23,699 research outputs found
Determine the galaxy bias factors on large scales using bispectrum method
We study whether the bias factors of galaxies can be unbiasedly recovered
from their power spectra and bispectra. We use a set of numerical N-body
simulations and construct large mock galaxy catalogs based upon the
semi-analytical model of Croton et al. (2006). We measure the reduced bispectra
for galaxies of different luminosity, and determine the linear and first
nonlinear bias factors from their bispectra. We find that on large scales down
to that of the wavenumber k=0.1h/Mpc, the bias factors b1 and b2 are nearly
constant, and b1 obtained with the bispectrum method agrees very well with the
expected value. The nonlinear bias factor b2 is negative, except for the most
luminous galaxies with M<-23 which have a positive b2. The behavior of b2 of
galaxies is consistent with the b2 mass dependence of their host halos. We show
that it is essential to have an accurate estimation of the dark matter
bispectrum in order to have an unbiased measurement of b1 and b2. We also test
the analytical approach of incorporating halo occupation distribution to model
the galaxy power spectrum and bispectrum. The halo model predictions do not fit
the simulation results well on the precision requirement of current
cosmological studies.Comment: 9 pages, 8 figures, accepted for publication in Ap
Band Gap of Strained Graphene Nanoribbons
The band structures of strained graphene nanoribbons (GNRs) are examined by a
tight binding Hamiltonian that is directly related to the type and strength of
strains. Compared to the two-dimensional graphene whose band gap remains close
to zero even if a large strain is applied, the band gap of graphene nanoribbon
(GNR) is sensitive to both uniaxial and shears strains. The effect of strain on
the electronic structure of a GNR strongly depends on its edge shape and
structural indices. For an armchair GNR, uniaxial weak strain changes the band
gap in a linear fashion, and for a large strain, it results in periodic
oscillation of the band gap. On the other hand, shear strain always tend to
reduce the band gap. For a zigzag GNR, the effect of strain is to change the
spin polarization at the edges of GNR, thereby modulate the band gap. A simple
analytical model is proposed to interpret the band gap responds to strain in
armchair GNR, which agrees with the numerical results.Comment: 30 pages,10 figure
Inelastic Phonon Scattering in Graphene FETs
Inelastic phonon scattering in graphene field-effect transistors (FETs) is
studied by numerically solving the Boltzmann transport equation in three
dimensional real and phase spaces (x, kx, ky). A kink behavior due to ambipolar
transport agreeing with experiments is observed. While low field behavior has
previously been mostly attributed to elastic impurity scattering in earlier
studies, it is found in the study that even low field mobility is affected by
inelastic phonon scattering in recent graphene FET experiments reporting high
mobilities . As the FET is biased in the saturation regime, the average carrier
injection velocity at the source end of the device is found to remain almost
constant with regard to the applied gate voltage over a wide voltage range,
which results in significantly improved transistor linearity compared to what a
simpler model would predict. Physical mechanisms for good linearity are
explained, showing the potential of graphene FETs for analogue electronics
applications
Unconventional behavior of Dirac fermions in three-dimensional gauge theory
We study the unconventional behavior of massless Dirac fermions due to
interaction with a U(1) gauge field in two spatial dimensions. At zero chemical
potential, the longitudinal and transverse components of gauge interaction are
both long-ranged. There is no fermion velocity renormalization since the system
respects Lorentz invariance. At finite chemical potential, the Lorentz
invariance is explicitly broken by the finite Fermi surface. The longitudinal
gauge interaction is statically screened and becomes unimportant, whereas the
transverse gauge interaction remains long-ranged and leads to singular
renormalization of fermion velocity. The anomalous dimension of fermion
velocity is calculated by means of the renormalization group method. We then
examine the influence of singular velocity renormalization on several physical
quantities, and show that they exhibit different behavior at zero and finite
chemical potential.Comment: 9 pages, 4 figure
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