Filled skutterudite superconductor CaOs4P12 prepared by high-pressure synthesis

Abstract

In this paper, we report the transport, thermodynamic, and superconducting properties of a new filled skutterudite ${\mathrm{CaOs}}_4{\mathrm{P}}_{12}$ synthesized under high pressure and high temperature. The electrical resistivity of 3.4–4.$8\mathrm{m}\mathrm{\Omega }\mathrm{cm}$, carrier concentration of 3.8–6.$1\times {10}^{20}{\mathrm{cm}}^{-3}$, and positive Hall coefficient suggest that ${\mathrm{CaOs}}_4{\mathrm{P}}_{12}$ is a semimetal with hole carriers. An anomaly due to low-energy optical modes corresponding to an Einstein temperature of 150 K was observed in the specific heat. Resistivity, dc magnetic susceptibility, and specific heat measurements indicate bulk superconductivity below 2.5 K. The specific heat anomaly at $T_{\mathrm{c}},\mathrm{\Delta }C/\gamma T_{\mathrm{c}}\approx 1.4$, is in agreement with the Bardeen-Cooper-Schrieffer (BCS) value of 1.43. The electron-phonon coupling constant ${\lambda }_{ep}$ is estimated to be 0.47. ${\mathrm{CaOs}}_4{\mathrm{P}}_{12}$ is classified as a BCS-type, weakly coupled type-II superconductor with an upper critical field of $H_{\mathrm{c}2}\approx 22$ kOe and Ginzburg-Landau coherence length of $\xi \approx 12$ nm.