Disappearance of the hexatic phase in a binary mixture of hard disks

Recent studies of melting in hard disks have confirmed the existence of a hexatic phase occurring in a narrow window of density which is separated from the isotropic liquid phase by a first-order transition, and from the solid phase by a continuous transition. However, little is known concerning the melting scenario in mixtures of hard disks. Here we employ specialized Monte Carlo simulations to elucidate the phase behavior of a system of large ($l$) and small ($s$) disks with diameter ratio $\sigma_l/\sigma_s=1.4$. We find that as small disks are added to a system of large ones, the stability window of the hexatic phase shrinks progressively until the line of continuous transitions terminates at an end point beyond which melting becomes a first-order liquid-solid transition. This occurs at surprisingly low concentrations of the small disks, $c\lesssim 1\%$, emphasizing the fragility of the hexatic phase. We speculate that the change to the melting scenario is a consequence of strong fractionation effects, the nature of which we elucidate.