Effect of Linear Density of States on the Quasiparticle Dynamics and Small Electron-Phonon Coupling in Graphite

Abstract

We obtained the spectral function of very high quality natural graphite single crystals using angle resolved photoelectron spectroscopy (ARPES). A clear separation of non-bonding and bonding bands and asymmetric lineshape are observed. The asymmetric lineshapes 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.PACS numbers: 74.25.Jb, 63.20.Ls, Recent discoveries of novel physical properties in carbon-based materials such as superconductivity 1,2,3,4,5 and massless Dirac Fermions 6 brought renewed interest in the electronic structure of graphite 7,8 . The peculiarity of the electronic structure of graphite has two aspects: graphite is extremely two dimensional and is a semi-metal. These facts make it fundamentally interesting to study how dimensionality affects the dynamics of the doped carriers and how the carriers in graphite intercalated compounds (GICs) couple to the mediating bosons. In fact, there is a long standing issue in regards to the carrier dynamics in graphite, that is, whether the carriers are Fermi-liquid-like or not. This question motivates experimental studies of electronic structures of these materials by using, for example, angle-resolved photoemission spectroscopy (ARPES) and one can find a long history in the ARPES studies on graphite single crystals 7,8,9,10,11,12,13,14,15 . In addition, studies of graphite-related materials such as single 16 and bilayer graphene 17 and GICs 18,19 can be found.High quality ARPES data from graphite is difficult to obtain despite graphite's two dimensional, inert nature. The problems associated with ARPES experiments on graphite are due mostly to difficulty in proper surface preparation and to some extent to the low quality of the single crystals. For example, the extreme twodimensional nature of graphite inevitably produces small flakes (many are small enough to be seen only under microscopes) on the cleaved surfaces which ruins the momentum resolution in ARPES. Such difficulties prevented one from obtaining good quality data to extract reliable information on the many-body interactions such as electron-phonon coupling (EPC). Therefore, unless such difficulties are overcome, reliable information on manybody interactions can not be extracted from the data. As a result, the experimental data in regards to the electron lifetime have been obtained mostly by time-resolved photoelectron spectroscopy on highly oriented pyrolytic graphite 20,21 .Motivated by the renewed interest in the carrier dynamics in graphite, we have performed ARPES studies on graphite single crystals. Our goal was to extract reliable quantitative information on the EPC from ARPES data. To ...

Observation of inhibited electron-ion coupling in strongly heated graphite

“…Converting our values for the coupling coefficient gives λ ~ 0.06. This is considerably lower than the value for typical metals, however previous studies on weakly excited carbon have also found low coupling in room temperature graphite23 ( λ < 0.2), and graphene10 ( λ = 0.05 – 0.1). These results have been attributed to a small density of states at the Fermi energy in semimetals, an argument that becomes questionable for the high electron temperatures present in our experiment.…”

On the low-field Hall coefficient of graphite

“…For example, early data on the low-field Hall coefficient obtained in single-crystalline natural graphite (SCNG) samples showed that it is positive at fields below and negative above ∼ 0.5 T at a temperature T = 77 K, 1 suggesting that holes are the majority carriers. This result appears to be at odd to several other studies on the graphite band structure obtained in highly oriented pyrolytic graphite (HOPG) samples [2][3][4][5][6][7][8][9][10] that suggest that electrons are the majority carriers, unless one argues in terms of different mobilities of the majority carriers, an interpretation that was used indeed in the past.…”

“…5 Turns negative at large field for samples with imperfect structure. 6 system of screw dislocations or a system of edge dislocations if the misfit is in the c-direction with an angle θ c 0. A system of dislocations at the two-dimensional interfaces or topological line defects can have a large influence in the dispersion relation of the carriers 31,32 and trigger localized high-temperature superconductivity.…”

Anisotropic electron-phonon coupling investigated by ultrafast electron crystallography: Three-temperature model

“…While the electron-phonon coupling in graphite is believed to be smaller than normal metals 49 , a large coupling constant is usually required for explaining fs laser 31,45,50 and swift ion-beam irradiation experiments 51,52 . This discrepancy reveals a complex energy exchange in nonthermally heated graphite when compared to conventional metals.…”