8,734 research outputs found
Decay of scalar variance in isotropic turbulence in a bounded domain
The decay of scalar variance in isotropic turbulence in a bounded domain is
investigated. Extending the study of Touil, Bertoglio and Shao (2002; Journal
of Turbulence, 03, 49) to the case of a passive scalar, the effect of the
finite size of the domain on the lengthscales of turbulent eddies and scalar
structures is studied by truncating the infrared range of the wavenumber
spectra. Analytical arguments based on a simple model for the spectral
distributions show that the decay exponent for the variance of scalar
fluctuations is proportional to the ratio of the Kolmogorov constant to the
Corrsin-Obukhov constant. This result is verified by closure calculations in
which the Corrsin-Obukhov constant is artificially varied. Large-eddy
simulations provide support to the results and give an estimation of the value
of the decay exponent and of the scalar to velocity time scale ratio
Octet Baryon Magnetic Moments in the Chiral Quark Model with Configuration Mixing
The Coleman-Glashow sum-rule for magnetic moments is always fulfilled in the
chiral quark model, independently of SU(3) symmetry breaking. This is due to
the structure of the wave functions, coming from the non-relativistic quark
model. Experimentally, the Coleman-Glashow sum-rule is violated by about ten
standard deviations. To overcome this problem, two models of wave functions
with configuration mixing are studied. One of these models violates the
Coleman-Glashow sum-rule to the right degree and also reproduces the octet
baryon magnetic moments rather accurately.Comment: 22 pages, RevTe
Ferromagnetism below 10 K in Mn doped BiTe
Ferromagnetism is observed below 10 K in [Bi0.75Te0.125Mn0.125]Te. This
material has the BiTe structure, which is made from the stacking of two
Te-Bi-Te-Bi-Te blocks and one Bi-Bi block per unit cell. Crystal structure
analysis shows that Mn is localized in the Bi2 blocks, and is accompanied by an
equal amount of TeBi anti-site occupancy in the Bi2Te3 blocks. These TeBi
anti-site defects greatly enhance the Mn solubility. This is demonstrated by
comparison of the [Bi1-xMnx]Te and [Bi1-2xTexMnx]Te series; in the former, the
solubility is limited to x = 0.067, while the latter has xmax = 0.125. The
magnetism in [Bi1-xMnx]Te changes little with x, while that for
[Bi1-2xTexMnx]Te shows a clear variation, leading to ferromagnetism for x >
0.067. Magnetic hysteresis and the anomalous Hall Effect are observed for the
ferromagnetic samples.Comment: Accepted for publication in Phys. Rev.
Theoretical investigation of magnetic order in ReOFeAs, Re = Ce, Pr
Density functional theory (DFT) calculations are carried out on ReOFeAs, Re =
Ce, Pr, the parent compounds of the high-T superconductors
ReOFFeAs, in order to determine the magnetic order of the ground
state. It is found that the magnetic moments on the Fe sites adopt a collinear
antiferromagnetic order, similar to the case of LaOFeAs. Within the generalized
gradient approximation along with Coulomb onsite repulsion (GGA+U), we show
that the Re magnetic moments also adopt an antiferromagnetic order for which,
within the ReO layer, same spin Re sites lie along a zigzag line perpendicular
to the Fe spin stripes. While within GGA the Re 4f band crosses the Fermi
level, upon inclusion of onsite Coulomb interaction the 4f band splits and
moves away from the Fermi level, making ReOFeAs a Mott insulator.Comment: 5 pages, 4 figure
Octet Magnetic Moments with Null Instantons and Semibosonized Nambu-Jona-Lasinio Model
It is shown that the difference between the magnetic moment results in the
quark model with null instantons and semibosonized Nambu-Jona-Lasinio model
lies in the description of the magnetic moment of the -hyperon.Comment: RevTex, 6 pages, submitted to Progr.Theor.Phy
Decuplet baryon magnetic moments in a QCD-based quark model beyond quenched approximation
We study the decuplet baryon magnetic moments in a QCD-based quark model
beyond quenched approximation. Our approach for unquenching the theory is based
on the heavy baryon perturbation theory in which the axial couplings for baryon
- meson and the meson-meson-photon couplings from the chiral perturbation
theory are used together with the QM moment couplings. It also involves the
introduction of a form factor characterizing the structure of baryons
considered as composite particles. Using the parameters obtained from fitting
the octet baryon magnetic moments, we predict the decuplet baryon magnetic
moments. The magnetic moment is found to be in good agreement with
experiment: is predicted to be compared to the
experimental result of (2.02 0.05) .Comment: 19 pages, 2 figure
The Physical Projector and Topological Quantum Field Theories: U(1) Chern-Simons Theory in 2+1 Dimensions
The recently proposed physical projector approach to the quantisation of
gauge invariant systems is applied to the U(1) Chern-Simons theory in 2+1
dimensions as one of the simplest examples of a topological quantum field
theory. The physical projector is explicitely demonstrated to be capable of
effecting the required projection from the initially infinite number of degrees
of freedom to the finite set of gauge invariant physical states whose
properties are determined by the topology of the underlying manifold.Comment: 24 pages, no figures, plain LaTeX file; one more reference added.
Final version to appear in Jour. Phys.
The Lie Algebraic Significance of Symmetric Informationally Complete Measurements
Examples of symmetric informationally complete positive operator valued
measures (SIC-POVMs) have been constructed in every dimension less than or
equal to 67. However, it remains an open question whether they exist in all
finite dimensions. A SIC-POVM is usually thought of as a highly symmetric
structure in quantum state space. However, its elements can equally well be
regarded as a basis for the Lie algebra gl(d,C). In this paper we examine the
resulting structure constants, which are calculated from the traces of the
triple products of the SIC-POVM elements and which, it turns out, characterize
the SIC-POVM up to unitary equivalence. We show that the structure constants
have numerous remarkable properties. In particular we show that the existence
of a SIC-POVM in dimension d is equivalent to the existence of a certain
structure in the adjoint representation of gl(d,C). We hope that transforming
the problem in this way, from a question about quantum state space to a
question about Lie algebras, may help to make the existence problem tractable.Comment: 56 page
Field transformations and simple models illustrating the impossibility of measuring off-shell effects
In the context of simple models illustrating field transformations in
Lagrangian field theories we discuss the impossibility of measuring off-shell
effects in nucleon-nucleon bremsstrahlung, Compton scattering, and related
processes. To that end we introduce a simple phenomenological Lagrangian
describing nucleon-nucleon bremsstrahlung and perform an appropriate change of
variables leading to different off-shell behavior in the nucleon-nucleon
amplitude as well as the photon-nucleon vertex. As a result we obtain a class
of equivalent Lagrangians, generating identical S-matrix elements, of which the
original Lagrangian is but one representative. We make use of this property in
order to show that what appears as an off-shell effect in an S-matrix element
for one Lagrangian may originate in a contact term from an equivalent
Lagrangian. By explicit calculation we demonstrate for the case of
nucleon-nucleon bremsstrahlung as well as nucleon Compton scattering the
equivalence of observables from which we conclude that off-shell effects cannot
in any unambiguous way be extracted from an S-matrix element. Finally, we also
discuss some implications of introducing off-shell effects on a
phenomenological basis, resulting from the requirement that the description of
one process be consistent with that of other processes described by the same
Lagrangian.Comment: 19 pages, Latex, using RevTe
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