2,021 research outputs found
Genericness of inflation in isotropic loop quantum cosmology
Non-perturbative corrections from loop quantum cosmology (LQC) to the scalar
matter sector is already known to imply inflation. We prove that the LQC
modified scalar field generates exponential inflation in the small scale factor
regime, for all positive definite potentials, independent of initial conditions
and independent of ambiguity parameters. For positive semi-definite potentials
it is always possible to choose, without fine tuning, a value of one of the
ambiguity parameters such that exponential inflation results, provided zeros of
the potential are approached at most as a power law in the scale factor. In
conjunction with generic occurrence of bounce at small volumes, particle
horizon is absent thus eliminating the horizon problem of the standard Big Bang
model.Comment: 4 pages, revtex4, one figure. Only e-print archive numbers correctedi
in the second version. Reference added in the 3rd version. Final version to
appear in Phys. Rev. Lett. Explanations improve
Discreteness Corrections to the Effective Hamiltonian of Isotropic Loop Quantum Cosmology
One of the qualitatively distinct and robust implication of Loop Quantum
Gravity (LQG) is the underlying discrete structure. In the cosmological context
elucidated by Loop Quantum Cosmology (LQC), this is manifested by the
Hamiltonian constraint equation being a (partial) difference equation. One
obtains an effective Hamiltonian framework by making the continuum
approximation followed by a WKB approximation. In the large volume regime,
these lead to the usual classical Einstein equation which is independent of
both the Barbero-Immirzi parameter as well as . In this work we
present an alternative derivation of the effective Hamiltonian by-passing the
continuum approximation step. As a result, the effective Hamiltonian is
obtained as a close form expression in . These corrections to the
Einstein equation can be thought of as corrections due to the underlying
discrete (spatial) geometry with controlling the size of these
corrections. These corrections imply a bound on the rate of change of the
volume of the isotropic universe. In most cases these are perturbative in
nature but for cosmological constant dominated isotropic universe, there are
significant deviations.Comment: Revtex4, 24 pages, 3 figures. In version 2, one reference and a para
pertaining to it are added. In the version 3, some typos are corrected and
remark 4 in section III is revised. Final version to appear in Class. Quantum
Gra
Chemical passivation of unstable FeO - a Mossbauer study
Highly unstable FeO is chemically passivated by incorporating Cr3+ ions by solid solution technique and forming FexO:Cr3+ single phase material. XRD, chemical analysis and Mossbauer spectroscopy are used for the characterization of the freshly prepared as well as samples aged in the desiccator for nearly three months. Optimum concentration range - 0.25 to 0.75 mole% - of Cr2O3 has been found to be necessary for stabilizing FexO:Cr3+. x is determined by chemical analysis. Mossbauer and XRD studies have confirmed the chemical passivation of unstable FeO
Star-Triangle Relation for a Three Dimensional Model
The solvable -chiral Potts model can be interpreted as a
three-dimensional lattice model with local interactions. To within a minor
modification of the boundary conditions it is an Ising type model on the body
centered cubic lattice with two- and three-spin interactions. The corresponding
local Boltzmann weights obey a number of simple relations, including a
restricted star-triangle relation, which is a modified version of the
well-known star-triangle relation appearing in two-dimensional models. We show
that these relations lead to remarkable symmetry properties of the Boltzmann
weight function of an elementary cube of the lattice, related to spatial
symmetry group of the cubic lattice. These symmetry properties allow one to
prove the commutativity of the row-to-row transfer matrices, bypassing the
tetrahedron relation. The partition function per site for the infinite lattice
is calculated exactly.Comment: 20 pages, plain TeX, 3 figures, SMS-079-92/MRR-020-92. (corrupted
figures replaced
Supermatrix models and multi ZZ-brane partition functions in minimal superstring theories
We study (p,q)=(2,4k) minimal superstrings within the minimal superstring
field theory constructed in hep-th/0611045. We explicitly give a solution to
the W_{1+\infty} constraints by using charged D-instanton operators, and show
that the (m,n)-instanton sector with m positive-charged and n negative-charged
ZZ-branes is described by an (m+n)\times (m+n) supermatrix model. We argue that
the supermatrix model can be regarded as an open string field theory on the
multi ZZ-brane system.Comment: 15 pages, 1 figure, minor chang
Direct observation of the multiple spin gap excitations in two-dimensional dimer system SrCu2(BO3)2
Various spin gap excitations have been observed in the two-dimensional dimer
system SrCu_2(BO_3)_2 by means of submillimeter wave ESR. The zero-field energy
gap of the lowest spin gap excitation shows a splitting into two triplet modes
and the energy splitting clearly depends on the magnetic field orientation when
a field is rotated in the {\mib {ac}}-plane. A zero-field splitting is also
found between the S(_z)=+1 and S(_z)=-1 branches of each triplet. These
behaviors are qualitatively explained by considering the anisotropic exchange
coupling of inter-dimer and intra-dimer, respectively. The averaged value of
the lowest spin gap energy is determined to be 722 \pm 2 GHz(34.7 K). We have
also found the second spin gap excitation at 1140 GHz(54.7 K), which indicates
that the inter-dimer coupling is significantly strong. Besides these modes, a
number of gapped ESR absorption are found and we propose that these multiple
magnetic excitations are caused by the localized nature of the excited state in
the present system.Comment: 4pages 4figure
B\"{a}cklund transformations for the constrained dispersionless hierarchies and dispersionless hierarchies with self-consistent sources
The B\"{a}cklund transformations between the constrained dispersionless KP
hierarchy (cdKPH) and the constrained dispersionless mKP hieararchy (cdmKPH)
and between the dispersionless KP hieararchy with self-consistent sources
(dKPHSCS) and the dispersionless mKP hieararchy with self-consistent sources
(dmKPHSCS) are constructed. The auto-B\"{a}cklund transformations for the
cdmKPH and for the dmKPHSCS are also formulated.Comment: 11 page
Integrable theories and loop spaces: fundamentals, applications and new developments
We review our proposal to generalize the standard two-dimensional flatness
construction of Lax-Zakharov-Shabat to relativistic field theories in d+1
dimensions. The fundamentals from the theory of connections on loop spaces are
presented and clarified. These ideas are exposed using mathematical tools
familiar to physicists. We exhibit recent and new results that relate the
locality of the loop space curvature to the diffeomorphism invariance of the
loop space holonomy. These result are used to show that the holonomy is abelian
if the holonomy is diffeomorphism invariant.
These results justify in part and set the limitations of the local
implementations of the approach which has been worked out in the last decade.
We highlight very interesting applications like the construction and the
solution of an integrable four dimensional field theory with Hopf solitons, and
new integrability conditions which generalize BPS equations to systems such as
Skyrme theories. Applications of these ideas leading to new constructions are
implemented in theories that admit volume preserving diffeomorphisms of the
target space as symmetries. Applications to physically relevant systems like
Yang Mills theories are summarized. We also discuss other possibilities that
have not yet been explored.Comment: 64 pages, 8 figure
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