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Polarons and bipolarons in two-dimensional (2 D) crystals with a simple conduction band are considered in the work. A study of the potential of pair interaction for the singlet states of polarons was carried out in the region intermediate coupling. The effective functional of exchange-coupled pair of polarons was obtained by the Buimistrov-Pekar method. Variational calculations of the singlet states energies were carried out taking into account the direct dependence of the wave function of the system on the interelectron distance. The variational functions have been selected in the form of a multiparameter system of Gaussian functions with exponentially correlated multipliers. The region of existence of a two-dimensional bipolaron was determined as a function of the crystal parameters. It has been shown that a single-center configuration (a configuration similar to a two-dimensional helium atom) is stable. The minimum corresponding to the two-center configuration ("polaron molecule") is secondary extremum that disappears when the electronic correlations associated with the direct dependence of the wave function of the system on the distance between electrons are taken into account. The main energy minimum corresponds to single-center configuration. The study of the fulfillment of the virial theorem for polaron and bipolaron functionals was carried out. The results of variational calculations were compared with the calculations performed by other authors. Two-center configuration of a 2D bipolaron is unstable. The only minimum corresponds to a one-center 2D bipolaron. Exchange energy of two 2D polarons is antiferromagnetic. [ABSTRACT FROM AUTHOR]

It has been proposed by Müller and Shengelaya that underdoped ultrathin layers of copper-oxide high-temperature superconductors (HTSs) sandwiched between high-dielectric-constant insulator layers could manifest increased superconducting critical temperature T c . To check this hypothesis, we investigated structural and transport properties of YBa 2 Cu 3 O 7 - δ (YBCO) thin films sandwiched between SrTiO 3 (STO) layers. Scanning transmission electron microscopy (STEM) showed that a high-quality interface is formed between YBCO and the top STO layers. An increase of T c up to Δ T ≈ 20 K was observed at these interfaces in case of underdoped YBCO films. [ABSTRACT FROM AUTHOR]

The temperature and frequency dependence of complex dynamical conductivity σ=σ1+iσ2, penetration depth, and superfluid density of Ba(Fe1−xCox)2As2 are calculated in the framework of one- and two-gap models at terahertz frequencies for a temperature range of 2 K < T < TC (TC = 22 K). In the single-gap model, an isotropic s-wave gap without any node and a d-wave gap with possible nodes are considered. In the two-gap model, one of the gaps was considered isotropic s-wave gap with amplitude ΔA=3meV, while the other gap was supposed to be an anisotropic d-wave gap with possible nodes, and its rms amplitude was Δ0=8meV. The interaction between the gaps is waived, since it is small, and its description is problematic. Comparing the result with the experimental data proves that the two-gap model consistently describes the optical characteristics of Ba(Fe1−xCox)2As2. In the two-gap model, the temperature dependence of σ1 demonstrated a coherence peak at frequencies below 15 cm−1. The temperature dependence of the penetration depth calculated from σ2 shows power-law behavior at low temperatures, and the superfluid density varies steeply near TC. [ABSTRACT FROM AUTHOR]

The magnitude and the temperature dependence of the superconducting order parameter Δ( T) of single crystals of Cd2Re2O7 ( Tc = 1.02 K) was measured using point-contact spectroscopy. To fit the conductance spectra and to extract the order parameter at different temperatures we generalized the Blonder-Tinkham-Klapwijk theory by including the self-energy of the quasiparticles into the Bogoliubov equations. This modification enabled excellent fits of the conductance spectra. Δ( T) increases steeply below the superconducting transition temperature of 1.02 K and levels off below ∼0.8 K at a value of 0.22(1) meV, ≈40% larger than the BCS value. Our results indicate the presence of a strong electron-phonon interaction and an enhanced quasiparticle damping and may be related to a possible phase transition within the superconducting region at ∼0.8 K. [ABSTRACT FROM AUTHOR]

We study the incommensurate charge ordered states in the $t\mbox{--}{t}^{\prime }\mbox{--}J$ model using the Gutzwiller mean field theory on large systems. In particular, we explore the properties of incommensurate charge modulated states referred to as nodal pair density waves (nPDW) in the literature. nPDW states intertwine site and bond charge order with modulated d -wave pair order, and are characterized by a nonzero amplitude of uniform pairing; they also manifest a dominant intra-unit cell d -density wave form factor. To compare with a recent scanning tunneling microscopy (STM) study (Hamidian et al 2015 Nat. Phys. http://dx.doi.org/10.1038/nphys3519 12 http://dx.doi.org/10.1038/nphys3519 ) of the cuprate superconductor BSCCO-2212, we compute the continuum local density of states (LDOS) at a typical STM tip height using the Wannier function based approach. By Fourier transforming Cu and O sub-lattice LDOS we also obtain bias-dependent intra-unit cell form factors and spatial phase difference. We find that in the nPDW state the behavior of form factors and spatial phase difference as a function of energy agrees remarkably well with the experiment.This is in contrast to commensurate charge modulated states, which we show do not agree with experiment. We propose that the nPDW states are good candidates for the charge density wave phase observed in the superconducting state of underdoped cuprates.

The fully anisotropic two-leg spin- $\tfrac{1}{2}$ XXZ ladder model is studied in terms of an algorithm based on the tensor network (TN) representation of quantum many-body states as an adaptation of projected entangled pair states to the geometry of translationally invariant infinite-size quantum spin ladders. The TN algorithm provides an effective method to generate the groundstate wave function, which allows computation of the groundstate fidelity per lattice site, a universal marker to detect phase transitions in quantum many-body systems. The groundstate fidelity is used in conjunction with local order and string order parameters to systematically map out the groundstate phase diagram of the ladder model. The phase diagram exhibits a rich diversity of quantum phases. These are the ferromagnetic, stripe ferromagnetic, rung singlet, rung triplet, Néel, stripe Néel and Haldane phases, along with the two XY phases XY 1 and XY 2.

We studied the pairing mechanism of the heavily electron doped FeSe (HEDIS) systems, which commonly have one incipient hole band—a band top below the Fermi level by a finite energy distance ε _b —at Γ point and ordinary electron bands at M points in Brillouin zone (BZ). We found that the system allows two degenerate superconducting solutions with the exactly same T _c in clean limit: the incipient ${s}_{{he}}^{\pm }$ -gap ( ${{\rm{\Delta }}}_{h}^{-}\ne 0$ , ${{\rm{\Delta }}}_{e}^{+}\ne 0$ ) and ${s}_{{ee}}^{++}$ -gap (Δ _h = 0, ${{\rm{\Delta }}}_{e}^{+}\ne 0$ ) solutions with different pairing cutoffs, Λ _sf (spin fluctuation energy) and ε _b , respectively. The ${s}_{{ee}}^{++}$ -gap solution, in which the system dynamically renormalizes the original pairing cutoff Λ _sf to Λ _phys = ε _b (

The strong power law behavior of the specific heat jump ${\rm{\Delta }}C\;$ versus T _c $({\rm{\Delta }}C/{T}_{{\rm{c}}}\sim {T}_{{\rm{c}}}^{\alpha },\alpha \approx 2)$ , first observed by Bud’ko et al (2009 Phys. Rev. B http://dx.doi.org/10.1103/PhysRevB.79.220516 79 http://dx.doi.org/10.1103/PhysRevB.79.220516 ), has been confirmed with several families of the Fe-based superconducting compounds with various dopings. We have tested a minimal two band BCS model to understand this anomalous behavior and showed that this non-BCS relation between ${\rm{\Delta }}C\;$ versus T _c is a generic property of the multiband superconducting state paired by a dominant interband interaction ( ${V}_{\mathrm{inter}}\gt {V}_{\mathrm{intra}}$ ) reflecting the relation $\frac{{{\rm{\Delta }}}_{{\rm{h}}}}{{{\rm{\Delta }}}_{{\rm{e}}}}\sim \sqrt{\frac{{N}_{{\rm{e}}}}{{N}_{{\rm{h}}}}}$ near T _c , as in the ${S}_{\pm }$ -wave pairing state. We also found that this ${\rm{\Delta }}C\;$ versus T _c power law can continuously change from the ideal BNC scaling to a considerable deviation by a moderate variation of the impurity scattering rate ${{\rm{\Gamma }}}_{0}$ (non-pair-breaking). As a result, our model provides a consistent explanation why the electron-doped Fe-based superconductors follow the ideal BNC scaling very well while the hole-doped systems often show varying degree of deviations.

Abstract: Macroscopic Quantum effects such as the Meissner effect and magnetic flux trapping in massive, pure poly-crystalline lead samples and in a low purity crystalline sample having non-superconducting parts could be visualized and quantified by radiography with polarized neutrons. A uniform poly-crystalline Pb sample with a high purity of 1−1×10−4 weight% and one with a low purity of approx. 85 weight% Pb having non-superconducting parts showed a partial Meissner effect and for B ext = 0 and T c a non-uniform magnetic flux trapping. Based on radiographies with polarized neutrons the trapped and expelled magnetic fields could be calculated for the uniform lead sample, in the case of the sample which contains non-superconducting parts an asymmetric (also squeezed) field distribution yielded the best fit. [Copyright &y& Elsevier]

Well recognized empty core pseudopotential of Ashcroft is used to investigate the superconducting state parameters viz; electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T, isotope effect exponent α and effective interaction strength N V of some (NiZr)V ( x = 0, 0.05, 0.1, 0.15) bulk amorphous alloys. We have incorporated five different types of local field correction functions, proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) to show the effect of exchange and correlation on the aforesaid properties. Very strong influence of the various exchange and correlation functions is concluded from the present study. The T obtained from local field correction function proposed by Sarkar et al. (S) is in excellent agreement with available theoretical data. Quadratic T equation has been proposed providing successfully the T values of bulk amorphous alloys under consideration. Also, the present results are found in qualitative agreement with other such earlier reported data, which confirm the superconducting phase in the s bulk amorphous alloys. [ABSTRACT FROM AUTHOR]

The superconducting pairing instability - as determined by a divergence of the two-particle susceptibility - is obtained in the mean field (BCS) approximation in the thermodynamic limit. The usual practice is to examine this property for a finite lattice. We illustrate that, while the conclusions remain unchanged, the technical features are very different in the thermodynamic limit and conform more closely with the usual treatment of phase transitions encountered in, for example, the mean-field paramagnetic-ferromagnetic transition. Furthermore, by going to the extreme dilute limit, one can distinguish three dimensions from one and two dimensions, in which a pairing instability occurs even for two particles. [ABSTRACT FROM AUTHOR]

By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multigap superconductivity, it is shown that these systems strongly resemble MgB2, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similarly to MgB2 and in contrast to cuprates, the two superconducting gaps in the Fe-As based materials are isotropic s-wave gaps. [ABSTRACT FROM AUTHOR]

The competition between singlet and triplet superconductivity is examined in consideration of correlations on an extended Hubbard model. It is shown that the triplet superconductivity may not be included in the common Hubbard model since the strong correlation favors the singlet superconductivity, and thus the triplet superconductivity should be induced by the electron–phonon interaction and the ferromagnetic exchange interaction. We also present a superconducting qualification with which magnetism is unbeneficial to superconductivity. [ABSTRACT FROM AUTHOR]

Abstract: The momentum dependence of the low energy quasiparticle spectrum and the related Bogoliubov angle in cuprate superconductors are studied within the kinetic energy driven superconducting mechanism. By calculation of the ratio of the low energy quasiparticle spectra at positive and negative energies, it is shown that the Bogoliubov angle increases monotonically across the Fermi crossing point. The results also show that the superconducting coherence of the low energy quasiparticle peak is well described by a simple d-wave Bardeen–Cooper–Schrieffer formalism, although the pairing mechanism is driven by the kinetic energy by exchanging spin excitations. [Copyright &y& Elsevier]

Abstract: Multi-band pairing of effectively ultrarelativistic electrons and holes in asymmetrically biased graphene bilayer in strong coupling regime is considered. In this regime, the pairing affects both conduction and valence bands of the both graphene layers, and the order parameter is a matrix, which indices correspond to the bands. For band-diagonal s-wave pairing, we derive the system of multi-band gap equations for the gaps in the valence and conduction bands and solve it in the approximation of constant gaps and in the approximation of separable pairing potential. For a characteristic width of the pairing region of order of magnitude of the chemical potential, the gap values are not much different from single-band BCS estimations. However, if the pairing region is wider, then the gaps can be much larger and depend exponentially on its energy width. We also predict gapped and soliton-like oscillations of a relative phase of the gaps and unpairing of quarter-vortices at Kosterlitz–Thouless transition. [Copyright &y& Elsevier]

Abstract: According to recent experimental findings the leading pairing resides in the nodal (FS arcs) momentum region of hole doped cuprates. The pseudogap is an antinodal feature. A corresponding multiband model of the electronic background evolving with doping serves the usually presented phase diagram. The pairing is due by the pair-transfer between overlapping nodal defect (polaron) band and the itinerant band. A bare gap vanishing with extended doping between the antinodal defect subband and the itinerant band top leads to the formation of the pseudogap as a perturbative band-structure effect. The calculated behaviour of two superconducting gaps and of the pseudogap on the whole doping scale is in qualitative agreement with the observations. Arguments to include cuprates into the class of multiband–multigap superconductors are given by these results. [Copyright &y& Elsevier]

Dynamical gravitational and geodesic equations are derived for superfluid densities of nonlocal self-coherent particles. The geometrized gravitational particle is the r−4 distribution of inertial mass that balances Ricci curvatures in the Einstein equation without the right-hand side. The spatial energy integral of such an infinite radial particle is finite and determines its nonlocal gravimechanical charge for energy-to-energy interactions with other nonlocal particles. Non-empty space of the flat material world is filled continuously by overlapping energy-flows of all nonlocal particles and their fields. [ABSTRACT FROM AUTHOR]

Abstract: We consider thermal strongly-coupled SYM theory with fundamental matter at finite isospin chemical potential. Using gauge/gravity duality, i.e. a probe of two flavor D7-branes embedded in the AdS black hole background, we find a critical temperature at which the system undergoes a second order phase transition. The critical exponent of this transition is one half and coincides with the result from mean field theory. In the thermodynamically favored phase, a flavor current acquires a vev and breaks an Abelian symmetry spontaneously. This new phase shows signatures known from superconductivity, such as an infinite dc conductivity and a gap in the frequency-dependent conductivity. The gravity setup allows for an explicit identification of the degrees of freedom in the dual field theory, as well as for a dual string picture of the condensation process. [Copyright &y& Elsevier]

The study of the superconducting state parameters (SSP) viz. electron-phonon coupling strength In1 − xTl x, Coulomb pseudopotential μ*, transition temperature T C, isotope effect exponent and effective interaction strength of N0 V of In1 − xTl x (0 < x < 1) binary alloys have been made extensively in the present work using a model potential. A considerable influence of various exchange and correlation functions on λ and μ* is found from the present study. The present results of the SSP are found in qualitative agreement with the available experimental or theoretical data wherever exist. [ABSTRACT FROM AUTHOR]

In the present paper some superconducting (SC) state parameters of metals Ga, Cd and In have been studied through Harrison’s First Principle [HFP] pseudopotential technique using McMillan’s formalism. The impact of choosing two different sets of core energy eigenvalues viz. Herman-Skillman and Clementi (or Experimental) has been studied. It has been observed that the choice of eigenvalues has appreciable impact on the form factor and consequently on the SC state transition parameter viz. electron-phonon coupling strength (λ) and superconducting transition temperature T c . Hence the choice of suitable eigenvalues is essential for these computations. Reasonable agreement with experimental and theoretical data have been obtained. [ABSTRACT FROM AUTHOR]

Abstract: We propose a way of making graphene superconductive by putting on it small superconductive islands which cover a tiny fraction of graphene area. We show that the critical temperature, , can reach several Kelvins at the experimentally accessible range of parameters. At low temperatures, , and zero magnetic field, the density of states is characterized by a small gap resulting from the collective proximity effect. Transverse magnetic field is expected to destroy the spectral gap driving graphene layer to a kind of a superconductive glass state. Melting of the glass state into a metal occurs at a higher field . [Copyright &y& Elsevier]

Within the BCS theory of superconductivity we calculate the superconducting gap at a zero-temperature for metallic hydrogen–graphene system in order to estimate the superconducting critical temperature of quasi-two-dimensional highly oriented pyrolytic graphite. The obtained results are given as a function of the hydrogen-induced density of carriers n and their effective mass m ⋆. The obtained gap shows a Maxwell-like distribution with a maximum of ∼60 K at n∼3×1014 cm−2 and m ⋆/ m=1. The theoretical results are discussed taking into account recent experimental evidence for granular superconductivity in graphite. [ABSTRACT FROM AUTHOR]

Abstract: We propose a simple explanation of unconventional thermodynamical and magnetic properties observed for Sr2RuO4. Actually, our two-phase model of superconductivity, based on a straight generalization of the Ginzburg–Landau theory, does predict two jumps in the heat capacity as well as a double curve for the dependence of the critical temperature on an external magnetic field. Such theoretical previsions well agree with the currently available experimental data for Sr2RuO4. [Copyright &y& Elsevier]

Abstract: Special solutions of the Hartree–Fock (HF) problem for Coulomb interacting electrons described by a simple model of the Cu–O planes in La2CuO4 are presented. One of the mean field states obtained, is able to predict some of the most interesting properties of this material, such as its insulator character and the antiferromagnetic order. The natural appearance of pseudogaps in some states of this material is also indicated by another of the HF states obtained. These surprising results follow after eliminating spin and crystal symmetry restrictions usually imposed on the single particle HF orbitals, by employing the rotational invariant formulation of the HF scheme originally introduced by Dirac. Therefore, it is exemplified here, how up to know considered strong correlation effects, can be described by improving the HF solution of the considered system. In other words, it has been argued, that defining correlation effects as the ones shown by the system and not predicted by the HF best (lowest energy) solution, allows to explain important, up to know considered as strong correlation properties, as simple mean field ones. The discussion also helps to clarify the role of the antiferromagnetism and pseudogaps in the physical properties of the HTSC materials and indicates a promising way to start conciliating the Mott and Slater pictures in the physics of the transition metal oxides and other strongly correlated electron systems. [Copyright &y& Elsevier]

The mechanism of spin–phonon coupling (SPC) and possible consequences for the properties of high- T C copper oxides are presented. The results are based on ab initio LMTO band calculations and a nearly free-electron (NFE) model of the band near E F. Many observed properties are compatible with SPC, as for the relation between doping and $\vec{q}$ for spin excitations and their energy dependence. The main pseudogap is caused by SPC and waves along [1,0,0], but it is suggested that secondary waves, generated along [1,1,0], contribute to a ‘waterfall’ structure. Conditions for optimal T C, and the possibilities for spin enhancement at the surface are discussed. [ABSTRACT FROM AUTHOR]

We investigate the ground-state properties of a one-dimensional spin-1/2 fermionic atoms interacting through the attractive δ-function potential in an external magnetic field. By the thermal Bethe ansatz method, the integral equations for the dressed energies are formulated. Those integral equations at zero-temperature are solved in the power series forms of rapidity and momentum for both strong and weak coupling cases. The magnetization as a function of the coupling constant and the external field is also obtained explicitly. Based on the analytic results, quantum phase transitions are identified among three phases, an unpolarized fully paired state, a fully polarized normal ferromagnetic state and a mixed state of paired and unpaired atoms. [ABSTRACT FROM AUTHOR]

In the framework of four-band model of superconductivity in iron arsenides proposed by Barzykin and Gor’kov we analyze the gap ratios on hole—like and electron—like Fermi—surface cylinders. It is shown that experimentally observed (ARPES) gap ratios can be obtained only within rather strict limits on the values of pairing coupling constants. The difference of T c values in 1111 and 122 systems is reasonably explained by the relative values of partial densities of states. The multiple bands electronic structure of these systems leads to a significant enhancement of effective pairing coupling constant determining T c, so that high enough T c values can be achieved even for the case of rather small intraband and interband pairing interactions. [ABSTRACT FROM AUTHOR]

Abstract: We argue that the cosmological constant is exponentially suppressed in a candidate ground state of loop quantum gravity as a nonperturbative effect of a holographic Fermi-liquid theory living on a two-dimensional spacetime. Ashtekar connection components, corresponding to degenerate gravitational configurations breaking large gauge invariance and CP symmetry, behave as composite fermions that condense as in Bardeen–Cooper–Schrieffer theory of superconductivity. Cooper pairs admit a description as wormholes on a de Sitter boundary. [Copyright &y& Elsevier]

We propose a way of making graphene superconductive by putting on it small superconductive islands which cover a tiny fraction of graphene area. We show that the critical temperature, T c, can reach several Kelvins at the experimentally accessible range of parameters. At low temperatures, T[Figure not available: see fulltext.] T c, and zero magnetic field, the density of states is characterized by a small gap E g ≤ T c resulting from the collective proximity effect. Transverse magnetic field H g( T) ∝ E g is expected to destroy the spectral gap driving graphene layer to a kind of a superconductive glass state. Melting of the glass state into a metal occurs at a higher field H g2( T). [ABSTRACT FROM AUTHOR]

In this study we focus on the magnetic properties of a system consisting of Spin Density Wave (SDW) order, d-wave singlet and π-triplet superconductivity (SC) taken on the same footing within a mean-field treatment. The competition and coexistence of these order parameters manifest the H-T phase diagrams of all antiferromagnetic superconductors. An experimental probe to such a multicomponent state can be provided by magnetic measurements such as NMR or magnetization techniques. We have calculated, both analytically and numerically, the static spin susceptibility of our multicomponent system as a function of temperature and external magnetic field under particle–hole asymmetry. Our results can serve as a way to identify these novel multicomponent states. An application to the identification of the high-field low-temperature phase in CeCoIn5 is discussed. [ABSTRACT FROM AUTHOR]

The superconducting state with a large momentum of a pair in a doped insulator is controlled by the features of Coulomb pairing associated with the suppression of small momentum transfers during scattering owing to electron-phonon interaction and of large transfers due to redistribution of the spectral weight among the superconducting or the insulating branches of the elementary excitation spectrum. Superconductivity emerges when the characteristic energy of the pairing interaction has an upper bound. The solutions obtained for a self-consistent equation with a simple structure of the degenerate kernel are antisymmetric and symmetric relative to inversion of the momentum of relative motion of a pair. The orbital symmetry of the singlet superconducting order is analyzed. [ABSTRACT FROM AUTHOR]

Mean-field study of the stripe structures is conducted for a hole-doped Hubbard model. For bond-directed stripes, the Fermi surface consists of segments of an open surface and the boundaries of the hole pockets which appear in the diagonal region of momenta under certain conditions. Segments of the first type are due to one-dimensional bands of states localized on the domain walls. The relation of bands to the doping and temperature dependences of the Hall constant is discussed. In connection with the observation of quantum magnetic oscillations, a systematic search for the electron pockets has been carried out. It is shown that the formation of such pockets in bilayer models is quite possible. [ABSTRACT FROM AUTHOR]

Abstract: Inhomogeneity is introduced through random local interactions () in an attractive Hubbard model on a square lattice and studied using mean-field Bogoliubov–de Gennes formalism. Superconductivity is found to be suppressed by the random contrary to the results of a bimodal distribution of . The proximity effect of superconductivity is found to be strong, all sites develop non-zero pairing amplitude. The gap in the density of states is always non-zero and does not vanish even for strong disorder. When two such superconductors are coupled via a channel, the effect of one on the other is negligible. The length and width of the connector do not seem to have any noticeable effect on the superconductivity in either system. The superconducting blocks behave as independent entities and the introduction of the channel has no effect on them. [Copyright &y& Elsevier]

Abstract: We study quasi-one-dimensional organic superconductors with two-band dispersion. We use spin ladder approximation with the Hubbard model. We determine the symmetry of the Cooper pairing by calculating the free energy. Thanks to close values of singlet and triplet scatterings, the phase diagram reveals that, at high magnetic field, the triplet configuration is more favorable, indicating a transition from singlet superconducting state at low fields to triplet one. This result confirms similar ones in one-band system and can explain recent experimental results on (TMTSF)2ClO4. [Copyright &y& Elsevier]

Abstract: The issue of how superconductivity originate in the CuO2 planes believed to be crucial to understanding the high superconducting cuprates is still an going debate. In the wake of recent experimental observations of the Zhang–Rice singlet (ZRS), its formation and propagation need to be revisited especially by using a simple approach almost at a phenomenological level. Within a highly simplified correlated variational approach (HSCVA) in this Letter, a new formation of the ZRS as constituting the ground state of a single-band model of the CuO2 planes is developed. This formation is then used to demonstrate how the ZRS can be propagated as a probable Cooper channel in the CuO2 planes. [Copyright &y& Elsevier]

Pairing symmetry in oxypnictides, a new family of multiband high- T c superconductors, is partially imposed by the positions of multiple Fermi pockets, which itself can give rise to new order parameters, such as s+,− states or the state of the $$ d_{x^2 - y^2 } $$ symmetry. Other pairing states may appear on small pockets for long-range interactions, but they are expected to be sensitive to defects. We identify the competing antiferromagnetic order with the triplet exciton transition in the semimetallic background and discuss whether its coexistence with superconductivity explains the doping dependence of T c. [ABSTRACT FROM AUTHOR]

Abstract: The superconductivity mechanism of Pb thin film on a Si substrate in the weak interaction regime is investigated. A discrete Fermi surface is constructed depend on the film thickness and electron density and crystallographic orientation. We consider two types of Cooper pair interactions, Cooper pair interaction at thin film surfaces and Cooper pair in the thin film volume. We have chosen the surface Cooper pair interaction, of ultra thin films superconductor proportional to the inverse of thin film thickness, while the volume Cooper pair interaction has been considered as a constant. By these assumptions, we have found oscillation feature of critical temperature and energy gap Δ in terms of thin film thickness similar to the experimental results for thin film superconductor. However, by increasing number of Pb layers, the thin film goes to bulk . In contrast to the previous claimed constant value for the in bulk, we have found oscillation of this parameter in terms of thin film thickness similar to the oscillation. [Copyright &y& Elsevier]

A well-known pseudopotential is used to investigate the superconducting state parameters (SSP), viz., electronphonon coupling strength, Coulomb pseudopotential, transition temperature, isotope effect exponent and effective interaction strength for Ag x Zn1 − x and Ag x Al1 − x binary alloys theoretically for the first time. We have incorporated here five different types of the local field correction functions to show the effect of exchange and correlation on the aforesaid properties. Very strong influence of various exchange and correlation functions is concluded from the present study. Comparison with others such experimental values is encouraging, which confirms applicability of the model potential in explaining the superconducting state parameters of binary mixture. [ABSTRACT FROM AUTHOR]

The effect of spin crossovers in d n terms on the effective Hubbard parameter ( U eff) determining the gap between the lower and upper Hubbard bands is analyzed using a many-electron approach to describe the electron structure of Mott insulators. A new mechanism of the insulator-metal transition is established for d 5 ions, which is related to a decrease in U eff caused by spin crossover. For other ions, the U eff value is either independent of pressure ( d 2, d 4, d 7) or it exhibits nonmonotonic growth ( d 3, d 6, d 8). [ABSTRACT FROM AUTHOR]

The properties of Fermi surfaces and electron bands in electron-doped cuprates have been studied. The possible origins of a hole pocket in the nodal direction and a pseudogap at hot spots are discussed, including stripe phases and double bands in an antiferromagnetically correlated Fermi liquid. Within the framework of the mean field method, it is shown that both t-t′-t″-U Hubbard model solutions with a homogeneous antifer-romagnetic spin structure and those with a diagonal stripe structure can reproduce the fragmentar character of the Fermi surface. The appearance of hole pockets in various structures is related either to states in the lower Hubbard band or to states localized on domain walls. The behavior of a gap at the leading edge of the energy distribution of photoelectrons and its dependence on oxygen removal in the course of annealing are considered. [ABSTRACT FROM AUTHOR]

The theoretical computation of the superconducting state parameters (SSP) viz; electron-phonon coupling strength λ, Coulomb pseudopotential μ *, transition temperature T c , isotope effect exponent α and effective interaction strength N O V of some monovalent (Cu and Au), divalent (Ca, Sr, Ba, αHg, βHg and Ra) and polyvalent (Lu, Rh, Sc, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Ac, Th, Hf, Ru, Os, Ir, V, Ta, Pa, Cr, Mo, U, Re, Np and Pu) amorphous metals based on the different groups of the periodic table have been carried out for the first time using the well known Ashcroft’s empty core (EMC) model pseudopotential. Herein, we have employed five different types of local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) to study the exchange and correlation effects on the present investigations. A very strong influence of all the exchange and correlation functions have been observed in the present study. Our results are in fair agreement with documented theoretical as well as experimental data. A strong dependency of the SSP of amorphous metals on the valency Z was found. [ABSTRACT FROM AUTHOR]

The theoretical investigations of the superconducting state parameters (SSP) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T C , isotope effect exponent α and effective interaction strength N O V of six binary La100- C Ga C ( C = 16, 20, 22, 24, 26 and 28 at. %) metallic glasses have been reported using Ashcroft’s empty core (EMC) model potential for the first time. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used in the present investigation to study the screening influence on the aforesaid properties. It is observed that the electron-phonon coupling strength λ and the transition temperature T C are quite sensitive to the selection of the local field correction functions, whereas the Coulomb pseudopotential μ*, isotope effect exponent α and effective interaction strength N O V show weak dependences on the local field correction functions. The T C obtained from H-local field correction function are found in qualitative agreement with available experimental data and show almost linear nature with the concentration ( C) of ‘Ga’ element. A linear T C equation is proposed by fitting the present outcomes for H-local field correction function, which is in conformity with other results for the experimental data. Also, the present results are found to be in qualitative agreement with other such earlier reported data, which confirms the superconducting phase in the metallic glasses. [ABSTRACT FROM AUTHOR]