Interplay of superconductivity and antiferromagnetism in SmRh4B4

A microscopic theory of interplay of superconductivity and antiferromagnetism in rare earth ternary systems is developed from first principles for less than half filledf atomic shells. Self consistent equations for the superconducting order parameter Δ and magnetic order parameter Γ, are derived using a Green’s function technique and equation of motion method. The theory is applied to explain the experimental results in the antiferromagnetic superconductor SmRh₄B₄. The present model explains true coexistence of superconductivity and antiferromagnetism and the suppression of superconductivity by antiferromagnetism. The behaviour of superconducting order parameter (Δ), magnetic order parameter (Γ), the specific heat, the density of states, free energy and critical field (H _c) is also studied for the system SmRh₄B₄.     

Effect of hybridization and dispersion of quasiparticles on the coexistent state of superconductivity and antiferromagnetism in <Emphasis Type="Italic">R</Emphasis>Ni<Subscript>2</Subscript>B<Subscript>2</Subscript>C

The effect of hybridization of conduction electrons and f-level on superconductivity (SC) and antiferromagnetism (AFM) in the coexistent phase of rare-earth nickel borocarbide superconductors (RNi₂B₂C) is reported. The Hamiltonian of the system is a mean field one and has been solved by writing equations of motion for the single-particle Green functions. It is assumed that superconductivity arises due to BCS pairing mechanism in the presence of antiferromagnetism in nickel lattices of Ni₂B₂ plane. The expressions for superconducting and antiferromagnetic order parameters are derived using double time electron Green functions. The quasiparticle energy bands are plotted and the nature of band dispersion of the quasiparticles is studied.     

On superconductivity of high-spin transition metal compounds

The possibility of Cooper instability in transition metal compounds is established based of the concept of the strong interaction in the same unit cell. The multicomponent scattering amplitude of excitations is calculated. The superconductivity equations are derived for compounds of 3d transition metals. It is shown that in the pole approximation, the superconductivity equations can be reduced to the multicomponent superconductivity equations with preset BCS constants. A method is developed for calculating one-orbital constants and constants with different orbitals as functions of the total spin. The concentration ranges of superconducting ordering are obtained for one-orbital equations.     

Calculations of the superconducting isotope effect

The strong-coupling theory of superconductivity is used, without simplification, to compute the isotope effect exponent β in T_c ∝ M^?β for several metals. A comparison is made with the values obtained using two approximate equations for T_c.     

Reduction dependence of superconductivity in the end-member T′ cuprates

We have recently achieved superconductivity in T′-RE₂CuO₄ (RE: Pr, Nd, Sm, Eu, and Gd), using epitaxial thin films by metal organic decomposition. The key recipes to achieve superconductivity are low-P_O2 firing and subsequent vacuum reduction to minimize the amount of impurity oxygen atoms, which are very harmful to high-T_c superconductivity. In this article, we report our investigation on the reduction dependence of superconductivity of T′-RE₂CuO₄. For thin films, the amount of remnant O_ap atoms is difficult to evaluate but we propose that one good measure for this may be the c-axis lattice constant, which tells us whether the reduction is insufficient or excessive.     

Solutions of Eliashberg equations for an electron-phonon coupling with a cutoff

In this paper we discuss the Eliashberg equations for the case of an electron-phonon coupling with an energy cutoff. This cutoff is imposed either for the energy difference by means of a strip function, or for both energies, with a Cooper-like expression. The strip function cutoff requires explicit calculation of not only the frequency renormalization functionZ but also the energy renormalizationX. The physical origin of such cutoffs might lie in the very strong electron-electron interaction which seems typical for highT _c superconductivity. If such cutoffs are admitted, the hypothesis thatT _c is caused at least in part by a strong electron-phonon interaction can be reconsidered. We find that the combination of strong coupling and low-energy cutoff could produce highT _c with only small isotope effect and with little damping or pulling of the phonon modes. Correlation with other physical properties, such as specific heat, is reexamined in view to estimate the coupling constant . Some objections to the model using strong electron phonon interaction are removed and better agreement with observed properties is obtained     

Molecular viewpoint on high-temperature superconductivity Importance of orbital degeneracy

It is shown that the antisymmetrized geminal power wavefunction (AGP) in the macroscopic limit and the Bardeen-Cooper-Schrieffer (BCS) supercon-ductivity model with fixed mean number of electrons coincide to arbitrary order in deviations from the extreme-type function which is considered as the carrier of the superconductivity property. Variational equations for the AGP in the macroscopic limit are formulated in terms of two sets of parameters, ∈ _i and Δ_i , which under simplifying assumptions reduce to eigenvalues of the open-shell Roothaan one-electron Hamiltonian and to the BCS energy gap parameter, respectively. The superconducting state is shown to be stable for the solution of these equations with a macroscopic number of non-zero Δ_i and of degenerate ∈ _i =∈_F at the Fermi level ∈_F. The macroscopic contribution to the maximal pair occupation number which is responsible for the superconductivity is expressed as a mean value of Δ_i ²/[(∈ _i ?∈_F)²+Δ_i ²]. The formulated non-zero temperature version of the equations for ∈ _i , Δ_i is able to describe the superconducting phase transition. On this ground the necessary condition of stabilization of the superconducting state is formulated that is the existence of the macroscopic-fold near-degenerate and almost half-filled level. As is shown it is realized in the energy band structure of doped fullerides, copper oxide ceramics and perovskite-type crystals, e.g. BaBiO₃. The additional requirement of negativity of exchange interelectron-interaction integrals may be satisfied not only by the known vibronic mechanism but also, as is demonstrated, by the polarization potential of an environment in a plane layer of stratum structures.     

多声子强耦合超导理论

本文提出了多声子过程的强耦合超导理论,给出了能隙方程和T_c公式。结果发现,考虑电子多声子过程,高频声子对提高T_c有重要作用,单胞中对超导贡献的简并度数大,也有利于高T_c的出现。 关键词：     

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.     

Dispersion of quasi-particles in high Tc cuprates

The interplay between superconductivity (SC) and antiferromagnetism (AFM) is studied in strongly correlated systems of high T _c Cuprate superconductors. It is assumed that superconductivity arises due to BCS pairing mechanism in presence of AFM in Cu lattices of Cu-O planes. The total Hamiltonian of the system is mean field one and has been solved exactly by writing the equations of motion for the single particle Green’s functions. Equations for the appropriate single particle co-relation functions are derived and the order parameters corresponding to SC and AFM are determined. It is assumed that the Fermi energy ∈ _F = 0 and the renormalized localized f energy level coincide with the Fermi level. All the quantities in the final equation for h and Δ are made dimensionless by dividing by 2t, where t is the hopping integral. The temperature dependent values of staggered magnetic field (h) and SC gap (Δ) were determined by solving self-consistent equations for h and Δ. The quasiparticle energy bands are function of AFM gap (h), SC gap (Δ) and hybridization (V). Then the dispersion of quasi-particles are studied at different temperatures by considering temperature dependent values of h and Δ and varying other different model parameters.      

Superconductivity with s and p symmetries in an extended Hubbard model with correlated hopping

We consider a generalized Hubbard model with on-site and nearest-neighbour repulsions U and V respectively, and nearest-neighbour hopping for spin up (down) which depends on the total occupation n_b of spin down (up) electrons on both sites involved. The hopping parameters are t _AA , t_AB and t_BB for n _b =0,1,2 respectively. We briefly summarize results which support that the model exhibits s-wave superconductivity for certain parameters and extend them by studying the Berry phases. Using a generalized Hartree-Fock(HF) BCS decoupling of the two and three-body terms, we obtain that at half filling, for t _AB <t _AA =t _BB and sufficiently small U and V the model leads to triplet p-wave superconductivity for a simple cubic lattice in any dimension. In one dimension, the resulting phase diagram is compared with that obtained numerically using two quantized Berry phases (topological numbers) as order parameters. While this novel method supports the previous results, there are quantitative differences. Received: 2 February 1998 / Accepted: 17 March 1998     

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.     

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.     

A generalization of the standard formula for the electrical resistivity of normal rare-earth metals

A unified approach to superconductivity is presented using natural extensions of weak coupling BCS theory. Multiphonon-multiband interactions cause a multigap opening atT _c , with cooperative pairing in all bands incorporated via interband interactions. Strong increases inT _c are found due to interband-induced gap coupling. Model calculations for the experimentally observed gap distribution are presented.     

Kondo impurities in anisotropic superconductors. Eliashberg formulation

The Kondo effect in an anisotropic superconductor is studied within the framework of the Migdal-Eliashberg (ME) formulation of superconductivity. The renormalization of the ME equations arising from the scattering by the Kondo impurities is obtained on the basis of the Müller-Hartmann and Zittartz theory. Both the decrease of the transition temperature T_c and the specific heat jump at T_c are calculated.     

Synthesis and superconductivity of a new 1222-type T1-based layered cuprate (T1, Nb) Sr2(Nd,Ce)2Cu2O z

Synthesis and superconductivity of a new 1222-type layered cuprate (Tl_1–x Nb _x ) Sr₂(Nd_1–y Ce _y )₂Cu₂O _z have been studied. The structure of this cuprate is directly related to that of Nb-1222 NbSr₂(Nd, Ce)₂Cu₂O _z with tetragonal body-center lattice. Partial substitution of Tl for Nb in Nb-1222 phase improves its superconductivity. (Tl_1–x Nb _x ) Sr₂(Nd_0.75Ce_0.25)₂Cu₂O _z samples prepared by the typical procedure exhibit superconductivity withT _c of 30–40 K. Effects of Tl and Nb on superconductivity of this cuprate are briefly discussed.     

Superconductivity in Bi2.2Ca2.8−x Sr x Cu2O y system and effect of rare earth substitution

The oxide system Bi_2.2Ca_2.8−x Sr _x Cu₂O _y has been investigated for superconductivity as a function of Ca/Sr ratio withx=0.7, 1.0, 1.4 and 1.8. All these compositions are found to be superconducting with onset temperature in the range of 80–90 K implying a large homogeneity range between Sr and Ca for the superconductivity to occur in this system. Effect of partial replacement of Bi by rare earths has been studied and it is observed that 20% replacement of Bi by Nd and Eu does not significantly affect the superconducting transition temperature.     

Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2Si2 versus YbRh2Si2

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu₂Si₂, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu₂Si₂. In YbRh₂Si₂, superconductivity appears to be suppressed at T???10?mK by AF order (T_N?=?70?mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at T_A slightly above 2?mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at T_c?=?2?mK. Like the pressure – induced QCP in CeRhIn₅, the magnetic field – induced one in YbRh₂Si₂ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavy – fermion superconductors and other families of unconventional superconductors with higher T_cs, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.     

Oxygen deficiency and superconductivity in T-, T'- and T∗-Ln2CuO4

Abstract

Effect of oxygen deficiency on superconductivity is discussed for three well-known systems, T?, T'?, and T?-Ln₂CuO₄. Although their crystal structures are closely related to each other, the oxygen-deficiency effect on superconductivity is different from system to system. In T-(La, Sr)₂CuO₄ and T?-(Nd,Ce,Sr)₂-CuO₄, oxygen defects suppress T _c while they enhance T _c in T'-(Nd,Ce)₂CuO₄. In T-(La,Ba)₂CuO₄, oxygen defects have, for x ～ 0.065, a positive effect on superconductivity as well. This positive effect is discussed related to a low-temperature structural phase transition found in the Ba-system.     

Formation of superconductivity through interparticle interactions in ferrimagnetic-like Sn nanoparticle assemblies

We report on the observation of the development of superconductivity through interparticle interactions in 3, 5, 7, and 23 nm ferrimagnetic-like Sn nanoparticle assemblies. The Sn nanoparticles are fabricated using the gas condensation method. Each sample consists of a macroscopic amount of individual Sn nanoparticles without a capping molecule. Ferrimagnetism is found but no sign of superconductivity can be detected when the 3 nm particles are very loosely assembled. A reduction in the mean particle moment results when the packing fraction of the assembly is increased. Superconductivity occurs when a critical packing fraction is reached. Beyond this, the superconducting transition temperature T _C continues to increase and noticeably exceeds that of the bulk T _C. The enhancement of superconductivity by interparticle interactions has also been observed in 5, 7, and 23 nm particle assemblies, with the effect becoming less significant in larger particles. We attribute these observations to the transfer of electrons between the surface and the core regions of the nanoparticles triggered by finite size effects and interparticle interactions.