Spin-valley dynamics of interlayer excitons in heterobilayers Mo$_x$W$_{1-x}$Se$_2$/WSe$_2$

We study spin-valley relaxation dynamics in two-dimensional Mo$_x$W$_{1-x}$Se$_2$/WSe$_2$ heterobilayers with different relative Mo/W concentration $x$ in the monolayer alloy. Three types of heterobilayers with $x$ = 1.00, 0.50, 0.33 are studied in time-resolved Kerr rotation experiments for different wavelengths and temperatures. The spin-valley relaxation times are found to decrease from $\sim$10 nanoseconds for $x$ = 1.00 to $\sim$50 picoseconds for $x$ = 0.33. The observed relaxation times are limited by the recombination of indirect excitons formed in the heterobilayers. Our results demonstrate that spin-valley relaxation in alloy-based van der Waals heterostructures can be controlled via their chemical composition.