Ultrafast carrier-coupled interlayer contraction, coherent intralayer motions, and phonon thermalization dynamics of black phosphorus

TL;DR

This study reveals ultrafast, anisotropic lattice contractions and vibrational dynamics in black phosphorus following photoexcitation, highlighting strong carrier-phonon coupling and complex thermalization processes within a few picoseconds.

Contribution

It uncovers the ultrafast coupled interlayer and intralayer motions in black phosphorus using electron diffraction, demonstrating directional atomic motions driven by photocarriers without phase transition.

Findings

Interlayer contraction occurs within a few picoseconds.

Coherent intralayer vibrations are observed concurrently.

Phonon thermalization reaches quasi-equilibrium in about 50 ps.

Abstract

Black phosphorus (BP) exhibits highly anisotropic properties and dynamical behavior that are unique even among two-dimensional and van der Waals (vdW) layered materials. Here, we show that an interlayer lattice contraction and concerted, symmetric intralayer vibrations take place concurrently within few picoseconds following the photoinjection and relaxation of carriers, using ultrafast electron diffraction in the reflection geometry to probe the out-of-plane motions. A strong coupling between the photocarriers and BP's puckered structure, with the alignment of the electronic band structure, is at work for such directional atomic motions without a photoinduced phase transition. Three temporal regimes can be identified for the phonon thermalization dynamics where a quasi-equilibrium without anisotropy is reached in about 50 ps, followed by propagation of coherent acoustic phonons and heat diffusion into the bulk. The early-time out-of-plane dynamics reported here have important implications for single- and few-layer BP and other vdW materials with strong electronic-lattice correlations.