Dynamical Phonon Anharmonicity Drives Charge-Density-Wave Softening and Electronic Quasiparticle Decoherence in Monolayer 1H-NbSe2

Abstract

Charge-density-wave (CDW) instabilities in two-dimensional materials are governed by strongly anharmonic soft phonons, whose dynamical behavior remains poorly understood beyond the statically renormalized vibrations. Here, using monolayer 1H-NbSe2, we investigate how frequency-dependent three- and four-phonon interactions reshape phonon spectral functions and influence electronic excitations. We find that three-phonon interactions induce substantial dynamical softening of CDW modes, increasing the estimated transition temperature relative to static self-consistent phonon theory. Four-phonon interactions further promote such dynamical softening and enhance spectral broadening, producing non-Lorentzian lineshapes that challenge the quasiparticle picture. Incorporating dynamically renormalized phonon frequencies and electron-phonon coupling into Fan-Migdal electron self-energy calculations reveals enhanced electronic kinks, satellite features, and momentum-dependent quasiparticle decoherence, driven by the strong momentum dependence of the electron-phonon coupling matrix elements. These findings establish dynamical anharmonicity as a critical link between soft-mode lattice fluctuations and electronic spectral signatures, providing insights into coupled lattice-electron dynamics in two-dimensional quantum materials.