Surface states and reconstruction on Ge(001)

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Abstract
High-resolution angle-resolved photoemission and low-energy electron diffraction (LEED) studies are reported which investigate the relationship between a recently discovered metallic or nearly metallic surface state and an order-disorder transition on Ge(001). Careful and systematic studies of this state’s angle- and temperature-dependent photoemission intensity lead to conclusions concerning the real-space character of the elementary excitation in this transition and the driving force for formation of the ordered c(4×2) low-temperature state. Comparisons to changes in the LEED pattern relate the onset of the metallic intensity to the formation of a phase in which surface dimers are randomly oriented with respect to each other. The data are most consistent with the flipping of a single dimer as the elementary excitation in the disordering transition. Evidence for a short-range driving force, probably involving interactions between dangling bonds on nearest neighboring dimers, is presented. The conclusions allow direct measurement of the fraction of flipped dimers on the surface as a function of temperature.