M. Mumtaz

Frequency dependent dielectric properties of Cu0.5Tl0.5Ba2Ca2Cu3−yZnyO10−δ (y=0, 1.0, 1.5,2.0, 2.5) superconductors

We have studied the dielectric properties of Cu0.5Tl0.5Ba2Ca2Cu3−yZnyO10−δ (y=0, 1.0, 1.5, 2.0, 2.5) superconductors to investigate the effects of Zn atoms and specifically their role in modifying dielectric properties, such as dielectric loss (tanu2009δ) and ac-conductivity (σac). These dielectric properties have been investigated in the normal state of the samples at room temperature and in their superconducting state. From these analyses, we have determined the excess conductivity in the superconducting state (lower energy state) of the system and suggested its possible role in the mechanism of superconductivity. The lower thermal agitation at 79 K may enhance the polarizability of atoms and hence their dielectric constants (e′,u2002e″). A comparison with the dielectric properties of Tl2Ba2Ca2Cu2Ox has shown that our Cu0.5Tl0.5Ba2Ca2Cu3−yZnyO10−δ material has lower losses and higher ac-conductivity at 290 and 79 K. The decreased dielectric loss with increased Zn doping strongly suggested that the polarization ...

A new Cu0.5Tl0.5Ba2Ca2Cu3−yZnyO10−δ high-temperature superconductor with three ZnO2 planes

A new Zn-rich high-temperature superconductor Cu 0.5 Tl 0.5 Ba 2 Ca 2 Cu 0.35 Zn 2.65 O 10-δ has been synthesized. The structure of this compound is tetragonal, with a- and c-axes lengths 3.354 and 14.402 A, respectively. The substitution of Zn at the CuO 2 planar site in the formula unit Cu 0.5 Tl 0.5 Ba 2 Ca 2 Cu 3-y Zn y O 10-δ gives superconductivity for all the doping concentrations with y = 0.75, 1.5, 2.25, and 2.65. The zero-resistivity critical temperature [T c (R = 0)], quantity of diamagnetism and critical current density [J c (H = 0)] are found to increase with the increased concentration of Zn in the final compound. The onset temperature of superconductivity in these samples was observed at 127 K and the zero-resistivity critical temperature was at 121 K. The material prepared by this method is extremely reproducible.

The effect of grain size on the fluctuation-induced conductivity of Cu1−xTlxBa2Ca3Cu4O12−δ superconductor thin films

The high temperature superconductor thin films Cu1−xTlxBa2Ca3Cu4O12−δ (Cu1−xTlx- 1234) are post-annealed in a nitrogen atmosphere. The zero-resistivity critical temperature (Tc(R = 0)) of these thin films is increased from 92.3 to 104xa0K. The grain size is enhanced and their morphology is improved with the post-annealing. The enlargement of grain size is linked to fluctuation-induced conductivity (FIC) in the light of Aslamazov–Larkin (AL) theory. The FIC measurements have shown that the cross-over of three-dimensional (3D) to two-dimensional (2D) behaviour of fluctuations is shifted to higher temperature values with an increase of post-annealing temperature. These results have shown that the removal of oxygen and the increased grain size are the most likely sources of the increase in the cross-over temperature, T* to higher values.

Improved interplane and intergranular coupling by Mg doping at Ca site in Cu0.5Tl0.5Ba2Ca2(Cu0.5Zn2.5)O10−δ superconductor

Improvement in the interplane and intergranular coupling achieved by Mg doping at Ca site in high temperature Cu0.5Tl0.5Ba2Ca2(Cu0.5Zn2.5)O10−δ superconductor is studied by infield magnetic measurements. The decrease of c-axis length and volume of the unit cell of Cu0.5Tl0.5Ba2(Ca2−yMgy) (Cu0.5Zn2.5)O10−δ (y=0, 0.5, 1.0, and 1.5) superconductor with the increase of Mg doping shows the improved interplane coupling among CuO2∕ZnO2 planes. The presence of Mg at the termination ends of the crystals of Cu0.5Tl0.5Ba2(Ca2−yMgy) (Cu0.5Zn2.5)O10−δ compound has been found to improve intergrain coupling, which is evident from enhanced critical current density (Jc) of these samples. The exponent of temperature variation of thermodynamic critical field Hc∼(1−TP∕Tc)n suggests that the weak links form superconductor-insulator-superconductor type of junctions in all the samples. The presence of insulating material at the intergrain sites possibly increases the area of shielding current flow, which promotes the enhancemen...

Studies of the phonon modes and superconducting properties of Cu0.5Tl0.5Ba2Ca2Cu3−yZnyO10−δ (y = 0, 1.5, 2.5)

The phonon modes and superconducting properties of Cu0.5Tl0.5Ba2Ca2Cu3?yZnyO10?? (y = 0, 1.5, 2.5) have been investigated. To optimize the carriers in the unit cell, annealing experiments were performed at different temperatures and for different annealing times. The resistivity and ac-susceptibility measurements revealed that the samples become optimally doped when they were annealed at 500??C for 6?h. The optimum concentration of carriers in the conducting ZnO2/CuO2 planes gives the desired Fermi velocity VF that is necessary for enhanced superconductivity. The volume of the unit cell observed through x-ray diffraction scans is found to decrease with the increase of Zn doping, promoting an increase in the Fermi vector KF and effective density of states which results in enhanced superconductivity parameters. These annealing experiments manifested the importance of the apical oxygen modes of type Cu(1)?OA?M(2) and Tl?OA?M(2) and the CuO2 planar mode M(2)?Op?M(2) (where M = Zn/Cu) for superconductivity. The Fourier transform infrared (FTIR) absorption studies are consistent with the resistivity and ac-susceptibility measurements.

Frequency dependent dielectric properties of Cu0.5Tl0.5Ba2Ca2(Cu3-yMy)O10-δ superconductor

The frequency dependent dielectric properties such as dielectric constants (ɛ/r, ɛ//r), absolute dielectric loss |tanδ|, and ac-conductivity (σac) of Cu0.5Tl0.5Ba2Ca2(Cu3-yMy)O10-δ (Mu2009=u2009Si, Ge, Sn, yu2009=u20090, 1) superconductor have been investigated by means of capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 KHz to 10u2009MHz at various temperatures from superconducting state to normal conducting state. The negative capacitance has been observed in all Cu0.5Tl0.5Ba2Ca2(Cu3-yMy)O10-δ samples. The large values of negative dielectric constant (ɛ/r) at lower frequencies and temperatures are linked with reduced thermal vibrations of the atoms due to which polarizability has been enhanced. The decreased value of dielectric constant observed in the Ge-doped samples may possibly be linked with its greater electronegativity (EN) and less polarization. The electronegativity of Si and Sn has approximately the same values as that of Cu, so almost all the dielectric properties d...

Optimization of carriers by self-doping in Cu0.5Tl0.5Ba2Ca2Cu3−yMyO10−δ superconductor

The elements of fourth group (IV-A) of the periodic table have been doped in Cu0.5Tl0.5Ba2Ca2Cu3−yMyO10−δ (M=Si, Ge, Sn, and y=0, 1) samples to explore any possible electron-phonon interaction mechanism in oxide high temperature superconductors. We have observed that doped atoms with masses significantly different from that of Cu atom suppress the superconducting properties of the final compound. The postannealing experiments in flowing oxygen were also carried out to optimize the carriers density in the unit cell. After postannealing in oxygen the superconducting properties of all the samples are improved, which is most likely due to the optimization of carriers density in the material. It is observed from Fourier transform infrared absorption measurements that the CuO2 planar mode is hardened with the doping of Si, whereas, it is softened with the incorporation of Ge and Sn in the unit cell. The apical oxygen modes are softened with Si doping but these modes are hardened with the incorporation of Ge and...

Dielectric properties of (CuO, CaO2, and BaO)y/CuTl-1223 composites

We synthesized (CuO, CaO2, and BaO)y/Cu0.5Tl0.5Ba2Ca2Cu3O10−δ (yu2009=u20090, 5%, 10%, 15%) composites by solid-state reaction and characterized them by x-ray diffraction, scanning electron microscopy, dc-resistivity, and Fourier transform infrared spectroscopy. Frequency and temperature dependent dielectric properties, such as real and imaginary parts of the dielectric constant, dielectric loss, and ac-conductivity of these composites were studied by capacitance and conductance measurements as a function of frequency (10 kHz to 10 MHz) and temperature (78 to 300u2009K). X-ray diffraction analysis reveals that the characteristic behavior of the superconductor phase and the structure of Cu0.5Tl0.5Ba2Ca2Cu3O10−δ are nearly undisturbed by doping with nanoparticles. Scanning electron microscopy images show the improvement in the intergranular linking between the superconducting grains occurring with increasing nanoparticle concentration. Microcracks are healed up with these nanoparticles, and superconducting volume fract...

Intergranular coupling of the CU0.5Tl0.5Ba2Ca2CU0.5Zn2.5O10-δ superconductor

The intergranular coupling of the Cu0.5Tl0.5Ba2Ca2Cu0.5Zn2.5O10?? superconductor has been studied by resistivity and AC magnetic susceptibility measurements. The main objective of the present study is to enhance the intergranular coupling of this compound by post-annealing the samples in nitrogen, oxygen and air atmospheres. Any improvement to the intergrain coupling may be promoted by a change of oxygen content in the final compound. The AC susceptibility measurements have shown that the intergranular coupling peak observed in the out-of-phase component (?) is shifted to higher temperatures after post-annealing in an air atmosphere, which is evidence of enhanced intergrain coupling promoted by oxygen diffusion. Enhanced intergranular coupling and flux pinning is observed in nitrogen and oxygen post-annealed samples in field AC susceptibility measurements.

Suppression of activation energy and superconductivity by the addition of Al2O3 nanoparticles in CuTl-1223 matrix

Low anisotropic (Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (CuTl-1223) high Tc superconducting matrix was synthesized by solid-state reaction and Al2O3 nanoparticles were prepared separately by co-precipitation method. Al2O3 nanoparticles were added with different concentrations during the final sintering cycle of CuTl-1223 superconducting matrix to get the required (Al2O3)y/CuTl-1223, yu2009=u20090.0, 0.5, 0.7, 1.0, and 1.5u2009wt.u2009%, composites. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive X-ray, and dc-resistivity (ρ) measurements. The activation energy and superconductivity were suppressed with increasing concentration of Al2O3 nanoparticles in (CuTl-1223) matrix. The XRD analysis showed that the addition of Al2O3 nanoparticles did not affect the crystal structure of the parent CuTl-1223 superconducting phase. The suppression of activation energy and superconducting properties is most probably due to weak flux pinning in the samples. The possible reason of weak flux p...