Evidence of asymmetric dimers down to 40 K at the clean Si(100) surface

We have studied by synchrotron-radiation photoelectron spectroscopy the apparently p(2X1) structure recently imaged by scanning-tunneling microscopy at low temperatures in the form of seemingly symmetric dimers. Yet we demonstrate that the surface is semiconducting as in the c(4X2) phase and that most dimers remain asymmetric. The p (2X1) structure seen at low temperature may result either from a ferromagneticlike arrangement of static buckled dimers or, eventually, from an artifact of local tip-surface interactions.