Real-space measurement of surface roughening

We use variable-temperature scanning tunneling microscopy (STM) combined wi th pattern recognition of STM images to demonstrate that the vicinal Ag(115 ) surface undergoes a true Kosterlitz-Thouless(KT)-type roughening transiti on. As predicted, the height correlation function remains finite below the roughening temperature and it evolves to a logarithmically diverging functi on at and above T-R. From a quantitative analysis of the height fluctuation s we derive a roughening temperature of T-R = 465 +/- 25 K. Recent diffract ion experiments could not confirm a KT-type roughening transition on vicina l surfaces. We attribute this to small residual errors in local surface ori entation, which overshadow the real thermally excited height fluctuations.