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Samples (Cu0.5Tl0.5)Ba2Ca2Cu3-xKxO10-δ (x = 0, 1, 2, 2.5, 3) and Tl1.0Ba2Ca2K3O10-δ were synthesized to study the effect of electron donor atoms at the copper planar sites CuO2 to better understand process of superconductivity in oxides. The samples were synthesized following the two-step solid-state reaction method, and analyzed using x-ray diffraction (XRD), scanning electron microscopy (SEM), x-ray fluorescence spectroscopy (XRF), resistivity (RT), and Fourier-transform infrared (FTIR) absorption measurements. By using the software VESTA (visualization for electronic and structural analysis), we have successfully drawn the crystal structures of all the samples. From room temperature to the onset of superconductivity, our samples showed metallic variation in resistivity, with zero resistivity at the critical temperature (Tc, R = 0) staying around 93 K. In the Tl1.0Ba2Ca2K3O10-δ samples, copper Cu (3d9) atoms were entirely removed from the charge reservoir layer (i.e., Tl1.0Ba2O4-δ charge reservoir layer) the samples showed semiconducting behavior with variable range Mott hopping (VRH) conductivity with an energy gap of 2.41 meV. The FTIR absorption spectrum demonstrated the planar oxygen mode, as well as the apical oxygen modes. Tl-OA-Cu(2) and Cu(1)-OA-Cu(2) were stiffened with the doping of K(4s1) at the CuO2 planar sites, confirming intrinsic doping of K atoms. Excess conductivity studies (FIC) of the Cu0.5Tl0.5Ba2Ca2Cu3-xKxO10-δ (x = 1, 2, 2.5, 3) samples revealed an increase in the coherence length along the c-axis ξc(0), the Fermi velocity of superconducting carriers VF, inter-layer coupling J, and the energy needed to break apart the Cooper pairs when compared with the un-doped Cu0.5Tl0.5Ba2Ca2Cu3O10-δ, sample. Doped alkali atoms provide a larger density of free carriers in the superconducting planes rising superconductivity parameters. Higher free carrier density provided by the doped alkali atoms at room temperature to the conducting planes enhanced the electron–hole recombination process which brings them to the optimal doping level, thereby promoting the critical temperature and superconducting parameters to superior values. The Turning of the Tl1Ba2Ca2K3O10-δ samples semiconducting has shown that the presence of a spin-bearing entity in the unit cell is important for the occurrence of high Tc superconductivity, and the interplay of spin and charge density waves induces superconductivity in oxide samples at high temperatures. [ABSTRACT FROM AUTHOR]

(Cu0.5Tl0.5)Ba2(Ca3-xMgx)(Cu4-yZny)O12-δ (x = 0, 2; y = 0, 3) and (Cu0.5Tl0.5)Ba2(Ca3-xSrx)Cu4O12-δ (x = 0, 1, 2, 3) samples have been synthesized at normal pressure by two-steps solid state reaction method and characterized by X-ray diffraction (XRD), resistivity measurements, and Fourier transform infrared (FTIR) absorption measurements techniques. These studies have shown that higher order CuTl-1234 phases cannot be stabilized in Mg doped samples. However, higher order CuO2 plane phases can be stabilized with the doping of Sr at the Ca sites in (Cu0.5Tl0.5)Ba2(Ca3-xSrx)Cu4O12-δ (x = 0, 1, 2, 3) superconductors. Doping of Sr at the Ca sites promoted decoupling of CuO2 planes and resulted into new exciting high Tc phases. The XRD analysis of diffraction scans of the samples have shown increase in c-axis lengths with increased Sr doping concentration which evidenced the enhanced inter-planar distance and hence enhanced inter-planar decoupling. The onset temperature of superconductivity and zero resistivity critical temperature suppressed with the increased Sr doping in (Cu0.5Tl0.5)Ba2(Ca3-xSrx)Cu4O12-δ (x = 0, 1, 2, 3) samples. Intrinsic doping of Sr in (Cu0.5Tl0.5)Ba2(Ca3-xSrx)Cu4O12-δ (x = 0, 1, 2, 3) unit cell is confirmed by softening of apical oxygen mode of type Tl-OA-Cu(2) and hardening of apical oxygen mode of type Cu(1)-OA-Cu(2) and CuO2 planar oxygen mode. The decoupling of the CuO2 planes is also confirmed by the decrease in value of real and imaginary parts of dielectric constant and ac-conductivity with increased Sr concentration in the final compound. In the excess conductivity analysis of conductivity data, the coherence length along c-axis ξc, inter-layer-coupling J, and the Fermi-velocity vF of the carriers are enhanced whereas the London penetration depth (except of samples with x = 1), Bc(T), Bc1(T), Bc2(T), Jc suppressed with increased Sr doping. Ginzburg–Landau parameter κ, however increased with the doping of Sr in the final compound. The decrease in the values of these parameters expressed the suppression of flux pinning characteristics of Sr-doped samples. The energy required to break-apart the Cooper-pairs increased but the the phase relaxation time of the carriers suppressed with increased Sr incorporation in the final compound. [ABSTRACT FROM AUTHOR]

(Cu0.5Tl0.5)(Ba2–yCay)(CaMg)(Cu3–xZnx)O10–δ (y = 0, 1; x = 0, 2, 2.5, 2.8, 3) and (MxTl1–x)(BaCa)(CaMg) Zn3O10–δ (M = Ag, K; x = 0, 0.5) superconductors are synthesized for the studies of role of spin density waves in the mechanism of high Tc superconductivity. Such compounds are synthesized by reducing the thickness of the charge reservoir layers and distance among various CuO2/ZnO2 planes; decreasing the thickness of charge reservoir layers (CRLs) and distance among the various CuO2/ZnO2 planes increase the spin–spin interaction. All such samples were prepared by a two-step solid-state reaction (SSR) method at 860 °C and have shown orthorhombic crystal structure in which volume (V) of the unit cell suppresses with the doping of Ca, showing that partial substitution of Ca at the Ba site suppresses the thickness of CRL that in turn promotes a decrease in c-axis length and volume of the unit cell. The phonon modes related to the vibrations of apical and planar oxygen atoms are softened with doping of Ca and Zn in the final compounds. Metallic variations of resistivity (ρ) from room temperature down to the onset of superconductivity are a typical feature of these samples in which the Tc(onset) and Tc(R = 0) suppress with the doping of Ca, Mg, and Zn in the final compounds. Surprisingly, the samples with all the CuO2 planes replaced by ZnO2 planes have shown superconducting properties. To confirm the role of Cu(3d9) atoms in superconductivity, we have synthesized (MxTl1–x)(BaCa)(CaMg) Zn3O10–δ (M = Ag, K; x = 0, 0.5) samples. To our surprise, such samples have shown semiconducting behavior and no sign of superconductivity down to 77 K which demonstrated that the presence of Cu(3d9) atoms either in CRLs or ZnO2 planes is essential for the mechanism of high Tc superconductivity. These studies have shown the unequivocal role of small spin-carrying Cu-atoms in the mechanism of high Tc superconductivity. Fluctuating spins of Cu(3d9) atoms in the CuO2 plane generate spin waves, and hence, their interaction with charge density waves develop long-range phase coherence among the carriers that result in high-temperature superconductivity in oxides. [ABSTRACT FROM AUTHOR]

High quality electrical resistivity ρ(T) versus temperature data of as-prepared and O2-annealed Cu0.5Tl0.5Ba2Ca2Cu3-ySnyO10-δ (y=0, 0.25, 0.5, 0.75, 1.0, 1.25, and 1.5) superconductors has been studied for fluctuation-induced phenomena setting in at temperatures well above the critical temperature [Tc(R=0)]. The analysis of the data is done by using Aslamazov–Larkin (AL) and Lawrence–Doniach models for the excess conductivity. We have estimated several physical parameters, including coherence length, interplane coupling strength, exponents, and dimensionality of the fluctuations. The as-prepared and oxygen postannealed samples have shown a cross-over temperature associated with two distinct exponents and the excess conductivity data fits well with the two-dimensional and three-dimensional AL equations. The coherence length along the c-axis [ξc(0)] and the interlayer coupling strength (J) are found to decrease with increased Sn doping. These values are increased after annealing the samples in oxygen atmosphere, which is most likely associated with the approach of carrier concentration in the conducting CuO2/SnO2 planes to the optimum value. The Tc(R=0) and the peak temperature (TP) as determined from the dρ/dT versus temperature plots are also found to decrease with increased Sn substitution, however, these temperatures are improved to higher values after annealing the samples in oxygen atmosphere. The decreased values of Tc(R=0) and TP with increased Sn substitution in the as-prepared samples are most likely arising from the suppression of carrier’s density promoted by the increased volume of unit cell and the localization of the carriers at the Sn4+ sites. [ABSTRACT FROM AUTHOR]

We report the enhanced fluctuation-induced conductivity of Be-doped Cu0.5Tl0.5Ba2Ca3-xBex Cu4O12-δ (x=0,0.5,0.75,1.0,1.25,1.5) samples. The analysis has been done with partial substitution of Be in place of Ca. In each case excess conductivity has been analyzed and we tried to make a fit with the Aslamasov–Larkin (AL) and Lawrence–Doniach equations. It is observed that our data fit well with the three-dimensional (3D) AL equation, and a crossover from two dimensions to three dimensions has been found in our samples. We have also employed this transition to estimate the Josephson coupling strength in our samples. This interlayer coupling strength J, which controls the superconducting transition, has been found to improve with increased Be content. The Ginzburg–Landau coherence lengths ξc(0) for all cases have also been calculated. The Fourier transform infrared spectroscopy (FTIR) absorption measurements also provide a clue for the enhanced 3D fluctuations. The higher electronegativity, as well as smaller ionic size, of Be compared to Ca is suggested to be the possible source of promoting enhanced 3D character in Be-doped samples. [ABSTRACT FROM AUTHOR]

High quality electrical resistivity [rho](T) versus temperature data of as-prepared and [O.sub.2]-annealed [Cu.sub.0.5][Tl.sub.0.5][Ba.sub.2][Ca.sub.2][Cu.sub.3-y][Sn.sub.y][O.sub.10-[delta]] superconductors is analyzed for fluctuation-induced phenomena setting in at temperatures well above the critical temperatures. The critical temperature and the peak temperature are found to decrease with increased Sn substitution however these temperatures are improved to higher values after annealing the samples in oxygen atmosphere.

The enhanced three-dimensional excess conductivity observed in the Be-doped [Cu.sub.0.5][Tl.sub.0.5][Ba.sub.2][Ca.sub.3-x][Be.sub.x] [Cu.sub.4][O.sub.12-[delta]] (x=0,0.5,0.75,1.0,1.25,1.5) superconductors is analyzed. An increase in the Be content of the complexes is shown to lead to a significant enhancement in the interlayer coupling strength, J.

Cu0.5K0.25Tl0.25Ba2Ca3Cu4O12-δ superconductor samples were synthesized and their dielectric properties were measured between 80 K and 290 K by means of capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 KHz to 4 MHz. A negative capacitance (NC) occurrence was observed, which most likely arose from the superior Fermi level of ceramic superconductor samples than metal electrodes. Also the NC may be due to the space charge situated at the multiple insulator-superconductor interfaces (grain boundaries) in the materials. The negative dielectric constant (ε′) and loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 80 K may boost the polarizability and hence the dielectric constants (ε′ and ε″). [ABSTRACT FROM AUTHOR]