Anomalous in-plane lattice thermal conductivity in an atomically thin two-dimensional α-GeTe layer

Low lattice thermal conductivity is one of the most important physical quantities for phononic device applications. Thus, we investigated the in-plane lattice thermal conductivity of mono- and bi-layer α-GeTe systems. The lattice thermal conductivity of the monolayer system along the zigzag direction was 0.43 (W mK-1) while it was 0.21 (W mK-1) along the armchair direction at 300 K, and the lattice thermal conductivity mostly originated from the out-of-plane acoustic mode. In the bilayer system, it was significantly suppressed to 0.044 (W mK-1) and 0.047 (W mK-1) along the zigzag and armchair directions, respectively, at 300 K. Particularly, the out-of-plane acoustic mode in the bilayer had a tremendous Grüneisen parameter and this led to an ultralow in-plane lattice thermal conductivity in the bilayer, and the optical mode dominated the contribution to the lattice thermal conductivity. Our findings may raise intriguing issues regarding the thermoelectric effect, heat insulators, and phononic device applications, and stimulate further experimental studies to verify our theoretical predictions.