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Register a new userDiscovery of a new nontoxic cuprate superconducting system Ga-Ba-Ca-Cu-O
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Superconductivity is observed in a new nontoxic cuprate system Ga-Ba-Ca-Cu-O, with Tc = 82K for GaBa2Ca5Cu6O14+{delta} (Ga-1256) and Tc = 116K probably for GaBa2Ca3Cu4O10+{delta} (Ga-1234) or GaBa2Ca2Cu3O8+{delta} (Ga-1223), respectively. All compounds are fabricated by solid state reaction method under high pressure and high temperature. Samples are characterized by resistivity, magnetization and X-ray diffraction (XRD) measurements. The temperature dependence of magnetization measured in both zero-field-cooled and field-cooled processes on one sample (S1) shows two superconducting transitions at about 82K and 113K. The estimated shielding fraction for the phase with Tc of 82K is about 67%, while the fraction for another phase with Tc of 113K is quite small. The XRD Rietveld refinement for S1 indicates two main phases existing in the sample, Ga-1256 with fraction of about 58% and non-superconducting Ca0.85CuO2 with fraction of about 42%, respectively. Thus, we can conclude the superconducting phase with transition temperature of 82K is due to Ga-1256. The resistivity measurement also confirms the superconductivity for S1, and the resistivity reaches zero at about 82K. The temperature dependence of magnetization for another sample (S2) shows much higher superconducting shielding fraction for the phase with Tc of 116K, which may be a promising prospective for the synthesis of Ga-1234 or Ga-1223 phase.
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We report the fabrication of high Tc superconducting wires by photodoping a GdBa2Cu3O{6.5} thin film. An optical near-field probe was used to locally excite carriers in the system at room temperature. Trapping of the photogenerated electrons define a confining potential for the conducting holes in the CuO planes. Spatially resolved reflectance measurements show the photogenerated nanowires to be ~ 250 nm wide. Electron diffusion, before electron capture, is believed to be responsible for the observed width of the wires.
The results of EXAFS measurements at 300 K for the superconducting compounds Tl$_{0.75}$Cu$_{0.25}$Ba$_{2}$Ca$_{3}$Cu$_4$O$_{y}$ [Tl-1234], TlBa$_{2}$Ca$_{3}$Cu$_{4}$O$_{y}$ [Tl-1212], and CuBa$_{2}$Ca$_{3}$Cu$_{4}$O$_{y}$ [Cu-1234]. are reported. We have measured the EXAFS spectrum for Tl$_{0.75}$Cu$_{0.25}$Ba$_{2}$Ca$_{3}$Cu$_4$O$_{y}$ in the range 10K-300K, however here we limit our discussion to the spectrum at 300 K. This material is prepared under high pressure [3.5 GPa] from precursors with small carbon concentrations and exhibits a T$_c$ of $~127$ K. We have also performed ``aging study by looking at XRD for this material after approximately one year. The XRD results at 300 K are ``unchanged. It is of interest to compare the EXAFS spectrum of this compound with the corresponding compound Cu-1234. Remarks on the choice of appropriate EXAFS standard for this and related compounds are also given. Based on our data analysis we quantify disorder in these systems. By using the Cu-O in-plane distance we give values for the microstrain parameter, which can be related to the charge ordering transition.
Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on a multi-layered cuprate superconductor Ba$_2$Ca$_5$Cu$_6$O$_{12}$(O$_{1-x}$,F$_x$)$_2$. STM topography revealed random spot structures with the characteristic length $le 0.5$ nm. The conductance spectra dI/dV(V) show the coexistence of smaller gaps $Delta_S$ and large gaps (pseudogaps) $Delta_L$. The pseudogap-related features in the superconducting state were traced with the spatial resolution of $sim$ 0.07 nm. Here, $I$ and $V$ are the tunnel current and bias voltage, respectively. The temperature, $T$, dependence of $Delta_S$ follows the reduced Bardeen-Cooper-Schrieffer (BCS) dependence. The hallmark ratio 2$Delta_{S}(T=0)/k_B T_c$ equals to 4.9, which is smaller than those of other cuprate superconductors. Here, $T_c$ is the superconducting critical temperature and $k_B$ is the Boltzmann constant. The larger gap $Delta_L$ survives in the normal state and even increases with $T$ above $T_c$. The $T$ dependences of the spatial distributions for both relevant gaps ($Delta$ map), as well as for each gap separately ($Delta_S$ and $Delta_L$) were obtained. From the histogram of $Delta$ map, the averaged gap values were found to be $bar Delta_S = sim 24$ meV and $bar Delta_L = sim 79$ meV. The smaller gap $Delta_S$ shows a spatially homogeneous distribution while the larger gap $Delta_L$ is quite inhomogeneous, indicating that rather homogeneous superconductivity coexists with the patchy distributed pseudogap. The spatial variation length $xi_{Delta_L}$ of $Delta_L$ correlates with the scale of the topography spot structures, being approximately 0.4 nm. This value is considerably smaller than the coherence length of this class of superconductors, suggesting that $Delta_L$ is strongly affected by the disorder of the apical O/F.
The structure and physical properties of superconducting compounds Y(La)-Ba(Sr)-Cu-O are studied, the compounds being prepared by the method of cryogenic dispersion of a charge consisting of premix oxides and carbonates. Electrical conductivity and critical current density of the superconductors are measured over a wide temperature range of 10~$mK$ to 300~$K$. Degradation of the superconductor critical parameters in time and structural characteristics, magnetic susceptibility, heat capacity and acoustic properties are studied, and current-voltage characteristics are determined.
Magneto-optical imaging and magnetization measurements were applied to investigate local formation of superconducting phase effected by a random neck synthesis in Y-Ba-Cu-O system. Polished pellets of strongly inhomogeneous ceramic samples show clearly the appearance of superconducting material in the intergrain zones of binary primary particles reacted under different conditions. Susceptibility measurements allows evaluation of superconducting critical temperature, which turned out to be close to that of optimally doped YBCO.
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