Influence of anisotropic fluctuations on the electrical resistivity in uniaxial ferromagnetic metals near Tc

The generalized Ornstein-Zernike correlation function appropriate to a uniaxial ferromagnet is applied to study the influence of anisotropic fluctuations on the electrical resistivity in uniaxial ferromagnetic metals near T_c. It is shown that both the anisotropic fluctuations and the Fermi surface anisotropy contribute on the same footing to an anisotropy of the spin-fluctuation resistivity. The results are consistent with those obtained for the isotropic system in the same approximation and may be useful for interpreting recent spin-fluctuation resistivity data obtained experimentally for uniaxial systems.     

A possible spin-fluctuation mechanism in the emergence of superconductivity in a region of magnetic instability, using the example of PuCoGa5

The spin-fluctuation superconductivity mechanism of strongly correlated d-f electrons is considered within the Hubbard model. It is shown that the emergence of superconductivity is possible under conditions of strong spin anharmonicity. Evaluations of the superconductivity temperature in the model developed for the electron structure of PuCoGa₅ agree with the experimental data.     

Magnetic quantum critical point and superconductivity in UPt3 doped with Pd

Transverse-field muon spin relaxation measurements have been carried out on the heavy-fermion superconductor UPt (3) doped with small amounts of Pd. We find that the critical Pd concentration for the emergence of the large-moment antiferromagnetic phase is approximately 0.6 at. %Pd. At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity, provides evidence for ferromagnetic spin-fluctuation mediated odd-parity superconductivity, which competes with antiferromagnetic order.     

Unconventional pairing originating from the disconnected Fermi surfaces of superconducting LaFeAsO1-xFx

For a newly discovered iron-based high T_{c} superconductor LaFeAsO1-xFx, we have constructed a minimal model, where inclusion of all five Fe d bands is found to be necessary. The random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin-fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.     

Magnetism and superconductivity in intermetallic uranium compounds

Superconductivity, spin-fluctuation effects and magnetic order are reviewed for intermetallic compounds of uranium with d-transition elements. Large values for the effective mass of the pairing electrons are deduced from critical field studies on the superconducting compounds. Magnetism in the 5f-electron compounds is strongly anisotropic even for paramagnetic materials. Magnetic order does not frequently occur in these intermetallics, spin fluctuations are often observed. A unique combination of spin-fluctuation phenomena and superconductivity is met in UPt₃.     

Kinematic spin-fluctuation mechanism of high-temperature superconductivity

We study d-wave superconductivity in the extended Hubbard model in the strong correlation limit for a large intersite Coulomb repulsion V. We argue that in the Mott-Hubbard regime with two Hubbard subbands, there emerges a new energy scale for the spin-fluctuation coupling of electrons of the order of the electron kinetic energy W much larger than the exchange energy J. This coupling is induced by the kinematic interaction for the Hubbard operators, which results in the kinematic spin-fluctuation pairing mechanism for V ? W. The theory is based on the Mori projection technique in the equation of motion method for the Green’s functions in terms of the Hubbard operators. The doping dependence of the superconductivity temperature T _c is calculated for various values of U and V.     

Spin fluctuations and unconventional superconducting pairing in iron-based superconductors

In this article, we review the recent theoretical works on the spin fluctuations and superconductivity in iron-based superconductors. Using the fluctuation exchange approximation and multi-orbital tight-binding models, we study the char- acteristics of the spin fluctuations and the symmetries of the superconducting gaps for different iron-based superconductors. We explore the systems with both electron-like and hole-like Fermi surfaces （FS） and the systems with only the electron-like FS. We argue that the spin-fluctuation theories are successful in explaining at least the essential part of the problems, indicating that the spin fluctuation is the common origin of superconductivity in iron-based superconductors.     

Spinfluctuations versus phonons: The case of Heavy-Fermion superconductivity

Superconductivity of Heavy-Fermions, with an emphasis on hexagonal UPt₃, is investigated microscopically on the basis of the LNCA-approximation for strongly correlated electrons in a Kondo lattice and of Eliashberg theory for anisotropic singlet and triplet superconducting order. The effective interaction kernel incorporates exchange of spin fluctuations carried by strongly renormalized particle-hole excitations as well as exchange of phonons generated by the breathing mechanism. Particular care is taken to include realistic anisotropies in band structure and couplings and to expand interaction kernel and order parameter in appropriate Fermi surface harmonics. Separate interactions and corresponding gap equations are used for pseudo-spin-singlet and -triplet channel, with a restriction to the case of weak spin-orbit coupling. Our main results include a detailed study of phonon- and spin-fluctuation mechanisms as a possible source for superconductivity and, in combination, their mutual influence and competition. Furthermore, the questions regarding singlet or triplet order, conventional or unconventional symmetry, order parameter zeros and phase transitions between different superconducting states are addressed. We propose a possible scenario of Heavy-Fermion superconductivity.     

Exchange and spin-fluctuation mechanisms of superconductivity in cuprates

A microscopic theory of superconductivity is considered in the framework of the Hubbard p-d model for the CuO₂ plane. The Dyson equation is derived in the nonintersecting diagram approximation using the projection technique for the matrix Green function of the Hubbard operator. The solution of the equation for the superconducting gap shows that interband transitions for Hubbard subbands lead to antiferromagnetic exchange pairing as in the t-J model, while intraband transitions additionally lead to spin-fluctuation pairing of the d-wave type. The calculated dependences of the superconducting transition temperature on the hole concentration and of the gap on the wave vector are in qualitative agreement with experiments.     

Superconductivity in CeCoIn5-xSnx: veil over an ordered state or novel quantum critical point?

Measurements of specific heat and electrical resistivity in magnetic fields up to 9 T along [001] and temperatures down to 50 mK of Sn-substituted CeCoIn5 are reported. The maximal -ln(T) divergence of the specific heat at the upper critical field Hc2 down to the lowest temperature characteristic of non-Fermi-liquid systems at the quantum critical point (QCP), the universal scaling of the Sommerfeld coefficient, and agreement of the data with spin-fluctuation theory provide strong evidence for quantum criticality at Hc2 for all x< or =0.12 in CeCoIn5-xSnx. These results indicate the "accidental" coincidence of the QCP located near Hc2 in pure CeCoIn5, in actuality, constitute a novel quantum critical point associated with unconventional superconductivity.     

Edge states of Sr2RuO4 detected by in-plane tunneling spectroscopy

Tunneling spectroscopy has been performed on Sr(2)RuO(4) searching for the edge states peculiar to topological superconductivity. Conductance spectra exhibit broad humps with three types of peak shape: domelike peak, split peak, and two-step peak. By comparing the experiments with predictions for unconventional superconductivity, these varieties are shown to originate from multiband chiral p-wave symmetry with weak anisotropy of pair amplitude. The broad hump in the conductance spectrum is a direct manifestation of the edge state due to chiral p-wave superconductivity.     

Hc2-Anisotropy in TaN single crystals

Experiments on the anisotropy of H_c2 in a tantalum single crystal doped with nitrogen in order to change its behaviour from type-I to type-II superconductivity are reported. The results show a disappearance of the anisotropy coefficient a₆ and are compared to the anisotropy in niobium single crystals.     

Effect of crystal anisotropy on superconductivity in zinc

The effect of crystal anisotropy on superconductivity in zinc has been studied making use of the recently observed highly anisotropic vibrations of a zinc atom in its crystal. The effect of crystal anisotropy on superconductivity turns out to be quite involved and significant.     

Localized 5f electrons in superconducting PuCoIn?: consequences for superconductivity in PuCoGa?

The physical properties of the first In analog of the PuMGa(5) (M = Co, Rh) family of superconductors, PuCoIn(5), are reported. With its unit cell volume being 28% larger than that of PuCoGa(5), the characteristic spin-fluctuation energy scale of PuCoIn(5) is three to four times smaller than that of PuCoGa(5), which suggests that the Pu 5f electrons are in a more localized state relative to PuCoGa(5). This raises the possibility that the high superconducting transition temperature T(c) = 18.5 K of PuCoGa(5) stems from the proximity to a valence instability, while the superconductivity at T(c) = 2.5 K of PuCoIn(5) is mediated by antiferromagnetic spin fluctuations associated with a quantum critical point.     

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 Hamiltonian and properties of the normal and superconducting phases of <Emphasis Type="Italic">n</Emphasis>-type cuprates

An effective low-energy Hamiltonian is derived from a microscopic multiband p-d model in the regime of strong electron correlations. The parameters of the p-d model are determined by comparison with the ARPES data for undoped Nd₂CuO₄. The Hamiltonian is the t-J* model in which hopping and exchange slowly decay with distance and are taken into account up to the fifth coordination sphere. The quasiparticle band structure is calculated as a function of the doping concentration with regard to short-range magnetic order, and the superconductivity theory with the spin-fluctuation pairing mechanism is constructed. Assuming that the parameters of the model do not depend on the doping level, we obtained quantitative agreement with the properties observed experimentally for the normal and superconducting phases without introducing fitting parameters.     

Altering the superconductor transition temperature by domain-wall arrangements in hybrid ferromagnet-superconductor structures

The [Co/Pt]n/Nb/[Co/Pt]n hybrids with perpendicular magnetic anisotropy reveal enhanced superconductivity with the presence, and the arrangements, of domain walls, where superconductivity persists. An in-plane field can manipulate the domain walls from labyrinth to stripe patterns and drive the hybrids from normal to superconducting. We observe anisotropic superconductivity in hybrids with stripe domains, along which enhanced superconductivity is realized.     

The galvanomagnetic properties of short-period superlattices

(001) oriented superlattices (SLs) with the period in the range of 9–60 nm are grown. The temperature dependence of the conductivity, the Hall coefficient and the magnetoresistance anisotropy are investigated in the SLs in the temperature range of 1.5–300 K and magnetic fields up to 1.5 T. The SLs are of n-type with the apparent electron concentration in the range of 10¹⁹–10²⁰ cm⁻³. The type II band ordering is concluded for the SLs. Below 6 K the SLs exhibit superconductivity. The superconductivity is suggested to have a local character and to be associated with the strained pseudomorphic SnTe layers.     

Effects of exchange interaction and spin-fluctuation on the magnetic and magneto-optic properties of NdF3

The exchange interaction between the electrons in the different magnetic ions and the spin-fluctuation of the magnetic ions exist in the paramagnetic media NdF3. The exchange interaction between the electrons in the different magnetic ions may be equivalent to an effective field Hin that is in direct proportion to the magnetization M. The spin-fluctuation of the magnetic ions leads the coefficient of the effective field to vary with temperature. The effective field is given as Hin = -（0.75 ＋ 0.22T） × 10^-5M in NdF3. When the secondary crystal field effect is taken into account, the magnetic susceptibility and Verdet constant are calculated for NdF3 by means of the effective field Hin and the applied field He. The calculated results are in agreement with the measured ones.     

Anisotropic superconductivity in UPt3

A consistent picture of conventional superconductivity in UPt₃ is presented which explains qualitatively the gap anisotropy, the dependence of T_c on crystal perfection, the low value of the specific heat discontinuity at T_c and the induced magnetic form factor. The model obtained is based on results of a local density band calculation for UPt₃ which are in excellent agreement with photoemission experiments and on estimates of the strength and anisotropy of the electron-phonon interaction. The key feature is anisotropy of the superconducting energy gap which arises from the variation of the electronic mass on the Fermi surface.