Spectroscopic evidence of topological phase transition in the three-dimensional Dirac semimetal Cd3(As1−xPx)2

We study the low-energy electronic structure of three-dimensional Dirac semimetal, ${\mathrm{Cd}}₃{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_x)}₂$ [$x=0$ and 0.34(3)], by employing angle-resolved photoemission spectroscopy (ARPES). We observe that the bulk Dirac states in ${\mathrm{Cd}}₃{({\mathrm{As}}_0.66{\mathrm{P}}_0.34)}₂$ are gapped out with an energy of 0.23 eV, contrary to the parent ${\mathrm{Cd}}₃{\mathrm{As}}₂$ in which the gapless Dirac states have been observed. Thus, our results confirm the earlier predicted topological phase transition in ${\mathrm{Cd}}₃{\mathrm{As}}₂$ with perturbation. We further notice that the critical P substitution concentration, at which the two Dirac points that are spread along the $c$-axis in ${\mathrm{Cd}}₃{\mathrm{As}}₂$ form a single Dirac point at $\mathrm{\ensuremath{\Gamma}}$, is much lower [${x}_c(\mathrm{P})<0.34(3)$] than the predicted value of ${x}_c(\mathrm{P})=0.9.$ Therefore, our results suggest that the nontrivial band topology of ${\mathrm{Cd}}₃{\mathrm{As}}₂$ is remarkably sensitive to the P substitution and can only survive over a narrow substitution range, i.e., $0\ensuremath{\le}x(\text{P})<0.34(3)$.