Direct Observation of Symmetry-Dependent Electron-Phonon Coupling in Black Phosphorus

Electron-phonon coupling in two-dimensional nanomaterials plays a fundamental role in determining their physical properties. Such interplay is particularly intriguing in semiconducting black phosphorus (BP) due to the highly anisotropic nature of its electronic structure and phonon dispersions. Here we report the direct observation of symmetry-dependent electron-phonon coupling in BP by performing the polarization-selective resonance Raman measurement in the visible and ultraviolet regimes, focusing on the out-of-plane A_g ¹ and in-plane A_g ² phonon modes. Their intrinsic resonance Raman excitation profiles (REPs) were extracted and quantitatively compared. The in-plane A_g ² mode exhibits remarkably strong resonance enhancement across the excitation wavelengths when the excitation polarization is parallel to the armchair (A_g ²//AC) direction. In contrast, a dramatically weak resonance effect was observed for the same mode with the polarization parallel to zigzag (A_g ²//ZZ) direction and for the out-of-plane A_g ¹ mode (A_g ¹//AC and A_g ¹//ZZ). Analysis on quantum perturbation theory and first-principles calculations on the anisotropic electron distributions in BP demonstrated that electron-phonon coupling considering the symmetry of the involved excited states and phonon vibration patterns is responsible for this phenomenon. Further analysis of the polarization-dependent REPs for A_g phonons allows us to resolve the existing controversies on the physical origin of Raman anomaly in BP and its dependence on excitation energy, sample thickness, phonon modes, and crystalline orientation. Our study gives deep insights into the underlying interplay between electrons and phonons in BP and paves the way for manipulating the electron-phonon coupling in anisotropic nanomaterials for future device applications.