We have investigated atomic arrangements and their electronic properties of the well-ordered thallium overlayer structures formed on the Si͑111͒-7ϫ7 surface. As for other trivalent atoms, Tl is found to form a well-defined ͱ3ϫͱ3 surface, indicating the absence of a so-called ''inert pair effect'' considered only for Tl.Another well ordered 1ϫ1 surface at 1.0 monolayer appears to be semiconducting in our angle-resolved photoemission spectra dominated by a unique dispersive surface band near the Fermi level. Our theoretical calculations using density-functional theory show that Tl adatoms occupy the T 4 sites and saturate all the dangling bonds of surface Si atoms to make the surface semiconducting with a band gap of 0.34 eV. The filled surface band observed has been well reproduced in our band calculations.The interaction of group III elements with Si surfaces has been extensively studied mainly because of their technological relevance as dopant materials and the presence of various adsorbate-induced phenomena at semiconductor surfaces. 1 The adsorption of thallium ͑Tl͒ on the Si͑111͒-7ϫ7 surface, in particular, is interesting since Tl has been known to behave quite differently from other group III elements in forming stable ordered surface structures; 2 the only ordered structure formed by Tl has been reported to be a 1ϫ1 surface ͓hereafter denoted as a Tl/Si͑111͒-1ϫ1 surface͔ at 1.0 monolayer͑ML͒ in sharp contrast with a variety of different structures found for other group III elements such as Al and In. 1 The absence of other ordered structures by Tl has been considered to exhibit a so-called ''inert pair effect'' of a Tl atom where the 6s 2 electrons are assumed to be inactive in chemical bonding with Si atoms. Tl, therefore, has been thought to act as a monovalent rather than trivalent atom on the Si͑111͒ surface in a sense chemically close to the alkali metals or novel metals. 2 Because of the large atomic radius and the effectively monovalent character of Tl, the on-top site T 1 directly above the Si first layer of the unreconstructed 1ϫ1 surface has been preferred as the binding site for Tl on the Si͑111͒ surface. 3,4 In addition to the presence of interesting structures such as the superlattice of metallic nanodots at low coverage ϳ0.2 ML 5 and the rotational epitaxy of an incommensurate Tl metallic overlayer, 4 the extremely inert nature of the Tl/Si͑111͒-1ϫ1 surface is another reason why it is so interesting since it may serve as a stable substrate surface to form other atomic structures.In this Brief Report we report results of combined studies for the atomic structures and their electronic properties of the ordered Tl overlayer structures focussed especially on the Tl/Si͑111͒-1ϫ1 surface. We have determined surface bands of the 1ϫ1 irreducible surface Brillouin zone ͑SBZ͒ from our angle-resolved photoemission spectroscopy ͑ARPES͒ measurements. The surface bands and the atomic structure of the Tl/Si͑111͒-1ϫ1 surface have been well reproduced in our theoretical calculations using the density function...

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We report that the π band of graphene sensitively changes as a function of an external potential induced by Na especially when the potential becomes periodic at low temperature. We have measured the band structures from the graphene layers formed on the 6H-SiC(0001) substrate using angle-resolved photoemission spectroscopy with synchrotron photons. With increasing Na dose, the π band appears to be quickly diffused into background at 85 K whereas it becomes significantly enhanced its spectral intensity at room temperature (RT). A new parabolic band centered at k ∼1.15 A −1 also forms near Fermi energy with Na at 85 K while no such a band observed at RT. Such changes in the band structure are found to be reversible with temperature. Analysis based on our first principles calculations suggests that the changes of the π band of graphene be mainly driven by the Na-induced potential especially at low temperature where the potential becomes periodic due to the crystallized Na overlayer. The new parabolic band turns to be the π band of the underlying buffer layer partially filled by the charge transfer from Na adatoms. The five orders of magnitude increased hopping rate of Na adatoms at RT preventing such a charge transfer explains the absence of the new band at RT.