Article

# Chirality-induced dynamic kohn anomalies in graphene.

Condensed Matter Theory Center, Department of Physics, University of Maryland, College Park, Maryland 20742, USA.

Physical Review Letters (Impact Factor: 7.73). 09/2008; 101(6):066401. DOI: 10.1103/PhysRevLett.101.066401 Source: PubMed

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**ABSTRACT:**We present a first-principles study of the electron-phonon (e-ph) interactions and their contributions to the linewidths for the optical-phonon modes at Γ and K in one-layer to three-layer graphene. It is found that, due to the interlayer coupling and the stacking geometry, the high-frequency optical-phonon modes in few-layer graphene couple with different valence and conduction bands, giving rise to different e-ph interaction strengths for these modes. Some of the multilayer optical modes derived from the Γ-E2g mode of monolayer graphene exhibit slightly higher frequencies and much reduced linewidths. In addition, the linewidths of K-A1′ related modes in multilayers depend on the stacking pattern and decrease with increasing layer numbers.Physical review. B, Condensed matter 02/2009; 79(11). · 3.66 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We study the properties of out-of-plane phonons in the framework of the many-body theory of graphene. We investigate, in particular, the way in which the coupling to electron–hole excitations renormalizes the dispersion of the acoustic branch of out-of-plane phonons. We show that the effect of the charge polarization cuts off the quadratic dispersion at low energies, implying the absence of long-wavelength flexural phonons. This result holds in the low-energy Dirac theory of graphene, and it is confirmed by an analysis of the corrections to the interaction vertex beyond the random phase approximation (RPA). Furthermore, we show that the acoustic branch of out-of-plane phonons presents near the K point a strong Kohn anomaly, which is much more pronounced than in the case of the in-plane phonons. The origin of the strong softening of the dispersion lies in the singular behaviour of the intervalley polarization at the threshold of electron–hole formation. This leads to a new branch of hybrid modes below the electron–hole continuum, with the potential to induce significant effects in the transport properties of graphene in the low-temperature regime.New Journal of Physics 09/2009; 11(9):095015. · 3.67 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Dirac electrons in graphene in the presence of Coulomb interactions of strength $\beta$ have been shown to display power law behavior with $\beta$ dependent exponents in certain correlation functions, which we call the mass susceptibilities of the system. In this work, we first discuss how this phenomenon is intimately related to the excitonic insulator transition, showing the explicit relation between the gap equation and response function approaches to this problem. We then provide a general computation of these mass susceptibilities in the ladder approximation, and present an analytical computation of the static exponent within a simplified kernel model, obtaining $\eta_0 =\sqrt{1-\beta/\beta_c}$ . Finally we emphasize that the behavior of these susceptibilities provides new experimental signatures of interactions, such as power law Kohn anomalies in the dispersion of several phonons, which could potentially be used as a measurement of $\beta$.Solid State Communications 02/2012; 152(15). · 1.70 Impact Factor

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