Giant quartic-phonon decay in PVD-grown α-MoO₃ flakes

Elementary excitations, such as in-plane anisotropic phonons and phonon polaritons (PhPs), in α-MoO₃ play key roles in its outstanding physical properties like high carrier mobility and ultralow phonon thermal conductivity ( ${\kappa }_{\mathrm{P}}$). Understanding the excitation mechanisms like phonon–phonon interactions is the most fundamental step to further applications. Here, we report on the systematic Raman investigations on phonon anisotropy and anharmonicity of representative Mo–O stretching vibration phonon modes (SVPMs) in physical vapor deposition (PVD)-grown α-MoO₃ flakes. Polarizations of SVPMs verify the phonon anisotropy. The abnormal temperature dependence of SVPMs reveals that giant quartic-phonon decay dominates the phonon anharmonicity in α-MoO₃. An ultrashort phonon lifetime of ~ 0.34 ps gives evidence of theoretically predicted ultralow ${\kappa }_{\mathrm{P}}$ in α-MoO₃. Our findings give deep insight into the phonon–phonon interactions in α-MoO₃ and provide an indicator for its extreme thermal device applications.