Dr. S. A. Jafari, Department of Physics, Isfahan University of Technology, Isfahan, Iran

Jeudi 25 février à 16 h 00 Amphi Howleck, Esc. C, 1er étage

Resonating Valence Bond point of view and spin excitations in Graphene

In this talk I will start with random phase approximation analysis of the excitation spectrum
of 2D graphene in the triplet particle-hole channel for un-doped case [1,2]. This analysis
suggests that lowest excitations in graphene are neutral triplet bosons. This excitation is
interpreted in terms of the two-spinon bound state on top of a long range Resonating Valence
Bond (RVB) ground state [1]. This is further substantiated by variational Monte Carlo
(VMC) study of the strong coupling limit of graphene [3], which suggests a long range RVB
state for undoped graphene [4]. Doping graphene with electrons extends the region of particlehole
continuum thereby causing the Landau damping of the excitations in some parts of the
Brillouin zone. We interpret the result in terms of shortening the resonating valence bonds [5].
We speculate that such a collective mode can account [6] for the missing decay channel in time
resolved photoemission spectroscopy of HOPG [7].

[1] G. Baskaran and S. A. Jafari, Phys. Rev. Lett. 89 (2002) 016402 ; ibid, 92 (2004) 199702.
[2] S. A. Jafari and G. Baskaran, Eur. Phys. J. B, 43 (2005) 175.
[3] S. A. Jafari, Eur. Phys. J. B. 68 (2009) 537.
[4] Z. Noorbakhsh, F. Shahbazi, S. A. Jafari, G. Baskaran, J. Phys. Soc. Jpn. 78
(2009) 054701.
[5] M. Ebrahimkhas, S. A. Jafari, G. Baskaran (under preparation)
[6] M. Ebrahimkhas, S. A. Jafari, Phys. Rev. B, 79 (2009) 205425.
[7] G. Moos, C. Gahl, R. Fasel, M. Wolf, T. Hertel, Phys. Rev. Lett. 87 (2001) 267402.
Pour tout renseignement :
N. Bergeal : 01 40 79 44 83 – nicolas.bergeal (arobase) espci.fr
B. Fauqué 01 40 79 58 14 – benoit.fauque (arobase) espci.fr

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