Pressure-induced phonon softening and superconductivity in the topological semimetal ZrP2

In this work, we have systematically investigated the structural, vibrational, and electrical transport properties of topological semimetal $\mathrm{Zr}{\mathrm{P}}_{2}$ under pressure using synchrotron x-ray diffraction (XRD), Raman spectroscopy, electrical resistance, and Hall-effect measurements, in combination with theoretical calculations. The XRD data show that while no evident structural transitions occur with pressures up to 43.1 GPa, the lattice-parameter ratio ($a/b$) reveals a diplike anomaly at ${P}_{\mathrm{C}}\ensuremath{\sim}25$ GPa. Concomitantly, one Raman-active phonon mode (\ensuremath{\sim}113.3 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) becomes increasingly softening with pressure and then beyond ${P}_{\mathrm{C}}$ shows a hardening behavior, at various fixed temperatures from 300 K down to 10 K. This soft mode can be assigned to the ${A}^{1}{}_{\mathrm{g}}$ mode and refers to $ac$-plane vibrations of Zr and P atoms with ``vortexlike'' eigenvectors. Strikingly, superconductivity (SC) was observed to emerge around ${P}_{\mathrm{C}}$, with the onset critical temperature of \ensuremath{\sim}0.6 K increasing gradually to \ensuremath{\sim}1.3 K at the highest measured pressure of 49.5 GPa. In addition, a hole- to electron-dominated carrier-type crossover takes place again around ${P}_{\mathrm{c}}$, involving the Fermi-level crossing of higher-energy electron-like pockets along the \ensuremath{\Gamma}-$Y$ path. Analysis suggests that the emergence of SC may be triggered by the phonon-softening related enhancement of ionic polarizability. The present system thus can serve as an ideal platform to study the interplay of phonon softening and SC.