Collective modes in strongly coupled binary liquids

Complete text of publication follows. While a great deal of information - from experiments, simulations, as well as from theory - is available on the collective mode structure of a strongly coupled single component liquid (2D and 3D Yukawa systems in particular), very little is known on the behavior of mixtures in this regard (Note, though, the existence of a large body of literature on the somewhat related subject of binary alloys). We have undertaken a systematic MD and theoretical study of 2D and 3D binary Yukawa liquids in order to map the collective mode structures of such systems over the parameter space of the 2 relevant mixing parameters c2Z2 / c1Z1 and m1Z2 / m2Z1 , and over a wide range of G values, including the crystalline and disordered lattice phases. Here we report on the first phase of the study, where we have concentrated on mixtures of components with different masses (m1,m2) and with different concentrations (c1, c2), but with identical charges. We observe the formation of acoustic and optic modes: the latter being the hallmark of the presence of more than one component. In the binary liquid there is one optic mode; in the binary crystal, however the number of optic modes also depends on the concentration ratios and on the crystal structure that can accommodate the chosen concentrations. We show that in all cases the mode frequencies satisfy a generalized Kohn sum rule. The k→0 sound speeds (sL, sT) associated with the longitudinal and transverse acoustic modes exhibit a remarkable G-dependence. For weak coupling, the RPA predicted dominance of the lighter mass species should prevail: s2 proportional to[(c1 / m1) + (c2 / m2)]. For higher coupling however, the QLCA predicts the reversal of roles and the dominance of the heavy mass species: s2 proportional to 1/[c1m1 + c2m2]. MD simulations show clearly the transition from the intermediate coupling to the strong coupling regime both in the liquid and in the crystalline or disordered solid phase and fully corroborate the QLCA result.