Thermal Conductivity Of Monolayer Hexagonal Boron Nitride: Four-Phonon Scattering And Quantum Sampling Effects

Monolayer hexagonal boron nitride is a prototypical planar 2-dimensional
system material and has been the subject of many investigations of its
exceptional vibrational, spectroscopic and transport properties. The lattice
thermal conductivity remains quite uncertain, with theoretical and experimental
reports varying between 218 and 1060 Wm-1K-1. It has a strong temperature
evolution and is sensitive to strain effects and isotope concentrations. While
the impact of isotope scattering has been widely studied and is well
understood, nuclear quantum effects and 4-phonon scattering have so far been
neglected. Monolayer hexagonal boron nitride is composed of light elements, and
further has its 3-phonon scattering phase space restricted by mirror plane
symmetry, so these effects may be of similar order as isotope scattering, and
would lead to a completely different understanding of the fundamental processes
limiting the lattice thermal conductivity for this system. In this work, we use
both classical and path-integral molecular dynamics, in conjunction with the
Temperature Dependent Effective Potential method, to compute
temperature-dependent renormalized phonons including isotope scattering,
3-phonon scattering, 4-phonon scattering and nuclear quantum effects. We show
the impact of the latter two on the lattice thermal conductivity for a large
temperature range, as well as their impact on the phonon lifetimes. Overall,
our work provides a robust framework for calculations of the lattice thermal
conductivity in solids, providing quantitative improvements and physical
understanding that help explain the variety of results found in the literature.