118,005 research outputs found
Hardcore bosons on checkerboard lattices near half filling: geometric frustration, vanishing charge order and fractional phase
We study a spinless hardcore boson model on checkerboard lattices by Green
function Monte Carlo method. At half filling, the ground state energy is
obtained up to lattice and extrapolated to infinite size, the
staggered pseudospin magnetization is found to vanish in the thermodynamic
limit. Thus the charge order is absent in this system. Away from
half filling, two defects induced by each hole (particle) may carry fractional
charge (). For one hole case, we study how the defect-defect
correlation changes with , which is the ratio between the hopping integral
and cyclic exchange, equals to when . Moreover, we argue that
these fractional defects may propagate independently when the concentration of
holes (or defects) is large enough
Molecular emission near metal interfaces: the polaritonic regime
The strong coupling of a dense layer of molecular excitons with
surface-plasmon modes in a metal gives rise to polaritons (hybrid light-matter
states) called plexcitons. Surface plasmons cannot directly emit into (or be
excited by) free-space photons due to the fact that energy and momentum
conservation cannot be simultaneously satisfied in photoluminescence. Most
plexcitons are also formally non-emissive, even though they can radiate via
molecules upon localization due to disorder and decoherence. However, a
fraction of them are bright even in the presence of such deleterious processes.
In this letter, we theoretically discuss the superradiant emission properties
of these bright plexcitons, which belong to the upper energy branch and reveal
huge photoluminescence enhancements compared to bare excitons. Our study
generalizes the well-known problem of molecular emission next to a metal
interface to collective molecular states and provides new design principles for
the control of photophysical properties of molecular aggregates using
polaritonic strategies.Comment: Replaced previous version, noticing that van Hove anomalies are only
observed in the direct and reflected contributions of photoluminescence, but
they cancel out when added up in the total photoluminescence. The correct
phenomenology is that enhancements of photoluminescence are still huge (not
infinite) and are near (not exactly at) the critical poin
Free energies and critical exponents of the A_1^{(1)}, B_n^{(1)}, C_n^{(1)} and D_n^{(1)} face models
We obtain the free energies and critical exponents of models associated with
elliptic solutions of the star-triangle relation and reflection equation. The
models considered are related to the affine Lie algebras A_1^{(1)},
B_n^{(1)},C_n^{(1)} and D_n^{(1)}. The bulk and surface specific heat exponents
are seen to satisfy the scaling relation 2\alpha_s = \alpha_b + 2. It follows
from scaling relations that in regime III the correlation length exponent \nu
is given by \nu=(l+g)/2g, where l is the level and g is the dual Coxeter
number. In regime II we find \nu=(l+g)/2l.Comment: 9 pages, Latex, no figure
Theory of Weiss oscillations in the magnetoplasmon spectrum of Dirac electrons in graphene
We present the collective excitations spectrum (magnetoplasmon spectrum) of
Dirac electrons in a weakly modulated single graphene layer in the presence
of a uniform magnetic field. We consider electric modulation in one-dimension
and the magnetic field applied perpendicular to graphene.We derive analytical
results for the intra-Landau band plasmon spectrum within the
self-consistent-field approach. We find Weiss oscillations in the
magnetoplasmon spectrum which is the primary focus of this work. Results are
presented for the intra-Landau band magnetoplasmon spectrum as a function of
inverse magnetic field. These results are also compared with those of
conventional 2DEG. We have found that the Weiss oscillations in the
magnetoplasmon spectrum are larger in amplitude compared to those in
conventional 2DEG for the same modulation strength, period of modulation and
electron density.Comment: 9 pages, 1 figure Phys. Rev. B (accepted for publication
A non-local vector calculus,non-local volume-constrained problems,and non-local balance laws
A vector calculus for nonlocal operators is developed, including the definition of nonlocal divergence, gradient, and curl operators and the derivation of the corresponding adjoints operators. Nonlocal analogs of several theorems and identities of the vector calculus for differential operators are also presented. Relationships between the nonlocal operators and their differential counterparts are established, first in a distributional sense and then in a weak sense by considering weighted integrals of the nonlocal adjoint operators. The nonlocal calculus gives rise to volume-constrained problems that are analogous to elliptic boundary-value problems for differential operators; this is demonstrated via some examples. Another application is posing abstract nonlocal balance laws and deriving the corresponding nonlocal field equations
Calculation of renormalized viscosity and resistivity in magnetohydrodynamic turbulence
A self-consistent renormalization (RG) scheme has been applied to nonhelical
magnetohydrodynamic turbulence with normalized cross helicity and
. Kolmogorov's 5/3 powerlaw is assumed in order to compute the
renormalized parameters. It has been shown that the RG fixed point is stable
for . The renormalized viscosity and resistivity
have been calculated, and they are found to be positive for all
parameter regimes. For and large Alfv\'{e}n ratio (ratio of
kinetic and magnetic energies) , and . As
is decreased, increases and decreases, untill where both and are approximately zero. For large ,
both and vary as . The renormalized parameters for
the case are also reported.Comment: 19 pages REVTEX, 3 ps files (Phys. Plasmas, v8, 3945, 2001
Long-range frustration in T=0 first-step replica-symmetry-broken solutions of finite-connectivity spin glasses
In a finite-connectivity spin-glass at the zero-temperature limit, long-range
correlations exist among the unfrozen vertices (whose spin values being
non-fixed). Such long-range frustrations are partially removed through the
first-step replica-symmetry-broken (1RSB) cavity theory, but residual
long-range frustrations may still persist in this mean-field solution. By way
of population dynamics, here we perform a perturbation-percolation analysis to
calculate the magnitude of long-range frustrations in the 1RSB solution of a
given spin-glass system. We study two well-studied model systems, the minimal
vertex-cover problem and the maximal 2-satisfiability problem. This work points
to a possible way of improving the zero-temperature 1RSB mean-field theory of
spin-glasses.Comment: 5 pages, two figures. To be published in JSTA
MM Algorithms for Geometric and Signomial Programming
This paper derives new algorithms for signomial programming, a generalization
of geometric programming. The algorithms are based on a generic principle for
optimization called the MM algorithm. In this setting, one can apply the
geometric-arithmetic mean inequality and a supporting hyperplane inequality to
create a surrogate function with parameters separated. Thus, unconstrained
signomial programming reduces to a sequence of one-dimensional minimization
problems. Simple examples demonstrate that the MM algorithm derived can
converge to a boundary point or to one point of a continuum of minimum points.
Conditions under which the minimum point is unique or occurs in the interior of
parameter space are proved for geometric programming. Convergence to an
interior point occurs at a linear rate. Finally, the MM framework easily
accommodates equality and inequality constraints of signomial type. For the
most important special case, constrained quadratic programming, the MM
algorithm involves very simple updates.Comment: 16 pages, 1 figur
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