Tl-based Superconductors Research Articles

The effect of O2 pressure on phase formation and superconductivity properties of Tl-based HTS

Thallium-based high-temperature superconductors exhibit excellent superconductivity performance (Tc, Jc). Formation of the Tl-based superconducting materials critically depends on the used precursor and synthesis conditions. In the present investigation, we used the in situ polymerization method for synthesizing high-purity and reaction-ability precursors. We find the critical dependence of Tl-based superconductor phase formation on the quantity of applied oxygen pressure. The experimental results showed that the Tl-1223 phase formed at an oxygen gas pressure at 1 to 2 atm, Tl-2223 phase at 3 atm, and Tl-2212 phase at 4 to 8 atm.

The dependence of the critical temperature on pressure

The aim of this research is to study the critical temperature depending on the pressure of one-band superconductor. We derive the exact equation of the critical temperature [Formula: see text] by using the BCS-like model. The effect of pressure and pseudogap on critical temperature has been investigated. The analytic form and the approximation of the critical temperature are shown. First, we consider the effect of pressure on the critical temperature and find that the critical temperature increased as pressure is increased which fits well with the experimental data of Tl-based and Bi-based superconductors. Second, the effect of the pseudogap on the critical temperature is considered. We found that the critical temperature is decreased as pressure increased which agrees to the data of [Formula: see text], the iron chalcogenides [Formula: see text], intermetallic compounds [Formula: see text] and [Formula: see text] superconductors.

Probing the nature of the diamagnetic transition in Tl-based superconductors

We summarize here some of our magnetization measurements performed around Tc in Tl2Ba2Ca2Cu3O10 and Tl0.5Pb0.5Sr2CaCu2O7 single crystals with narrow Meissner transitions (which guarantees the absence of appreciable Tc inhomogeneities). The experimental data are quantitative and consistently explained in the different regions of the T – H diagram in terms of the conventional Ginzburg-Landau approaches for layered superconductors.

Microstructure dependent microwave properties of spray deposited Tl–Ba–Ca–Cu–O films

Tl-based superconductors are found to be the potential candidate for enormous applications especially in field of microwave devices. Thick Tl-based single phase (2 2 2 3) superconducting films have been prepared successfully on CeO 2 buffered Al 2O 3 substrates by two-step technique consisting of spray pyrolysis followed by thalliation. The modifications in structure and morphology by changing reaction conditions were studied. The microstructures were found to be sensitive to synthesis conditions and in turn their dependence on surface resistance of films. The microwave absorption properties were studied in the frequency range from 1 GHz to 10 GHz. The surface resistance of Tl–Ba–Ca–Cu–O films at room temperature was measured with the help of stripe line technique and found to be 14.9 mΩ/cm 2. The effects of microstructure on microwave absorption peaks are studied and results are discussed at the length in this paper.

Doped high- superconductors under pressure

Abstract Within the BCS framework generalized Fermi surface topologies, modeled with band overlapping, are proposed as a mechanism to enhance the electronic density of states at the Fermi level. This band overlapping allows the high- T c values observed in cuprate superconductors at different pressures. With this approach the correlation between the critical temperature, the charge carrier density and the pressure is obtained in terms of the coupling constant and the band overlapping parameter. In order to obtain numerical results the model is applied to a Tl-based superconductor.

Influence of the reaction conditions on the formation of Tl(Re)-Ba-Ca-Cu-O superconducting thin films by thallination in open system

Abstract Rhenium avoids air degradation of the target as well as of precursor thin films during the preparation of high temperature Tl-based superconductors. Addition of Re has been used during synthesis of oriented Tl(Re)-Ba-Ca-Cu-O films. High temperature superconducting Tl-based thin films were prepared by a two step method combining RF magnetron sputtering and ex-situ thallination. Precursors with a composition of Re0.1Ba2Ca2Cu3Ox were deposited on a CeO2 buffered R-plane sapphire substrate. The sputtered films were prepared at room temperature in an Ar atmosphere. The thallination of the precursor films was performed in a one zone configuration, where both the pellet and precursor film were kept at the same temperature. The thallination temperature varied in the range of 850–880°C and samples were held for 30, 45 and 60 min at this temperature in a flowing oxygen atmosphere. Besides Tl-2212, X-ray diffraction reveals the possibility of also preparing the Tl-2223 phase, which was until now not reported in a Re doped form.

Out-of-plane temperature-dependent resistivity studies on Tl-based superconductors

Temperature-dependent resistivities in theab-planeand c-axis of Tl-based cuprates have been measured. Unlike theab-plane properties,which are metallic, c-axis transport is semiconductor-like in the normal state forTl2Ba2Ca2Cu3Ox (Tl-2223) andTl2Ba2CaCu2Ox (Tl-2212). Incontrast, for Tl2Ba2CuOx (Tl-2201), transport is metal-like in both the in-plane and thec-axis. For multi-layered cuprates, transport properties along thec-axis could be described by a tunnelling model, whereas for single-layered compoundTl-2201 it would be easier for the out-of-plane transport behaviour to be coherentsince the there are no insulating Ca layers in its structure. Moreover, combiningthe studies on Bi-2201, which has an insulating behaviour for the out-of-planeresistivity, we suggest that the Tl–O layers in Tl-based superconductors couldbe conducting, unlike the weakly correlated Bi–O layers in Bi-based cuprates.

Vortex imaging in Tl-based superconductors with a scanning SQUID microscopy

Vortex images of superconducting material Tl-1223 thin film taken with a scanning SQUID microscope are presented. Among the vortices images, coexistence of vortex–antivortex pairs is observed at low field region. Short distances of the pairs indicate the existence of strong pinning centers in this material.

Vortex observation in Tl-based superconductors with a scanning SQUID microscopy

Pinning properties of superconducting material Tl-1223 thin film are investigated. Vortex images are compared between a pristine and high-energy gold ion irradiated sample. This comparison implies which pinning center is effective for this material between large pinning centers and columnar defect. Large pinning centers are better for this material.

Temperature and magnetic field effects on the carrier density and Hall mobility of boron-doped Tl–Ba–Ca–Cu–O superconductor

Boron-doped Tl-based superconductor was prepared by adding an amount of 1 wt.% B to the Tl 1.8Ba 2Ca 2.2Cu 3O x compound. The usual solid-state reaction method has been applied under optimum conditions. The X-ray data of the sample show a tetragonal structure with a high ratio of Tl-2223 superconducting phase. The sample showed a transition at 125 K and the zero resistance was observed at 120 K. The magnetic field and temperature effects on the normal state electrical resistivity, carrier density, and Hall mobility have been investigated. Both temperature and magnetic field significantly affect the resistivity behavior. The zero field resistivity was found to vary exponentially with temperature with a slope revealing activation energy of 27.5 meV. When the magnetic field is applied, the resistivity varied up-normally in the temperature region of 160–240 K. The temperature dependent carrier concentration calculated from the Hall coefficient data varied linearly with the applied magnetic field. This effect was attributed to the increase in the hole effective mass upon field increment. The temperature-dependent carrier concentration data at several applied fields were analyzed by the single donor–single acceptor model to obtain the values of effective masses. The temperature and magnetic field dependent normal state Hall mobility was found to be limited by the scattering of acoustic phonons.

Synthesis, X‐Ray Data, and Hall Effect Measurements of Li‐Doped Tl—Ba—Ca—Cu—O Superconductor.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of B2O3 addition on the formation and properties of Tl-2212 and Tl-2223 superconductors

We have studied the effect of B2O3 addition on the formation and properties of the Tl-based superconductors. Polycrystalline Tl-2212 and Tl-2223 samples were synthesized by adding B2O3 to the chemicals at the first and at the second stage of two stages sintering processes (3+1 h) at 860 °C. X-ray powder diffraction patterns, resistance versus temperature behaviours and the micrographs of the B2O3 free and B2O3 added samples have been analysed. The compositions and the critical temperatures of the samples indicate that about 1.0 weight percent B2O3 added at the first stage of sintering stabilize the Tl-2212 phase and at the second stage promote the formation of the Tl-2223 phase. For both of the phases, the a-lattice param-eter remains the same, but the c-parameter slightly decreases with B2O3 additions. Few small peaks due to the impurity phases appear in the diffraction patterns but none due to B2O3 or any other phase involving B atoms. We suggest that, with their small size B atoms may partially substitute Cu atoms and disperse at interstitial positions and grain boundaries. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microwave Josephson absorption of high-temperature superconductors

Magnetic field dependences of the direct and the magnetically modulated microwave absorption of a number of Y-, Bi- and Tl-based high-temperature superconductors were measured at 4.2 and 77 K by X-band microwave spectrometer. Correlation between value of the low field maximum of the absorption and the inter-grain critical currents of Josephson junctions was found. Results were analyzed and compared to theoretical models of microwave absorption available in the literature yielding good agreement.

Properties of boron doped Tl–Ba–Ca–Cu–O superconductors

The effects of boron doping on the formation and properties of the Tl-based superconductors have been studied. Up to 10 wt.% boron has been added to the oxides having the nominal composition, Tl 1.8Ba 2Ca 2.2Cu 3O x , by the usual solid-state reaction method. Boron additions in the range 0.8–1.0% increase the fraction of the Tl-2223 phase and significantly improve the critical temperature of the samples. Higher amounts of boron additions eliminate the Tl-2223 phase, reduce the fraction of Tl-2212 phase and cause separate non-superconducting phases to be formed. The formation of non-superconducting phases does not allow us to obtain pure Tl-2212 phase.

Atomic structures of surface and interface in (Hg,Tl,Pb)-based superconductors studied by high-resolution electron microscopy

Atomic structures of surface and interface in (Hg,Tl,Pb)-based superconductors were investigated by high-resolution electron microscopy. The interfaces observed in Tl-based superconductors have the disordered region with ∼1 nm thickness, which would be suitable for barrier layers for tunneling junctions. Although the {001} planes of (Hg,Tl)-based oxides are fairly stable, most of the {100} planes are unstable, and atomic rearrangement was often observed. However, the HREM image of Hg 0.5Tl 0.5Ba 2CuO 5 {100} showed the stable Ba–O–Ba layer at the surface, and oxygen atom positions were determined. The present study indicates that the surfaces of the Hg- and Tl-based superconductors are strongly dependent on the crystallographic planes and crystal structures.

The CaO–CuO–TlO 1.5 system and its key role in the formation of the Tl-based superconductors

It has been shown recently that the formation of Tl 2Ba 2Ca 2Cu 3O z is conditioned by an equilibrium plane with the Tl-rich limit of the (Ca,Tl) 1− x CuO 2 solid solution as one of the apices. The parent phase for the latter solid solution is the binary compound Ca 0.80CuO 1.93. We report here on the phase relations in the CaO–CuO and the CaO–CuO–TlO 1.5 systems. The emphasis is also put on the thermodynamic stability and the crystallographic properties of Ca 0.80CuO 1.93 (space group P2/c, a=10.9456(4) Å, b=6.3192(2) Å, c=16.8408 Å and β=104.952(2)°). The kinetics for phase formation and decomposition have been studied. In particular, we show that the decomposition reaction into Ca 2CuO 3 and CuO which occurs at 870 °C in p(O 2)=1 bar and at 720 °C in vacuum, is rate limited by a two-dimensional process consistent with structural considerations and has an activation energy of 656 kJ/mol.

The effect of rare earth oxide substitution on the magnetic properties of Tl-based superconductors

The effects of the heterovalent substitution of the rare earth ions La 3+, Sm 3+, Pr 3+, Dy 3+ and of Y 3+ into the Ca 2+ separating layers on the magnetic properties of (Tl,Pb)-1223 and (Tl,Pb)-1212 superconductors are reported. Nominal compositions of (Tl 0.6Pb 0.5)(Sr 0.8Ba 0.2) 2(Ca 1− x M x ) 2Cu 3O 8+ δ , where M=La, Sm, Pr, Dy, Y and x=0.0, 0.05, 0.1, 0.15 and 0.2 were investigated. Increasing contents of the rare earth elements enlarged the superconducting Tl-1212 phase in the samples. This led to a decrease of the T c values. AC and virgin volume magnetization characteristics of all fabricated samples were measured at 77.3 K in magnetic fields ranging from 10 −1 to 10 5 Am −1. A decrease in the content of the Tl-1223 phase affected the magnetization mainly in the range of stronger fields around 10 5 Am −1. For the lower substitution levels ( x=0.05, 0.1) an improvement of the intergrain links was found.

Effect of lithium doping on the properties of Tl-basedsuperconductors

The effects of lithium doping on the formation and properties ofthe Tl-based superconductors have been studied. Lithium atoms up to around3 mol.% have been added to the oxides of nominal compositionTl1.8Ba2Ca2.2Cu3Ox, and the usual solid-state reactionmethod has been applied. Lithium additions in the range 0.23-0.29 mol.%increase the fraction of the Tl-2223 phase and significantly improve thecritical temperature of the samples. Higher amounts of lithium additionsdiminish the Tl-2223 phase, reduce the fraction of the Tl-2212 phase, andcause separate non-superconducting phases to be formed.

Electrodeposition of biaxially aligned Tl-based superconductors on Ag tapes

An alternative preparation method of Tl(1212) and Tl(1223) biaxially aligned tapes on Ag was developed: electrodeposition. This method is based on the co-deposition of the metals by an electrochemical process followed by a high temperature heat treatment in order to form the superconducting phase. Nitrates of the metals are dissolved in DMSO and the deposition on conducting substrates is performed in a standard three electrodes cell. The thermal treatment is performed in a high isostatic pressure furnace (1 bar O/sub 2//50 bar He) in order to prevent Tl evaporation. The main advantage of this low cost technique is that it is very fast, (/spl sim/1 /spl mu/m/min) and can be easily scaled up to produce long length tapes in a continuous process. The desired stoichiometry is obtained by varying iteratively the concentration of nitrates in the starting solution. Moreover, the reaction temperature and time of the thermal treatment are drastically reduced with respect to the powder synthesis due to the amorphous form of the precursor. Excellent biaxial alignment is observed on both {110} Ag single crystal and textured Ag tapes, while epitaxial growth is observed on SrTiO/sub 3/ single crystal. T/sub c/ of 118 K and 60 K were measured resistively for Tl(1223) and Tl(1212) samples, respectively.

ELECTRODEPOSITION OF BIAXIALLY ALIGNED TL-BASED SUPERCONDUCTORS ON AG TAPES

An alternative preparation method of Tl based superconducting tapes is discussed. We developed Ag textured tapes as substrates: this material can be used without any buffer layer owing to its weak chemical interaction with the HTSC. Electrodeposition proved to be a fast, cheap and suitable technique to deposit a precursor that can be subsequently transformed in the superconducting phase by an opportune thermal treatment. The solution is formed with nitrates of all the metals dissolved in DMSO. The as-electrodeposited precursor is a highly reactive mixture at atomic scale of all the metals; a rapid (~30 min) thermal treatment is performed in a high isostatic pressure furnace (1 bar O2/50 bar He) in order to prevent Tl evaporation. Epitaxial growth of Tl-based superconductor was achieved by this technique on <110> textured Ag, both on single crystals and on oriented tapes. Superconducting transitions are observed at ~60 K and ~118 K for Tl(1212) and Tl(1223), respectively.