True First-Order Surface Phase Transition without Nanoscale Phase Separation

Nanoscale phase separation is common in many materials ranging from correlated electron systems to semiconductor surfaces undergoing phase transitions. On solid surfaces, nanoscale phase separations are known to occur over an extended temperature range during temperature-driven first-order surface phase transitions, precluding the true first-order transitions in thermodynamics. Here, we report the case of a surface phase transition very close to a true first-order transition. An array of indium wires on Si(111) undergoes a first-order charge-density-wave (CDW) transition with surprisingly little or no phase separation when prepared free of indium adatom impurities. The lack of phase separation was attributed to the small difference in strain with the substrate between the two competing normal and CDW phases. Indium adatom impurities cause phase separation and blur the transition, making it gradual and incomplete. These experimental observations provide insight into the surface phase transition at the nanoscale level.