8,725 research outputs found
Ground-State Phase Diagram of the XXZ Model on a Railroad-Trestle Lattice with Asymmetric Leg Interactions
Using the bosonization and level spectroscopy methods, we study the
ground-state phase diagram of a XXZ antiferromagnet on a railroad-trestle
lattice with asymmetric leg interactions. It is shown that the asymmetry does
not change the dimer/Neel transition line significantly, which agrees with the
expectation based on a naive bosonization procedure, but it does change the
dimer/spin-fluid transition line. To understand this observation, we analyze
eigenvectors of the ground state, dimer excitation, doublet excitation and Neel
excitation, and find that only the doublet excitation is affected by the
asymmetric interaction.Comment: 6 pages, 11 Postscript figures, use jpsj2.cl
Covariant - tensor method for quantum groups and applications I:
A covariant - tensor method for is described. This tensor method
is used to calculate q - deformed Clebsch - Gordan coefficients. The connection
with covariant oscillators and irreducible tensor operators is established.
This approach can be extended to other quantum groups.Comment: 18 page
Higgsless Theory of Electroweak Symmetry Breaking from Warped Space
We study a theory of electroweak symmetry breaking without a Higgs boson,
recently suggested by Csaki et al. The theory is formulated in 5D warped space
with the gauge bosons and matter fields propagating in the bulk. In the 4D dual
picture, the theory appears as the standard model without a Higgs field, but
with an extra gauge group G which becomes strong at the TeV scale. The strong
dynamics of G breaks the electroweak symmetry, giving the masses for the W and
Z bosons and the quarks and leptons. We study corrections in 5D which are
logarithmically enhanced by the large mass ratio between the Planck and weak
scales, and show that they do not destroy the structure of the electroweak
gauge sector at the leading order. We introduce a new parameter, the ratio
between the two bulk gauge couplings, into the theory and find that it allows
us to control the scale of new physics. We also present a potentially realistic
theory accommodating quarks and leptons and discuss its implications, including
the violation of universality in the W and Z boson couplings to matter and the
spectrum of the Kaluza-Klein excitations of the gauge bosons. The theory
reproduces many successful features of the standard model, although some
cancellations may still be needed to satisfy constraints from the precision
electroweak data.Comment: 17 pages, Latex; important correction in discussions on effects from
brane terms, reference adde
Dust Size Growth and Settling in a Protoplanetary Disk
We have studied dust evolution in a quiescent or turbulent protoplanetary
disk by numerically solving coagulation equation for settling dust particles,
using the minimum mass solar nebular model. As a result, if we assume an
ideally quiescent disk, the dust particles settle toward the disk midplane to
form a gravitationally unstable layer within 2x10^3 - 4x10^4 yr at 1 - 30 AU,
which is in good agreement with an analytic calculation by Nakagawa, Sekiya, &
Hayashi (1986) although they did not take into account the particle size
distribution explicitly. In an opposite extreme case of a globally turbulent
disk, on the other hand, the dust particles fluctuate owing to turbulent motion
of the gas and most particles become large enough to move inward very rapidly
within 70 - 3x10^4 yr at 1 - 30 AU, depending on the strength of turbulence.
Our result suggests that global turbulent motion should cease for the
planetesimal formation in protoplanetary disks.Comment: 27 pages, 8 figures, accepted for publication in the Ap
Breakdown of the Luttinger sum-rule at the Mott-Hubbard transition in the one-dimensional t1-t2 Hubbard model
We investigate the momentum distribution function near the Mott-Hubbard
transition in the one-dimensional t1-t2 Hubbard model (the zig-zag Hubbard
chain), with the density-matrix renormalization-group technique. We show that
for strong interactions the Mott-Hubbard transition occurs between the
metallic-phase and an insulating dimerized phase with incommensurate spin
excitations, suggesting a decoupling of magnetic and charge excitations not
present in weak coupling. We illustrate the signatures for the Mott-Hubbard
transition and the commensurate-incommensurate transition in the insulating
spin-gapped state in their respective ground-state momentum distribution
functions
Algorithm for Linear Response Functions at Finite Temperatures: Application to ESR spectrum of s=1/2 Antiferromagnet Cu benzoate
We introduce an efficient and numerically stable method for calculating
linear response functions of quantum systems at finite
temperatures. The method is a combination of numerical solution of the
time-dependent Schroedinger equation, random vector representation of trace,
and Chebyshev polynomial expansion of Boltzmann operator. This method should be
very useful for a wide range of strongly correlated quantum systems at finite
temperatures. We present an application to the ESR spectrum of s=1/2
antiferromagnet Cu benzoate.Comment: 4 pages, 4 figure
Fast algorithm for calculating two-photon absorption spectra
We report a numerical calculation of the two-photon absorption coefficient of
electrons in a binding potential using the real-time real-space higher-order
difference method. By introducing random vector averaging for the intermediate
state, the task of evaluating the two-dimensional time integral is reduced to
calculating two one-dimensional integrals. This allows the reduction of the
computation load down to the same order as that for the linear response
function. The relative advantage of the method compared to the straightforward
multi-dimensional time integration is greater for the calculation of non-linear
response functions of higher order at higher energy resolution.Comment: 4 pages, 2 figures. It will be published in Phys. Rev. E on 1, March,
199
Finite-Field Ground State of the S=1 Antiferromagnetic-Ferromagnetic Bond-Alternating Chain
We investigate the finite-field ground state of the S=1
antiferromagnetic-ferromagnetic bond-alternating chain described by the
Hamiltonian
{\calH}=\sum\nolimits_{\ell}\bigl\{\vecS_{2\ell-1}\cdot\vecS_{2\ell}
+J\vecS_{2\ell}\cdot\vecS_{2\ell+1}\bigr\} +D\sum\nolimits_{\ell}
\bigl(S_{\ell}^z)^2 -H\textstyle\sum\nolimits_\ell S_\ell^z, where
\hbox{} and \hbox{}. We find that two kinds of
magnetization plateaux at a half of the saturation magnetization, the
1/2-plateaux, appear in the ground-state magnetization curve; one of them is of
the Haldane type and the other is of the large--type. We determine the
1/2-plateau phase diagram on the versus plane, applying the
twisted-boundary-condition level spectroscopy methods developed by Kitazawa and
Nomura. We also calculate the ground-state magnetization curves and the
magnetization phase diagrams by means of the density-matrix
renormalization-group method
Extended supersymmetry and its reduction on a circle with point singularities
We investigate -extended supersymmetry in one-dimensional quantum
mechanics on a circle with point singularities. For any integer ,
supercharges are explicitly constructed in terms of discrete transformations,
and a class of singularities compatible with supersymmetry is clarified. In our
formulation, the supersymmetry can be reduced to -extended supersymmetry for
any integer . The degeneracy of the spectrum and spontaneous supersymmetry
breaking are also studied.Comment: 36 pages, 5 figures, 2 table
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