  Time dependent STM has been used to evaluate step fluctuations as a function
of temperature (300-450 K) on Ag(111) films grown on mica. The temporal
correlation function scales as a power law in time, t^1/n with measured values
of 1/n varying over a range of 0.19 pm 0.04 to 0.29 pm 0.04 with no dependence
on temperature. The average value of 1/n = 0.24 pm 0.01 is consistent with
step-edge diffusion limited fluctuations (n = z = 4, conserved noise). The
magnitude of the time correlation function and the width of the fluctuations
both scale with temperature with the same apparent activation energy of Eeff =
0.21 pm 0.02 eV, indicating that the correlation time is at most weakly
temperature dependent. Direct analysis of the autocorrelation function confirms
that the correlation time is at most weakly temperature dependent, and thus the
apparent correlation length is strongly temperature dependent. This behavior
can be reproduced by assuming that the apparent correlation length is governed
by the longest wavelength of step fluctuations that can be sampled in the
measurement time interval. Evaluation of the correlation time for previous
measurements for Al/Si(111) (z =2) yields the same conclusion about measurement
time interval. In both cases the ratio of the measurement time to the effective
correlation time is on the order of 10.
