We thank Y. H. Sanejouand and F. Tama for making the program package RTB available to us and F. Tama for helpful discussion for developing the nonlinear approach. This work has been funded by the National Science Foundation-sponsored Center for Theoretical Physics (Grants PHY0216576 and 0225630). O.M. acknowledges the La Jolla Interfaces in Science postdoctoral training program supported by the Burroughs Wellcome Fund. Computations were carried out at the University of California, San Diego Keck II computing facility (partially supported by the National Science Foundation, Division of Molecular and Cellular Biosciences) and the San Diego Supercomputer Center.