Polarons in Two-Dimensional Materials—Boron Nitride and Beyond

The polaron is an emergent quasiparticle that consists of an electron dressed by a cloud virtual phonons. Polarons arise from the interaction between electrons and phonons in solids, and are responsible for a plethora of unusual properties. Very recently, new spectroscopic capabilities have enabled the direct observation of polarons in several important 3D materials, shedding light on quantum many-body effects that were previously inaccessible. In contrast with these advances in the physics of polarons in 3D bulk materials, very little is known about polarons in 2D atomic crystals. It is currently unknown whether polarons can form in 2D materials, whether they are localized and to what extent, how they respond to probes such as electric and magnetic fields, and whether they exhibit collective phenomena such as Bose-Einstein condensation and superconductivity.<br/>In this talk I will present recent work aimed at answering some of these questions by developing an <i>ab initio</i> theory of polarons in 2D materials. I will show that the physics of polarons in 2D is fundamentally different from their 3D counterparts as the effective Coulomb interaction between electrons and phonons becomes short-ranged. This effect alters the energetics and localization of polarons and gives rise to a critical condition for the existence of polarons in 2D which has no counterpart in bulk 3D materials.<br/>To illustrate these concepts, I will first discuss <i>ab initio</i> calculations of polarons on bulk and monolayer hexagonal boron nitride (h-BN). Then I will present an <i>ab initio</i>-inspired model of polarons in realistic 2D materials which links widely available materials properties, such as dielectric constants and carrier effective masses, to the atomic-scale properties of polarons. This theoretical framework naturally identifies the most important materials descriptors of polaronic properties in 2D, and is suitable for high-throughput screening of two-dimensional polarons.