2,991 research outputs found
Analysis of the total 12C(α,γ)16O cross section based on available angular distributions and other primary data
Because a knowledge of the 12C/16O ratio is crucial to the understanding of the later evolution of massive stars, new R- and K-matrix fits have been completed using the available angular distribution data from radiative α capture and elastic α scattering on 12C. Estimates of the total 12C(α,γ)16O rate at stellar energies are reported. In contrast with previous work, the analyses generally involve R- and K-matrix fits directly to the primary data, i.e., the energy- and angle-dependent differential yields, with all relevant partial waves fitted simultaneously (referred to here as surface fits). It is shown that, while the E1 part of the reaction is well constrained by a recent experiment on the β-delayed α-particle decay of 16N, only upper limits can be placed on the E2 ground state cross section factor which we take conservatively as SE2(300)<140 keV b. Simulations were then carried out to explore what kind of new data could lead to better restrictions on SE2(300). We find that improved elastic scattering data may be the best short-term candidate for such restrictions while significantly improving S(300) with new radiative capture data may require a longer-term effort. Theoretical models and estimates from α-transfer reactions for the E2 part of 12C(α,γ)16O are then discussed for comparison with the R- and K-matrix fits of the present work
Phases of a two dimensional large N gauge theory on a torus
We consider two-dimensional large N gauge theory with D adjoint scalars on a
torus, which is obtained from a D+2 dimensional pure Yang-Mills theory on
T^{D+2} with D small radii. The two dimensional model has various phases
characterized by the holonomy of the gauge field around non-contractible cycles
of the 2-torus. We determine the phase boundaries and derive the order of the
phase transitions using a method, developed in an earlier work
(arxiv:0910.4526), which is nonperturbative in the 'tHooft coupling and uses a
1/D expansion. We embed our phase diagram in the more extensive phase structure
of the D+2 dimensional Yang-Mills theory and match with the picture of a
cascade of phase transitions found earlier in lattice calculations
(arxiv:0710.0098). We also propose a dual gravity system based on a
Scherk-Schwarz compactification of a D2 brane wrapped on a 3-torus and find a
phase structure which is similar to the phase diagram found in the gauge theory
calculation.Comment: 28 pages (+ 17 pages of appendix + 6 pages of ref.); 8 figures; (v2)
LaTeX Showkeys command deleted; (v3) refs and minor clarifications added;
emphasized the new proposal for applying holography to nonsupersymmetric
gauge theory; (v4) modified the arguments about holography; (v5) minor
corrections, version appeared in PR
A Plaquette Basis for the Study of Heisenberg Ladders
We employ a plaquette basis-generated by coupling the four spins in a
lattice to a well-defined total angular momentum-for the study of
Heisenberg ladders with antiferromagnetic coupling. Matrix elements of the
Hamiltonian in this basis are evaluated using standard techniques in
angular-momentum (Racah) algebra. We show by exact diagonalization of small
( and ) systems that in excess of 90% of the ground-state
probability is contained in a very small number of basis states. These few
basis states can be used to define a severely truncated basis which we use to
approximate low-lying exact eigenstates. We show how, in this low-energy basis,
the isotropic spin-1/2 Heisenberg ladder can be mapped onto an anisotropic
spin-1 ladder for which the coupling along the rungs is much stronger than the
coupling between the rungs. The mapping thereby generates two distinct energy
scales which greatly facilitates understanding the dynamics of the original
spin-1/2 ladder. Moreover, we use these insights to define an effective
low-energy Hamiltonian in accordance to the newly developed COntractor
REnormalization group (CORE) method. We show how a simple range-2 CORE
approximation to the effective Hamiltonian to be used with our truncated basis
reproduces the low-energy spectrum of the exact theory at the \alt
1% level.Comment: 12 pages with two postscript figure
Low-Energy Properties of Regularly Depleted Spin Ladders
We investigate a model for the regularly depleted two-leg spin ladder
systems. By using Lieb-Schultz-Mattis theorem, it is rigorously shown that this
model realizes massless excitations or, alternatively, a degenerate ground
state, although the original spin ladder system has a spin gap and a unique
ground state. The ground state of the depleted model is either a spin singlet
or partially ferromagnetic reflecting topological properties of the depleted
sites. In order to show that the low-energy excitations are indeed massless, we
proceed our analysis in two different ways by resorting to effective field
theories. We first investigate an effective weak-coupling model in terms of
renormalization group methods. Although the tendency to massless spin
excitations is seen in the strong coupling regime, it turns out that the model
is still massive for any finite coupling, implying that a conventional
weak-coupling approach is not efficient to describe massless modes in our
model. To overcome this difficulty, we further study low-energy properties of
the depleted spin model by mapping on the non-linear sigma model, and confirm
that the massless spin excitation indeed occur.Comment: 13 pages, revtex, 7 figures, to appear in PRB. A new section is
added, in which we derived the spin-wave dispersion relation and analyzed the
quantum fluctuation around it by means of the O(3) nonlinear sigma mode
An Alternative Parameterization of R-matrix Theory
An alternative parameterization of R-matrix theory is presented which is
mathematically equivalent to the standard approach, but possesses features
which simplify the fitting of experimental data. In particular there are no
level shifts and no boundary-condition constants which allows the positions and
partial widths of an arbitrary number levels to be easily fixed in an analysis.
These alternative parameters can be converted to standard R-matrix parameters
by a straightforward matrix diagonalization procedure. In addition it is
possible to express the collision matrix directly in terms of the alternative
parameters.Comment: 8 pages; accepted for publication in Phys. Rev. C; expanded Sec. IV,
added Sec. VI, added Appendix, corrected typo
Photoemission Spectra in t-J Ladders with Two Legs
Photoemission spectra for the isotropic two-leg t-J ladder are calculated at
various hole-doping levels using exact diagonalization techniques. Low-energy
sharp features caused by short-range antiferromagnetic correlations are
observed at finite doping levels close to half-filling, above the naive Fermi
momentum. These features should be observable in angle-resolved photoemission
experiments. In addition, the formation of a d-wave pairing condensate as the
ratio J/t is increased leads to dynamically generated spectral weight for
momenta close to where the -order parameter is large.Comment: 9 pages, RevTex, to be published in Phys. Rev. B (RC
63Cu NQR evidence of dimensional crossover to anisotropic 2d regime in S= 1/2 three-leg ladder Sr2Cu3O5
We probed spin-spin correlations up to 725 K with 63Cu NQR in the S= 1/2
three-leg ladder Sr2Cu3O5. We present experimental evidence that below 300 K,
weak inter-ladder coupling causes dimensional crossover of the spin-spin
correlation length \xi from quasi-1d (\xi ~ 1/T) to anisotropic 2d regime (\xi
\~ exp[2\pi\rho_{s}/T], where 2\pi\rho_{s} = 290 +/- 30 K is the effective spin
stiffness). This is the first experimental verification of the renormalized
classical behavior of the anisotropic non-linear sigma model in 2d, which has
been recently proposed for the striped phase in high T_{c} cuprates.Comment: 4 pages, 3 figure
Chern-Simons matrix model: coherent states and relation to Laughlin wavefunctions
Using a coherent state representation we derive many-body probability
distributions and wavefunctions for the Chern-Simons matrix model proposed by
Polychronakos and compare them to the Laughlin ones. We analyze two different
coherent state representations, corresponding to different choices for electron
coordinate bases. In both cases we find that the resulting probability
distributions do not quite agree with the Laughlin ones. There is agreement on
the long distance behavior, but the short distance behavior is different.Comment: 15 pages, LaTeX; one reference added, abstract and section 5
expanded, typos correcte
Recurrent Variational Approach to the Two-Leg Hubbard Ladder
We applied the Recurrent Variational Approach to the two-leg Hubbard ladder.
At half-filling, our variational Ansatz was a generalization of the resonating
valence bond state. At finite doping, hole pairs were allowed to move in the
resonating valence bond background. The results obtained by the Recurrent
Variational Approach were compared with results from Density Matrix
Renormalization Group.Comment: 10 pages, 14 Postscript figure
Preliminary Report of the AMS analysis of tsunami deposits in Tohoku – Japan – 18 th to the 21 st Century
Sedimentary records of tsunamis are a precious tool to assess the occurrence of past events, as attested by an abundant literature, which has seen a particular 'boom' in the aftermath of the 2004 Indian Ocean tsunami and the 2011 Tohoku tsunami. Despite an extensive literature, there is very little to no understanding of the role that the changing coastal environment is playing on the record of a tsunami, and for a given location, it is still unclear whether the largest tsunamis leave the largest amount of deposits. To research this question, the present study took place in Japan, in the Tohoku Region at Agawa-pond, because the pond act as a sediment trap. Using a sediment-slicer, a 1 m thick deposit was retrieved, from which 4 tsunami sequences were identified, including the latest 2011 tsunami. Using a series of sedimentary proxies: the AMS (Anisotropy of Magnetic Susceptibility), grain size analysis, quartz morphoscopy (morphology and surface characteristics) and the analysis of microfossils, disparities between the tsunami deposits were identified and most importantly a clear thinning of the tsunami deposit towards the top. Provided the present evidences, the authors discuss that the upward fining is due to at least two components that are seldom assessed in tsunami research (1) a modification of the depositional environment, with the progressive anthropization of the coast, providing less sediments to remobilize; and (2) a progressive filling of the Agawa pond, which progressively loses its ability to trap tsunami materials
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