Composition, Microstructure, and Superconducting and Ferroelectric Properties of Bi–Sr–Ca–Cu–O/Pb–Ti–Zr–O Heterostructures

Ferroelectric–superconductor heterostructures consisting of a ferroelectric Pb–Ti–Zr–O film on a superconducting Bi–Sr–Ca–Cu–O ceramic substrate or a superconducting Bi–Sr–Ca–Cu–O film on a ferroelectric Pb–Ti–Zr–O ceramic substrate with Ag, Au, Pd, and Pt barrier layers are prepared and investigated. Thin-film Bi₂Sr₂CaCu₂O_{8 + x} /ZrO₂/Pb(Ti_0.6Zr_0.4)O₃ (superconductor/barrier/ferroelectric) heterostructures on LaAlO₃ substrates are for the first time produced and characterized. The superconducting and ferroelectric transition temperatures in the heterostructures are 80 and 638 K, respectively.     

Effect of cyclic annealing on the static and high-frequency characteristics of YBCO-film-based resonant structures

We have studied correlations between the static parameters and high-frequency Q values of megahertz-range resonant LC circuits representing planar coils with self-capacitance manufactured on the basis of thin films of YBa₂Cu₃O_{7 − δ} (YBCO) high-temperature superconductor. It is established that the static characteristics of resonant circuits reversibly vary in the course of cyclic annealing, while the Q value does not exhibit such changes.     

Coupling Across an Intimate Ceprate-Manganite Interface

Electronic and magnetic coupling across the interface between a ferromagnetic La_2-xBa_xMnO₄ (“ colossal magnetoresistance” CMR) layer and a La_1.85Ba_0.15CuO₄ (high-temperature superconductor) layer are studied in a self-consistent virtual crystal approximation using the local spin density functional approach. The manganite material dopes the first cuprate layer, with holes ifx = 2/3 and with electrons if x = 0. Forx near the CMR regime the ferromagnetic manganite slab should be half-metallic. Such multilayer systems may be useful as magnetic field-controlled current switches, or as a relatively high-temperature superconducting material with small critical field.     

Preparation and characterization of non-random YBa2Cu3O7-Ag composite

Silver addition to the highT _c superconductor, YBa₂Cu₃O₇ (YBCO) is known to improve its mechanical properties and critical current (J _c) characteristics. We have synthesized the superconductor-metal composite YBCO-Ag by an electroless process at room temperature (using sucrose or hydrazine hydrate in an alkaline medium). The latter method produces a non-random coating whereby each grain of YBCO is coated uniformly by silver. The intrinsic highT _c of 90 K of YBCO can be recovered by compaction of the composite and subsequent heat treatment (900°C and 600°C) in oxygen.     

Superconducting gap function and upper critical field in the presence of inhomogeneous magnetic order

Rare earth ternary superconductors are known to exhibit oscillatory magnetic orders below their superconducting transition temperatures. The study of behaviour of superconducting electrons in an inhomogeneous magnetic field is therefore important for such systems. We report here the results of our theoretical study of superconducting gap function Δ(T) and upper critical fieldH _c2 (T). The results are applied to analyse and explain the variation of Δ(T) andH _c2 (T) in case of NdRh₄B₄.     

Influence of Vertical Vibrations on an Array of Bulk HTSC Above the Permanent Magnet Guideway

The dynamic response of high-temperature superconducting (HTS) maglev vehicle system, which consists of a high-temperature superconductor (HTSC) array of seven YBa₂Cu₃O_7−x bulks above permanent magnet guideway (PMG), was investigated experimentally. The dynamic stiffness and displacements and the levitation force of the bulk HTSCs were investigated as function of the external excitation amplitudes in the range of 1–4 mm and frequencies in the range of 1–400 Hz. The levitation force and the resonance frequency and the dynamic stiffness of the bulk HTSCs are correlative to reduce with the excitation frequency lower than 30 Hz and the applied excitation amplitude higher than 3 mm due to the energy loss. However, the experimental results show the reduction occurred in the first several cycles, which proves that the array of the bulk HTSCs can sustain the external excitation vibration in the vertical direction due to its strong dynamic stiffness. The experimental results are helpful in the application of the HTS maglev vehicle in the future.     

Multimode Sapphire Resonator for the Measurement of Surface Resistance of HTS Films

A multimode sapphire resonator is developed for multiple frequency surface resistance measurement of a single piece of high-temperature superconductor thin film. Simple formulas are used for the calculations of surface resistances. On this sapphire resonator, three modes, TE_011+ε , TE_012+δ and TE_021+ξ (0<ε,δ,ξ<1), which had resonances at 11.84, 17.49 and 31.99 GHz were used simultaneously and the measured surface resistances at these frequencies were 0.553, 1.32 and 3.75 mΩ, respectively. The measured result matches the theory very well.     

Superconducting Bands Stabilizing Superconductivity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> and MgB<Subscript>2</Subscript>

It is shown that the ceramic superconductor YBa₂Cu₃O₇ as well as the superconducting intermetallic compound MgB₂ possesses a narrow, partly filled “superconducting band” with Wannier functions of special symmetry in their band structures. This result corroborates previous observations about the band structures of numerous superconductors and non-superconductors showing that evidently superconductivity is always connected with such superconducting bands. These findings are interpreted in the framework of a nonadiabatic extension of the Heisenberg model. Within this new group-theoretical model of correlated systems, Cooper pairs are stabilized by a nonadiabatic mechanism of constraining forces effective in narrow superconducting bands. The formation of Cooper pairs in a superconducting band is mediated by the energetically lowest boson excitations in the considered material that carry the crystal-spin angular momentum 1⋅ℏ. These crystal-spin-1 bosons are proposed to determine whether the material is a conventional low-T _c or a high-T _c superconductor. This interpretation provides the electron–phonon mechanism that enters the BCS theory in conventional superconductors.     

Self-doping Effects on the Superconducting Properties of Cu0.5Tl0.25M0.25Ba2Ca2Cu3O10−δ (M = Bi, Hg, Nb, Pd, Li, Na, K)

The effect of self-doping and substitution of elements of higher and lower electronegativity, such as Bi, Hg, Nb, Pd, Li, Na, K, on the superconducting properties of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ with x=0.25 is investigated. These experiments demonstrated that the elements of lower electronegativity such as Li, Na, and K can easily liberate their outer most s-electron that could be supplied to the conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ superconductor, and as a result, we get enhanced superconducting properties. However, highly electro-negative elements hinder the transfer of carriers from charge reservoir layer to the conducting CuO₂ planes and promote inferior superconducting properties. In the present studies, we have investigated the effect of post-annealing in nitrogen and oxygen atmospheres for optimizing the carriers in conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ (M=Bi,Hg,Nb,Pd,Li,Na,K) superconductor. These studies are important since the density of carriers in the conducting CuO₂ planes determines the Fermi-vector k _F and Fermi velocity v _F of the carriers, which ultimately brings about the final superconducting state of the system.