子 声子瓶颈     

BCS-van Hove方案与高温超导电性

基于BCS—van Hove机制,研究了非磁杂质对高温超导电性的抑制效应,考虑到双层耦合效应和三维耦合效应,将van Hove方案推广到多带BCS模型,计算了超导转变温度与化学势的关系,并与实验结果进行了比较。     

Exact quantum partition function of the BCS model

The exact calculation of the reduced BCS model quantum partition function (QPF) in the thermodynamic limit is carried out by the path integration method. The expression for the QPF and the phase transition temperatureT _c in the regular phase coincide with the results of Bogolyubov. In the nonregular phase a temperature singularity appears in the expression for the QPF: the QPF diverges in the region of temperaturesT _c which are smaller than some critical temperatureT _c ^* , and it turns out that in all casesT _c ^* > T _c and the differenceT _c ^* –T _c is not small. The interpretation of the temperatureT _c ^* is given.     

The superconductivity and metallurgy of C15 materials: Hf-V and Zr-V

Variation of theT _c as a function of composition for the C15 materials of the systems Hf−V and Zr−V was investigated. Samples in the as cast and annealed conditions were examined by x-ray and optical metallography. None of the samples was found to be of single phase. For the Hf−V system,T _c increases to a constant value of about 9 K for the compositions between 36 and 60 at. %V. A slight increase ofT _c between 60 and 70 at. %V and a degradation inT _c above 80 at. %V were observed. As cast materials show the maximumT _c value of 9.4 K. Deviations from stoichiometry and heat treatments produce considerable changes in the microstructures but have little effect onT _c . A slight enhancement ofT _c for the alloy containing 80 at. %V is explained by the pressure on V₂Hf due to V phase by the differential thermal contraction. For the Zr−V system, maximumT _c value of 8–6 K was observed for alloys having 20–66 at. %V in as cast condition. Heat treatments cause the lowering ofT _c . Results indicate that the preparation of ZrV₂ superconducting material will need more careful control than would be necessary for HfV₂.     

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.     

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.     

Superconductivity of BCC Zr and Zr-Nb alloys at high pressures

The superconducting transition temperature T _c of bcc Zr is measured at pressures to 64 GPa. The T _c value gradually decreases as pressure is increased. For the Zr-Nb alloys, there are found anomalies in the T _c (P) dependences at rather low pressures. The anomalies are discussed within the available theoretical models. We assume on the basis of the T _c (P) experimental data for the Zr-Nb alloys that the T _c (P) curve for bcc zirconium has a maximum in the metastability region.