72,741 research outputs found
Differential quadrature method for space-fractional diffusion equations on 2D irregular domains
In mathematical physics, the space-fractional diffusion equations are of
particular interest in the studies of physical phenomena modelled by L\'{e}vy
processes, which are sometimes called super-diffusion equations. In this
article, we develop the differential quadrature (DQ) methods for solving the 2D
space-fractional diffusion equations on irregular domains. The methods in
presence reduce the original equation into a set of ordinary differential
equations (ODEs) by introducing valid DQ formulations to fractional directional
derivatives based on the functional values at scattered nodal points on problem
domain. The required weighted coefficients are calculated by using radial basis
functions (RBFs) as trial functions, and the resultant ODEs are discretized by
the Crank-Nicolson scheme. The main advantages of our methods lie in their
flexibility and applicability to arbitrary domains. A series of illustrated
examples are finally provided to support these points.Comment: 25 pages, 25 figures, 7 table
Laser Mode Bifurcations Induced by -Breaking Exceptional Points
A laser consisting of two independently-pumped resonators can exhibit mode
bifurcations that evolve out of the exceptional points (EPs) of the linear
system at threshold. The EPs are non-Hermitian degeneracies occurring at the
parity/time-reversal () symmetry breaking points of the threshold
system. Above threshold, the EPs become bifurcations of the nonlinear
zero-detuned laser modes, which can be most easily observed by making the gain
saturation intensities in the two resonators substantially different. Small
pump variations can then switch abruptly between different laser behaviors,
e.g. between below-threshold and -broken single-mode operation.Comment: 4 pages, 3 figure
Transport in gapped bilayer graphene: the role of potential fluctuations
We employ a dual-gated geometry to control the band gap \Delta in bilayer
graphene and study the temperature dependence of the resistance at the charge
neutrality point, RNP(T), from 220 to 1.5 K. Above 5 K, RNP(T) is dominated by
two thermally activated processes in different temperature regimes and exhibits
exp(T3/T)^{1/3} below 5 K. We develop a simple model to account for the
experimental observations, which highlights the crucial role of localized
states produced by potential fluctuations. The high temperature conduction is
attributed to thermal activation to the mobility edge. The activation energy
approaches \Delta /2 at large band gap. At intermediate and low temperatures,
the dominant conduction mechanisms are nearest neighbor hopping and
variable-range hopping through localized states. Our systematic study provides
a coherent understanding of transport in gapped bilayer graphene.Comment: to appear in Physical Review B: Rapid Com
Bloch Oscillation under a Bichromatic Laser: Quasi-Miniband Formation, Collapse, and Dynamical Delocalization and Localization
A novel DC and AC driving configuration is proposed for semiconductor
superlattices, in which the THz AC driving is provided by an intense
bichromatic cw laser. The two components of the laser, usually in the visible
light range, are near but not exactly resonant with interband Wannier-Stark
transitions, and their frequency difference equals the Wannier-Stark ladder
spacing. Multi-photon processes with the intermediate states in the conduction
(valence) band cause dynamical delocalization and localization of valence
(conduction) electrons, and the corresponding formation and collapse of the
quasi-minibands.Comment: 4 pages, 3 figure
Electron-phonon bound states in graphene in a perpendicular magnetic field
The spectrum of electron-phonon complexes in a monolayer graphene is
investigated in the presence of a perpendicular quantizing magnetic field.
Despite the small electron-phonon coupling, usual perturbation theory is
inapplicable for calculation of the scattering amplitude near the threshold of
the optical phonon emission. Our findings beyond perturbation theory show that
the true spectrum near the phonon emission threshold is completely governed by
new branches, corresponding to bound states of an electron and an optical
phonon with a binding energy of the order of where
is the electron-phonon coupling and the phonon energy.Comment: To be published in Phys. Rev. Lett., 5 pages, 3 figures, 1 tabl
Probing non-Abelian statistics of Majorana fermions in ultracold atomic superfluid
We propose an experiment to directly probe the non-Abelian statistics of
Majorana fermions by braiding them in an s-wave superfluid of ultracold atoms.
We show different orders of braiding operations give orthogonal output states
that can be distinguished through Raman spectroscopy. Realization of Majorana
bound states in an s-wave superfluid requires strong spin-orbital coupling and
a controllable Zeeman field in the perpendicular direction. We present a simple
laser configuration to generate the artificial spin-orbital coupling and the
required Zeeman field in the dark state subspace.Comment: 4 pages; Add detailed discussion of feasibility of the scheme;add
ref
A distinct sortase SrtB anchors and processes a streptococcal adhesin AbpA with a novel structural property.
Surface display of proteins by sortases in Gram-positive bacteria is crucial for bacterial fitness and virulence. We found a unique gene locus encoding an amylase-binding adhesin AbpA and a sortase B in oral streptococci. AbpA possesses a new distinct C-terminal cell wall sorting signal. We demonstrated that this C-terminal motif is required for anchoring AbpA to cell wall. In vitro and in vivo studies revealed that SrtB has dual functions, anchoring AbpA to the cell wall and processing AbpA into a ladder profile. Solution structure of AbpA determined by NMR reveals a novel structure comprising a small globular α/β domain and an extended coiled-coil heliacal domain. Structural and biochemical studies identified key residues that are crucial for amylase binding. Taken together, our studies document a unique sortase/adhesion substrate system in streptococci adapted to the oral environment rich in salivary amylase
Neutron scattering study of commensurate magnetic ordering in single crystal CeSb
Temperature and field-dependent magnetization measurements and
neutron scattering study of a single crystal CeSb are presented. Several
anomalies in the magnetization curves have been confirmed at low magnetic
field, i.e., 15.6 K, 12 K, and 9.8 K. These three transitions are all
metamagnetic transitions (MMT), which shift to lower temperatures as the
magnetic field increases. The anomaly at 15.6 K has been suggested as
paramagnetic (PM) to ferromagnetic (FM) phase transition. The anomaly located
at around 12 K is antiferromagnetic-like transition, and this turning point
will clearly split into two when the magnetic field T. Neutron
scattering study reveals that the low temperature ground state of CeSb
orders antiferromagnetically with commensurate propagation wave vectors
and , with N\'eel
temperature K. This transition is of first-order, as shown in the
hysteresis loop observed by the field cooled cooling (FCC) and field cooled
warming (FCW) processes.Comment: 7 pages,9 figure
Long-term Evolution of Protostellar and Protoplanetary Disks. I. Outbursts
As an initial investigation into the long-term evolution of protostellar
disks, we explore the conditions required to explain the large outbursts of
disk accretion seen in some young stellar objects. We use one-dimensional
time-dependent disk models with a phenomenological treatment of the
magnetorotational instability (MRI) and gravitational torques to follow disk
evolution over long timescales. Comparison with our previous two-dimensional
disk model calculations (Zhu et al. 2009b, Z2009b) indicates that the neglect
of radial effects and two-dimensional disk structure in the one-dimensional
case makes only modest differences in the results; this allows us to use the
simpler models to explore parameter space efficiently. We find that the mass
infall rates typically estimated for low-mass protostars generally result in
AU-scale disk accretion outbursts, as predicted by our previous analysis (Zhu
et al. 2009a,Z2009a). We also confirm quasi-steady accretion behavior for high
mass infall rates if the values of -parameter for the magnetorotational
instability is small, while at this high accretion rate convection from the
thermal instability may lead to some variations. We further constrain the
combinations of the -parameter and the MRI critical temperature, which
can reproduce observed outburst behavior. Our results suggest that dust
sublimation may be connected with full activation of the MRI. This is
consistent with the idea that small dust captures ions and electrons to
suppress the MRI. In a later paper we will explore both long-term outburst and
disk evolution with this model, allowing for infall from protostellar envelopes
with differing angular momenta.Comment: Accepted to publish in Ap
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