Electrons in cuprates: A consistent ARPES view

Angle resolved photoemission spectroscopy (ARPES) has been playing a crucial role in understanding of physics behind high-temperature superconductivity. Our ARPES investigation of superconducting cuprates, performed over a decade and accomplished by very recent results, suggests a consistent view of electronic interactions in cuprates which provides natural explanation of both the origin of the pseudogap state and the mechanism for high-temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. In view of these results and their projection to numerous other materials, two general questions are arising: is the normal state in 2D metals ever stable and how does this intrinsic instability interplay with superconductivity?     

Superconductivity and magnetism in Ir-doped GdFeAsO

The 5d-transition metal, Ir has successfully been doped at Fe site and induced superconductivity in GdFeAsO at T_c = 18.9 K and ∼20 atom%. The Ir-doping shortened the c-axis length and stretched the a-axis one, which led to enhance the coupling between the FeAs- and SmO-layer, and to weaken the bonding between Fe and As atom. Paramagnetism was observed in all of the samples, which was resulted from the magnetic Gd ion as in the F-doped GdFeAsO. An upper critical field of GdFe_0.8Ir_0.2AsO was extrapolated to around 24 T, much smaller than that of F-doped GdFeAsO owing to a relatively low T_c and small value of dH_c2/dT.     

Direct visualization of the vortex distributions in a superconducting film with a Penrose array of magnetic pinning centers: Symmetry induced giant vortex state

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. We demonstrate that stable vortex configurations can be found for fields H ∼ 0.8H₁, H₁ and 1.6H₁, where H₁ corresponds to one flux quantum per pinning site. The vortex pattern at 0.8H₁ corresponds to one vacancy in one of the vertices of the thin tiles whereas at 1.6H₁ the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H = 1.6H₁ interstitial and pinned vortices arrange themselves in ring-like structures (“vortex corrals”) which favor the formation of a giant vortex state at their center.     

Fractal superconductivity near localization threshold

We develop a semi-quantitative theory of electron pairing and resulting superconductivity in bulk “poor conductors” in which Fermi energy E_F is located in the region of localized states not so far from the Anderson mobility edge E_c. We assume attractive interaction between electrons near the Fermi surface. We review the existing theories and experimental data and argue that a large class of disordered films is described by this model.Our theoretical analysis is based on analytical treatment of pairing correlations, described in the basis of the exact single-particle eigenstates of the 3D Anderson model, which we combine with numerical data on eigenfunction correlations. Fractal nature of critical wavefunction's correlations is shown to be crucial for the physics of these systems.We identify three distinct phases: ‘critical’ superconductive state formed at E_F = E_c, superconducting state with a strong pseudo-gap, realized due to pairing of weakly localized electrons and insulating state realized at E_F still deeper inside a localized band. The ‘critical’ superconducting phase is characterized by the enhancement of the transition temperature with respect to BCS result, by the inhomogeneous spatial distribution of superconductive order parameter and local density of states. The major new feature of the pseudo-gapped state is the presence of two independent energy scales: superconducting gap Δ, that is due to many-body correlations and a new “pseudo-gap” energy scale Δ_P which characterizes typical binding energy of localized electron pairs and leads to the insulating behavior of the resistivity as a function of temperature above superconductive T_c. Two gap nature of the pseudo-gapped superconductor is shown to lead to specific features seen in scanning tunneling spectroscopy and point-contact Andreev spectroscopy. We predict that pseudo-gapped superconducting state demonstrates anomalous behavior of the optical spectral weight. The insulating state is realized due to the presence of local pairing gap but without superconducting correlations; it is characterized by a hard insulating gap in the density of single electrons and by purely activated low-temperature resistivity ln R(T) ∼ 1/T.Based on these results we propose a new “pseudo-spin” scenario of superconductor-insulator transition and argue that it is realized in a particular class of disordered superconducting films. We conclude by the discussion of the experimental predictions of the theory and the theoretical issues that remain unsolved.     

Fuzzy-AHP-TOPSIS法在多属性方案优选中的应用——基于油气田开发项目方案优选的研究

基于集成创新的思想,将多属性决策中模糊层次分析法和灰色—正负理想点逼近法优势集成,创建Fuzzy-AHP-TOPSIS法对方案进行最终优劣排序.方法综合考虑了决策者的主观权重和投资项目各影响因素间固有的客观权重、投资风险和收益等诸多因素,为企业投资项目的比选提供了一种新的可靠的决策方法.最后以某气田开发项目-中石油某气田增压开采方案的优选为实例加以说明.     

Investigation of the Rb-W-O system in connexion with the superconducting properties of the hexagonal tungsten bronzes

The previous studies of the superconducting properties of the hexagonal tungsten bronzes have repeatedly come up against the lack of reproducibility of the data and, at the first place, of the data relating to the stability of superconducting state. We revisited this problem and identified the main causes of these contradictory data in Rb_xWO₃, among which the major one is the ordering of the alkali atoms. Our study has emerged onto a determination of the x dependence of the order-disorder transition, of the lattice parameters and of the superconducting transition temperature T_c. We have also clarified the crystal structure and examined the mechanisms involved in the oxidation and reduction of this compound. Finally, we have assembled our data to draw a plausible Rb-W-O phase diagram.     

A free boundary problem of type-I superconductivity

1.IntroductionSuperconductorsofTypeIarematerialswhicharecapableofchangingfromthephaseofbeingnormalconductorstoaphasewherethereisnoresistancetothemotionoffreeelections.InnormalconductorphasethenormalizedMaxwellequations(neglectingdisplacementcurrents)aretogetherwithOhm'slawj~acEwhereuistheelectricconductivity.InasuperconductingphaseOhm'slawisnolongervalidandMaxwell'sequationsaresupplemelltedbytheGinzburg-Landaufieldequationsll].Underisothermalconditions,thechangeofphasefromsuperconductingto…     

Supraleitung in Seltenerdmetall-Carbidhalogeniden des Typs SE2X2C2

Superconductivity in Rare Earth Metal Carbide Halides of the Type SE₂X₂C₂ The metallic nature of the carbide halides Y₂X₂C₂ is due to Y? C covalency. The superconductivity of the compounds is attributed to a pairwise attraction of conduction electrons by C₂-π* states at the Fermi level. The hypothesis is followed by experiments and band structure calculations. – Neutron powder diffraction reveals d(C? C) = 128(1) pm for Y₂Br₂C₂. X-ray single crystal investigations on Y₂Br₂C₂ and Y₂I_1.5Br_0.5C₂ show a characteristic variation of the coordination of the C₂ unit. Systematic changes of the average halide radius in Y₂(X,X′)₂C₂ (X,X′ = Br, Cl I, Cl and I, Br) lead to a monotonic increase of T_c = 2.3 K (X = Cl) via T_c = 5.05 K (X = Br) to a maximum T_c = 11.2 K for Y₂I_1.6Br_0.4C₂. No isotope effect for ¹²C/¹³C could be detected. Photoelectron spectra of Y₂Br₂C₂ (excitation energies between 40 and 140 eV) are compared with the results of band structure calculations (LMTO, E.H.). The electronic structure reveals two bands crossing the Fermi level. One of them has C₂-π*-Y-d_xz,yz character and exhibits a saddle-point at E_F. The other intersects the Fermi level with large dispersion and has exclusively Y-d character at the crossing point. The results are discussed with respect to theoretical models (van Hove singularity, local pairs and itinerant electrons).