H.A. Borges

Magnetism and superconductivity in Ce_2RhIn_8

We report the discovery of pressure-induced superconductivity, with T_c=2 K, in the heavy-fermion antiferromagnet Ce_2RhIn_8, where superconductivity and magnetic order coexist over an extended pressure interval. A T-linear resistivity in the normal state, accessed by an applied magnetic field, does not appear to derive from the existence of a 2D quantum-critical spin-density wave.

Pressure study of the paraconductivity of high Tc superconductors

Abstract We measure the resistivity of several high T c ceramic superconductors ( REBa 2 Cu 3 O 7− δ , RE = Y, Yb, Gd and Er) under applied pressure. An analysis of the temperature derivative of the resistivity close to the critical temperature shows that the transition to the superconducting state occurs in two steps. We discuss the origin of this double transition and obtain the exponents associated with the paraconductivity which give information on the dimensionality of the critical fluctuations.

Magnetism in CeRu2Ge2 and CeRh2Ge2 and its pressure dependence

Abstract We have studied magnetism in CeRu 2 Ge 2 and CeRh 2 Ge 2 through specific heat in zero and applied magnetic fields, DC magnetization, thermal expansion and electrical resistance measurements at pressures to 21 kbar. Both compounds have two magnetic transitions; although we argue that the higher one in CeRu 2 Ge 2 is due to residual stress in the polycrystalline sample. The overall systematics are consistent with those found at ambient pressure in pseudoternaries formed with Si substitution for Ge.

Processing of bulk Bi-2223 high-temperature superconductor

The Bi2Sr2Ca2Cu3 O10+x (Bi-2223) is one of the main high temperature superconductors for applications. One of these applications is the Superconductor Fault Current Limiter (SCFCL), which is a very promising high temperature superconducting device. SCFCLs can be improved by using bulk superconductors with high critical currents, which requires a sufficiently dense and textured material. In the present work, a process for improving the microstructure of Bi-2223 bulk samples is investigated. Pressed precursor blocks are processed by sintering with a further partial melting step, in order to enhance the Bi-2223 grain texture and to healing cracks induced by pressing. In order to improve the microstructure, the precursor is mixed with silver powder before pressing. Samples with and without silver powder have been studied, with the aim of investigating the influence of silver on the microstructure evolution. The phase contents and the microstructure obtained have been analyzed through XRD and SEM/EDS. The electromagnetic characterization has been performed by Magnetic Susceptibility Analysis. We present and discuss the process and the properties of the superconducting blocks. High fractions of textured Bi-2223 grains have been obtained.

Preparation of Bi-2223/Ag tapes by controlling precursor powders

Abstract The effects of the precursor powder on Bi-2223 phase formation and the superconducting properties of Ag sheathed Bi-2223 tapes were studied. Three precursors were prepared. One was made by the “one-powder” method and the other two were prepared by the “two-powder” method, in which two compounds were mixed. The results showed that not only the phase assemblage, but also the microstructure of a precursor could influence the 2223 formation and the final morphology of the tapes.

Investigation of Pb-free Bi-2223 high temperature sintering

The Bi-2223 performance could be significantly improved by melt processing. However, it is a very difficult task, partly due to the volatility of Pb, its main dopant. On the other hand, it is also a hard task to obtain high Pb-free 2223 amounts, even by sinterization using a primary phase field composition (Bi/sub 2.5/Sr/sub 1.9/Ca/sub 2.1/Cu/sub 3/O/sub x/). In the present work, we investigate the sinterization of Pb-free 2223 in the presence of appreciable liquid fractions. The samples were analyzed by XRD, SEM/EDS and Magnetic Susceptibility. The results show significant Pb-free 2223 amounts after sintering at temperatures close to the melting. Such investigation represents an additional step toward the elucidation of the Pb-free 2223/melt equilibrium.

Effect of pressure of the magnetic transitions in CeRh2Ge2 and CeRu2Ge2

We have studied the effect of pressure on the magnetic transition in CeRh 2 Ge 2 and CeRu 2 Ge 2 by electrical resistance measurements in the temperature range between 1.2 K and 300 K. Both samples show two magnetic transitions below 30 K. In CeRh 2 Ge 2 , the magnetic transition temperatures increase with increasing pressure. In the case of CeRu 2 Ge 2 , the upper magnetic transition temperature increases with pressure, whereas the lower one decreases with increasing pressure