Metal-insulator transitions at surfaces.pdfVIP

a r X i v : c o n d - m a t / 0 2 0 4 1 3 5 v 1 [ c o n d - m a t .s t r - e l ] 5 A p r 2 0 0 2 Metal-Insulator Transitions at Surfaces Michael Potthoff Lehrstuhl Festko?rpertheorie, Institut fu?r Physik, Humboldt-Universita?t zu Berlin, Germany Abstract. Various types of metal-insulator transitions are discussed to find condi- tions for which an ideal surface of a bulk insulator is metallic. It is argued that for the correlation-driven Mott metal-insulator transition the surface phase diagram should be expected to have the same topology as the phase diagram for magnetic order at surfaces: The corresponding linearized mean-field descriptions, a simplified dynamical mean-field theory of the Hubbard model and the Weiss mean-field the- ory for the Ising model, are found to be formally equivalent. A new kind of surface state appears in the low-energy part of the one-particle excitation spectrum as a precursor effect of the Mott transition. The Mott metal-insulator transition at a crystal surface is a subject that touches different areas in solid-state theory which are usually treated as being disjoined: many-body theory of the correlation-driven metal-insulator tran- sition, the general theory of surface phase transitions, and the theory of electronic surface states. It is the intention of the present paper to show that a corresponding combination of different concepts can be fruitful and allows for some new theoretical predictions. 1 Surface Phase Transitions The large variety of novel and interesting phenomena in surface physics is closely related with the occurrence of surface phase transitions. As has been pointed out by Mills [1], the surface of a system may undergo a phase tran- sition at a critical temperature Tc,s being different from the bulk critical temperature Tc, i.e. the surface may undergo its own phase transition. Criti- cal exponents, for example, can be defined and determined which are specific for the transition at the surface and which canno