Upper Critical Field and Hall effect in superconducting perovskite

Upper critical field H c2 and Hall coefficient R H in superconducting per-ovskite M gCN i 3 have been studied. The normal state ρ(T) behavior is similar to that observed in oxide perovskite (Ba, K)BiO 3 (BKBO), in which ρ(T) fits well curve predicted by Bloch-Grüneisen theory consistently with electron-phonon scattering. H c2 (T), determined from the onset of the resis-tive superconducting transition under magnetic fields, can be described by an expression H c2 (T) = H c2 (0)[1 − (T /T c) 2 ] with the zero-temperature critical field H c2 (0) = 8.1 T followed by conventional type-II superconductors. It is found that R H of M gCN i 3 is negative and its magnitude increases as temperature decreases. At T = 100 K, R H = −4.3 × 10 −10 m 3 /C, and the calculated carrier density is 1.4 × 10 22 /cm 3 , which is comparable with that in perovskite (Ba, K)BiO 3 , and less than that of the metallic binary M gB 2. The recent discovery of superconductivity in the simple intermetallic compound MgB 2 [1] has attracted great attention because of its relatively high transition temperature (T c 1 = 39 K) and the highly promising potential application. It suggests that intermetallic compounds with simple structure types are worth serious reconsideration as sources of new superconducting materials. More recently, the observation of superconductivity at 8 K in the perovskite structure intermetallic compound MgCNi 3 [2] indicates that MgB 2 will not be the only one of its kind within the chemical paradigm for new superconducting materials. The variable stoichiometry compound MgC x Ni 3 , for 0.5 < x < 1.25, has been reported and assigned to the perovskite structure type by analogy in 1950's [3,4], but neither its crystal structure nor its physical properties had been determined. By using powder neutron diffraction, He et al. [2] has found that the superconducting phase in nominal composition MgC 1.25 Ni 3 is MgC 0.96 Ni 3 with the classical cubic perovskite structure, space group P m − 3m. They also determined the electron-phonon coupling constant λ ph ∼ 0.77 by specific heat measurements. Although this value of λ ph is in the range of conventional phonon, more properties of both normal state and superconducting state need to be clarified to determind the microscopic mechanism of superconductivity in this compound. A complete structural …