On the Role of Polarization of Oxygen Ions and Mechanism of Superconductivity in Cuprate Superconductors

A new approach to the valence electron state in cuprate superconductors leading to a new mechanism of high-temperature superconductivity is considered. The approach takes into account specific features of the outer electron shells of the compound-forming elements and polarization of anions by cations. The polarization of anions by copper cations results in sharing of a valence electron by every two neighboring ions, which gives rise to a highly correlated state of the valence electrons and formation of a localized spin-ordered electron lattice responsible for dielectric and magnetic properties of an undoped material. It is also shown that asymmetric polarization of an anion by cations with inert gas shells transforms an unshared anion electron pair into a superconducting one by exciting it into a hybrid state. The superconducting state of the material itself results from formation and delocalization of one-dimensional Wigner crystals of the excited pairs.     

Theoretical Study of Interplay of Superconductivity and Ferromagnetism in Hybrid Rutheno–Cuprate Superconductors RuSr2RCu2O8

We consider the extended two-band s–f model with additional terms, describing intersite Cooper pairs’ interaction between 4f (5f) and conduction electrons. Following Green’s function technique and equation of motion method, self-consistent equations for superconducting order parameter (Δ) and magnetic order parameter (m _f ) are derived. The expressions for specific heat, density of states, and free energy are also derived. The theory has been applied to explain the coexistence of superconductivity and ferromagnetism in hybrid rutheno-cuprate superconductors RuSr₂RECu₂O₈ (RE = Gd, Eu). The theory shows that it is possible to become superconducting if the system is already ferromagnetic. A study of specific heat, density of states and free energy is also presented. The agreement between theory and experimental observations is quite satisfactory.      

Coexistence of Positional and Superfluid Orders in Imbalanced Fermi Condensates

The so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is a possible inhomogeneous superfluid phase in population-imbalanced two-component Fermi gas systems. Here, a novel phase created by a population-imbalance to be called a Fermion supersolid will be proposed. In a Fermi gas with a short-ranged repulsive interaction, a positional order, corresponding to the antiferromagnetic (AFM) one in the electronic systems, may occur, although the ordinary superfluid phase usually suppresses it in population-balanced two component Fermi systems. However, we show that the coexistence between this “AFM” order and the FFLO superfluid with s-wave pairing is rather favored by the population-imbalance. This coexisting phase is the s-wave analog of the coupled AFM and FFLO orders detected in the d-wave paired superconductor CeCoIn₅.     

Coexistence of Superconductivity and Ferromagnetic Phases in YBa2Cu3O7−δ Nanoparticles

High-temperature superconductor YBa₂Cu₃O_7?δ (YBCO) nanopowders were synthesized by the citrate-gel route, which is a modification of the sol-gel method. The fine powders were calcinated at 860 and 900 °C. They were of small size, in the range of 30–35 nm. X-ray diffraction (XRD) patterns verified production of the orthorhombic superconducting phase in all samples. Measuring the magnetic properties of these nanoparticles at room temperature, via a vibrating sample magnetometer (VSM), indicated ferromagnetism behavior in the YBa₂Cu₃O_7?δ nanoparticles. As the size of the nanoparticles decreased, the magnetic saturation of all samples increased. The development of the ferromagnetism effect was attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. Thus, in an innovative work, the produced samples were annealed at 700 °C for 5 h under 0.8–0.9 bar of air atmosphere. The results showed that a small increase in the nanoparticle size provided a dramatic increase of magnetic saturation in all samples. Thus, we can say that the annealing process at vacuum improves the ferromagnetic properties of YBCO nanoparticles.     

Cooperative Two-Quanta Phase Transitions in Quantum Optics and Superconductivity

The behavior of an electronic subsystem in strong interaction with phonon subsystem, or quantified electromagnetic field (QEF) is discussed. In this case the correlation effect between first and second order electron–phonon interaction (or atom–QEF interaction) takes place. It is shown that the temperature dependence of two-quanta exchange between Fermi sub-system and thermal reservoir gives a non-linear behavior of the order parameter in superconductivity and super-radiance, accompanied by an increase of the electron correlations with increasing temperature. The same effect is considered for two-quanta scattering processes, in which one quantum is absorbed and another is emitted. It is demonstrated, that the order parameter in such a system firstly increases with temperature achieving the maximal value. After that it decreases as in traditional phase transition effects.     

Hard-Wired Dopant Networks and the Prediction of High Transition Temperatures in Ceramic Superconductors

I review the multiple successes of the discrete hard-wired dopant network model ZZIP, and comment on the equally numerous failures of continuum models, in describing and predicting the properties of ceramic superconductors. The prediction of transition temperatures can be regarded in several ways, either as an exacting test of theory, or as a tool for identifying theoretical rules for defining new homology models. Popular “first principle” methods for predicting transition temperatures in conventional crystalline superconductors have failed for cuprate HTSC, as have parameterized models based on CuO₂ planes (with or without apical oxygen). Following a path suggested by Bayesian probability, we find that the glassy, self-organized dopant-network percolative model is so successful that it defines a new homology class appropriate to ceramic superconductors. The reasons for this success in an exponentially complex (Non-Polynomial Complete, NPC) problem are discussed, and a critical comparison is made with previous polynomial (PC) theories. The predictions are successful for the superfamily of all ceramics, including new non-cuprates based on FeAs in place of CuO₂.     

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

Standard models for simple metals and insulators often fail for systems based on elements with unstable d- or f-electron shells, where strong electronic correlations can generate new and unexpected states of matter. Such a scenario can often be induced when a magnetic phase transition is tuned to absolute zero temperature by an external control parameter such as chemical composition, pressure or magnetic field. At the resulting quantum critical point (QCP), emergent phenomena, such as unconventional superconductivity and novel magnetic phases are frequently observed. The temperature and energy dependences of the physical properties are also found to deviate from expectations for a simple Fermi liquid. This “non-Fermi-liquid” (NFL) behavior is commonly manifested as weak power laws and logarithmic divergences in the physical properties at low temperatures and is often found in a V-shaped region near a QCP, which has become the “classic” QCP phase diagram. However, there is also a growing number of materials where the NFL behavior either occurs far away from the QCP, within an ordered phase, or may not be associated with any putative QCP. Thus, after nearly 20 years of research, it remains unknown whether NFL physics is universal, or if a multitude of unique subclasses exist. In this article, we review research that has primarily been carried out in our laboratory on systems that exhibit NFL behavior that does not conform to the “classic” QCP scenario.     

Point-Contact Andreev-Reflection Spectroscopy in Fe-Based Superconductors: Multigap Superconductivity and Strong Electron–Boson Interaction

We performed point-contact Andreev-reflection measurements in Ba(Fe_1?x Co _x )₂As₂ single crystals (x=0.1, T _c =24.5?K) and SmFeAsO_1?x F _x polycrystals (x=0.2, T _c =52?K). The spectra indicate the presence of two superconducting gaps with no line nodes on the Fermi surface, but also feature additional structures related to the electron?Cboson interaction (EBI). From the spectra, it is possible to extract the characteristic energy ?? ₀ of the mediating boson. In Co-doped Ba-122, we obtain ?? ₀=12?meV that coincides with the spin-resonance energy observed in neutron-scattering experiments. In Sm-1111, ?? ₀=20?meV fulfils the relation ?? ₀=4.65k _B T _c inferred from neutron-scattering results on other Fe-based superconductors. The strong electron?Cboson coupling may also explain some anomalies in the PCAR conductance curves (e.g., the excess conductance at high energy) which sometimes prevent a good fit of the curves with models based on constant, BCS-like order parameters.     

Experimental Studies to Realize Josephson Junctions and Qubits in Cuprate and Fe-based Superconductors

Cuprate superconductors are very promising in terms of Josephson junction device because of the large energy scale of high-T _c superconductivity. In particular, fabrication of qubits attracts lots of attention because of its primary importance for future computer technology. We will present some of our recent activities pointing to this purpose. (1) We succeeded in fabricating Josephson junction of cuprate without making any extra oxide barrier layer, rather putting Fe islands on the small strip of cuprate superconductors. This is very promising, since the fabrication of good Josephson junction was unsuccessful so far. Together with the introduction of the data, we discuss the possible mechanism of the weak-link fabrication in this structure. (2) We investigated the switching events in the I–V characteristics of the intrinsic Josephson junctions of Bi-cuprate superconductor, where macroscopic quantum tunneling (MQT) observation is well established recently. In addition to confirm the MQT for the first switching at 1 K in the multiple-branched current–voltage characteristics, we found that the temperature independence of the switching distribution for the second switching up to higher temperatures (10 K) is not due to the trivial Joule heating. We discuss the mechanism of the phenomena, including the possibility of MQT. (3) New Fe-based superconductors are also promising in terms of the application of superconductivity, since the anisotropy looks rather weak, in contrast to cuprates. We will introduce our trial to fabricate epitaxial thin films as the initial step to fabricate Josephson junction of this material.     

First-order Field-Induced Magnetization Processes and Magnetostriction in Tb2Co12Fe5

We report the results of magnetization and magnetostriction measurements performed by pulsed-magnetic-field techniques on the intermetallic compound Tb₂Co₁₂Fe₅. The temperature dependence of the critical field at which the first-order magnetization process takes place was measured by a singular point detection technique on aligned powders. Magnetostriction measurements were performed by a strain-gauge-based technique at different temperatures. The obtained results are compared with observations recently made for the closely related compound Pr₂Co₁₂Fe₅, allowing to clarify the crucial role of both the rare-earth and the transition-metal sublattices in the peculiar behaviour of the latter.     

Effect of Compaction Pressure on Structural and Superconducting Properties of Bi-2223 Superconductors

In this work, effects of compaction pressure on the structural and superconducting properties of BSCCO ceramic superconductors were investigated. The study was carried out on two systems which were, System I: Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _y and System II: Bi_1.6Pb_0.3Ag_0.1Sr₂Ca₂Cu₃O _y , respectively. Ceramic powders were prepared by conventional solid-state reaction method and sintered at 850°C after compaction at five different pressures in the 150–750 MPa range. Critical temperatures of samples were determined by resistivity-temperature determinations made by four-point probe method in liquid nitrogen conditions. XRD analysis was conducted by powder X-ray diffraction method. Morphology of the grains present in the samples were determined by using scanning electron microscope (SEM) photographs at 2 K× and 2.5 K× magnifications for System I and System II, respectively. Sintered densities of the superconducting ceramics were measured by Archimedes water displacement method and unit cell parameters were additionally obtained from XRD data. T _c values for System I was determined to be in the 109–115 K with sample D having the highest T _c of 115 K while T _c varied in the 104–109 K range and sample B had the highest T _c value of 109 K for System II. The transition width, which is a sign of the purity of the samples, was determined to be narrow for both systems. The data obtained from X-ray diffraction measurements have shown that 2223 high-T _c phase was dominant in both systems. The determination of the optimum pellet compaction pressure for BSCCO ceramic superconductors was the main purpose of this work. The results of this work indicated that compaction at around 450 MPa improves the superconducting and structural properties of the BSCCO ceramic superconductors.     

Numerical Calculation of Trapped Magnetic Field for Single and Multiple Bulk Superconductors

In this work, the trapped magnetic field was calculated using the sand-pile model and Biot–Savart law. The numerical simulation of the trapped magnetic field was performed varying the sample shape and dimensions, distance between the sample surface and observation point. In addition, the magnetic field characteristics of superconducting bulk arrangement in an array was determined. The trapped magnetic field was found to be depending on the sample shape and dimensions and distance between the sample and observation point. It is observed that, when the distance from the surface is larger, the value of the trapped magnetic field decreases, but the variation of the trapped magnetic field becomes small due to averaging the distribution of the trapped magnetic field.     

Phase Diagram of Vortices in Type-II Superconductors with Periodic Square Columnar Pins

The phase diagram of a two-dimensional vortex system with periodic square columnar pins is studied. For the case of vortex number matching pinning number, we find two scenarios for the freezing transitions. For weak pinning where vortex–vortex interactions dominate, the vortex liquid is frozen into triangular lattice via a first-order phase transition. For strong pinning, the vortex liquid is frozen into a square lattice with all vortices trapped by pins via a second-order phase transition.     

Effect of Boron Substitution for Bi on Superconductivity of the Bi-2212 Phase in the Bi-Sr-Ca-Cu-O System

Possibility of boron substitution for Bi and the substitution effect on superconductivity is investigated for the Bi-2212 phase of Bi-Sr-Ca-Cu-O. From X-ray diffraction study, it is found that samples in the (Bi_2?x B _x )Sr₂CaCu₂O _z system are mainly of the single 2212 phase in a composition range of 0.0≤x≤0.6, and both of the lattice parameters a and c change with increasing x up to 0.6. From measurements of the magnetic susceptibility and the electrical resistivity, the superconducting transition temperature is found to increase up to 0.6 with increasing x. These results are considered to show that boron is substitutable for Bi up to x=0.6 in the (Bi_2?x B _x )Sr₂CaCu₂O _z system and that the boron substitution causes the number of hole-carriers to decrease in this system.     

Doping Dependence of Magnetic Field Penetration Depth in Kinetic Energy Driven Cuprate Superconductors

The doping and temperature dependence of the magnetic field penetration depth in cuprate superconductors is studied based on the kinetic energy driven superconducting mechanism. It is shown that the magnetic field penetration depth shows a crossover from the linear temperature dependence at low temperatures to a nonlinear one in the extremely low temperatures.     

Magnetic Rare-Earth Impurity Resonance Bound States in Iron-Based Superconductors

Starting from an Anderson model for two-orbital model of electron and hole conduction bands which hybridized with the localized impurity spin, we investigate the effect of magnetic impurities on the local quasiparticle density of states (LDOS) in iron-based superconductors. We consider extended s-wave (s ⁺⁻) superconducting gap symmetry with higher harmonic correction. The impurity-induced bound states are a probe for the nodal structure of the extended s-wave symmetry in ferropnictides.     

Electron-Conduction Mechanism and Specific Heat Above Transition Temperature in LaFeAsO and BaFe2As2 Superconductors

The ionization energy theory is used to calculate the evolution of the resistivity and specific heat curves with respect to different doping elements in the recently discovered superconducting pnictide materials. Electron-conduction mechanism in the pnictides above the structural transition temperature is explained unambiguously, which is also consistent with other strongly correlated materials, such as cuprates, manganites, titanates and magnetic semiconductors.