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Register a new userSnapshots of non-equilibrium Dirac carrier distributions in graphene
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The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a semiconducting bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy and momentum dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme ultra-violet (EUV) pulses at 31.5 eV photon energy to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. While excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find indication for carrier multiplication, questioning the applicability of graphene for light harvesting. Time-resolved photoemission spectroscopy in the EUV emerges as the technique of choice to assess the suitability of new materials for optoelectronics, providing quantitatively accurate measurements of non-equilibrium carriers at all energies and wavevectors.
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Graphene is an ideal material to study fundamental Coulomb- and phonon-induced carrier scattering processes. Its remarkable gapless and linear band structure opens up new carrier relaxation channels. In particular, Auger scattering bridging the valence and the conduction band changes the number of charge carriers and gives rise to a significant carrier multiplication - an ultrafast many-particle phenomenon that is promising for the design of highly efficient photodetectors. Furthermore, the vanishing density of states at the Dirac point combined with ultrafast phonon-induced intraband scattering results in an accumulation of carriers and a population inversion suggesting the design of graphene-based terahertz lasers. Here, we review our work on the ultrafast carrier dynamics in graphene and Landau-quantized graphene is presented providing a microscopic view on the appearance of carrier multiplication and population inversion.
We performed infrared transmission experiment on ion-gel gated graphene and measured carrier scattering rate g as function of carrier density n over wide range up to n=2E13 cm-2. The g exhibits a rapid decreases along with the gating followed by persistent increases on further carrier doping. This behavior of g(n) demonstrates that carrier is scattered dominantly by the two scattering mechanisms, namely, charged impurity (CI) scattering and short-range disorder (SR) scattering, with additional minor scattering from substrate phonon (SPP). We can determine the absolute strengths of all the scattering channels by fitting the g(n) data and unveils the complete n-dependent map of the scattering mechanisms g(n)=gCI(n)+gSR(n)+gSPP(n). The gCI(n) and gSR(n) are larger than those of SiO2$-gated graphene by 1.8 times, which elucidates the dual role of the ion-gel layer as a CI-scatterer and simultaneously a SR-scatterer to graphene. Additionally we show that freezing of IG at low-T (~200 K) does not cause any change to the carrier scattering.
We report a multiband transport study of bilayer graphene at high carrier densities. Employing a poly(ethylene)oxide-CsClO$_4$ solid polymer electrolyte gate we demonstrate the filling of the high energy subbands in bilayer graphene samples at carrier densities $|n|geq2.4times 10^{13}$ cm$^{-2}$. We observe a sudden increase of resistance and the onset of a second family of Shubnikov de Haas (SdH) oscillations as these high energy subbands are populated. From simultaneous Hall and magnetoresistance measurements together with SdH oscillations in the multiband conduction regime, we deduce the carrier densities and mobilities for the higher energy bands separately and find the mobilities to be at least a factor of two higher than those in the low energy bands.
We discuss plasmons of biased twisted bilayer graphene when the Fermi level lies inside the gap. The collective excitations are a network of chiral edge plasmons (CEP) entirely composed of excitations in the topological electronic edge states (EES) that appear at the AB-BA interfaces. The CEP form an hexagonal network with an unique energy scale $epsilon_p=frac{e^2}{epsilon_0epsilon t_0}$ with $t_0$ the moire lattice constant and $epsilon$ the dielectric constant. From the dielectric matrix we obtain the plasmon spectra that has two main characteristics: (i) a diverging density of states at zero energy, and (ii) the presence of a plasmonic Dirac cone at $hbaromegasimepsilon_p/2$ with sound velocity $v_D=0.0075c$, which is formed by zigzag and armchair current oscillations. A network model reveals that the antisymmetry of the plasmon bands implies that CEP scatter at the hexagon vertices maximally in the deflected chiral outgoing directions, with a current ratio of 4/9 into each of the deflected directions and 1/9 into the forward one. We show that scanning near-field microscopy should be able to observe the predicted plasmonic Dirac cone and its broken symmetry phases.
It is widely assumed that the dominant source of scattering in graphene is charged impurities in a substrate. We have tested this conjecture by studying graphene placed on various substrates and in high-k media. Unexpectedly, we have found no significant changes in carrier mobility either for different substrates or by using glycerol, ethanol and water as a top dielectric layer. This suggests that Coulomb impurities are not the scattering mechanism that limits the mean free path currently attainable for graphene on a substrate.
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