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.$8\mathrm{m}\mathrm{\ensuremath{\Omega}}\mathrm{cm}$, carrier concentration of 3.8--6.$1\ifmmode\times\else\texttimes\fi{}\phantom{\rule{4pt}{0ex}}{10}^{20}{\mathrm{cm}}^{\ensuremath{-}3}$, 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 ${T}_{\mathrm{c}},\mathrm{\ensuremath{\Delta}}C/\ensuremath{\gamma}{T}_{\mathrm{c}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}1.4$, is in agreement with the Bardeen-Cooper-Schrieffer (BCS) value of 1.43. The electron-phonon coupling constant ${\ensuremath{\lambda}}_{ep}$ 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 ${H}_{\mathrm{c}2}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}22$ kOe and Ginzburg-Landau coherence length of $\ensuremath{\xi}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}12$ nm.