The ground state of Sn/Si(111) and Sn/Ge(111) surface $\alpha$-phases is reexamined theoretically, based on $ab-initio$ calculations where correlations are ap\proximately included through the orbital dependence of the Coulomb interaction (LDA+U). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state,[1] in agreement with very recent experiments.[2,3] Antiferromagnetic exchange is proposed to be responsible for the observed $\Gamma$-point photoemission intensity, [4] as well as for the partial metallization observed above 60 K.[3] Extrinsic metallization of Sn/Si(111) by, $e.g.$ alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces, such as Si-terminated SiC(001).[5]
References:
[1] G. Profeta and E. Tosatti, Phys. Rev. Lett. 98, 086401 (2007)
[2] R. Cortés, et al., Phys. Rev. Lett. 96, 126103 (2006)
[3] S. Modesti, et al., to appear in Phys. Rev. Lett., (2007)
[4] R. I. G. Uhrberg, et al., Phys. Rev. B 62, 8082 (2000)
[5] L.I. Johansson, et al., Phys. Rev. B 53, 13803 (1996)
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