Nonperturbative determination of isotope-induced anomalous vibrational physics*

In general, lattice vibrational spectra has been well described by quantum perturbation theory (QPT) to provide footprint characteristics for common crystals. However, despite weak phonon anharmonicity, the recently discovered cubic crystals (boron arsenide and boron phosphide) have shown anomalous vibrational dynamics with elusive fundamental origin. Here, we developed a nonperturbative ab initio approach, together with spectroscopy and high-pressure experiments, to successfully determine the exact dynamic evolutions of the vibrational physics. We found that the local disorders and coupled isotopes significantly dictate the vibrational spectra, through the Brillouin zone folding that has previously been ignored in literature. By decomposing vibrational spectra into individual isotope eigenvectors, we observed both positive and negative contributions to Raman intensity from constitutional atoms (10B, 11B, 75As, or 31P). Importantly, our nonperturbative theory predicts that a vibrational resonance appears at high hydrostatic pressure due to broken translational symmetry, which was indeed verified by experimental measurement under a pressure up to 31.5 GPa. In this paper, we develop fundamental understandings for the anomalous lattice physics under the failure of QPT and provide an approach in exploring transport phenomena for materials of extreme properties.