Nawazish A. Khan

Enhanced superconducting properties of Cu1-xTlxBa2Ca2-yMgyCu3O10-δ (y=0, 0.5, 1.0, and 1.5)

In the present studies, the interplane couplings in Cu0.5Tl0.5Ba2Ca2Cu3O10-δ superconductor have been enhanced by Mg doping. With the improved interplane coupling a- and c-axis lengths are decreased while critical current density [Jc(H=0)] is enhanced by two orders of magnitude and the zero resistivity critical temperature [Tc(R=0)] is increased by 10 K. Predominant single phase of Cu1-xTlxBa2Ca2-yMgyCu3O10-δ (y=0, 0.5, 1.0, and 1.5) was prepared by the solid state reaction of Ba(NO3)2, Ca(NO3)2, MgO, and Cu(CN). The material was checked by x-ray diffraction for crystallinity and was found to be the predominant single phase of Cu1-xTlxBa2Ca2-yMgyCu3O10-δ (y=0, 0.5, 1.0, and 1.5) with a- and c-axes lengths 3.840 and 14.20 A, respectively. The c-axis length decreases with the increased concentration of Mg in the compound. Fourier transform infrared absorption measurements have shown softening of apical oxygen modes with increased Mg doping and improved interplane coupling.

Superconducting properties of Cu/sub 1-x/Tl/sub x/-1223 [Cu/sub 1-x/Tl/sub x/(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10-y/] thin films

The bi-axially-oriented thin films of Cu/sub 1-x/Tl/sub x/(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10-y/(Cu/sub 1-x/Tl/sub x/-1223: x=0.2-1.0) were prepared by an amorphous phase epitaxy (APE) technique on SrTiO/sub 3/ substrates. The amorphous phase was deposited by sputtering on the substrate and annealed in a thallium atmosphere at /spl sim/885/spl deg/C for /spl sim/50 min. Their T/sub c/ values were 100-113 K and higher than those of the YBa/sub 2/Cu/sub 3/O/sub 7-y/ (Y-123, that is, Cu-1212) system. Their J/sub c/ values were 2/spl times/10/sup 7/ A/cm/sup 2/ (77 K, 0 T) and 4/spl times/10/sup 5/ A/cm/sup 2/ (77 K, 10 T) at their maximum for x/spl sim/0.5. The J/sub c/ values were higher than the maximum values of Y-123 thin films by a factor of 2. The high J/sub c/ values are consistent with the low superconducting anisotropy (/spl gamma/=/spl xi//sub ab///spl xi//sub c/=2.5-4) of Cu/sub 1-x/Tl/sub x/-1223 and its high T/sub c/.

Simple method for direct synthesis of YBa2Cu4O8 at atmospheric oxygen pressure

A simple method for the synthesis of YBa2Cu4O8 superconductor at oxygen pressure of 1 atm is reported. The starting copper compound Cu2 (CN)2 makes it possible to synthesize near single phase material directly, without the use of reaction rate enhancers or wet chemical methods previously considered necessary. Resistivity and magnetic susceptibility measurements confirm the presence of bulk superconductivity at 81 K previously achieved only in YBa2Cu4O8 material synthesized at high oxygen pressure.

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 (tan δ) 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′, e″). 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 ...

Preparation of biaxially oriented TlCu-1234 thin films

Abstract The single phase of TlCu-1234 superconductor thin films is prepared for the first time by the amorphous phase epitaxy (APE) method, which is thallium treatment of sputtered amorphous phase at 900°C for 1 h. The amorphous phase is prepared by sputtering from the stoichiometric target composition CuBa 2 Ca 3 Cu 4 O 12− y . The films on the SrTiO 3 substrate are aligned biaxially after the thallium treatment. Highly reproducible TlCu-1234 films are prepared by this method. The XRD reflected a predominant single phase with the c -axis lattice constant of 18.74 A. This lattice constant value is in between that of Cu-1234 (17.99 A) and Tl-1234 (19.11 A). The pole figure measurements of (103) reflection of the films showed a -axis-oriented crystals with Δ φ =0.8°. The composition of the films after Energy Dispersive X-ray (EDX) measurements is Tl 0.8 Cu 0.2 Ba 2 Ca 3 Cu 4 O 12− y . From the resistivity measurements, the T c is 113 K. Preliminary J c measurements showed a current density of 1.0×10 6 A/cm 2 (77 K, 0 T).

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.

Growth kinetics of Cu1−xTlxBa2Ca3Cu4O12−y thin films

Cu1−xTlxBa2Ca3Cu4O12−y (Cu1−xTlx-1234) thin films have been found to be very attractive candidates in the cuprate family due to their low superconductor anisotropy, long coherence length and, consequently, high Jc. The method of preparation has been reported previously, however the kinetics of their formation has not yet been studied. In this paper, we report on the growth kinetics of superconducting Cu1−xTlxBa2Ca3Cu4O12−y thin films. In the preparation, we use the amorphous phase epitaxy method, which is a thallium treatment of the amorphous phase at elevated temperatures. The amorphous phase was deposited on a SrTiO3 substrate by rf-sputtering from a stoichiometric target with a composition of CuBa2Ca3Cu4Oy. The thallium treatment of the amorphous phase was carried out in a gold capsule for 45 min. The mechanism of the growth kinetics has shown that the formation of Cu1−xTlxBa2Ca3Cu4O12−y thin films was accomplished from Cu1−xTlxBa2Ca1Cu2O8−y (Cu1−xTlx-1212) and Cu1−xTlxBa2Ca2Cu3O10−y (Cu1−xTlx-1223) by the successive introduction of CuO2 planes in these phases. We also studied the effect of the time and temperature of the thallium treatment on the growth of Cu1−xTlx-1234 films. The best synthesis temperature for Cu1−xTlx-1234 films was found to be 910 °C, but this phase could also be isolated as a single phase at lower temperatures (~890 °C). However, the low-temperature synthesis results in a higher thallium content in the final compound. From the x-ray diffraction measurements the c-axis length was found to increase with the increase of the thallium content. The pole figure measurements of the (103) reflection of the films have shown a-axis oriented crystals with Δ = 0.8°. The observed critical temperature (Tc) for Cu1−xTlx-1212, Cu1−xTlx-1223 and Cu1−xTlx-1234 are 78 K, 103 K and 110 K respectively. Current density measurements have shown a maximum Jc ~ 2 × 106 A cm−2.

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...

Effect of Mg doping on the superconducting properties of Cu/sub 1-x/Tl/sub x/Ba/sub 2/Ca/sub 3-y/Mg/sub y/Cu/sub 4/O/sub 12-/spl delta//

Superconducting properties of Cu_1-xTl_xBa₂Ca_3-yMg_yCu₄O_12-delta (Cu_1-xTl_xMg_y-1234) material have been studied in the composition range y=0,1.5,2.25. The zero resistivity critical temperature [T_c(R=0)] was found to increase with the increased concentration of Mg in the unit cell; for y=1.5 [T_c(R=0)]=131 K was achieved which is hitherto highest in Cu_1-xTl_x-based superconductors. The X-ray diffraction analyses have shown the formation of a predominant single phase of Cu_0.5Tl_0.5Ba₂Ca_3-yMg_yCu₄O_12-delta superconductor with an inclusion of impurity phase. It is observed from the convex shape of the resistivity versus temperature measurements that our as-prepared material was in the region of carrier over-doping, and the number of carriers was optimized by postannealing experiments in air at 400degC, 500degC, and 600degC. The T_c(R=0) was found to increase with postannealing and the best postannealing temperature was found to be 600degC. The mechanism of increased T_c(R=0) is understood by carrying out infrared absorption measurements. It was observed through softening of Cu(2)-O_A-Tl apical oxygen mode that improved interplane coupling was a possible source of enhancement of T_c(R=0) to 131 K

Doping of the CuO2 planes of Cu1−xTlxBa2Ca2Cu3O10−y superconductor via light and heavier ions

Abstract The superconducting properties of Cu 1− x Tl x Ba 2 Ca 2 Cu 3 O 10− y thin films prepared by amorphous phase epitaxial (APE) method have been studied, by resistivity measurements, X-ray diffraction, electron microscopy and infrared spectroscopy. The main emphasis of this research work is on the doping of the charge reservoir layer, which ultimately controls carrier concentration in the CuO 2 conducting planes. The phenomena of the superconductivity, takes place in the conducting CuO 2 planes, however, we can control it via charge concentration in the CuO 2 planes. We have done this by carrying out annealing experiments in different conditions i.e. in air, nitrogen atmosphere and in vacuum. In these annealing experiments the doping is carried out by intercalation of the light O 2 in the CuO chains and by the removal of heavy thallium from the CuO chain axis. The removal of thallium at 717 °C from the CuO chains shifted the T c ( R =0) to higher values and the doping of oxygen and nitrogen at 450 °C increased the semiconductivity and shifted the T c (onset) to lower values. The removal of oxygen from CuO chains by vacuum annealing at 450 °C has also increased the degree of semiconductivity and shifted the T c (onset) to lower temperatures. The XRD of the samples of Cu 1− x Tl x -1223 phase showed the predominant c -axis oriented material with c -axis lattice constant 15.53 A. The electron micrograph of these samples showed elongated grains with the grains ends fused together. The surface roughness of the samples was below 0.2 μm. The infrared absorption measurements have shown the softening of 451 cm −1 mode after removal of oxygen from the unit cell, which shows that oxygen doping controls the carrier concentration in the charge reservoir layer and hence the superconductivity in the material.