R. Janes

Synthesis of YBa2Cu4O8 at one atmosphere oxygen pressure by the modified citrate gel process

High Tc superconducting YBa2Cu4O8 was successfully synthesized via a modified citrate gel process at one atmosphere oxygen pressure. The resulting material had a stable and reproducible superconducting transition temperature around 80 K, confirmed by both resistivity and magnetic measurements. This technique can be employed in a large‐scale preparation of the YBa2Cu4O8 powders.

Superconductivity up to 105K in the new high-Tc TlVSrCaCuO and TlVSrCaYCuO systems

Abstract Superconductivity up to 105K has been observed in the Tlue5f8Vue5f8Srue5f8Caue5f8Cuue5f8O and Tlue5f8Vue5f8Srue5f8Caue5f8Yue5f8Cuue5f8O systems using both electrical resistivity and magnetisation measurements. From the results of X-ray diffraction, energy dispersive spectrometry and selected area electron diffraction, we identify the phases responsible for the superconductivity to be (Tl0.72V0.28)Sr2CaCu2Oy (with Tc=80K) and (Tl0.72V0.28)Sr2(Ca0.86Y0.14)Cu2Oy (with Tc=105K). We propose that the partial substitution of vanadium for thallium in the crystal lattice stabilizes the TlSr2CaCu2Oy phase and enhances superconductivity up to 80K. Importantly, 20% of Y3+ replacement of Ca2+ sites in the (Tl, V)Sr2CaCu2Oy compound reduces the excess hole concentration and increases superconductivity by about 20K.

On the absence of EPR in cuprate superconductors and their antiferromagnetic antecedents

Abstract We propose here an explanation for the absence of intrinsic Electron Paramagnetic Resonance (EPR) in high-T c ceramic cuprate superconductors in their normal state. This phenomenon is compared to and contrasted with the absence of a Cu 2+ resonance in the related semiconducting, antiferromagnetic cuprates such as La 2 CuO 4 . We report structural data, and the absence of EPR, for the series La 2 Cu 1−x Zn x O 4 and Nd 2 Cu 1−x Zn x O 4 for x=0 to 0.1. It has previously been reported that antiferromagnetic order is rapidly suppressed by the substitution of Zn onto Cu sites in La 2 CuO 4 . These Zn substituted materials (and the related Nd 2 Cu 1−x Zn x O 4 series) exhibit no EPR signals from 77K to room temperature. Most notably, the absence of EPR for these cuprates in which the Neel temperature is less than, or comparable to, the observation temperature reflects the strong 2D antiferromagnetic coupling within the CuO 2 planes.

Critical current densities of superconducting YBa2Cu4O8 synthesised by the citrate gel process

Abstract High-T c superconducting YBa 2 Cu 4 O 8 submicron powders with transition temperature around 80 K were successfully synthesized via citrate gel process at one atmosphere oxygen pressure. An intragrain critical current density of up to 1.3×10 7 A / cm 2 at 6 K and H=1 T has been measured by magnetisation method; this value is comparable to that found in sintered YBa 2 Cu 3 O 7−δ . The integrain criticalcurrent density, as measured by an a.c. inductive technique, was 10 3 A/cm 2 at 4.2 K; this is in excess of typical values for YBa 2 Cu 3 O 7−δ .

The solid solution phase Tl2-xCuxO3 and its properties

Abstract Several nominal compositions in the Tl 2 O 3 ue5f8CuO binary system have been prepared and investigated by X-ray powder diffractometry, electron diffractometry and energy dispersive X-ray spectrometry. Only the solid solution phase Tl 2− x Cu x O 3 ( x ⩽ 0. 12) was observed. Lattice distortion, resistivity and electron spin resonance measurements are discussed.

An investigation of the new high-Tc Tl-V-Sr-Ca-Cu-O and Tl-V-Sr-Ca-Y-Cu-O superconductors

Superconductivity up to 105K has been achieved in the Tl-V-Sr-Ca-Cu-O and Tl-V-Sr-Ca-Y-Cu-O systems. The partial substitution of vanadium for thallium in the crystal lattice stabilizes the TlSr2CaCu2O7 phase and enhances superconductivity up to 80K. Importantly, 20% of Y3+ replacement of Ca2+ sites in the (Tl,V)Sr2CaCu2Oy compound reduces the excess hole concentration and increases superconductivity by about 20K.

Synthesis of high critical current YBa2Cu4O8 submicron powders at one atmosphere pressure by the citrate-gel process

High-Tc superconducting YBa2Cu4O8 submicron powders was successfully synthesised via citrate-gel process at one atmosphere oxygen pressure. The resulting material had a stable and reproducible superconducting transition temperature around 80K. The intra-grain critical current density has been up to 1.3×107 A/cm2 at 6K and H=IT. This technique can be employed in a large-scale preparation of the YBa2Cu4O8 powders.

Preparation of Ti-Ca-Ba-Cu-O single crystals with Tc up to 118 K by the step-cooling method

Single crystals of the Tl-Ca-Ba-Cu-O system with superconducting transition temperature (Tc) up to 118 K have been successfully prepared by the step-cooling method. Energy-dispersive X-ray spectrometry, selected area electron diffraction and high-resolution electron microscopy were used to determine the composition and crystal structure of the superconducting phase. The crystals with chemical composition Tl2Ca2Ba2Cu3OY are based on a body-centred tetragonal unit cell with a=3.85 AA and c=35.9 AA. Large-scale single crystals of Tl2Ca3Ba2Cu4OY, containing some intergrowths with Tl2Ca2Ba2Cu3OY and Tl2Ca4Ba2Cu5OY, were also observed. Electron spin resonance measurements revealed a low-field microwave absorption below Tc.

Magnetic properties of the high-Tc superconductor Tl0.5Pb0.5Ca0.8Er0.2Sr2Cu2 oxide

The magnetic properties of the monophasic superconductor Tl0.5Pb0.5Ca0.8Er0.2Sr2Cu2Oy with a TlCaBa2Cu2O7-type (1122) structure have been studied using a combination of magnetisation and ESR (Electron Spin Resonance) measurements. This new material exhibits a superconductive transition (Tc) around 110 K, above which the temperature dependence of the magnetic susceptibility can be described by a Curie-Weiss law. Below Tc the co-existence of superconductivity and paramagnetism was observed.

Electron spin resonance studies of some cuprate(II)systems

The electron spin resonance (ESR) of a number of ternary oxides of copper (II) has been studied at both 77 K and 300 K. Materials investigated include those which might be considered 1-dimensional analogues of La2CuO4, namely A2CuO3 (A=Ca,Sr, Ba) plus the related material SrCuO2. The two oxygen rich derivatives of Ba2CuO3 have also been studied in view of their different oxygen environments around copper. Also other alkaline earth cuprates such as the recently described Sr14Cu24O41-x and Bi2Cu2O5, both pure and doped with Sr2+ are included. Compounds containing extended Cu-O chains are found to be essentially ESR silent both at room temperature and 77 K, regardless of whether the chains are one-dimensional or form two-dimensional sheets as in Ca(Sr)CuO2. In Sr14Cu24O41−x, which contains both Cu-O chains and chains of edge-shared CuO4 square-planar units there is an asymmetric signal which sharpens on cooling.