Exceptionally low thermal conductivity in simple two-dimensional SiS: anomalous emergence of rattling phonon modes in non-caged materials

Abstract

The two-dimensional orthorhombic phase of SiS exhibits low particle-like thermal conductivity (the propagation term) along both in-plane directions. This unique behavior is due to the Coulomb interaction among lone pair electrons at adjacent S atoms, which induces low-lying rattling optical modes with ultralow frequency. These rattling modes interact with acoustic modes, significantly softening them due to anti-overlapping effects. Consequently, the group velocities of acoustic modes decrease, while their scattering rates increase, dramatically reducing their contribution to the overall thermal conductivity. Surprisingly, optical modes become the dominant heat carriers in particle-like thermal transport, deviating from typical behavior observed in most materials. Strong phonon scattering also causes the wave-like thermal conductivity (the coherence term) to be nearly an order of magnitude larger than the particle-like thermal conductivity. Additionally, the anisotropic structure of SiS results in orientation-dependent phonon group velocities, leading to an interesting anisotropy in thermal conductivity. The combination of low thermal conductivity and superior electronic transport properties make 2D SiS a promising candidate for thermoelectric applications. Our study suggests that lowering the frequencies of optical phonons could serve as an effective strategy to further suppress the thermal conductivity of 2D materials.