Origin of localized states in graphite: Indirect photoemission processes or impurities? M.E. Da ´vila a, * , M.A. Valbuena a , V. Pantı ´n a , J. Avila b , P. Esquinazi c , M.C. Asensio b a Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı ´ficas (CSIC), C/ Sor Juana Ine ´s de la Cruz, 3, 28049 Madrid, Spain b Synchrotron SOLEIL, Orme des Merisiers, Saint Aubin BP 48, 91192 Gif sur Yvette Cedex, France c Department of Superconductivity and Magnetism, Leipzig University, Germany Available online 20 July 2007 Abstract The electronic band structure of different types of graphite samples have been investigated in order to identify the origin of non-dispersive density of states recently reported in the literature. A systematic series of synchrotron radiation angle resolved photoemission spectroscopy (ARPES) measurements on graphite single crystal, highly oriented graphite (HOPG) and epitaxial grown graphite single crystal on 6H– SiC(0 0 0 1) samples, have been carried out as well as compared with theoretical tight binding calculations. Our results indicate that these localized states are present in all the graphite-investigated samples showing the same non-dispersive character and at the same binding energies. The photoemission data taken at several photon energies demonstrate that these states are not surface states nor due to indirect photoemission processes. It seems that they are closely related to the level of impurities present in the studied samples. # 2007 Elsevier B.V. All rights reserved. PACS : 71.20.b; 71.23.k Keywords: Photoelectron spectroscopies; Electronic structure; Low-dimensional solids; Graphite 1. Introduction The interest on graphite has been renewed due to the unexpected properties (magnetic, transport, etc.) [1–4] recently reported of new allotropic carbon forms [5,6], i.e. nanotubes, foams, which have been evidenced thanks to the development of advanced techniques allowing the visualization of novel exotic properties, unknown until now. In fact, recent PES and STS/STM studies on graphite-based systems have reported a particular signature characterized by the presence of localized state near E F in the density of states (DOS) [7–10]. These peculiar electronic states have been related to a wide variety of magnetic or transport properties [11,12] of graphite materials, which are still not fully understood. As we have recently indicated, HOPG samples show dramatic decrease in their conductivity as a function of the level of defects and order in the investigated materials [7]. The better ordered HOPG samples displayed a metallic behaviour, while poorly ordered samples exhibited a semiconductor character. Magnetic measurements have also showed a probable ferromagnetism and/or superconducting-like behaviour in HOPG depending on the existence of topological defects observed in the investigated samples [13]. This subject has been initially investigated by Lanzara’s group [14], followed by many others [15–17]. To better understand the origin of such anomalous properties, a large set of different graphite samples have been systematically investigated by ARPES. The valence band electronic structure of metallic and semiconductor HOPG graphite samples, graphite single crystals as well as epitaxial graphite single crystals grown on SiC(0 0 0 1) substrates have been measured and compared with tight binding theoretical calculations. This paper is organized as follows. In Section 2 the experimental details are given. In Section 3 our results and discussion are presented and Section 4 summarizes our conclusions. www.elsevier.com/locate/apsusc Applied Surface Science 254 (2007) 55–61 * Corresponding author. E-mail address: mdavila@icmm.csic.es (M.E. Da ´vila). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.119