Vibrational\nFermi Resonance in Atomically Thin Black\nPhosphorus

Fermi resonance is a phenomenon involving the hybridization\nof\ntwo coincidentally quasi-degenerate states that is observed in the\nvibrational or electronic spectra of molecules. Despite numerous examples\nin molecular systems, vibrational Fermi resonances in dispersive semiconducting\nsystems remain largely unexplored due to the rarity of occurrence.\nHere we report a vibrational Fermi resonance in atomically thin black\nphosphorus. The Fermi resonance arises via anharmonic mixing of a\nfundamental Raman mode and a Davydov component of an infrared mode,\nleading to a doublet with mixed character. The extent of Fermi coupling\ncan be modulated by the application of external biaxial strain. The\nconsequences of Fermi hybridization are revealed by electronic resonance\neffects in the thickness-dependent and excitation-wavelength-dependent\nRaman spectrum, which is predicted by <i>ab initio</i> hybrid\nfunctional simulations including excitonic interactions. This work\nreveals new insight into electron–phonon coupling in black\nphosphorus and demonstrates a novel method for modulating Fermi resonances\nin 2D semiconductors.