25,819 research outputs found
Impurity State and Variable Range Hopping Conduction in Graphene
The variable range hopping theory, as formulated for exponentially localized
impurity states, does not necessarily apply in the case of graphene with
covalently attached impurities. We analyze the localization of impurity states
in graphene using the nearest-neighbor, tight-binding model of an
adatom-graphene system with Green's function perturbation methods. The
amplitude of the impurity state wave function is determined to decay as a power
law with exponents depending on sublattice, direction, and the impurity
species. We revisit the variable range hopping theory in view of this result
and find that the conductivity depends as a power law of the temperature with
an exponent related to the localization of the wave function. We show that this
temperature dependence is in agreement with available experimental results
Systematic study of GaInAs self-assembled quantum wires with different interfacial strain relaxation
A systematic theoretical study of the electronic and optical properties of
GaInAs self-assembled quantum-wires (QWR's) made of short-period
superlattices (SPS) with strain-induced lateral ordering is presented. The
theory is based on the effective bond-orbital model (EBOM) combined with a
valence-force field (VFF) model. Valence-band anisotropy, band mixing, and
effects due to local strain distribution at the atomistic level are all taken
into account. Several structure models with varying degrees of alloy mixing for
lateral modulation are considered. A valence force field model is used to find
the equilibrium atomic positions in the QWR structure by minimizing the lattice
energy. The strain tensor at each atomic (In or Ga) site is then obtained and
included in the calculation of electronic states and optical properties. It is
found that different local arrangement of atoms leads to very different strain
distribution, which in turn alters the optical properties. In particular, we
found that in model structures with thick capping layer the electron and hole
are confined in the Ga-rich region and the optical anisotropy can be reversed
due to the variation of lateral alloying mixing, while for model structures
with thin capping layer the electron and hole are confined in the In-rich
region, and the optical anisotropy is much less sensitive to the lateral alloy
mixing.Comment: 23 pages, and 8 figure
A Cosmology-Independent Calibration of Gamma-Ray Burst Luminosity Relations and the Hubble Diagram
An important concern in the application of gamma-ray bursts (GRBs) to
cosmology is that the calibration of GRB luminosity/energy relations depends on
the cosmological model, due to the lack of a sufficient low-redshift GRB
sample. In this paper, we present a new method to calibrate GRB relations in a
cosmology-independent way. Since objects at the same redshift should have the
same luminosity distance and since the distance moduli of Type Ia supernovae
(SNe Ia) obtained directly from observations are completely cosmology
independent, we obtain the distance modulus of a GRB at a given redshift by
interpolating from the Hubble diagram of SNe Ia. Then we calibrate seven GRB
relations without assuming a particular cosmological model and construct a GRB
Hubble diagram to constrain cosmological parameters. From the 42 GRBs at
, we obtain ,
for the flat CDM model, and
for the dark energy model with a constant equation of state
, which is consistent with the concordance model in
a 1- confidence region.Comment: 7 pages, 3 figures, 1 table, now matches the editorially revised
version; accepted for publication in ApJ (vol 685)
Ultra-broadband wavelength-swept Tm-doped fiber laser using wavelength-combined gain stages
A wavelength-swept thulium-doped fiber laser system employing two parallel cavities with two different fiber gain stages is reported. The fiber gain stages were tailored to provide emission in complementary bands with external wavelength-dependent feedback cavities sharing a common rotating polygon mirror for wavelength scanning. The wavelength-swept laser outputs from the fiber gain elements were spectrally combined by means of a dichroic mirror and yielded over 500 mW of output with a scanning range from ~1740 nm to ~2070 nm for a scanning frequency of ~340 Hz
Theoretical prediction of topological insulator in ternary rare earth chalcogenides
A new class of three-dimensional topological insulator, ternary rare earth
chalcogenides, is theoretically investigated with ab initio calculations. Based
on both bulk band structure analysis and the direct calculation of topological
surface states, we demonstrate that LaBiTe3 is a topological insulator. La can
be substituted by other rare earth elements, which provide candidates for novel
topological states such as quantum anomalous Hall insulator, axionic insulator
and topological Kondo insulator. Moreover, YBiTe3 and YSbTe3 are found to be
normal insulators. They can be used as protecting barrier materials for both
LaBiTe3 and Bi2Te3 families of topological insulators for their well matched
lattice constants and chemical composition.Comment: 5 pages, 3 figures and 1 tabl
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