  Concerted experimental and numerical studies of step bunching on vicinal
crystal surfaces resulting from step-down electromigration of partially charged
adatoms, confirmed the theoretical prediction of scaling dependence of the
minimal bunch distance $l_{\rm min}$ on the bunch size $N$: $l_{\rm min} \sim
N^{-\gamma}$, with $\gamma = 2/3$. The value of the so called size-scaling
exponent $\gamma$ was observed in experiments on vicinal surfaces of
semiconducting, metallic, and dielectric materials. Careful theoretical
investigations and numerical calculations predict a second value of $\gamma =
1/2$. However, this value is still not been reported from experiments. And we
report here experimental observation of step bunching in the universality class
relative to $\gamma = 1/2$. This is achieved by monitoring step flow during
sublimation of Si(111)-vicinals heated by a direct step-down current at
~1200$^\circ$C. In the experiment we also measure other characteristic for the
bunching quantities, such as the mean total number of steps in the bunch $N$
and the mean bunch width $W$. We then compare our findings with published
experimental and numerical data to arrive at a theoretically consistent
framework in terms of universality classes. The ultimate benefit of our study
is not only to advance fundamental knowledge but also to provide further
guidance for bottom-up synthesis of vicinal nanotemplates.
