Model Of High-temperature Superconductivity Research Articles

Real space inhomogeneities in high temperature superconductors: the perspective of thetwo-component model

The two-component model of high temperature superconductors in its real space versionhas been solved using Bogoliubov–de Gennes equations. The disorder in the electron andboson subsystem has been taken into account. It strongly modifies the superconductingproperties and leads to local variations of the gap parameter and density of states. Theassumption that the impurities mainly modify boson energies offers a naturalexplanation of the puzzling positive correlation between the positions of impurities andthe values of the order parameter found in the scanning tunneling microscopyexperiments.

Ripple current loss measurement with DC bias condition for high temperature superconducting power cable using calorimetry method

The authors calculated the loss of the High Temperature Superconducting (HTS) model cable using Norris ellipse formula, and measured the loss of the model cable experimentally. Two kinds of measuring method are used. One is the electrical method, and the other is the calorimetric method. The electrical method can be used only in AC condition. But the calorimetric method can be used in both AC and DC bias conditions. In order to propose an effective measuring approach for Ripple Dependent Loss (RDL) under DC bias condition using the calorimetric method, Bismuth Strontium Calcium Copper Oxide (BSCCO) wires were used for the HTS model cable, and the SUS tapes were used as a heating tape to make the same pattern of the temperature profiles as in the electrical method without the transport current. The temperature-loss relations were obtained by the electrical method, and then applied to the calorimetric method by which the RDL under DC bias condition was well estimated.

Characterization of the Superconducting to Normal Transition of HTS Coated Conductors by Fast Pulsed Current Measurements

In this paper, we present measurements of the superconducting to normal transition (extended <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> curves) of commercial coated conductors with and without stabilizing copper layer. These measurements were realized with a custom developed pulsed current measurement (PCM) system. Currents between 5 to 10 times the critical current of commercial wires (up to 1000 A) can be applied for a period as short as 50-80 mus, limiting the energy released in the sample to a fraction of a Joule. For such short pulses, the temperature rise in the sample is relatively small, which allows characterizing the electrical resistivity of high temperature superconductors (HTS) at high current densities and electric fields. The data obtained will be used to develop more accurate models of HTS in the over-critical current regime, which is major issue for allowing the development of quality simulation tools for optimizing the design of superconducting fault current limiters. The PCM technique is also a very powerful tool for investigating the transient thermal behavior of coated conductors, whose better understanding is required in order to devise reliable fault current limiters based on this technology. So far, the measurements have been restricted to 77 K and self-field, but further works will extend the range of measurements to higher fields and temperatures.

The Role of In-plane Oxygens in Optimally Doped NCCO

Within the framework of the three-band Hubbard (Emery) model of high-temperature superconductors, we find a new physical regime for the electron-doped compound NCCO at optimal doping. It is characterized by a strong renormalization of the bare copper–oxygen hopping, similar to the one on the hole-doped side, and a low value of the effective copper–oxygen splitting. After a zeroth order fit of the Fermi surface is achieved, ARPES data are reproduced by adding a simple phenomenological weak-coupling antiferromagnetic perturbation of the conduction electrons. The experimental situation corresponds to a pseudogap across the whole Fermi surface. The wide dispersive features in NCCO ARPES spectra are analogous to the hump in BSCCO. The oxygen degree of freedom dominates the band structure of both NCCO and BSCCO, however, in NCCO this cannot be demonstrated without simultaneously accounting for the antiferromagnetic scattering. The difference in Tc of the two compounds may be related in part to the difference in the calculated densities of states.

Fluctuating Cu–O–Cu bond model of high-temperature superconductivity

Twenty years of research have yet to produce a consensus on the origin of high-temperature superconductivity (HTS). However, several generic characteristics of the copper oxide superconductors have emerged as the essential ingredients of and/or constraints on any viable microscopic model of HTS. Besides a critical temperature Tc of the order of 100 K, they include a d-wave superconducting gap with Fermi liquid nodal excitations, a pseudogap with d-symmetry and the characteristic temperature scale T*, an anomalous doping-dependent oxygen isotope shift, nanometre-scale gap inhomogeneity and so on. The isotope shift implies a key role for oxygen vibrations, but conventional Bardeen–Cooper–Schrieffer single-phonon coupling is essentially forbidden by symmetry and by the on-site Coulomb interaction U. Here we invoke the nonlinear modulation of the Cu–Cu bond by planar oxygen vibrations. The Fermi liquid nature of the d-wave superconducting ground state supports a weak-coupling treatment of this modulation. The dominant fluctuations are manifested in a pattern of oxygen vibrational square amplitudes with quadrupolar symmetry around a given Cu site. On the basis of such bond fluctuations, both dynamic and static, we can understand the salient features of HTS.

Peculiarities of pair interaction in the four-band Hubbard model

We consider the four-band model of high-temperature superconductors with copper \(3d_{x^2 - y^2 } \) and 4s electrons and oxygen 2px and 2py electrons. We assume that the Coulomb repulsion between the copper 3d electrons is strong. We diagonalize the oxygen subsystem and pass to the Wannier representation for the CuO2 cluster. We study the properties of a local cell model and find its low-energy states such as the one-particle doublet and the two-particle singlet and triplet states. We consider the cell fermion delocalization because of their tunneling between the copper sites. We obtain a system of Dyson-type equations for renormalized one-particle Green’s functions and discuss whether it can be simplified.

Superlight High Tc Bipolarons in Cuprates

Most of the proposed models of high-temperature superconductivity (HTSC) are based on the short-range electron-electron correlations or/and on a short-range electron-phonon interaction. However, in the cuprates the screening is poor due to a low carrier density, layered crystal structure, and high ionicity of the lattice. We develop further the bipolaron model of HTSC, which explicitly takes into account the long-range origin of all interactions. The long-range electron-phonon (Fröhlich) interaction binds carriers into real space pairs (small bipolarons) with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repulsion keeps them from forming larger clusters. The model has explained many key features of cuprates. It is shown here that real-space pairing takes place in many cuprates at variance with some (incorrect) criteria of the BCS-BEC crossover.

The Fröhlich-Coulomb model ofhigh-temperature superconductivity and charge segregation inthe cuprates

We introduce a generic Fröhlich-Coulomb model of the oxides,which also includes infinite on-site (Hubbard) repulsion, anddescribe a simple analytical method of solving the multi-polaronproblem in complex lattice structures. Two particular lattices,a zigzag ladder and a perovskite layer, are studied. We findthat, depending on the relative strength of the Fröhlich andCoulomb interactions, these systems are either polaronic Fermi(or Luttinger) liquids, bipolaronic superconductors, or charge-segregated insulators. In the superconducting phase the carriersare superlight mobile bipolarons. The model describes keyfeatures of the cuprates such as their Tc-values, the isotopeeffects, the normal-state diamagnetism, the pseudogap, and spectralfunctions measured in tunnelling and photoemission. We arguethat a low Fermi energy and strong coupling of carriers withhigh-frequency phonons is the cause of high criticaltemperatures in novel superconductors.

Universality of the intermediate phase and high-temperature superconductivity in C 60

With the discovery of Tc s over 100K, continuously biased hole-doped semiconductive C60+y has joined the family of high-temperature superconductors (HTSC) previously dominated by the ceramic cuprates. The phase diagrams of these two quite different materials are remarkably similar, despite the absence from undoped C60+y of magnetic order. This supports percolative models of HTSC based on strong electron-phonon interactions. These models explain observed fine structure in the C60+y phase diagrams, and furthermore suggest novel paths to still higher Tc s.

Universality of the intermediate phase and high-temperature superconductivity in C60

Abstract With the discovery of T c values over 100 K, continuously biased hole-doped semiconductive C+y 60 has joined the family of high-temperature superconductors (HTSCs) previously dominated by the ceramic cuprates. The phase diagrams of these two quite different materials are remarkably similar, despite the absence from undoped C+y 60 of magnetic order. This supports percolative models of HTSCs based on strong electron-phonon interactions. These models explain observed fine structure in the C+y 60 phase diagrams, and furthermore suggest novel paths to still higher T c values.

THE PHASE SEPARATION EFFECTS IN A PSEUDOSPIN-ELECTRON MODEL

The two-sublattice pseudospin-electron model of high temperature superconductors is studied with respect to phase separations. Such a model can be used for the description of dielectric properties of YBaCuO-type crystals along c -axis (the pseudospins represent an anharmonic motions of apical oxygen O4). The model is treated within different approximations. It is shown that the model has got phase transitions to the phase with pseudospin ordering as well as phase separation. The effect of separation on the appearance of the ordered state is discussed.

Superlight Bipolarons Explain the High Superconducting Temperature

Over the last decade, several competing models of high-temperature superconductivity (HTSC) were proposed, none of which have succeeded to explain high values of the superconducting critical temperature T c without adjustable parameters. Most of the proposed models are based on the short-range electron-electron interaction or/and on a short-range electron-phonon interaction. Here we propose a model of HTSC which explicitly takes into account the long-range origin of both types of interaction. Remarkably, the long-range electron-phonon interaction binds carriers into superlight real-space pairs-bipolarons-with a relatively low mass but sufficient binding energy. The model quantitatively explains high T c values without any fitting parameters and other key features of the cuprates.

COOPER–PAIR SUPERFLUID STATE IN HIGH-TEMPERATURE SUPERCONDUCTORS

We propose a lattice model of high-temperature superconductors based on their intrinsic granular-like structure. Due to the shortness of the coherent length in high-temperature superconductors, the superconducting order parameter can be well defined locally only in small regions, in which the number and the phase of the Cooper–pairs will experience strong quantum fluctuations. The "superconducting order parameter" in terms of the local number and phase operators of the Cooper–pairs is then regarded as an effective quantized bosonic field described by a discretized Ginzberg–Landau-type effective Hamiltonian, first proposed for granular superconductors and recently for high-temperature superconductors. The effective Hamiltonian also describes the quantum fluctuations of the superconducting order parameter. The global superconducting order sets in when the Cooper–pairs are in the superfluid state. This is in contrast to the previous picture, in which the phase coherence drives the system into the superconducting state when only the phase fluctuations of the superconducting order parameter are considered.

The neutron peak in the extended saddle point model of high temperature superconductors

An explanation is proposed for the maximum at 41 meV in the inelastic spin-flip neutron scattering from YBa 2Cu 3O 7− δ , based on the extended saddle point model developed by the author in his previous works. It is shown that, for appearance of the maximum in the imaginary part of the spin susceptibility, close proximity of the Fermi energy to the extended saddle point is necessary. The energy of the maximum is then close to 2 Δ max, in agreement with experiment. Theoretical and experimental evidence concerning the energy of the extended saddle point (flat region) is discussed. Different limiting cases are calculated. A proof is given within the present model that interaction in the final state is small, and hence no collective modes are formed. A general discussion of the experimental situation is presented.

Empirical model of the microwave properties of high-temperature superconductors

This paper presents simple correct models of high-temperature superconductor (HTS) film parameters at microwave frequencies. The models are based on the enhanced two-fluid model of a superconductor. The quasi-particle scattering and peculiarities of the normal conductivity of the HTS at microwaves, including residual resistance of a material, are taken into account. The difference between the known Gorter and Casimir two-fluid model of low-temperature superconductor and the enhanced two-fluid model of HTS is proven. A simple quasi-static model of current distribution across the microstrip line and coplanar waveguide is used for a simulation of a contribution of the superconductor transport processes into impedance per unit length of the transmission lines considered. The models developed were applied to a simulation of microstrip line and coplanar waveguide resonators. Good agreement of simulated results and measurements in a wide temperature range has been demonstrated. The model presented can be considered as a starting point for the formation of the computer-aided design (CAD) package of HTS microwave components.

Neutron peak in the extended-saddle-point model of high-temperature superconductors

An explanation is proposed of the maximum at 41 meV in the inelastic spin-flip neutron scattering from YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, based on the extended-saddle-point model developed by the author in his previous works. It is shown that for appearance of the maximum in the imaginary part of the spin susceptibility a close proximity of the Fermi energy to the extended saddle point is necessary. The energy of the maximum is then close to 2{Delta}{sub max} in agreement with experiment. Theoretical and experimental evidence concerning the energy of the extended saddle point (flat region) is discussed. Different limiting cases are calculated. A proof is given within the present model that interaction in the final state is small, and hence, no collective modes are formed. A general discussion of the experimental situation is presented. {copyright} {ital 1998} {ital The American Physical Society}

The Neutron Peak in the Interlayer Tunneling Model of High Temperature Superconductors

Recent neutron scattering experiments in YBCO exhibit an unusual magnetic peak that appears only below the superconducting transition temperature. The experimental observations are explained within the context of the interlayer tunneling theory of high temperature superconductors.

Novel 1/N Expansion for Self-Energy and Correlation Functions of the Hubbard Model

The novel slave-free (i.e. without slave bosons and fermions) approach to the problem of strong correlations is used in studying the Hubbard (t-J) model, where the self-energy is expressed in terms of charge and spin vertex functions of Hubbard operators. A systematic 1/N expansion (N is the number of spin components) of the self-energy is carried out. In O(1) it gives the same quasiparticle spectrum as the slave boson theory. The next order O(1/N) of the self-energy Σ(k, ω) is calculated exactly by solving analytically the integral equations for the charge and spin vertices. As an example for the feasibility of the proposed method explicit results for Σ(k, ω) are given in the case J=0. Dealing only with observable quantities our method is transparent and easily applicable to models which exclude double occupancy on lattice sites, such as the Anderson model and the Emery model for high temperature superconductors.

Magnetic and pair correlations of the Hubbard model with next-nearest-neighbor hopping.

A combination of analytical approaches and quantum Monte Carlo simulations is used to study both magnetic and pairing correlations for a version of the Hubbard model that includes second-neighbor hopping $t^{\prime }=-0.35t$ as a model for high-temperature superconductors. Magnetic properties are analyzed using the Two-Particle Self-Consistent approach. The maximum in magnetic susceptibility as a function of doping appears both at finite $% t^{\prime }$ and at $t^{\prime }=0$ but for two totally different physical reasons. When $t^{\prime }=0$, it is induced by antiferromagnetic correlations while at $t^{\prime }=-0.35t$ it is a band structure effect amplified by interactions. Finally, pairing fluctuations are compared with $% T $-matrix results to disentangle the effects of van Hove singularity and of nesting on superconducting correlations. The addition of antiferromagnetic fluctuations increases slightly the $d$-wave superconducting correlations despite the presence of a van Hove singularity which tends to decrease them in the repulsive model. Some aspects of the phase diagram and some subtleties of finite-size scaling in Monte Carlo simulations, such as inverted finite-size dependence, are also discussed.

Mechanisms and models of high temperature superconductors

Mechanisms and models of high temperature superconductors are discussed with an emphasis on a selfconsistent description both normal and superconducting states in these materials. It is shown that there is a strong support for a Fermi-liquid based approach to these systems both from theoretical investigations and experimental evidences. Moreover it is claimed that the electronic properties of these materials at moderate and high energies of excitations can be well described in terms of the usual band - structure approach like for conventional metals. It is demonstrated that “noncanonical” Fermi-liquid behavior of these systems in a normal state can be easily explained in terms of the usual strong electron-phonon interaction. It is also shown that the same electron-phonon interaction can explain the values of T c and some peculiarities of superconducting state.