In contrast to standard thermodynamic models, we observe phase coexistence over an extended temperature range at a first-order surface phase transition. We have measured the domain evolution of the Si(111)-( $7\times 7$) to ( $1\times 1$) phase transition with temperature, using low-energy electron microscopy. Comparison with detailed, quantitative theoretical predictions shows that coexistence is due to long-range elastic and electrostatic domain interactions. Phase coexistence is predicted to be a universal feature of surface phase transitions.

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