3,335 research outputs found
Semiconducting-to-metallic photoconductivity crossover and temperature-dependent Drude weight in graphene
We investigated the transient photoconductivity of graphene at various
gate-tuned carrier densities by optical-pump terahertz-probe spectroscopy. We
demonstrated that graphene exhibits semiconducting positive photoconductivity
near zero carrier density, which crosses over to metallic negative
photoconductivity at high carrier density. Our observations are accounted for
by considering the interplay between photo-induced changes of both the Drude
weight and the carrier scattering rate. Notably, we observed multiple sign
changes in the temporal photoconductivity dynamics at low carrier density. This
behavior reflects the non-monotonic temperature dependence of the Drude weight,
a unique property of massless Dirac fermions
Effect of next-nearest neighbor coupling on the optical spectra in bilayer graphene
We investigate the dependence of the optical conductivity of bilayer graphene
(BLG) on the intra- and inter-layer interactions using the most complete model
to date. We show that the next nearest-neighbor intralayer coupling introduces
new features in the low-energy spectrum that are highly sensitive to sample
doping, changing significantly the ``universal'' conductance. Further, its
interplay with interlayer couplings leads to an anisotropy in conductance in
the ultraviolet range. We propose that experimental measurement of the optical
conductivity of intrinsic and doped BLG will provide a good benchmark for the
relative importance of intra- and inter-layer couplings at different doping
levels.Comment: 5 pages, 5 figure
Trion induced negative photoconductivity in monolayer MoS2
Optical excitation typically enhances electrical conduction and low-frequency
radiation absorption in semiconductors. We have, however, observed a pronounced
transient decrease of conductivity in doped monolayer molybdenum disulfide
(MoS2), a two-dimensional (2D) semiconductor, under femtosecond laser
excitation. In particular, the conductivity is reduced dramatically down to
only 30% of its equilibrium value with high pump fluence. This anomalous
phenomenon arises from the strong many-body interactions in the system, where
photoexcited electron-hole pairs join the doping-induced charges to form
trions, bound states of two electrons and one hole. The resultant increase of
the carrier effective mass substantially diminishes the carrier conductivity
Competing Ultrafast Energy Relaxation Pathways in Photoexcited Graphene
For most optoelectronic applications of graphene a thorough understanding of
the processes that govern energy relaxation of photoexcited carriers is
essential. The ultrafast energy relaxation in graphene occurs through two
competing pathways: carrier-carrier scattering -- creating an elevated carrier
temperature -- and optical phonon emission. At present, it is not clear what
determines the dominating relaxation pathway. Here we reach a unifying picture
of the ultrafast energy relaxation by investigating the terahertz
photoconductivity, while varying the Fermi energy, photon energy, and fluence
over a wide range. We find that sufficiently low fluence ( 4
J/cm) in conjunction with sufficiently high Fermi energy (
0.1 eV) gives rise to energy relaxation that is dominated by carrier-carrier
scattering, which leads to efficient carrier heating. Upon increasing the
fluence or decreasing the Fermi energy, the carrier heating efficiency
decreases, presumably due to energy relaxation that becomes increasingly
dominated by phonon emission. Carrier heating through carrier-carrier
scattering accounts for the negative photoconductivity for doped graphene
observed at terahertz frequencies. We present a simple model that reproduces
the data for a wide range of Fermi levels and excitation energies, and allows
us to qualitatively assess how the branching ratio between the two distinct
relaxation pathways depends on excitation fluence and Fermi energy.Comment: Nano Letters 201
Generator Coordinate Method Calculations for Ground and First Excited Collective States in He, O and Ca Nuclei
The main characteristics of the ground and, in particular, the first excited
monopole state in the He, O and Ca nuclei are studied
within the generator coordinate method using Skyrme-type effective forces and
three construction potentials, namely the harmonic-oscillator, the square-well
and Woods-Saxon potentials. Calculations of density distributions, radii,
nucleon momentum distributions, natural orbitals, occupation numbers and
depletions of the Fermi sea, as well as of pair density and momentum
distributions are carried out. A comparison of these quantities for both ground
and first excited monopole states with the available empirical data and with
the results of other theoretical methods are given and discussed in detail.Comment: 15 pages, LaTeX, 6 Postscript figures, submitted to EPJ
Asymptotic Normalization Coefficients for 13C+p->14N
The proton exchange reaction has been measured
at an incident energy of 162 MeV. Angular distributions were obtained for
proton transfer to the ground and low lying excited states in . Elastic
scattering of on also was measured out to the rainbow angle
region in order to find reliable optical model potentials. Asymptotic
normalization coefficients for the system have been
found for the ground state and the excited states at 2.313, 3.948, 5.106 and
5.834 MeV in . These asymptotic normalization coefficients will be used
in a determination of the S-factor for at solar
energies from a measurement of the proton transfer reaction
.Comment: 5 pages, 6 figure
#Bieber + #Blast = #BieberBlast: Early Prediction of Popular Hashtag Compounds
Compounding of natural language units is a very common phenomena. In this
paper, we show, for the first time, that Twitter hashtags which, could be
considered as correlates of such linguistic units, undergo compounding. We
identify reasons for this compounding and propose a prediction model that can
identify with 77.07% accuracy if a pair of hashtags compounding in the near
future (i.e., 2 months after compounding) shall become popular. At longer times
T = 6, 10 months the accuracies are 77.52% and 79.13% respectively. This
technique has strong implications to trending hashtag recommendation since
newly formed hashtag compounds can be recommended early, even before the
compounding has taken place. Further, humans can predict compounds with an
overall accuracy of only 48.7% (treated as baseline). Notably, while humans can
discriminate the relatively easier cases, the automatic framework is successful
in classifying the relatively harder cases.Comment: 14 pages, 4 figures, 9 tables, published in CSCW (Computer-Supported
Cooperative Work and Social Computing) 2016. in Proceedings of 19th ACM
conference on Computer-Supported Cooperative Work and Social Computing (CSCW
2016
Magnetic properties of Mn doped ZnO tetrapod structures
Author name used in this publication: C. Surya2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe
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