Weak antilocalization effect and triply degenerate state in Cu-doped CaAuAs

The effect of 50% Cu doping at the Au site in the topological Dirac semimetal CaAuAs is investigated through electronic band structure calculations, electrical resistivity, and magnetotransport measurements. Electronic structure calculations suggest a broken-symmetry-driven topological phase transition from the Dirac to triple-point state in CaAuAs via alloy engineering. The electrical resistivity of both the CaAuAs and ${\mathrm{CaAu}}_0.5{\mathrm{Cu}}_0.5\mathrm{As}$ compounds shows metallic behavior. Nonsaturating quasilinear magnetoresistance (MR) behavior is observed in CaAuAs. On the other hand, MR of the doped compound shows a pronounced cusplike feature in the low-field regime. Such behavior of MR in ${\mathrm{CaAu}}_0.5{\mathrm{Cu}}_0.5\mathrm{As}$ is attributed to the weak antilocalization (WAL) effect. The WAL effect is analyzed using different theoretical models, including the semiclassical $\ensuremath{\sim}\sqrt{B}$ one which accounts for the three-dimensional WAL and the modified Hikami-Larkin-Nagaoka model. A strong WAL effect is also observed in the longitudinal MR, which is well described by the generalized Altshuler-Aronov model. Our study suggests that the WAL effect originates from weak disorder and the spin-orbit coupled bulk state. Interestingly, we have also observed the signature of chiral anomaly in longitudinal MR when both the current and field are applied along the $c$ axis. The Hall resistivity measurements indicate that the charge conduction mechanism in these compounds is dominated by the holes with a concentration $\ensuremath{\sim}{10}²⁰\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and mobility $\ensuremath{\sim}{10}²\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}²\phantom{\rule{4pt}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$.