Free-Electron Interlayer States in Pure and Li-Intercalated Graphite

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M. POSTERNAK Institut de Physique Appliqu4e, EPFL, CH-1015 Lausanne, Switzerland A. BALDERESCHI Institut de Physique Appliqu~e, EPFL, CH-1015 Lausanne, Switzerland, and Istituto di Fisica Teorica, Universit& di Trieste, Trieste, Italy A.J. FREEMAN, E. WIMMER [a] AND M. WEINERT [b] "Department of Physics, Northwestern University, Evanston, Illinois 60201, USA ABSTRACT First-principle SCF-FLAPW calculations prove the existence in graphite of a new kind of electron states. They are interlayer states, resulting from the bonding combination of states bound to single graphite layers, and give rise to a still undetected energy band just above the Fermi energy. This band also occurs at slightly lower energy in Li intercalated graphite, and has been incorrectly interpreted as arising mainly from Li 2s orbitals. INTRODUCTION The commonly accepted energy-band structures of pure graphite and of the LiC6 compound show that (i) in graphite the sequence of bands is a- and itbonding, it- and a-antibonding, with the Fermi level EF separating the two kinds of ffbands [1,2]; (ii) in LiC6 an additional parabolic band, the so-called Li-band, is present whose bottom is no2eV above EF [1,3]. In this work we study the three model 2-D systems displayed in Fig.l, i.e. (a) a graphite monolayer (C6 ), (b) two graphite layers with the same crystallographic arrangement as two consecutive C layem in bulk LiC6 (C6-C6), and (c) a Li layer sandwiched between the two above graphite layers (C6 -Li-C 6 ). Electronic energies and wavefunctions were calculated with the self-consistent full-potential linearized APW method (SCF-FLAPW) for thin films [4]. Our results indicate that an additional band which originates from free-electron interlayer states is present above EF in pure graphite. They also provide for LiC6 a new understanding of the origin of the band previously interpreted incorrectly as arising mostly from the Li 2s orbitals. RESULTS FOR GRAPHITE MONOLAYER (C6 ) The energy levels that we obtain for the isolated graphite monolayer are given in Fig. 2. The zero of energy being placed at vacuum zero, the spectrum is continuous at positive energy, corresponding to the free electron states existing in the half spaces on each side of the monolayer. The energy bands

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Fig. 1. Side view of the crystallographic structure of the three 2-D model systems studied in this work. at negative energy correspond to the well known a bonding states (full lines), and to the 7r-bonding and iT-antibonding states (broken lines) separated by the Fermi energy. Two parabolic bands are also present just below vacuum zero and close to the zone center. These two bands have not been reported in previous LCAO calculations of two-dimensional graphite based on minimal sets of basis

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SCF-FLAPW band structure of a graphite monolayer.

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