Enhanced long-range quadrupole effects in 2D MSi₂N₄: impacts on electric and thermal transport

Long-range higher-order multipolar electron–phonon (e-ph) interactions beyond the dipole-like Fröhlich interactions have long been neglected in the description of various physical properties. Here we demonstrate the contribution from quadrupole effect to the electric and thermal transport properties of monolayer MSi₂N₄ (M = Mo/W) systems. The quadrupole effect reduces the electron and hole mobilities at 300 K by 25.4%, 12.8% for MoSi₂N₄, and by 19.2%, 52.3% for WSi₂N₄, respectively. For n- and p-type monolayers with modest dopings by fixing the carrier concentration to 1.0 × 10¹⁴ cm⁻², the dipole-like e-ph interaction decreases the three-phonon-limited lattice thermal conductivities κ_l by 17.9% and 43.5% for monolayer MoSi₂N₄ and WSi₂N₄, respectively. However, further considerations of quadrupole e-ph interaction shrink such reductions of three-phonon-limited κ_l to only 3.6% and 2.4%, respectively due to the cancellation effects. Our results highlight the potential of MSi₂N₄ monolayers as promising candidates for advanced micro-electronic applications.