1,950 research outputs found
Doping quantum dimer models on the square lattice
A family of models is proposed to describe the motion of holes in a
fluctuating quantum dimer background on the square lattice. Following
Castelnovo et al. [Ann. Phys. (NY) 318, 316 (2005)], a generalized
Rokhsar-Kivelson Hamiltonian at **finite doping** which can be mapped on a
**doped** interacting classical dimer model is constructed. A simple physical
extension of this model is also considered. Using numerical computations and
simple considerations based on the above exact mapping, we determine the phase
diagram of the model showing a number of quantum phases typical of a doped Mott
insulator. The two-hole correlation function generically exhibits short-range
or long-range algebraic correlations in the solid (columnar) and liquid
(critical) phases of the model, respectively. Evidence for an extended region
of a doped VBS phase exhibiting holon pairing but **no** phase separation is
given. In contrast, we show that hole deconfinement occurs in the staggered
dimer phase.Comment: 5 page
Unraveling radial dependency effects in fiber thermal drawing
Fiber-based devices with advanced functionalities are emerging as promising
solutions for various applications in flexible electronics and bioengineering.
Multimaterial thermal drawing, in particular, has attracted strong interest for
its ability to generate fibers with complex architectures. Thus far, however,
the understanding of its fluid dynamics has only been applied to single
material preforms for which higher order effects, such as the radial dependency
of the axial velocity, could be neglected. With complex multimaterial preforms,
such effects must be taken into account, as they can affect the architecture
and the functional properties of the resulting fiber device. Here, we propose a
versatile model of the thermal drawing of fibers, which takes into account a
radially varying axial velocity. Unlike the commonly used cross section
averaged approach, our model is capable of predicting radial variations of
functional properties caused by the deformation during drawing. This is
demonstrated for two effects observed, namely, by unraveling the deformation of
initially straight, transversal lines in the preform and the dependence on the
draw ratio and radial position of the in-fiber electrical conductivity of
polymer nanocomposites, an important class of materials for emerging fiber
devices. This work sets a thus far missing theoretical and practical
understanding of multimaterial fiber processing to better engineer advanced
fibers and textiles for sensing, health care, robotics, or bioengineering
applications
On methods to determine bounds on the Q-factor for a given directivity
This paper revisit and extend the interesting case of bounds on the Q-factor
for a given directivity for a small antenna of arbitrary shape. A higher
directivity in a small antenna is closely connected with a narrow impedance
bandwidth. The relation between bandwidth and a desired directivity is still
not fully understood, not even for small antennas. Initial investigations in
this direction has related the radius of a circumscribing sphere to the
directivity, and bounds on the Q-factor has also been derived for a partial
directivity in a given direction. In this paper we derive lower bounds on the
Q-factor for a total desired directivity for an arbitrarily shaped antenna in a
given direction as a convex problem using semi-definite relaxation techniques
(SDR). We also show that the relaxed solution is also a solution of the
original problem of determining the lower Q-factor bound for a total desired
directivity.
SDR can also be used to relax a class of other interesting non-convex
constraints in antenna optimization such as tuning, losses, front-to-back
ratio. We compare two different new methods to determine the lowest Q-factor
for arbitrary shaped antennas for a given total directivity. We also compare
our results with full EM-simulations of a parasitic element antenna with high
directivity.Comment: Correct some minor typos in the previous versio
Relativistic corrections for two- and three-body flux tube model
We generalize the relativistic flux tube model for arbitrary two- or
three-body systems. The spin-independent and spin-dependent contributions of
the flux tube to the total Hamiltonian are computed in perturbation. In
particular, we show that the spin-dependent part exhibits a universal
spin-orbit form: It does not depend on the nature of the confined particles.
The general equations we present, being well-defined for light particles, can
thus be applied to usual as well as exotic hadrons such as hybrid mesons and
glueballs.Comment: 10 pages; v2 accepted for publication (minor changes
A systematic benchmark of the ab initio Bethe-Salpeter equation approach for low-lying optical excitations of small organic molecules
The predictive power of the ab initio Bethe-Salpeter equation (BSE) approach,
rigorously based on many-body Green's function theory but incorporating
information from density functional theory, has already been demonstrated for
the optical gaps and spectra of solid-state systems. Interest in photoactive
hybrid organic/inorganic systems has recently increased, and so has the use of
the BSE for computing neutral excitations of organic molecules. However, no
systematic benchmarks of the BSE for neutral electronic excitations of organic
molecules exist. Here, we study the performance of the BSE for the 28 small
molecules in Thiel's widely-used time-dependent density functional theory
benchmark set [M. Schreiber et al. J. Chem. Phys. 128, 134110 (2008)]. We
observe that the BSE produces results that depend critically on the mean-field
starting point employed in the perturbative approach. We find that this
starting point dependence is mainly introduced through the quasiparticle
energies obtained at the intermediate GW step, and that with a judicious choice
of starting mean-field, singlet excitation energies obtained from BSE are in
excellent quantitative agreement with higher-level wavefunction methods. The
quality of the triplet excitations is slightly less satisfactory
Sorption-induced Static Bending of Microcantilevers Coated with Viscoelastic Material
Absorption of a chemical analyte into a polymercoating results in an expansion governed by the concentration and type of analyte that has diffused into the bulk of the coating. When the coating is attached to a microcantilever, this expansion results in bending of the device. Assuming that absorption (i.e., diffusion across the surface barrier into the bulk of the coating) is Fickian, with a rate of absorption that is proportional to the difference between the absorbed concentration and the equilibrium concentration, and the coating is elastic, the bending response of the coated device should exhibit a first-order behavior. However, for polymercoatings, complex behaviors exhibiting an overshoot that slowly decays to the steady-state value have been observed. A theoretical model of absorption-induced static bending of a microcantilever coated with a viscoelastic material is presented, starting from the general stress/strain relationship for a viscoelastic material. The model accounts for viscoelasticstress relaxation and possible coating plasticization. Calculated responses show that the model is capable of reproducing the same transient behavior exhibited in the experimental data. The theory presented can also be used for extracting viscoelasticproperties of the coating from the measured bending data
Optimal Planar Electric Dipole Antenna
Considerable time is often spent optimizing antennas to meet specific design
metrics. Rarely, however, are the resulting antenna designs compared to
rigorous physical bounds on those metrics. Here we study the performance of
optimized planar meander line antennas with respect to such bounds. Results
show that these simple structures meet the lower bound on radiation Q-factor
(maximizing single resonance fractional bandwidth), but are far from reaching
the associated physical bounds on efficiency. The relative performance of other
canonical antenna designs is compared in similar ways, and the quantitative
results are connected to intuitions from small antenna design, physical bounds,
and matching network design.Comment: 10 pages, 15 figures, 2 tables, 4 boxe
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