Scaling and universality in models of step bunching: The "c ⁺-C ^-" model

We recently introduced a novel model of step flow crystal growth - the so-called "C ⁺-C ^-" model [B. Ranguelov et al., C.R. Acad. Bulgare Sci. 60, 389 (2007)]. In this paper we aim to develop a complete picture of the model's behaviour in the framework of the notion of universality classes. The basic assumption of the model is that the reference ("equilibrium") densities used to compute the supersaturation might be different on either side of a step, so C _L/C _R ≠ 1 (L/R stands for left/right in a step train descending from left to right), and that this will eventually cause destabilization of the regular step train. Linear stability analysis considering perturbation of the whole step train shows that the vicinal is always unstable when the condition C _L /C _R >1 is fulfilled. Numerical integration of the equations of step motion combined with an original monitoring scheme(s) results in obtaining the exact size- and time- scaling of the step bunches in the limit of long times (including the numerical prefactors). Over a broad range of parameters the surface morphology is characterized by the appearance of the minimal interstep distance at the beginning of the bunches (at the trailing edge of the bunch) and may be described by a single universality class, different from those already generated by continuum theories [A. Pimpinelli et al., Phys. Rev. Lett. 88, 206103 (2002), J. Krug et al., Phys. Rev. B 71, 045412 (2005)]. In particular, the scaling of the minimal interstep distance l _min in the new universality class is shown to be l _min = (S _n /N) ^1/(n+1), where N is the number of steps in the bunch, n is the exponent in the step-step repulsion law U ∼ 1/d ₀ ⁿ for two steps placed a distance d ₀ apart and S _n is a combination of the model parameters. It is also shown that N scales with time with universal exponent 1/2 independent of n. For the regime of slow diffusion it is obtained for the first time that the time scaling depends only on the destabilization parameter C _L/C _R. The bunching outside the parameter region where the above scaling exists cannot be assigned to a specific universality class and thus should be considered non-universal.