Anomalous quasiparticle lifetime in graphite: band structure effects

Enhanced optical conductivity and many-body effects in strongly-driven photo-excited semi-metallic graphite

The excitation of quasi-particles near the extrema of the electronic band structure is a gateway to electronic phase transitions in condensed matter. In a many-body system, quasi-particle dynamics are strongly influenced by the electronic single-particle structure and have been extensively studied in the weak optical excitation regime. Yet, under strong optical excitation, where light fields coherently drive carriers, the dynamics of many-body interactions that can lead to new quantum phases remain largely unresolved. Here, we induce such a highly non-equilibrium many-body state through strong optical excitation of charge carriers near the van Hove singularity in graphite. We investigate the system's evolution into a strongly-driven photo-excited state with attosecond soft X-ray core-level spectroscopy. We find an enhancement of the optical conductivity of nearly ten times the quantum conductivity and pinpoint it to carrier excitations in flat bands. This interaction regime is robust against carrier-carrier interaction with coherent optical phonons acting as an attractive force reminiscent of superconductivity. The strongly-driven non-equilibrium state is markedly different from the single-particle structure and macroscopic conductivity and is a consequence of the non-adiabatic many-body state.

Coulomb decay rates in monolayer doped graphene

Excited conduction electrons, conduction holes, and valence holes in monolayer electron-doped graphene exhibit unusual Coulomb decay rates.

Importance of σ Bonding Electrons for the Accurate Description of Electron Correlation in Graphene

Electron correlation in graphene is unique because of the interplay between the Dirac cone dispersion of π electrons and long-range Coulomb interaction. Because of the zero density of states at Fermi level, the random phase approximation predicts no metallic screening at long distance and low energy, so one might expect that graphene should be a poorly screened system. However, empirically graphene is a weakly interacting semimetal, which leads to the question of how electron correlations take place in graphene at different length scales. We address this question by computing the equal time and dynamic structure factor S(q) and S(q,ω) of freestanding graphene using ab initio fixed-node diffusion Monte Carlo simulations and the random phase approximation. We find that the σ electrons contribute strongly to S(q,ω) for relevant experimental values of ω even at distances up to around 80 Å. These findings illustrate how the emergent physics from underlying Coulomb interactions results in the observed weakly correlated semimetal.

Ultrafast electron-optical phonon scattering and quasiparticle lifetime in CVD-grown graphene

Ultrafast quasiparticle dynamics in graphene grown by chemical vapor deposition (CVD) has been studied by UV pump/white-light probe spectroscopy. Transient differential transmission spectra of monolayer graphene are observed in the visible probe range (400-650 nm). Kinetics of the quasiparticle (i.e., low-energy single-particle excitation with renormalized energy due to electron-electron Coulomb, electron-optical phonon (e-op), and optical phonon-acoustic phonon (op-ap) interactions) was monitored with 50 fs resolution. Extending the probe range to near-infrared, we find the evolution of quasiparticle relaxation channels from monoexponential e-op scattering to double exponential decay due to e-op and op-ap scattering. Moreover, quasiparticle lifetimes of mono- and randomly stacked graphene films are obtained for the probe photon energies continuously from 1.9 to 2.3 eV. Dependence of quasiparticle decay rate on the probe energy is linear for 10-layer stacked graphene films. This is due to the dominant e-op intervalley scattering and the linear density of states in the probed electronic band. A dimensionless coupling constant W is derived, which characterizes the scattering strength of quasiparticles by lattice points in graphene.

Electronic properties of a biased graphene bilayer

We study, within the tight-binding approximation, the electronic properties of a graphene bilayer in the presence of an external electric field applied perpendicular to the system-a biased bilayer. The effect of the perpendicular electric field is included through a parallel plate capacitor model, with screening correction at the Hartree level. The full tight-binding description is compared with its four-band and two-band continuum approximations, and the four-band model is shown to always be a suitable approximation for the conditions realized in experiments. The model is applied to real biased bilayer devices, made out of either SiC or exfoliated graphene, and good agreement with experimental results is found, indicating that the model is capturing the key ingredients, and that a finite gap is effectively being controlled externally. Analysis of experimental results regarding the electrical noise and cyclotron resonance further suggests that the model can be seen as a good starting point for understanding the electronic properties of graphene bilayer. Also, we study the effect of electron-hole asymmetry terms, such as the second-nearest-neighbour hopping energies t' (in-plane) and γ(4) (inter-layer), and the on-site energy Δ.

Interaction of coherent phonons with defects and elementary excitations

We present an overview of the feasibility of using coherent phonon spectroscopy to study interaction dynamics of excited lattice vibrations with their environments. By exploiting the features of coherent phonons with a pump-probe technique, one can study lattice motions in a sub-picosecond time range. The dephasing properties tell us not only about interaction dynamics with carriers (electrons and holes) or thermal phonons but also about point defects in crystals. Modulations of the coherent phonon amplitude by more than two modes are closely related to phonon-carrier or phonon-phonon interferences. Related to this phenomenon, formation of coherent phonons at higher harmonics gives direct evidence for phonon-phonon couplings. A combined study of coherent phonons and ultrafast carrier response can be useful for understanding phonon-carrier interaction dynamics. For metals like zinc, nonequilibrium electrons may dominate the dynamics of both relaxation and generation of coherent phonons. The frequency chirp of coherent phonons can be a direct measure of how and when phonon-phonon and phonon-carrier couplings occur. Carbon nanotubes show some complicated behavior due to the existence of many modes with different symmetries, resulting in superposition or interference. To illustrate one of the most interesting applications, the selective excitation of specific phonon modes through the use of a pulse train technique is shown.

First-principles study of electron linewidths in graphene

We present first-principles calculations of the linewidths of low-energy quasiparticles in n-doped graphene arising from both the electron-electron and the electron-phonon interactions. The contribution to the electron linewidth arising from the electron-electron interactions varies significantly with wave vector at fixed energy; in contrast, the electron-phonon contribution is virtually wave vector independent. These two contributions are comparable in magnitude at a binding energy of approximately 0.2 eV, corresponding to the optical phonon energy. The calculated linewidths, with both electron-electron and electron-phonon interactions included, explain to a large extent the linewidths seen in recent photoemission experiments.

Electron-phonon interactions in graphene, bilayer graphene, and graphite

Using first-principles techniques, we calculate the renormalization of the electron Fermi velocity and the vibrational lifetimes arising from electron-phonon interactions in doped bilayer graphene and in graphite and compare the results with the corresponding quantities in graphene. For similar levels of doping, the Fermi velocity renormalization in bilayer graphene and in graphite is found to be approximately 30% larger than that in graphene. In the case of bilayer graphene, this difference is shown to arise from the interlayer interaction. We discuss our findings in the light of recent photoemission and Raman spectroscopy experiments.

Sharp contrasts in low-energy quasiparticle dynamics of graphite between Brillouin zone K and H points

The low-energy quasiparticle (QP) dynamics of graphite are governed by a coupling with the E(2g) longitudinal optical phonon of omega(LO) approximately 200 meV, which is found to dramatically depend on the electronic band dispersion epsilon(k). A discontinuity of the QP linewidth develops near omega(LO) for a linear band with a quadratic band top [near the Brillouin zone (BZ) K point], while it disappears for a pure linear band (near the BZ H point). It is also found that the effective electron-phonon coupling near the K point is stronger than near the H point by more than 50%. This finding makes possible a consistent understanding of recent angle-resolved photoemission observations near the K point.

Electron-electron correlation in graphite: a combined angle-resolved photoemission and first-principles study

The full three-dimensional dispersion of the pi bands, Fermi velocities, and effective masses are measured with angle-resolved photoemission spectroscopy and compared to first-principles calculations. The band structure by density-functional theory underestimates the slope of the bands and the trigonal warping effect. Including electron-electron correlation on the level of the GW approximation, however, yields remarkable improvement in the vicinity of the Fermi level. This demonstrates the breakdown of the independent electron picture in semimetallic graphite and points toward a pronounced role of electron correlation for the interpretation of transport experiments and double-resonant Raman scattering for a wide range of carbon based materials.

Effect of linear density of states on the quasiparticle dynamics and small electron-phonon coupling in graphite

We obtained the spectral function of very high quality natural graphite single crystals using angle-resolved photoelectron spectroscopy. A clear separation of nonbonding and bonding bands and asymmetric lineshape are observed. The asymmetric line shapes are well accounted for by the finite photoelectron escape depth and the band structure. The extracted width of the spectral function (inverse of the photohole life time) near the K point is, beyond the maximum phonon energy, approximately proportional to the energy as expected from the linear density of states near the Fermi energy. The upper bound for the electron-phonon coupling constant is about 0.2, a much smaller value than the previously reported one.

Anomalous quasiparticle lifetime and strong electron-phonon coupling in graphite

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on high-quality single crystals of graphite to elucidate the character of low-energy excitations. We found evidence for a well-defined quasiparticle (QP) peak in the close vicinity of the Fermi level comparable to the nodal QP in high-T(c) cuprates, together with the mass renormalization of the band at an extremely narrow momentum region around the K(H) point. Analysis of the QP lifetime demonstrates the presence of strong electron-phonon coupling and linear energy dependence of the QP scattering rate indicative of a marked deviation from the conventional Fermi-liquid theory.

Real-time ab initio simulations of excited carrier dynamics in carbon nanotubes

Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a (3,3) carbon nanotube at different temperatures. Following an hnu=6.8 eV photoexcitation, the carrier decay is initially dominated by efficient coupling to electronic degrees of freedom. At room temperature, the excitation gap is reduced to nearly half its initial value after approximately 230 fs, where coupling to ionic motion starts dominating the decay. We show that the onset point and damping rate in the phonon regime change with initial ion velocities, a manifestation of temperature-dependent coupling between electronic and ionic degrees of freedom.

Electric field modulation of galvanomagnetic properties of mesoscopic graphite

Electric field effect devices based on mesoscopic graphite are fabricated for galvanomagnetic measurements. Strong modulation of magnetoresistance and Hall resistance as a function of the gate voltage is observed as the sample thickness approaches the screening length. Electric field dependent Landau level formation is detected from Shubnikov-de Haas oscillations. The effective mass of electron and hole carriers has been measured from the temperature dependent behavior of these oscillations.

Anisotropy and interplane interactions in the dielectric response of graphite

We determined the anisotropic dielectric response of graphite by means of time-dependent density-functional theory and high-resolution valence electron energy-loss spectroscopy. The calculated loss function was in very good agreement with the experiment for a wide range of momentum-transfer orientations with respect to the graphitic basal planes, provided that local-field effects were included in the response. The calculations also showed strong effects of the interlayer Coulomb interaction on the total pi+sigma plasmon. This finding must be taken into account for the explanation of recent loss spectra of carbon nanotube materials.