Double Correlations in HTSC

The Nambu–Gorkov generalized Hartree–Fock theory of superconductivity is further generalized to treat simultaneously two kinds of correlations: the correlations that lead to superconductivity, as well as the correlations that produce the pseudogap in the normal state. The treatment of these simultaneous double correlations is done by enlarging the dimension of the propagator and self-energy matrices, from two to four. The off-diagonal self-energies and the DOS are calculated. The results are compatible with tunneling and PES experiments.     

D-Wave Superconductivity and Coulomb Correlations in the Two-Dimensional Hubbard Lattice

We considered anisotropic superconductivity within the two-dimensional Hubbard model extended by pairing correlations originating from the electron–phonon interaction. To discuss the onset of superconductivity close to the insulator–metal transition, we used the Hubbard I approximation to account for the formation of the insulating gap and see the role of Coulomb correlations for superconducting pairing. It has been shown that the Hubbard I approximation reflects effective pairing interactions genuine for correlated electron systems and leads to the stabilization of the superconductivity in the d-wave channel. One may expect the cooperation of phonon-free and phonon-induced mechanism in the formation of thed-wave superconducting state.     

Anisotropic superconductivity in the two-dimensional Hubbard model

We address the problem of anisotropic superconductivity in the two-dimensional Hubbard model. The Eliashberg equations have been generalized to the case which accounts for the anisotropy of the order parameter. Strong local correlations are treated within the mean field slave boson approximation. The superconducting transition temperatureT _c is evaluated as a function of the occupation number. Our results indicate that thed-wave state is the most likely channel for superconductivity for small concentration of holes. We have also derived an approximate analytical formula forT _c valid for any value of the occupation number. In addition, the influence of strong correlations on the electron-phonon coupling function is also discussed.     

Analysis of Response Functions of a Spin-Polarized Electron Gas

We present a spin analysis of local-field corrections and various susceptibility functions of a spin-polarized electron gas (SPEG). With a use of spin-resolved pair correlation functions of the SPEG, including the carrier correlations, we evaluate the local-field correction within a generalized random-phase approximation (RPA) and examine spin-polarization dependences of various susceptibility functions generalized in terms of spin-dependent local-field corrections. A pronounced maximum in spin-resolved local-field correction is observed and the location of the peaks are found to depend strongly on the values of spin polarization. For a system with vanishing spin polarization, the charge–spin mixed susceptibilities vanish. However, in the SPEG of finite spin polarization, the mixed susceptibilities become finite and are as important as the usual charge–charge, and spin–spin susceptibilities.     

A New Approach to Transport Theory of High Tc Bismuth-Based Ceramic Superconductors

A new transport theory for high temperature superconductors is propos#ed and supported with extensive calculations.The Cooper-pairs while remaining on a pseudo Fermi surface behave like weak Fermions. When the Cooper-pairs become free they behave as spinless bosons. The polarons and bipolarons are highly localized quantum states with virtually no gap existing in between them and are considered as bosons with nondegenerate spining. The binding energies of Cooper-pairs both in a weak Fermion system and for a spinless boson are calculated.A kind of a semimetallic softening transition is responsible for high temperature superconductivity and is temperature independent. There is no electron–phonon coupling for high temperature superconductors. High temperature ceramic superconductors before the onset of superconductivity are found to be Mott–Hubbard dielectric insulating materials.     

Flux Phase and Its Competition with Superconductivity in the t–J Model

The phase diagram of the t–J model is investigated within the X-operator formalism using the Baym–Kadanoff theory and a 1/N expansion. In this way, no auxiliary fields are introduced. For finite Coulomb repulsion, the system shows a strong competition between d-wave superconductivity and d-wave flux phase, leading to a strong suppression of superconductivity in the flux state. The underdoped region is characterized by flux order in the normal state, with a d-wave gap in the excitation spectrum, and by coexistence of both superconductivity and flux phases below the superconducting transition temperature T _c. The balance between the two phases is determined by the short-range Coulomb repulsion, while the long-range part of Coulomb interaction prevents phase separation and leads to incommensurate charge-density-wave state far away from the superconducting region.     

Very low-temperature search for superconductivity in semiconducting KTaO3

A series of attempts have been made to detect the presence of superconductivity in semiconducting potassium tantalate (KTaO₃) single crystals. Semiconducting potassium tantalate has a number of properties in common with semiconducting SrTiO₃, which is superconducting below 0.3 K, with a critical temperatureT _c that varies as a function of the carrier concentration. Both KTaO₃ and SrTiO₃ are perovskite-structure oxides and both materials are so-called incipient ferroelectrics that are characterized by high dielectric constants at low temperature. These common properties suggest that superconductivity might also be observed in semiconducting potassium tantalate. In the temperature ange from 0.01 to 4.0 K, however, no evidence was found for superconductivity in KTaO₃ in the presence of magnetic fields of 10^–5–10^–4 T (i.e., 0.1–1 Oe). Below 1.5 K, the search for superconductivity in KTaO₃ was carried out using a³He-⁴He dilution refrigerator equipped with a SQUID magnetometer and an ac magnetometer. The system response was verified by measuring the paramagnetic susceptibility of Dy₂O₃-doped KTaO₃. The failure to observe superconductivity in KTaO₃, while SrTiO₃ is an established superconducting material, may be related to the fact that the latter substance assumes a tetragonal symmetry phase at 105 K, while KTaO₃ remains cubic to low temperatures.Operated by Union Carbide Corporation for the U.S. Department of Energy under contract w-7405-eng-26.     

Phonon Mechanism of High-Tc Superconductivity

For a long time the majority view has been that phonons are irrelevant to the mechanism of high-temperature superconductivity. However, recent experimental results, including the neutron inelastic scattering measurements reviewed here, seriously challenge this view. We point out that the electron–phonon coupling in the cuprates can be substantially different from that in simple metals because of the covalency and strong electron correlation. In particular certain phonon branches induce substantial intersite charge transfer that can result in a negative electronic dielectric susceptibility. Such a strong electron–phonon coupling of certain modes could form the basis for the phonon mechanism of superconductivity in the cuprates.     

Rare-Earths as Probes of High-Temperature Superconductivity

Using only two principles: (i) high-temperature superconductivity requires hypercharged oxygen, and (ii) the superconducting condensates are located in those parts of the crystal structures where they are unaffected by magnetic pair breaking, we are able to explain why certain rare-earth ions R are compatible with superconductivity and others are not, in the compounds RBa₂Cu₃O₇, RBa₂Cu₄O₈, RBa₂Cu₂NbO₈, R_{2 – z} Ce _z CuO₄, and R_{2 – z} Ce _z Sr₂Cu₂NbO₁₀. Various defects are proposed as having central roles in the superconductivity or the suppression of superconductivity in these compounds. Many experiments for testing this physical picture are suggested.     

The Mechanism of High-Tc Superconductivity: “Nonlinear” Superconductivity

The main purpose of the paper is to present an overview of the current situation in the development of the understanding of the mechanism of high-T _c superconductivity which arises due to moderately strong, nonlinear electron–phonon interactions and due to spin fluctuations. The former are responsible for electron pairing, and the latter mediate the phase coherence. In addition, a key experiment for superconductivity in cuprates is proposed.     

Superconductivity and magnetism in 11-structure iron chalcogenides in relation to the iron pnictides

This is a review of the magnetism and superconductivity in ‘11’-type Fe chalcogenides, as compared to the Fe-pnictide materials. The chalcogenides show many differences from the pnictides, as might be anticipated from their very varied chemistries. These differences include stronger renormalizations that might imply stronger correlation effects as well as different magnetic ordering patterns. Nevertheless the superconducting state and mechanism for superconductivity are apparently similar for the two classes of materials. Unanswered questions and challenges to theory are emphasized.     

Effects of Non-Magnetic Impurities on Cu-Spin Dynamics and Superconductivity in La2−x Sr x Cu1−y Zn y O4 Around x = 0.115

Muon-spin-relaxation (SR) and magnetic susceptibility measurements have been carried out in La_2–x Sr _x Cu_1–y Zn _y O₄ with x=0.13 changing y finely up to 0.10, with the aim at clarifying effects of the non-magnetic impurity Zn on the Cu-spin dynamics and superconductivity. It has been found that the y dependence of the volume fraction of the superconducting region estimated from susceptibility measurements highly correlates with that of the magnetically ordered region estimated from the SR results. The rapid decrease in the superconducting region and the rapid increase in the magnetically ordered region by the doping of a small amount of Zn can be interpreted as follows; Zn pins the dynamical spin correlation or the dynamical stripe correlations and hence the superconductivity is destroyed around itself.     

Synergetic Phonon–Spin–Charge Mechanism of High-Temperature Superconductivity

For a long time the majority opinion in the field of high-temperature superconductivity (HTSC) has been that it is purely an electronic phenomenon involving spin, and could be explained by a t–J Hamiltonian. Phonons and local distortion were regarded as irrelevant or harmful to HTSC. However, various experimental results indicate strong phonon involvement and ubiquitous presence of local spin–charge–lattice inhomogeneity. We suggest that the electron–phonon (e–p) coupling in the cuprate is unconventional, and a synergetic coupling of spin, charge, and phonon could explain the HTSC phenomenon. In our view the spin–charge–lattice inhomogeneity is a signature of such a coupling and an important component of the HTSC mechanism.     

Magnetic superconductors: A review

The recent discovery of superconductivity in some rare earth ternary and actinide compounds has led to considerable experimental and theoretical activity in the field of magnetic superconductors. The ternary compounds ErRh₄B₄ and HoMo₆S₈ exhibit reentrant superconductivity. Experimental results indicate that superconductivity and long-range ferromagnetic order coexist in a very narrow range of temperature in these two systems. Superconductivity and antiferromagnetic order truly coexist in several rare earth ternary compounds. The most important result is the anomalous behavior of the upper critical field in the vicinity of the Néel temperature. NdRh₄B₄ displays two antiferromagnetic transitions below the superconducting transition. The pseudoternary series Eu _x Sn_1–x Mo₆S₈ containing small amounts of Br or Se exhibits the novel phenomenon of magnetic field-induced superconductivity. The compound Y₉Co₇ exhibits very weak itinerant ferromagnetic superconductivity. Several cerium- and uranium-based intermetallic compounds, usually known as heavy-electron metals, have been found to exhibit heavy-fermion superconductivity, characterized by a specific heat at low temperature that is two to three orders of magnitude larger than in ordinary transition metals. Several phenomenological and microscopic theories have appeared to explain the observed exciting properties of these magnetic superconductors. A brief survey of experimental and theoretical activity in this field is presented.     

Thermodynamic Properties of Ammonia–Water Mixtures for Power Cycles

Power cycles with ammonia–water mixtures as working fluids have been shown to reach higher thermal efficiencies than the traditional steam turbine (Rankine) cycle with water as the working fluid. Different correlations for the thermo-dynamic properties of ammonia–water mixtures have been used in studies of ammonia–water mixture cycles described in the literature. Four of these correlations are compared in this paper. The differences in thermal efficiencies for a bottoming Kalina cycle when these four property correlations are used are in the range 0.5 to 3.3%. The properties for saturated liquid and vapor according to three of the correlations and available experimental data are also compared at high pressures and temperatures [up to 20 MPa and 337°C (610 K)]. The difference in saturation temperature for the different correlations is up to 20%, and the difference in saturation enthalpy is as high as 100% when the pressure is 20 MPa.     

Screened Optical-Phonon Pairing Mechanism in Electron Doped Cuprates

An effective two-dimensional dynamic interaction is developed which incorporates screening of electrons by plasmons and by optical phonons to discuss the nature of the pairing mechanism leading to superconductivity in layered electron doped cuprates. The system is treated as an ionic solid containing layers of electrons as carriers and a model dielectric function is set up which fulfils the appropriate sum rules on the electronic and ionic polarizabilities. Estimate of the Coulomb pseudo-potential (*=0.24), describing the screening effects on superconductivity is due to reduced electron density and large value of the optical dielectric constant as well the effective mass of electrons. The electron–phonon coupling constant () is evaluated as 1.5 which infers strong strength of coupling. Following strong coupling theory, the superconducting transition temperature of optimally doped Nd–Ce–CuO is estimated as 30 K and the energy gap ratio is larger than the BCS value. The isotope exponent, coherence length and magnetic penetration depth are also estimated. The implications of the model and its analysis are discussed.     

Thermophysical Properties of Ammonia–Water Mixtures for Prediction of Heat Transfer Areas in Power Cycles

In power cycles using ammonia–water mixtures as the working fluid, several heat exchangers are used. The influence of different correlations for predicting thermophysical properties on the calculations of the size of the heat exchangers is presented. Different correlations for predicting both the thermodynamic and the transport properties are included. The use of different correlations for the thermodynamic properties gives a difference in the total heat exchanger area of 7%, but for individual heat exchangers, the difference is up to 24%. Different correlations for the mixture transport properties give differences in the predicted heat exchanger areas that are, at most, about 10% for the individual heat exchangers. The influence on the total heat exchanger area is not larger than 3%. A difference in the total heat exchanger area of 7% would probably correspond to less than 2% of the total cost for the process equipment. Experimental data and correlations developed for the ammonia–water mixture transport properties are very scarce. The evaporation and condensation processes involving ammonia–water mixtures are also not fully understood.     

P+ implantation and annealing effects on theT c in BiSrCaCuO films

TheT _c changes related to the microstructure as a function of annealing temperature for the BiSrCaCuO (BSCCO) film implanted with 170 keV P⁺ at two different doses were studied. The BSCCO films were prepared by d.c. sputtering on MgO substrates. For the film implanted at a dose of 5×10¹⁵ cm^–2 post-implantation annealing at 600–800°C enabled theT _cs of the film to be completely recovered. For the film implanted at a dose of 1.0×10¹⁷cm^–2 theT _cs were only partly recovered after 600°C annealing. On further annealing at 700°C the superconductivity of the film disappeared. TEM examination showed that significant amount of CaP, Ca₃P₂, and some unknown phases were formed. It is considered that the significant amounts of these phases formed during post-implantation annealing renders the recovery of the superconductivity of the P⁺-implanted BSCCO film difficult.     

Crystal Structure and Superconductivity of MgB2 Thin Films Grown on Various Oxide Substrates

The crystal structure and superconductivity of MgB₂ thin films grown on various oxide substrates were investigated by X-ray diffraction and resistance measurement. The films were prepared by a two-step method, in which precursors B films were annealed in Mg vapor at 900C. The X-ray diffraction shows that the MgB₂ films grown on C–AL₂O₃, R–AL₂O₃, and MgO (001) are c-axis oriented while the films grown on SrTiO₃ (001), LaAlO₃ (001), and ZrO₂ (001) are aligned with the (101) direction normal to the substrate planes. All the grown films show superconductivity and their transition temperature varies with the substrates in the range of 34–39 K. We think that the transition temperature variation is probably due to the lattice matching between the film and the substrate, as well as the interdiffusion at the film/substrate interface. The experimental results suggest that if there is no severe interdiffusion at the film/substrate interface in the high temperature annealing process, more substrates could be used for the growth of MgB₂ films using the two-step method.     

Three-Parameter Corresponding-States Correlations for Joule–Thomson Inversion Curves

In the present work, the Lee–Kesler (LK) and Boublík–Alder–Chen–Kreglewski (BACK) equations of state were used to compute Joule–Thomson inversion curves for nonsimple fluids. Comparisons with available data showed that predictions were quite reliable and could be used in place of experimental values. Two sets of corresponding-states correlations were developed, giving reduced inversion pressures and densities as functions of reduced temperature and acentric factor. The LK-based correlations are valid for T _r4.0, giving an average absolute deviation (AAD) of 4.5% for pressures. The BACK-based correlations are valid up to the maximum inversion temperature and give a 6.7% AAD for pressures. Respective volume AADs are 12.0 and 8.0% in the high-density region.