Strong four-phonon effects and anomalous thermal transport behavior in the monolayer group-IVB transition metal dichalcogenides MX2 (M=Ti,Zr,Hf;X

The discovery of strong four-phonon (4ph) interactions in boron arsenide has spurred significant investigations into the influence of higher-order phonon anharmonic scattering on the thermal transport of materials. Considering the special relative regular residual feature and great potential applications in thermoelectrics, this study systematically investigates the anharmonic lattice dynamics and thermal transport behavior of monolayer group-IVB transition metal dichalcogenides $1T\ensuremath{-}M{X}_{2} (M=\mathrm{Ti},\mathrm{Zr},\mathrm{Hf};X=\mathrm{S},\mathrm{Se})$ via ab initio calculations with the self-consistent phonon and Boltzmann transport theories. The calculation results indicate that $1T\ensuremath{-}M{X}_{2}$ has strong 4ph anharmonic scattering and that the phonon dispersion exhibits a significant temperature dependence, which stems from the obvious in-plane quartic potential well vibrations of the transition metal atoms. When 4ph scattering is considered, a remarkable reduction of thermal conductivity is observed in $1T\ensuremath{-}M{X}_{2}$, especially for $1T\ensuremath{-}\mathrm{Ti}{\mathrm{S}}_{2}$ and $1T\ensuremath{-}\mathrm{Ti}{\mathrm{Se}}_{2}$, with their conductivity reduction reaching 66 and 62% at room temperature, respectively. Moreover, the thermal conductivity of $1T\ensuremath{-}M{X}_{2}$ anomalously increases when the $M$ atoms transition from $\mathrm{Ti}\ensuremath{\rightarrow}\mathrm{Zr}\ensuremath{\rightarrow}\mathrm{Hf}$, which is in contrast to the traditional intuitive impression where heavy atomic systems always exhibit poor thermal transport performance. Such abnormal mass-dependent behavior of thermal conductivity mainly arises from the enhancement of the bond energy (reduction of phonon anharmonicity) in the Zr and Hf systems. The study results shed light on the physical mechanism of the thermal transport in monolayer $1T\ensuremath{-}M{X}_{2}$ and provides helpful insights into the necessity of considering the influence of 4ph effects on thermal transport.