Accessing the Anisotropic Nonthermal Phonon Populations in Black Phosphorus.

Hélène Seiler,Christian Carbogno,Ralph Ernstorfer,Marios Zacharias,Patrick-Nigel Hildebrandt,Thomas Vasileiadis,Daniela Zahn,Yoav William Windsor,Claudia Draxl,Fabio Caruso,Yingpeng Qi

doi:10.1021/acs.nanolett.1c01786

Abstract

We combine ultrafast electron diffuse scattering experiments and first-principles calculations of the coupled electron–phonon dynamics to provide a detailed momentum-resolved picture of lattice thermalization in black phosphorus. The measurements reveal the emergence of highly anisotropic nonthermal phonon populations persisting for several picoseconds after exciting the electrons with a light pulse. Ultrafast dynamics simulations based on the time-dependent Boltzmann formalism are supplemented by calculations of the structure factor, defining an approach to reproduce the experimental signatures of nonequilibrium structural dynamics. The combination of experiments and theory enables us to identify highly anisotropic electron–phonon scattering processes as the primary driving force of the nonequilibrium lattice dynamics in black phosphorus. Our approach paves the way toward unravelling and controlling microscopic energy flows in two-dimensional materials and van der Waals heterostructures, and may be extended to other nonequilibrium phenomena involving coupled electron–phonon dynamics such as superconductivity, phase transitions, or polaron physics.

Highlights

We combine ultrafast electron diffuse scattering experiments and first-principles calculations of the coupled electron−phonon dynamics to provide a detailed momentumresolved picture of lattice thermalization in black phosphorus
To unravel the origin of the nonequilibrium lattice dynamics and its signatures in Ultrafast electron diffuse scattering (UEDS) experiments, we conduct first-principles calculations of the coupled electron−phonon dynamics based on the timedependent Boltzmann formalism, whereby electron−phonon and phonon−phonon scattering processes are explicitly taken into consideration
We have provided a comprehensive picture of the microscopic energy flows in the crystal lattice of Black phosphorus (BP) following photoexcitation of the electrons

Summary

Introduction

We combine ultrafast electron diffuse scattering experiments and first-principles calculations of the coupled electron−phonon dynamics to provide a detailed momentumresolved picture of lattice thermalization in black phosphorus. Black phosphorus (BP) exhibits a tunable band gap in the mid-IR,[1−3] high carrier mobilities,[4−6] and a layered crystal structure These features make it a versatile platform to explore novel device concepts, such as field-effect transistors, saturable absorbers, and polarization-sensitive photodetectors.[2,4,5,7−9] The pronounced crystal structure anisotropy of BP further gives rise to highly anisotropic macroscopic properties, as exemplified by its thermal[10−12] and electrical conductivities,[1,5,13,14] as well as its optical response.[5,15−17]. To unravel the origin of the nonequilibrium lattice dynamics and its signatures in UEDS experiments, we conduct first-principles calculations of the coupled electron−phonon dynamics based on the timedependent Boltzmann formalism, whereby electron−phonon and phonon−phonon scattering processes are explicitly taken into consideration.

Methods

Results

Conclusion