2,314 research outputs found
On Phase Transition of -Type Crystals by Cluster Variation Method
The Cluster Variation Method (CVM) is applied to the Ishibashi model for
ammonium dihydrogen phosphate () of a typical hydrogen
bonded anti-ferroelectric crystal. The staggered and the uniform susceptibility
without hysteresis are calculated at equilibrium. On the other hand, by making
use of the natural iteration method (NIM) for the CVM, hysteresis phenomena of
uniform susceptibility versus temperature observed in experiments is well
explained on the basis of local minimum in Landau type variational free energy.
The polarization curves against the uniform field is also calculated.Comment: 14 pages, 10 figure
Constraints and Period Relations in Bosonic Strings at Genus-g
We examine some of the implications of implementing the usual boundary
conditions on the closed bosonic string in the hamiltonian framework. Using the
KN formalism, it is shown that at the quantum level, the resulting constraints
lead to relations among the periods of the basis 1-forms. These are compared
with those of Riemanns' which arise from a different consideration.Comment: 16 pages, (Plain Tex), NUS/HEP/9320
Effect of Material Properties on Soil Liquefaction
Four material constants included in the pore-pressure buildup equation for saturated sands under earthquake loadings are determined as functions of grain size, soil angularity, coefficient of uniformity, and void ratio. This would allow engineers to readily calculate pore-pressure buildup as a function of time, and hence assess the liquefaction potential, for a given soil without conducting cyclic tests
Origin of second-harmonic generation in the incommensurate phase of K2SeO4
We show that a ferroelectric phase transition takes place in the
incommensurate phase of the K2SeO4 crystal. The ferroelectric character of the
IC phase explains the second-harmonic generation observed in the corresponding
temperature range.Comment: 5 pages, 1 figur
Modeling the momentum distributions of annihilating electron-positron pairs in solids
Measuring the Doppler broadening of the positron annihilation radiation or
the angular correlation between the two annihilation gamma quanta reflects the
momentum distribution of electrons seen by positrons in the
material.Vacancy-type defects in solids localize positrons and the measured
spectra are sensitive to the detailed chemical and geometric environments of
the defects. However, the measured information is indirect and when using it in
defect identification comparisons with theoretically predicted spectra is
indispensable. In this article we present a computational scheme for
calculating momentum distributions of electron-positron pairs annihilating in
solids. Valence electron states and their interaction with ion cores are
described using the all-electron projector augmented-wave method, and atomic
orbitals are used to describe the core states. We apply our numerical scheme to
selected systems and compare three different enhancement (electron-positron
correlation) schemes previously used in the calculation of momentum
distributions of annihilating electron-positron pairs within the
density-functional theory. We show that the use of a state-dependent
enhancement scheme leads to better results than a position-dependent
enhancement factor in the case of ratios of Doppler spectra between different
systems. Further, we demonstrate the applicability of our scheme for studying
vacancy-type defects in metals and semiconductors. Especially we study the
effect of forces due to a positron localized at a vacancy-type defect on the
ionic relaxations.Comment: Submitted to Physical Review B on September 1 2005. Revised
manuscript submitted on November 14 200
Excited state g-functions from the Truncated Conformal Space
In this paper we consider excited state g-functions, that is, overlaps
between boundary states and excited states in boundary conformal field theory.
We find a new method to calculate these overlaps numerically using a variation
of the truncated conformal space approach. We apply this method to the Lee-Yang
model for which the unique boundary perturbation is integrable and for which
the TBA system describing the boundary overlaps is known. Using the truncated
conformal space approach we obtain numerical results for the ground state and
the first three excited states which are in excellent agreement with the TBA
results. As a special case we can calculate the standard g-function which is
the overlap with the ground state and find that our new method is considerably
more accurate than the original method employed by Dorey et al.Comment: 21 pages, 6 figure
On The spectrum of a Noncommutative Formulation of the D=11 Supermembrane with Winding
A regularized model of the double compactified D=11 supermembrane with
nontrivial winding in terms of SU(N) valued maps is obtained. The condition of
nontrivial winding is described in terms of a nontrivial line bundle introduced
in the formulation of the compactified supermembrane. The multivalued
geometrical objects of the model related to the nontrivial wrapping are
described in terms of a SU(N) geometrical object which in the
limit, converges to the symplectic connection related to the area preserving
diffeomorphisms of the recently obtained non-commutative description of the
compactified D=11 supermembrane.(I. Martin, J.Ovalle, A. Restuccia. 2000,2001)
The SU(N) regularized canonical lagrangian is explicitly obtained. In the limit it converges to the lagrangian in (I.Martin, J.Ovalle,
A.Restuccia. 2000,2001) subject to the nontrivial winding condition. The
spectrum of the hamiltonian of the double compactified D=11 supermembrane is
discussed.
Generically, it contains local string like spikes with zero energy.
However the sector of the theory corresponding to a principle bundle
characterized by the winding number , described by the SU(N) model we
propose, is shown to have no local string-like spikes and hence the spectrum of
this sector should be discrete.Comment: 16 pages.Latex2
Properties of D-Branes in Matrix Model of IIB Superstring
We discuss properties of D-brane configurations in the matrix model of type
IIB superstring recently proposed by Ishibashi, Kawai, Kitazawa and Tsuchiya.
We calculate central charges in supersymmetry algebra at infinite N and
associate them with one- and five-branes present in IIB superstring theory. We
consider classical solutions associated with static three- and five-branes and
calculate their interactions at one loop in the matrix model. We discuss some
aspects of the matrix-model formulation of IIB superstring.Comment: 15pp., Latex, v2: a few typos corrected, v3: coefficient in Eq.(3.19)
correcte
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