F. A. B. Chaves

A New Noncentrosymmetric Superconducting Phase in the Li–Rh–B System

Superconductivity, at 2-3 K, was observed in a novel phase of the ternary Li-Rh-B system. The structural phase exhibits a large noncentrosymmetric cubic unit cell with the a-parameter being within 1.208 \leq a \leq 1.215 nm. This phase is stable over a wider compositional range of LixRhBy (0.6 < x < 2, 1 < y < 2). The superconductivity, as well as the unit cell volume, is sensitive to the Li/B content but it is manifested with Tc \geq 1.8 K over a wider compositional range: the highest Tc \approx 3 K occurs for x : y \approx 0.9:1.5 with a \approx 1.209 nm. The superconducting shielding fraction of most samples is almost 80% of that of Sn. The lower critical field, Hc1(0), is ~65 Oe while the upper one, Hc2(0) is determined from extrapolation to be higher than 14 kOe. We discuss the influence of pressure on Tc and also the influence of the lack of inversion symmetry on the superconducting properties.

Resistivity studies on the layered semi-metallic CaAl2Si2: evaluating its temperature-, field- and pressure-dependence

We studied the layered, hexagonal, semi-metal CaAl(2)Si(2) by magnetization, specific heat and resistivity measurements over a wide range of temperature, pressure and magnetic field. Both the Sommerfeld coefficient (γ = 1 mJ mol(-1) K(-2)) and the Debye temperature (θ(D) = 288 K) are in agreement with the values obtained from the band structure calculation. The resistivity shows a metallic character up to 200 K, followed by saturation and, afterwards, a weak decrease up to 840 K, at which it sharply rises reaching a local maximum at 847 ± 5 K. While the low-temperature thermal evolution was accounted for in terms of intrinsic and extrinsic effects, the additional high-temperature scattering was attributed, based on differential thermal analysis, to a first-order thermal event. No appreciable magnetoresistivity was observed at liquid helium temperatures even for fields up to 90 kOe, indicating an absence of coupling between the electronic and magnetic degrees of freedom. Finally, an externally applied pressure was found to induce a strong reduction in the resistivity following a second-order polynomial: this effect will be discussed in terms of the influence of pressure on the effective mobility and concentration of charge carriers.

Synthesis and magnetic characterization of TmCo2B2C

A new quaternary intermetallic borocarbide TmCo2B2C has been synthesized via a rapid-quench of an arc-melted ingot. Elemental and powder-diffraction analyses established its correct stoichiometry and single-phase character. The crystal structure is isomorphous to that of TmNi2B2C(I4/mmm) and is stable over the studied temperature range. Above 7 K, the paramagnetic state follows the modified Curie-Weiss behavior (X=C/(T-theta)+X0 wherein X0=0.008(1) emu/mole and the temperature-dependent term reflecting the paramagnetism of the Tm subsystem: ueff=7.6(2) uB [in agreement with the expected value for a free Tm3+ ion] and theta = -4.5(3) K. Long range ferromagnetic order of the Tm sublattice is observed to develop around ~1 K. No superconductivity is detected in TmCo2B2C down to 20 mK, a feature which is consistent with the general trend in the RCo2B2C series. Finally, the influence of the rapid-quench process on the magnetism (and superconductivity) of TmNi2B2C will be discussed and compared to that of TmCo2B2C.