43,739 research outputs found
Optical Resonator Analog of a Two-Dimensional Topological Insulator
A lattice of optical ring resonators can exhibit a topological insulator
phase, with the role of spin played by the direction of propagation of light
within each ring. Unlike the system studied by Hafezi et al., topological
protection is achieved without fine-tuning the inter-resonator couplings, which
are given the same periodicity as the underlying lattice. The topological
insulator phase occurs for strong couplings, when the tight-binding method is
inapplicable. Using the transfer matrix method, we derive the bandstructure and
phase diagram, and demonstrate the existence of robust edge states. When gain
and loss are introduced, the system functions as a diode for coupled resonator
modes.Comment: 10 pages, 9 figure
An efficient mixed variational reduced order model formulation for non-linear analyses of elastic shells
The Koiter-Newton method had recently demonstrated a superior performance for non-linear analyses of structures, compared to traditional path-following strategies. The method follows a predictor-corrector scheme to trace the entire equilibrium path. During a predictor step a reduced order model is constructed based on Koiter's asymptotic post-buckling theory which is followed by a Newton iteration in the corrector phase to regain the equilibrium of forces.
In this manuscript, we introduce a robust mixed solid-shell formulation to further enhance the efficiency of stability analyses in various aspects. We show that a Hellinger-Reissner variational formulation facilitates the reduced order model construction omitting an expensive evaluation of the inherent fourth order derivatives of the strain energy. We demonstrate that extremely large step sizes with a reasonable out-of-balance residual can be obtained with substantial impact on the total number of steps needed to trace the complete equilibrium path. More importantly, the numerical effort of the corrector phase involving a Newton iteration of the full order model is drastically reduced thus revealing the true strength of the proposed formulation. We study a number of problems from engineering and compare the results to the conventional approach in order to highlight the gain in numerical efficiency for stability problems
Modelling Time-varying Dark Energy with Constraints from Latest Observations
We introduce a set of two-parameter models for the dark energy equation of
state (EOS) to investigate time-varying dark energy. The models are
classified into two types according to their boundary behaviors at the redshift
and their local extremum properties. A joint analysis based on
four observations (SNe + BAO + CMB + ) is carried out to constrain all the
models. It is shown that all models get almost the same and the cosmological parameters with the
best-fit results , although the constraint results on two
parameters and the allowed regions for the EOS are
sensitive to different models and a given extra model parameter. For three of
Type I models which have similar functional behaviors with the so-called CPL
model, the constrained two parameters and have negative correlation
and are compatible with the ones in CPL model, and the allowed regions of
get a narrow node at . The best-fit results from the most
stringent constraints in Model Ia give which may compare with the best-fit results in the CPL model. For four of
Type II models which have logarithmic function forms and an extremum point, the
allowed regions of are found to be sensitive to different models and a
given extra parameter. It is interesting to obtain two models in which two
parameters and are strongly correlative and appropriately reduced
to one parameter by a linear relation .Comment: 30 pages, 7 figure
Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles
A formula suitable for a quantitative evaluation of the tunneling effect in a
ferromagnetic particle is derived with the help of the instanton method. The
tunneling between n-th degenerate states of neighboring wells is dominated by a
periodic pseudoparticle configuration. The low-lying level-splitting previously
obtained with the LSZ method in field theory in which the tunneling is viewed
as the transition of n bosons induced by the usual (vacuum) instanton is
recovered. The observation made with our new result is that the tunneling
effect increases at excited states. The results should be useful in analyzing
results of experimental tests of macroscopic quantum coherence in ferromagnetic
particles.Comment: 18 pages, LaTex, 1 figur
Non-Markov dynamics and phonon decoherence of a double quantum dot charge qubit
In this paper we investigate decoherence times of a double quantum dot (DQD)
charge qubit due to it coupling with acoustic phonon baths. We individually
consider the acoustic piezoelectric as well as deformation coupling phonon
baths in the qubit environment. The decoherence times are calculated with two
kinds of methods. One of them is based on the qusiadiabatic propagator path
integral (QUAPI) and the other is based on Bloch equations, and two kinds of
results are compared. It is shown that the theoretical decoherence times of the
DQD charge qubit are shorter than the experimental reported results. It implies
that the phonon couplings to the qubit play a subordinate role, resulting in
the decoherence of the qubit.Comment: 5 pages, 4 figure
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Experimental and Numerical Investigation on Progressive Collapse Resistance of Post-tensioned Precast Concrete Beam-Column Sub-assemblages
In this paper, four 1/2 scaled precast concrete (PC) beam-column sub-assemblages with high performance connection were tested under push-down loading procedure to study the load resisting mechanism of PC frames subjected to different column removal scenarios. The parameters investigated include the location of column removal and effective prestress in tendons. The test results indicated that the failure modes of unbonded post-tensioned precast concrete (PTPC) frames were different from that of reinforced concrete (RC) frames: no cracks formed in the beams and wide opening formed near the beam to column interfaces. For specimens without overhanging beams, the failure of side column was eccentric compression failure. Moreover, the load resisting mechanisms in PC frames were significantly different from that of RC frames: the compressive arch action (CAA) developed in concrete during column removal was mainly due to actively applied pre-compressive stress in the concrete; CAA will not vanish when severe crush in concrete occurred. Thus, it may provide negative contribution for load resistance when the displacement exceeds one-beam depth; the tensile force developed in the tendons could provide catenary action from the beginning of the test. Moreover, to deeper understand the behavior of tested specimens, numerical analyses were carried out. The effects of concrete strength, axial compression ratio at side columns, and loading approaches on the behavior of the sub-assemblages were also investigated based on validated numerical analysis
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