Session B2: Many-Body Effects for the Excited States of Graphene

Optical properties of single- and few-layer graphene: the role of interlayer and many-body interactions

Graphene, a single layer of carbon atoms, has attracted much attention in the past few years because of its unique 2D structure and linear dispersion relation near the K-point of the Brillouin zone. Optical spectroscopy provides a powerful tool for probing the electronic structure and interactions in graphene. In this talk we will discuss two types of interactions that affect the optical response -- those arising from interlayer coupling of electrons and those arising from many-body effects. The possibility of altering the low-energy band structure of graphene through the interlayer interactions in few-layer graphene (FLG) was recognized theoretically several years ago and was demonstrated experimentally recently by infrared absorption spectroscopy. Two distinct classes of IR absorption spectra for crystalline samples of the same number of layers, but different stacking order, were also observed. These findings demonstrate the pronounced effect of interlayer interaction and stacking order on the electronic structure of FLG. Furthermore, significant many-body effects are revealed in the optical conductivity spectra. These were manifested as excitonic modifications to optical absorption near the saddle- point singularities. The strong electron-hole interactions produce an asymmetric resonance, significantly red-shifted from the value predicted by ab-initio GW calculations for the band- to-band transitions. Our experiment also showed a weak dependence of the excitonic resonance in FLG on layer thickness. This result reflects the effective cancellation of the increasingly screened repulsive electron-electron and attractive electron-hole interactions.   

Band structures of expitaxial graphene: the role of many-body interactions

This abstract not available.   

Ultrafast Carrier Dynamics in Graphene and Few Layer Graphite

Graphene and its multilayer counterparts provide unique opportunities to study how the ultrafast carrier dynamics of layered systems evolve with layer number. We have carried out systematic investigations [1] of layered graphitic materials, from graphene to bulk graphite, exfoliated on to a Si/Silicon oxide substrate. The samples are excited using 150 fs, 800 nm pulses at room temperature and the time resolved reflectivity and transmission is probed using 150 fs, 1300 nm pulses. The response is governed by two times constants, one near 250 fs and the other near 3 ps, but both vary with the number of layers. The time constant are related to carrier cooling kinetics, interband transitions and hot phonon effects. The change in the first time constant with layer number is discussed in terms of alterations to the band structure with increasing number of layers over a few layers while the changes in the longer time constant over 10's of layers is related to substrate coupling effects. The results are compared with results from related experiments [2,3] using multilayer graphene, epitaxially grown on SiC, and also from results from experiments [4] using freestanding, thin graphite layers. *Work carried out with R.W. Newson and J.J. Dean. \\[4pt] [1] R.W. Newson, J. Dean, B. Schmidt and H.M. van Driel, Op. Ex. 17, 2326-33 (2009). \\[0pt] [2] D. Sun et al., Phys. Rev. Lett. 101, 157402 (2008). \\[0pt] [3] J.M. Dawlaty et al., Appl. Phys. Lett. 92, 043116 (2008). \\[0pt] [4] M. Breusing, C. Ropers and T. Elsaesser, Phys. Rev. Lett. 102, 086809 (2009).   

The Ordering and Electronic Structure of Multilayer Epitaxial Graphene on SiC

The structural definition of graphene as a single sheet of hexagonal carbon limits how we view this material. It is the electronic properties of a single isolated graphene sheet that actually defines and motivates current graphene research. Remarkably, the best example of the idealized band structure of graphene comes does not come from a single graphene layer but from multilayer films grown on SiC. Multilayer epitaxial graphene (MEG) not only shows all the 2D properties expected for an isolated graphene sheet, but it the scalability to large scale integrated carbon circuits. I will show that the reason for this remarkable property, i.e. that a multilayer graphene films behaving like a single graphene sheet, is due to MEG's unique stacking. MEG films have a quasi-ordered rotational stacking that breaks the Bernal stacking symmetry associated with graphite. Angle resolved photoemission spectroscopy (ARPES) data demonstrates that the bands are linear at the K-point of these films. We can also show that the rotated stacking is highly ordered and that less than 20\% of the graphene sheets in the film are Bernal stacked. I will also show that ARPES measurements on MEG films demonstrate serious inadequacies with both tight binding and ab initio formalisms. In particular the data shows no reductions in the Fermi velocity or the formation of Van Hove singularity that have been consistently predicted for this material.   

Raman spectroscopy of pristine, defected and strained graphene

Raman spectroscopy is the most common and informative characterization technique in graphene science and technology. It is used to determine the number of layers, doping, strain, defects, functional groups, quality and type of edges [1-15]. I will discuss the historical development of the identification of the main Raman bands in graphene, focussing on the 2D'' peak around 2450cm$^{-1}$, and its deep-UV Raman spectrum. I will then discuss the effects of defects, uniaxial and biaxial strain on the Raman spectrum. Combining strain and Raman measurements one can derive the constitutive relation for graphene, and gain insights in the resonant Raman process. The results on graphene are the basis to explain and unify analogous measurements on graphite, carbon fibres and carbon nanotubes reported over the past 30 years. \\[4pt] [1] A. C. Ferrari et al. Phys. Rev. Lett. 97, 187401 (2006).\\[0pt] [2] C. Casiraghi et al. Nano. Lett. 7, 2711 (2007).\\[0pt] [3] C. Casiraghi et al. Appl. Phys. Lett. 91, 233108 (2007).\\[0pt] [4] S. Pisana et al. Nat. Mater. 6, 198 (2007).\\[0pt] [5] S. Piscanec et al. Phys. Rev. Lett. 93, 185503 (2004).\\[0pt] [6] C. Casiraghi, et al. Nano Lett. 9, 1433 (2009).\\[0pt] [7] A. C. Ferrari, Solid State Comm. 143, 47 (2007).\\[0pt] [8] A. Das et al. Nature Nano. 3, 210 (2008).\\[0pt] [9] A. Das et al. Phys. Rev. B 79, 155417 (2009).\\[0pt] [10] T. M. G. Mohiuddin et al. Phys. Rev. B 79, 205433 (2009).\\[0pt] [11] J. Yan et al. Phys. Rev. Lett. 98, 166802 (2007).\\[0pt] [12] D Graf et al. Nano Lett. 7, 238 (2007).\\[0pt] [13] A. C Ferrari et al. Phys. Rev. B 61, 14095 (2000); 64, 075414 (2001).\\[0pt] [14] D. M. Basko et al. Phys Rev B 80, 165413 (2009).\\[0pt] [15] F. Schedin et al. ACS Nano 4, 5617 (2010)