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    Possible role of the alkaline-earth ions in high-temperature superconductivity
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    Abstract:
    In high-temperature superconductors with ${T}_{c}$ exceeding the temperature of liquid nitrogen, there is no conventional isotope shift. This suggests an unconventional pairing mechanism, but we notice a systematic upward "polarizability shift" in ${T}_{c}$. The shift in ${T}_{c}$ of about 20% occurs in the 1:2:3, 2:2:1:2, and 2:2:2:3 high-temperature superconducting compounds when the more polarizable Ba replaces Sr. A semiquantitative calculation enables us to interpret our result in the framework of the two-hole resonance mechanism. The role of the alkaline-earth ion is to partially screen the electron-electron Coulomb repulsion $U$, lowering the energy of the ${d}^{8}$ state and hence increasing ${T}_{c}$.
    Keywords:
    Alkaline earth metal
    Liquid nitrogen
    Superconducting transition temperature
    BCS theory

    (Bi, Pb)2Sr2Ca2Cu3O10 (Bi-2223) high temperature superconducting (HTS) tape is one of the high temperature superconducting wires for commercial applications recently. Its superconducting critical transition temperature is 105–110 K, which is higher than that of another commercially coated HTS REBa2Cu3O7 conductor tape (90–93 K). The Bi-2223 tape has a wide range of superconducting temperatures above the vaporization temperature of liquid nitrogen (77 K). The critical currents of two Bi-2223 tape specimens were systematically calculated, and the findings evaluated to understand the superconductive current-carrying characteristics of Bi-2223 tape in the temperature range above 77 K. To research their effects on the critical current of the specimens in this temperature range, we applied magnetic fields of different intensities and angles to the specimens. The results of the measurement are recorded in this paper in the temperature range of 77–92 K, the magnetic field strength range of 0–0.2 T and the magnetic field angle range of 0°–90°. The results show that the Bi-2223 tape can still bear the practical significance of the superconducting current at 92 K. If there is no external magnetic field, or the external magnetic field is almost parallel to the HTS tape surface and weaker than 0.1 T, the critical current in the self-field can still sustain 40%–50% of its critical current at 77 K. For example, the results of this study have a significant reference price for its use in this temperature range, providing basic information for the creation of cable applications that are superconductive.

    Liquid nitrogen
    Atmospheric temperature range
    Strontium oxide
    Vaporization
    Transition temperature
    Citations (1)
    Superconducting transition temperature
    Transition temperature
    Similarity (geometry)
    Citations (5)
    Abstract Since their discovery in 1986 [11], the high temperature superconducting (HTS) copper oxides have presented a continuing challenge to both experiment and theory. The identification of the underlying mechanism (or mechanisms) responsible for their superconductivity remains an unanswered question. Numerous theories have been proposed ranging from phonon-mediated pairing of the charge carriers, similar to the Bardeen–Cooper–Schrieffer (BCS) [2] theory developed for conventional low-temperature superconductors, to novel concepts independent of phonons [3–-l0]. For conventional superconductors the variation of the transition temperature Tc , with isotopic mass M (from BCS theory Tc ∼ M−a ) was an important verification of the contribution of electron-phonon interactions to electron pairing. Measurements of this effect of HTS cuprates resulted in isotope shifts much smaller than predicted by theory [ll-14], raising doubts about the role of phonons. However, Barbee [15] argued that the size of the isotope shift is not a unique indicator of phonon-mediated pairing. Since the HTS materials contain Cu ions with partially filled 3d shells, many of the alternative theories of HTS have focused on magnetic interactions and associated spin fluctuations [3–10]. The reader is referred to Ref. 16 for the details of other theories that have been proposed and to the article by Schrieffer and Anderson [17) for an overview discussion of the theory of high temperature superconductivity.
    BCS theory
    Transition temperature
    Citations (13)
    Rare earth barium copper oxide (REBCO) type materials are widely used in second-generation high-temperature superconductor tapes to carry high current volumes. These tapes can provide high critical currents along with good tolerance to perpendicular magnetic fields. However, REBCO tapes generally use a laminate structure that suffers from low peeling strength among its inner interfaces. Previously, we proposed a bending-peeling (BP) method and applied it to measurement of the interfacial strength of REBCO tapes at room temperature. All samples failed at the REBCO layer at room temperature. In this paper, we used the BP method to perform low-temperature interfacial strength measurements at liquid-nitrogen temperatures. There are two types of failure, where one involves peeling of the REBCO layer and the other involves peeling at the interface between the tape and the epoxy resin. The errors in the BP method were discussed in detail. The superconducting current was monitored during the BP experiment and showed that the effect of out-of-plane tensile stress on the superconducting current was not significant before the breaking of the tape. The degradation in the superconductivity is mainly caused by the structural failure of the REBCO layer.
    Liquid nitrogen
    Copper oxide
    Citations (6)
    It is well known and it has been shown experimental evidence that the critical transition temperature Tc of any type of superconductor depends among other criteria on the crystal structure. Very good examples are thallium, mercury, lanthanum, tin and gallium. Also in high temperature superconductor (HTSC) cuprates a relation between the geometry of the crystal and the transition temperature has been found. This investigation of HTSC's with the model of a resonance effect between the de Broglie wavelength of paired current carrying particles and the dimension of the solid state structure forming a quantum well, has been suggested by the authors in a previous paper. In this model the dimension is given by the distance of the atoms providing the current carrying particles and at the same time the structure acts as a resonator stimulating a coherent phase transition from an electron or particle gas to a condensate.
    Superconducting transition temperature
    Transition temperature
    An enormous number of scientific papers have been devoted to high-temperature superconductors. For this reason this report only touches upon certain topics. These topics include the history of the study of superconductivity and the discovery of high-Tc superconductivity, the calculation of the critical temperature Tc of the superconducting transition and ways of increasing this temperature, and the mechanism that provides for high Tc values. This report also examines the specific details of high-Tc superconductivity within the framework of the macroscopic theory of superconductivity and the question of thermocirculational thermoconductivity in high-Tc superconductors. Finally, a few comments are made concerning the future study of this topic.
    Superconducting transition temperature
    Room-temperature superconductor