16,683 research outputs found
Coherent transport of armchair graphene constrictions
The coherent transport properties of armchair graphene nanoconstrictions(GNC)
are studied using tight-binding approach and Green's function method. We find a
non-bonding state at zero Fermi energy which results in a zero conductance
valley, when a single vacancy locates at of a perfect metallic
armchair graphene nanoribbon(aGNR). However, the non-bonding state doesn't
exist when a vacancy locates at y=3n, and the conductance behavior of lowest
conducting channel will not be affected by the vacancy. For the square-shaped
armchair GNC consisting of three metallic aGNR segments, resonant tunneling
behavior is observed in the single channel energy region. We find that the
presence of localized edge state locating at the zigzag boundary can affect the
resonant tunneling severely. A simplified one dimensional model is put forward
at last, which explains the resonant tunneling behavior of armchair GNC very
well.Comment: 6 pages, 11 figure
Planck Constraints on Holographic Dark Energy
We perform a detailed investigation on the cosmological constraints on the
holographic dark energy (HDE) model by using the Planck data. HDE can provide a
good fit to Planck high-l (l>40) temperature power spectrum, while the
discrepancy at l=20-40 found in LCDM remains unsolved in HDE. The Planck data
alone can lead to strong and reliable constraint on the HDE parameter c. At 68%
CL, we get c=0.508+-0.207 with Planck+WP+lensing, favoring the present phantom
HDE at > 2sigma CL. Comparably, by using WMAP9 alone we cannot get interesting
constraint on c. By combining Planck+WP with the BAO measurements from
6dFGS+SDSS DR7(R)+BOSS DR9, the H0 measurement from HST, the SNLS3 and Union2.1
SNIa data sets, we get 68% CL constraints c=0.484+-0.070, 0.474+-0.049,
0.594+-0.051 and 0.642+-0.066. Constraints can be improved by 2%-15% if we
further add the Planck lensing data. Compared with the WMAP9 results, the
Planck results reduce the error by 30%-60%, and prefer a phantom-like HDE at
higher CL. We find no evident tension between Planck and BAO/HST. Especially,
the strong correlation between Omegam h^3 and dark energy parameters is helpful
in relieving the tension between Planck and HST. The residual
chi^2_{Planck+WP+HST}-chi^2_{Planck+WP} is 7.8 in LCDM, and is reduced to 1.0
or 0.3 if we switch dark energy to the w model or the holographic model. We
find SNLS3 is in tension with all other data sets; for Planck+WP, WMAP9 and
BAO+HST, the corresponding Delta chi^2 is 6.4, 3.5 and 4.1, respectively.
Comparably, Union2.1 is consistent with these data sets, but the combination
Union2.1+BAO+HST is in tension with Planck+WP+lensing, corresponding to a Delta
chi^2 8.6 (1.4% probability). Thus, it is not reasonable to perform an
all-combined (CMB+SNIa+BAO+HST) analysis for HDE when using the Planck data.
Our tightest self-consistent constraint is c=0.495+-0.039 obtained from
Planck+WP+BAO+HST+lensing.Comment: 29 pages, 11 figures, 3 tables; version accepted for publication in
JCA
Design of Ultra-compact Graphene-based Superscatterers
The energy-momentum dispersion relation is a fundamental property of
plasmonic systems. In this paper, we show that the method of dispersion
engineering can be used for the design of ultra-compact graphene-based
superscatterers. Based on the Bohr model, the dispersion relation of the
equivalent planar waveguide is engineered to enhance the scattering cross
section of a dielectric cylinder. Bohr conditions with different orders are
fulfilled in multiple dispersion curves at the same resonant frequency. Thus
the resonance peaks from the first and second order scattering terms are
overlapped in the deepsubwavelength scale by delicately tuning the gap
thickness between two graphene layers. Using this ultra-compact graphene-based
superscatterer, the scattering cross section of the dielectric cylinder can be
enhanced by five orders of magnitude.Comment: This paper has been accepted by IEEE Journal of Selected topics in
Quantum Electronic
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