High‐temperature superconductivity and long‐range order in strongly correlated electronic systems

A selective review of the question of how repulsive electron correlations might give rise to off‐diagonal long‐range order (ODLRO) in high‐temperature superconductors is presented. The article makes detailed explanations of the relevance to superconductivity of reduced electronic density matrices and how these can be used to understand whether ODLRO might arise from Coulombic repulsions in strongly correlated electronic systems. Time‐reversed electron pairs on alternant Cuprate and the iron‐based pnictide and chalcogenide lattices may have a weak long‐range attractive tail and much stronger short‐range repulsive Coulomb interaction. The long‐range attractive tail may find its origin in one of the many suggested proposals for high‐T_c superconductivity and thus has an uncertain origin. A phenomenological Hamiltonian is invoked whose model parameters are obtained by fitting to experimental data. A detailed summary is given of the arguments that such interacting electrons can cooperate to produce a superconducting state in which time‐reversed pairs of electrons effectively avoid the repulsive hard‐core of the Coulomb interaction but reside on average in the attractive well of the long‐range potential. Thus, the pairing of electrons itself provides an enhanced screening mechanism. The alternant lattice structure is the key to achieving robust high‐temperature superconductivity with dx²‐y² or sign alternating s‐wave or s± condensate symmetries in cuprates and iron‐based compounds. Some attention is also given to the question first raised by Leggett as to where the Coulombic energy is saved in the superconducting transition in cuprates. A mean‐field‐type model in which the condensate density serves as an order parameter is discussed. Many of the observed trends in the thermal properties of cuprate superconductors are reproduced giving strong support for the proposed model for high‐temperature superconductivity in such strongly correlated electronic systems. © 2015 Wiley Periodicals, Inc.     

Comparative XANES study on the two electron-doped high-Tc superconductor systems, (Sr,La)CuO2 and (Nd,Ce)2CuO4

Here we employ high-quality samples of (Sr_1−xLa_x)CuO₂ and (Nd_2−xCe_x)CuO₄ and XANES spectroscopy at O-K, Cu-L_2,3 and Ce-M_4,5 edges to gain comprehensive understanding of the electronic structure and doping in n-type high-T_c superconductors. Not only common but also slightly different features are revealed for the two systems. From O-K-edge spectra, the UHB is found essentially independent of the electron-doping level for both the systems, in line with our understanding that the doped electrons do not go to the O site in n-type copper-oxide superconductors. Another common observation is that the main Cu^II peak at the Cu-L₃ edge (due to transitions to the Cu^II-3d orbitals) systematically decreases in intensity upon electron doping, hence verifying the fact that the doped electrons go to the Cu site. The difference then between the two systems is that in (Sr_1−xLa_x)CuO₂ the weaker Cu^II peak due to transitions to the Cu^II-4s orbital depends on the degree of doping. Moreover, it was found that with increasing x, electron density increases much faster in (Sr_1−xLa_x)CuO₂ than in (Nd_2−xCe_x)CuO₄. This is a consequence of two phenomena: a tiny increase in oxygen content concomitant to the Ce^IV-for-Nd^III substitution and the somewhat lower Ce-valence value of +3.8 compared to the nominal tetravalent state.     

Energy Estimate for the Type-Ⅱ Superconducting Film

We study the minimizers of the Ginzburg-Landau model for variable thickness, superconducting, thin films with high k, placed in an applied magnetic field hex, when hex is of the order of the ＂first critical field＂, i.e. of the order of ｜lnε｜. We obtain the asymptotic estimates of minimal energy and describe the possible locations of the vortices.     

无源高温超导磁浮轴承两种方案比较

无源磁浮轴承的优点是结构简单、造价低廉、无需有源控制。文中探讨了两种无源磁浮轴承的方案 ;对两种方案的悬浮力、悬浮品质因数、悬浮刚度与悬浮稳定性进行了讨论、计算、比较 ;并且讨论了两种方案的优缺点及应用。比较结果表明 ,方案 1更适合于小型的飞轮储能系统 ,方案 2较适合于大中型飞轮储能系统。     

Effective interactions in the periodic Anderson model in the regime of mixed valency with strong correlations

For the periodic Anderson model in the strong correlation regime, we construct the effective Hamiltonian H _eff up to terms of the fourth order in the parameter V/U, where V is the hybridization interaction intensity and U is the intra-atom Coulomb repulsion strength. This Hamiltonian contains interactions inducing both magnetic ordering and Cooper instability under conditions of a mixed valency of rare-earth ions. Based on numerical calculations, we obtain information about the dependences of the effective interaction parameters on the distance between crystal lattice sites. We demonstrate that realizing exchange interactions corresponds to a strongly frustrated system of localized spin moments and facilitates the suppression of the antiferromagnetic order parameter with a possible transition to the state of a quantum spin liquid. It is essential that among the terms in H _eff inducing the transition to the superconductivity phase, there are terms resulting in the d-type symmetry of the superconductivity order parameter; such a symmetry is realized in many heavy-fermion compounds. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 157, No. 2, pp. 235–249, November, 2008.     

Fluctuations of order parameters in the highT c superconductors

Recently we have proposed a phenomenological approach in terms of two coexisting macroscopic order parameters corresponding to the superconducting and insulating states and have discussed the electrodynamical responses of the superconducting ceramics. In this paper we discuss the fluctuations of the order parameters both in static and dynamical situations in the mean field approach and obtain results for the electrical conductivity which possesses anomalies as in granular materials.     

Microscopic Superconductivity and Room Temperature Electronics of High-Tc Cuprates

Abstract This paper points out that the Landau criterion for macroscopic superfluidity of He H is only a criterion for microscopic superfluidity of ^4He, extends the Landau criterion to microscopic superconductivity in fermions （electron and hole） system and system with Cooper pairs without long-range phase coherence. This paper gives another three non-superconductive systems that are of microscopic superconductivity. This paper demonstrates that one application of microscopic superconductivity is to establish room temperature electronics of the high-To cuprates.     

Superconductivities and structural properties of ternary Tiben Zrben Ta alloys

Superconductivities and structural properties of Ti-Zr-Ta ternary alloys are extensively investigated. The TiZrTa sample has a cubic structure (β -phase) and shows a sharp superconducting transition at a critical temperature (T_c) of about 7.3 K. In addition, two series of Ti-Zr-Ta alloys, with nominal compositions of Ti_65-xZr₃₅Ta_x and Ti_xZr_65-xTa₃₅ respectively, are prepared, and their superconductivities and crystal structures change regularly with the chemical composition. Our experimental study also indicates that the annealing processing of this kind of material can cause the transition temperature to increase and the highest T_c is observed to be about 8.3 K in annealed samples.     

Physical insight of superconductivity of Nb2AlC: in situ Raman spectrometry investigation

Superconductivity of Nb₂AlC has been previously reported, but the origin is not clear. In this paper, in situ Raman spectra of Nb₂AlC are measured in the temperature range from 80 to 380 K at ambient pressure. The line‐width of E_2g (ω₁) mode increases with temperature which originates from the anharmonic phonon–phonon scattering. On the contrary the line‐widths of E_2g (ω₂) and A_1g (ω₄) modes decrease continuously at elevated temperature. The phenomenon is explained by the electron–phonon coupling. The origin of superconductivity is therefore interpreted by the coupling of Nb 4d electrons with E_2g (ω₂) and A_1g (ω₄) phonon modes. Copyright © 2013 John Wiley & Sons, Ltd.     

Superconductivity in ultrasmall metallic grains

We review recent experimental and theoretical work on superconductivity in ultrasmall metallic grains, i.e. grains sufficiently small that the conduction electron energy spectrum becomes discrete. The discrete excitation spectrum of an individual grain can be measured by the technique of single‐electron tunneling spectroscopy, and reveals parity effects indicative of pairing correlations in the grain. After introducing the discrete BCS model that has been used to model such grains, we review a phenomenological, grand‐canonical, variational BCS theory describing the paramagnetic breakdown of these pairing correlations with increasing magnetic field. We also review recent canonical theories that have been developed to describe how pairing correlations change during the crossover, with decreasing grain size, from the bulk limit to the limit of few electrons, and compare their results to those obtained using Richardson's exact solution of the discrete BCS model.