736 research outputs found
High Intrinsic Mobility and Ultrafast Carrier Dynamics in Multilayer Metal Dichalcogenide MoS2
The ultimate limitations on carrier mobilities in metal dichalcogenides, and
the dynamics associated with carrier relaxation, are unclear. We present
measurements of the frequency-dependent conductivity of multilayer
dichalcogenide MoS2 by optical-pump terahertz-probe spectroscopy. We find
mobilities in this material approaching 4200 cm2/Vs at low temperatures. The
temperature dependence of scattering indicates that the mobility, an order of
magnitude larger than previously reported for MoS2, is intrinsically limited by
acoustic phonon scattering at THz frequencies. Our measurements of carrier
relaxation reveal picosecond cooling times followed by recombination lasting
tens of nanoseconds and dominated by Auger scattering into defects. Our results
provide a useful context in which to understand and evaluate the performance of
MoS2-based electronic and optoelectronic devices.Comment: 13 pages, 8 figure
Ultrafast Carrier Recombination and Generation Rates for Plasmon Emission and Absorption in Graphene
Electron-hole generation and recombination rates for plasmon emission and
absorption in Graphene are presented. The recombination times of carriers due
to plasmon emission have been found to be in the tens of femtoseconds to
hundreds of picoseconds range. The recombination times depend sensitively on
the carrier energy, carrier density, temperature, and the plasmon dispersion.
Carriers near the Dirac point are found to have much longer lifetimes compared
to carriers at higher energies. Plasmons in a Graphene layer on a polar
substrate hybridize with the surface optical phonons and this hybridization
modifies the plasmon dispersion. We also present generation and recombination
rates of carriers due to plasmon emission and absorption in Graphene layers on
polar substrates.Comment: 7 Pages, 11 Figures, To appear in Phys. Rev. B (2011
Carrier Recombination and Generation Rates for Intravalley and Intervalley Phonon Scattering in Graphene
Electron-hole generation and recombination rates for intravalley and
intervalley phonon scattering in Graphene are presented. The transverse and the
longitudinal optical phonon modes (-modes) near the zone center
(-point) contribute to intravalley interband carrier scattering. At the
zone edge (-point), only the transverse optical phonon mode
(-mode) contributes significantly to intervalley interband scattering
with recombination rates faster than those due to zone center phonons. The
calculated recombination times range from less than a picosecond to more than
hundreds of picoseconds and are strong functions of temperature and electron
and hole densities. The theoretical calculations agree well with experimental
measurements of the recombination rates of photoexcited carriers in graphene.Comment: 6 pages, 9 figure
Ultrafast Relaxation Dynamics of Hot Optical Phonons in Graphene
Using ultrafast optical pump-probe spectroscopy, we study the relaxation
dynamics of hot optical phonons in few-layer and multi-layer graphene films
grown by epitaxy on silicon carbide substrates and by chemical vapor deposition
on nickel substrates. In the first few hundred femtoseconds after
photoexcitation, the hot carriers lose most of their energy to the generation
of hot optical phonons which then present the main bottleneck to subsequent
carrier cooling. Optical phonon cooling on short time scales is found to be
independent of the graphene growth technique, the number of layers, and the
type of the substrate. We find average phonon lifetimes in the 2.5-2.55 ps
range. We model the relaxation dynamics of the coupled carrier-phonon system
with rate equations and find a good agreement between the experimental data and
the theory. The extracted optical phonon lifetimes agree very well with the
theory based on anharmonic phonon interactions.Comment: 4 pages, 3 figure
Measurement of the Splitting Function in &ITpp &ITand Pb-Pb Collisions at root&ITsNN&IT=5.02 TeV
Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions, but it is not known where in the shower evolution the modifications occur. The momentum ratio of the two leading partons, resolved as subjets, provides information about the parton shower evolution. This substructure observable, known as the splitting function, reflects the process of a parton splitting into two other partons and has been measured for jets with transverse momentum between 140 and 500 GeV, in pp and PbPb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair. In central PbPb collisions, the splitting function indicates a more unbalanced momentum ratio, compared to peripheral PbPb and pp collisions.. The measurements are compared to various predictions from event generators and analytical calculations.Peer reviewe
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