  Bulk gallium oxide (Ga2O3) has been widely used in lasers, dielectric
coatings for solar cells, deep-ultraviolet transistor applications due to the
large band gap over 4.5 eV. With the miniaturization of electronic devices,
atomically thin Ga2O3 monolayer has been unveiled recently, which features an
asymmetric configuration with a quintuple-layer atomic structure. The superior
stability, the strain-tunable electronic properties, high carrier mobility and
optical absorption indicate the promising applications in the electronic and
photoelectronic devices. However, the strict investigation of lattice thermal
conductivity (kappa_L) of 2D Ga2O3 is still lacking, which has impeded the
widespread use in practical applications. Here, we report the computational
discovery of low kappa_L with a value of 10.28 W m-1 K-1 at 300 K in atomically
thin Ga2O3. Unexpectedly, two quasi-acoustic shear phonon modes contribute as
high as 27% to the kappa_L at 300 K, leading to 37% contribution of optical
phonon modes, much larger than many other 2D materials. We also find that the
quasi-acoustic shear mode can emerge in the system without van der Waals
interactions. This work provides new insight into the nature of thermal
transport in non-van der Waals monolayer materials and predicts a new low
kappa_L material of potential interest for thermal insulation in transistor
applications.
