Magnetic structure and critical behavior of GdRhIn5_{5}: resonant x-ray diffraction and renormalization group analysis

The magnetic structure and fluctuations of tetragonal GdRhIn5 were studied by resonant x-ray diffraction at the Gd LII and LIII edges, followed by a renormalization group analysis for this and other related Gd-based compounds, namely Gd2IrIn8 and GdIn3. These compounds are spin-only analogs of the isostructural Ce-based heavy-fermion superconductors. The ground state of GdRhIn5 shows a commensurate antiferromagnetic spin structure with propagation vector tau = (0,1/2, 1/2), corresponding to a parallel spin alignment along the a-direction and antiparallel alignment along b and c. A comparison between this magnetic structure and those of other members of the Rm(Co,Rh,Ir)n In3m+2n family (R =rare earth, n = 0, 1; m = 1, 2) indicates that, in general, tau is determined by a competition between first-(J1) and second-neighbor(J2) antiferromagnetic (AFM) interactions. While a large J1 /J2 ratio favors an antiparallel alignment along the three directions (the so-called G-AFM structure), a smaller ratio favors the magnetic structure of GdRhIn5 (C-AFM). In particular, it is inferred that the heavy-fermion superconductor CeRhIn5 is in a frontier between these two ground states, which may explain its non-collinear spiral magnetic structure. The critical behavior of GdRhIn5 close to the paramagnetic transition at TN = 39 K was also studied in detail. A typical second-order transition with the ordered magnetization critical parameter beta = 0.35 was experimentally found, and theoretically investigated by means of a renormalization group analysis.Comment: 22 pages, 4 figure

Two Superconducting Phases in CeRh_1-xIr_xIn_5

Pressure studies of CeRh_1-xIr_xIn_5 indicate two superconducting phases as a function of x, one with T_c >= 2 K for x < 0.9 and the other with T_c < 1.2 K for x > 0.9. The higher T_c phase, phase-1, emerges in proximity to an antiferromagnetic quantum-critical point; whereas, Cooper pairing in the lower T_c phase-2 is inferred to arise from fluctuations of a yet to be found magnetic state. The T-x-P phase diagram of CeRh_1-xIr_xIn_5, though qualitatively similar, is distinctly different from that of CeCu_2(Si_1-xGe_x)_2.Comment: 5 pages, 3 figure

CONVERSION AND GLOBAL REACTION RATE COEFFICIENT IN THE ABSORPTION OF SO2 BY DIFFERENT TYPES OF LIMESTONE IN A FLUIDIZED BED REACTOR

This work concerns the study of the effect of limestone type on SO2 absorption in a bench fluidized bed reactor plant. Conversion and global reaction rate coefficients were established for conditions typical to fluidized bed combustion of coal. The bench plant is a bubbling bed reactor 160 mm internal diameter using silica sand as bed material, fluidized by pre-heated air. In order to simulate conditions close to the fluidized bed coal combustion ambience, the fluidizing air is pre-heated at high temperature (850 oC) and SO2 is added to the fluidizing air in a concentration typical of the process (1000 ppm). All the particulate, i.e. silica sand and limestone particles, was fed to the bed in a narrow size distribution between two subsequent ASTM sieves (with 545 μm mean diameter). In transient batch experiments charges of limestone are quickly injected into the bed, while the consequent variations of the exit concentrations of SO2, CO2 and O2 are continuously recorded. Analysis were performed on the effects of the type of limestone in the process, taking into account possible reaction controlling resistances, and considering possible effects of the calcination on the sulfation process

Direct determination of the crystal field parameters of Dy, Er and Yb impurities in the skutterudite compound CeFe4_{4}P12_{12} by Electron Spin Resonance

Despite extensive research on the skutterudites for the last decade, their electric crystalline field ground state is still a matter of controversy. We show that Electron Spin Resonance (ESR) measurements can determine the full set of crystal field parameters (CFPs) for the Th cubic symmetry (Im3) of the Ce$_{1-x}$R$_{x}$Fe$_{4}$P$_{12}$ (R = Dy, Er, Yb, $x\lesssim 0.003$) skutterudite compounds. From the analysis of the ESR data the three CFPs, B4c, B6c and B6t were determined for each of these rare-earths at the Ce$^{3+}$ site. The field and temperature dependence of the measured magnetization for the doped crystals are in excellent agreement with the one predicted by the CFPs Bnm derived from ESR.Comment: 7 pages, 5 figures, to appear in PR