Fluctuations of Pb/Si(111) Domain Boundaries*

We have used variable-temperature scanning tunneling microscopy to study fluctuations of a 1D interface --- the boundary between two coexisting surface phases. We have prepared Pb/Si(111) surfaces that consist of high-density $(1 \times 1)$-Pb domains coexisting with a lower-density $\sqrt 3 \times \sqrt 3 -R30^{\circ}$-Pb phase. The domain boundaries between these phases fluctuate at moderate temperatures, allowing direct observation with STM. Measurement of the temporal correlation function for the fluctuating boundary between 525 K and 625 K allows determination of the mass transport mechanisms below and above the onset of thermal desorption of the $(1 \times 1)$ phase. In the desorption regime, our measured dynamical exponent of $1/2$ provides microscopic confirmation that fluctuations of the $(1 \times 1)$ boundaries occur via exchange of mass with a 2D adatom gas on the $\sqrt 3 \times \sqrt 3$ phase, consistent with the zeroth-order desorption kinetics inferred from macroscopic measurements.