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: 7.51). 09/2008; 101(6):066401. DOI: 10.1103/PhysRevLett.101.066401
Source: PubMed


We develop a theory for the renormalization of the phonon energy dispersion in graphene due to the combined effects of both Coulomb and electron-phonon (e-ph) interactions. We obtain the renormalized phonon energy spectrum by an exact analytic derivation of the phonon self-energy, finding three distinct Kohn anomalies (KAs) at the phonon wave vector q=omega/v, 2k_{F}+/-omega/v for LO phonons and one at q=omega/v for TO phonons. The presence of these new KAs in graphene, in contrast to the usual KA q=2k_{F} in ordinary metals, originates from the dynamical screening of e-ph interaction (with a concomitant breakdown of the Born-Oppenheimer approximation) and the peculiar chirality of the graphene e-ph coupling.

Download full-text


Available from: Ben Yu-Kuang Hu,
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We address the question of how large can the lifetime of electronic states be at low energies in graphene, below the scale of the optical phonon modes. For this purpose, we study the many-body effects at the K point of the spectrum, which induce a strong coupling between electron-hole pairs and out-of-plane phonons. We show the existence of a soft branch of hybrid states below the electron-hole continuum when graphene is close to the charge neutrality point, leading to an inverse lifetime proportional to the cube of the quasiparticle energy. This implies that a crossover should be observed in transport properties, from such a slow decay rate to the lower bound given at very low energies by the decay into acoustic phonons.
    Physical Review Letters 11/2008; 101(17):176802. DOI:10.1103/PhysRevLett.101.176802 · 7.51 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We develop a theory for the energy relaxation of hot Dirac fermions in graphene. We obtain a generic expression for the energy relaxation rate due to electron-phonon interaction and calculate the power loss due to both optical and acoustic phonon emission as a function of electron temperature $T_{\mathrm{e}}$ and density $n$. We find an intrinsic power loss weakly dependent on carrier density and non-vanishing at the Dirac point $n = 0$, originating from interband electron-optical phonon scattering by the intrinsic electrons in the graphene valence band. We obtain the total power loss per carrier $\sim 10^{-12} - 10^{-7} \mathrm{W}$ within the range of electron temperatures $\sim 20 - 1000 \mathrm{K}$. We find optical (acoustic) phonon emission to dominate the energy loss for $T_{\mathrm{e}} > (<) 200-300 \mathrm{K}$ in the density range $n = 10^{11}-10^{13} \mathrm{cm}^{-2}$. Comment: 5 pages
    Physical review. B, Condensed matter 12/2008; 79(23). DOI:10.1103/PhysRevB.79.235406 · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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). DOI:10.1103/PhysRevB.79.115443 · 3.66 Impact Factor
Show more