Crystal growth, characterization, and point-contact Andreev-reflection spectroscopy of the noncentrosymmetric superconductor Mo3Al2C

We report on the first successful growth of single crystals of the noncentrosymmetric superconductor ${\mathrm{Mo}}_{3}{\mathrm{Al}}_{2}\mathrm{C}$ obtained by means of a cubic-anvil, high-pressure, and high-temperature technique. Composition, structure, and normal-state transport properties of the crystals were studied by means of x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetic susceptibility, and resistivity measurements as a function of temperature. Variations in critical temperature (${T}_{c}$) between 8.6 and 9.3 K were observed, probably due to the slightly different carbon stoichiometry of the samples. Single-crystal x-ray refinement confirmed the high structural perfection of the grown crystals. Remarkably, the refined Flack parameter values for all the measured crystals using a $P{4}_{1}32$ space-group model were consistently close to either 0 or 1, hence indicating that the considered crystals belong to two enantiomorphic space groups, $P{4}_{1}32$ and $P{4}_{3}32$. An anomaly in the resistivity is observed at $\ensuremath{\simeq}130$ K, most likely associated with the onset of a charge-density-wave phase. The superconducting properties (and in particular the symmetry, the amplitude, and the temperature dependence of the superconducting gap) were studied by using point contact Andreev-reflection spectroscopy. The results confirm that ${\mathrm{Mo}}_{3}{\mathrm{Al}}_{2}\mathrm{C}$ is a moderately strongly-coupled superconductor with $2\mathrm{\ensuremath{\Delta}}/{k}_{\mathrm{B}}{T}_{c}\ensuremath{\simeq}4$ and unambiguously prove that the order parameter has an $s$-wave symmetry despite the asymmetric spin-orbit coupling arising from the lack of inversion symmetry.