Swelling-collapse transition of self-attracting walks

We study the structural properties of self-attracting walks in d dimensions using scaling arguments and Monte Carlo simulations. We find evidence for a transition analogous to the \Theta transition of polymers. Above a critical attractive interaction u_c, the walk collapses and the exponents \nu and k, characterising the scaling with time t of the mean square end-to-end distance ~ t^{2 \nu} and the average number of visited sites ~ t^k, are universal and given by \nu=1/(d+1) and k=d/(d+1). Below u_c, the walk swells and the exponents are as with no interaction, i.e. \nu=1/2 for all d, k=1/2 for d=1 and k=1 for d >= 2. At u_c, the exponents are found to be in a different universality class.Comment: 6 pages, 5 postscript figure

Possible mechanism for the room-temperature stabilization of the Ge(111) T > 300 deg.C phase by Ga

At low coverages, Ga on Ge(111) induces a hexagonal, domain wall modulated (2×2) adatom phase, stable at room temperature, that is characterized in low energy electron diffraction (LEED) by split 1/2-order reflections. This pattern closely resembles the one observed for a phase of clean Ge(111) appearing at temperatures above 300 °C (T>300 °C phase). We report scanning tunneling microscopy, LEED, as well as surface x-ray diffraction measurements on the Ga-induced room-temperature (RT) phase and compare it with a model for the T>300 °C phase of clean Ge(111). RT deposition of Ga yields a metastable c(2×8) structure which upon annealing transforms to the hexagonal (2×2) one. The transition occurs at considerably lower temperatures compared to clean Ge(111) and is irreversible due to pinning of adatom domains at Ga-induced defects, preventing the reordering of the adatoms and the correct stacking of the c(2×8) structure when cooling to RT. For the lowest Ga coverages investigated, a stablized phase is obtained that resembles a striped (2×2) rather than a hexagonal (2×2) structure. We discuss the possible existence of a striped (2×2) phase as an intermediate state in the transition from the c(2×8) of clean Ge(111) to the T>300 °C phase. Driven by entropy—and in the presence of Ga by defects—this intermediate phase transforms to a quasihexagonal (2×2) structure above 300 °C