Inelastic neutron scattering from Tl2CaBa2Cu2O8

Coherent inelastic neutron scattering measurements have been carried out on the high temperature superconductors Tl₂CaBa₂Cu₂O₈ (Tl-2122,T _c =107 K) and YBa₂Cu₃O₇ (Y-123,T _c =92 K), at the Dhruva reactor at Trombay. The density of phonon states in Tl-2122 is enhanced at 6–17 meV and reduced at 40–70meV compared to that in Y-123.     

Quantum tunneling of vortices in single crystal Tl2CaBa2Cu2O8 superconductors

Data are presented on the temperature-dependent time-logarithmic magnetic relaxation rate S(T) = ¦dM/d_in t¦ of the high-T_c superconductor Tl₂CaBa₂ Cu₂O₈. It is found that at low temperatures the relaxation rate has the form S(T) = A(H) exp[(T/T^*)²], which does not extrapolate to zero at T = 0, thus excluding conventional thermally activated flux creep and providing evidence of quantum vortex tunneling. From a quantum flux tunneling theory, it is shown that S(T) I/_er _p ² . The measurements of the relaxation rate thus provide information about the effective viscosity _e of fluxons.     

Synthesis and Processing of Doped Hg1Ba2Ca2Cu3O y Superconductors

A series of quenching experiments were conducted to understand the sequence of reactions that occur during the synthesis of doped Hg1223, (Hg, A)Ba₂Ca₂Cu₃O _y , A = Re, Bi, and Pb (HgA1223). The formation and decomposition of the intermediate phases during the high-temperature reaction were followed as a function of temperature. HgA1223 phase forms over a wide range of temperatures, 750–950°C, 750–880°C, and 840–880°C for A = Re, Pb, and Bi, respectively. At T<750°C, HgA1212 phase forms for A = Re and Pb. Based on the results of quenching experiments, heat treatment conditions were optimized for the synthesis of pure HgA1223 phase using commercial BaCaCuO precursor powders. A reduced-temperature annealing stage after the high-temperature reaction helps in grain growth and improves the microstructural characteristics of HgA1223 samples. Control of Hg pressure during the reaction is crucial for achieving phase purity, grain growth, and texture in the final products. A novel approach for the control of Hg pressure during the synthesis of HgA1223, which consists of using CaHgO₂ as an external Hg source, is reported. HgA1223 samples synthesized using the new synthesis protocol exhibit improved microstructural and superconducting properties.     

Bulk thermal expansion studies of BiCaSrCu2O x and Bi2CaSr2Cu2O x

The compounds BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x were prepared by ceramic techniques and characterized by X-ray powder diffractometry (XRD) and microthermogravimetry (TG) and their bulk thermal expansion measurements were carried out using dilatometry in the temperature range 298T1073 K in air. The results have been analyzed and are compared with those obtained earlier for YBa₂Cu₃O₇. The XRD analysis shows that both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x are single phase in nature, having an orthorhombic symmetry. The TG analysis carried out in oxygen, air, and nitrogen shows negligible weight loss (R~0.1%) on heating to 1073 K, indicating that these two compounds, unlike YBa₂Cu₃O₇, are quite stable. The analysis of bulk thermal expansion data reveals that the average linear thermal expansion coefficient ( ₁) for both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is almost the same ( ₁ 10.5×10^–6 K^–1) and is found to be nearly half of that for YBa₂Cu₃O₇ ( ₁ 18×10^–6 K^–1), suggesting that the interatomic bonding in both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is stronger as compared to YBa₂Cu₃O₇.     

Cu0.5Tl0.5Ba2Ca3Cu4−y Zn y O12−δ (y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO2 Planes

A new Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO₂ planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO₂ planar site was carried out following Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature T _c(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and T _c(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector K _F and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ material by this method is highly reproducible.      

Preparation of Tl2Ba2Ca1 − y CeyCu2O8 − x/2Fx Ce-and F-substituted superconductors

High-T _c superconducting ceramics with the compositions Tl₂Ba₂CaCu₂O_{8 ? x/2} (0 ≤ x ≤ 0.3), Tl₂Ba₂(Ca_{1 ? y} Ce_y)Cu₂O_x (0 ≤ y ≤ 0.2), and Tl₂Ba₂(Ca_0.9Ce_0.1)(Cu_1.98Fe_0.02)F_0.2O_8.01 are synthesized. Partial fluorine substitution for oxygen in Tl₂Ba₂CaCu₂O_{8 ? x/2} (0 ≤ x ≤ 0.1) alters the carrier density in the Cu-O planes of the material, raising its superconducting transition temperature from 106 to 110 K. Partial cerium substitution for calcium in Tl₂Ba₂(Ca_{1 ? y} Ce_y)Cu₂O_x (0 ≤ y ≤ 0.2) reduces T _c. The combined substitution in Tl₂Ba₂(Ca_0.9Ce_0.1)(Cu_1.98Fe_0.02)F_0.2O_8.01 has an insignificant effect on T _c.     

Superconductivity in Co and Li Substituted Cu0.5Tl0.5Ba2Ca2Cu3?x Co x O10?��

We have tried to explore the possible sources of suppression of superconducting properties in cobalt substituted Cu_0.5Tl_0.5Ba₂Ca₂Cu_3?x Co _x O_10??? (x=0,0.05,0.1,0.5) superconductor. It can be expected from these studies that Co substitution at Cu sites in the CuO₂ planes somehow localizes the carriers and decreases the mobile carriers density. The density of mobile carriers in the CuO₂ planes can be restored by partial substitution of Li at Tl sites in Cu_0.5Tl_0.5Ba₂O_4??? charge reservoir layer of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3?x Co _x O_10??? superconductor; since alkali metals are known to lose their outermost ??s-orbital?? electron easily, which can be supplied to the CuO₂ planes. The post-annealing experiments on these samples have been carried out in flowing oxygen and nitrogen and the motivation behind these extreme post-annealing conditions was to optimize the carriers density in the CuO₂ planes of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3?x Co _x O_10??? superconductor. The optimized free carrier density in the CuO₂ planes enhances the superconducting properties of the final compound.     

Preparation and properties of Tl2Ba2CaCu2O8 thin films

Anex situ process has been developed to produce thin superconducting Tl₂Ba₂CaCu₂O₈ films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103–107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO₃, LaAlO₃, and Y-ZrO₂ substrates. A critical current density,J _c, of 2×10⁶ A/cm² at 77 K was measured on LaAlO₃. Film morphology was studied by SEM, AFM, and STM.     

Effect of silver in Y1Ba2Cu3O7−x samples

Two techniques for preparing Y-Ba-Cu-O/Ag specimens are presented: (a) solid-state reaction, and (b) normal precursors dissolved in hot concentrated nitric acid and dried in air. The samples are characterized by scanning electron microscopy (SEM), scanning Auger, X-ray diffraction, optical microscopy and electrical resistivity measurements. In comparison with a normal 1-2-3 superconductor it was found that the normalized resistance [R(T)/R(280 K)] decreases for both preparation methods. The material retained a high initial temperature of superconductivity, T _c, for a maximum of 6% w/o Ag₂O during solid-state reaction and 6.5% w/o Ag₂O for samples dissolved in hot concentrated nitric acid and dried in air.     

Superconducting gap in Pr x Y1−x Ba2Cu4O8 and YBa2−y Sr y Cu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1?x Ba₂Cu₄O₈ and YBa_2?y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration atω～300 cm^?1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.     

Preparation and Characterization of HgO and AgO Added La2CaBa2Cu5O z Superconductors

We have prepared a series of La₂CaBa₂Cu₅O _z (La2125) compounds by the standard solid-state reaction method. The oxygen stoichiometry in this R-123 type compound has been homogenized by two different methods. The first method is heating the sample in flowing oxygen at 500 °C for 6 hours and then slowly cooling to room temperature. The second method is to provide internal oxygen while sintering the sample by adding HgO or AgO. The addition of HgO or AgO in preparing oxide superconductors is advantageous when preparing them in bulk quantities, as HgO or AgO provide internal oxygen which is a much essential ingredient of the structure of oxide superconductors. Such a method of synthesis eliminates the need of additional oxygen annealing, thus enabling the synthesis in huge quantities at once. The composites referred to as La2125+xHgO and La2125+xAgO respectively, have been characterized by XRD, SEM, resistivity and magnetic measurements for their structural and superconducting properties. We compare the properties of oxygen-annealed and HgO/AgO added compounds and discuss the results in the light of applicability of such method in synthesizing bulk quantities of high-T _c oxide superconductors.      

Effect of Dissipation on Quantum Tunneling of Vortices in Tl2Ba2Ca2Cu3O10+δSuperconductors

The temperature and magnetic field dependence of the magnetic relaxation rate has been investigated at low temperatures (1.8 < T < 10 K) on two Tl₂Ba₂Ca₂Cu₃O_10+δ samples (epitaxial thin film and sintered pellet). The temperature dependence gives evidence of a crossover in the mechanism of vortex motion, from classical thermal activation to quantum tunneling as temperature decreases. The field dependence of the relaxation rate indicates a crossover in the dimensionality of vortices, from three-dimensional flux lines to two-dimensional pancake vortices as field increases. For the thin film, the temperature dependence of the rate has been fitted to the theoretically predicted expressions for finite-temperature enhancement of the quantum rate in different regimes of dissipation. In spite of the similarity of the fits, the estimate of the ratio of Hall to viscous drag terms in the equation of motion indicates that quantum tunneling in this system occurs in an intermediate dissipative regime, where both terms contribute to the motion of vortices.     

Chemical Control of Underdoped and Overdoped States in Y(Ba2 − y Sr y )Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba_{2 ? y} Sr _y )Cu₃O_{6 + δ} (δ～0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba_2?y Sr _y )Cu₃O_{6 + δ} (δ～0.9) compounds gives rise to high hole concentrations within both the CuO₂ planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba_{2 ? y} Sr _y )Cu₃O_{6 + δ} (δ～0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.     

Crystal growth of NdBa2Cu3O7−y and Ba2Cu3O5+δ single crystals by a horizontal Bridgman like method

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigation of Y1Ba2Cu3O6.9 and Er1Ba2Cu3O6.8 during cyclic temperature variations

Gravimetric investigations of the superconductive ceramics Y₁Ba₂Cu₃O_6.9, Y₁Ba₂Cu_2.7Al_0.3O_7.1 and Er₁ Ba₂Cu₃O_6.8 during cycling heating cooling in the interval 293–1323 K at partial pressure of the oxygen 0.02 and 0.2 × 10⁵ Nm^–2 are presented. Intervals of intense weight changes during heating and cooling are determined. Oxygen loss was shown to be the main reason for the weight changes. Structural characteristics and resistivity of the yttrium ceramics in the above-mentioned temperature interval are investigated. Experimental data are discussed within the framework of the models of the quasi-equilibrium and essentially nonequilibrium processes. Binding energy and diffusional parameters of the evolving gas are estimated.     

Low Field High Harmonic Response of Tl2Ba2Ca2Cu3O10 and SnMo6S8 Polycrystalline Superconductors

High harmonic response of polycrystalline Tl₂Ba₂Ca₂Cu₃O₁₀ and SnMo₆S₈ superconductors are measured in the presence of small ac excitation field and dc magnetic field applied on it. A purpose of this work is to study the Josephson weak link behavior and make comparison between two systems in order to understand a granular nature of polycrystalline superconductors. Critical state models are used to explain the nonlinear magnetic response in polycrystalline superconductors. For High-T _c superconductor the Anderson-Kim critical state model and for Chevrel-phase superconductors a field independent Bean’s model, are observed to work reasonably well.