7,967 research outputs found
Studying the Variation of the Fine Structure Constant Using Emission Line Multiplets
As an extension of the method by Bahcall et al. (2004) to investigate the
time dependence of the fine structure constant, we describe an approach based
on new observations of forbidden line multiplets from different ionic species.
We obtain optical spectra of fine structure transitions in [Ne III], [Ne V], [O
III], [OI], and [SII] multiplets from a sample of 14 Seyfert 1.5 galaxies in
the low-z range 0.035 < z < 0.281. Each source and each multiplet is
independently analyzed to ascertain possible errors. Averaging over our sample,
we obtain a conservative value alpha^2(t)/\alpha^2(0) = 1.0030+-0.0014.
However, our sample is limited in size and our fitting technique simplistic as
we primarily intend to illustrate the scope and strengths of emission line
studies of the time variation of the fine structure constant. The approach can
be further extended and generalized to a "many-multiplet emission line method"
analogous in principle to the corresponding method using absorption lines. With
that aim, we note that the theoretical limits on emission line ratios of
selected ions are precisely known, and provide well constrained selection
criteria. We also discuss several other forbidden and allowed lines that may
constitute the basis for a more rigorous study using high-resolution
instruments on the next generation of 8 m class telescopes.Comment: 20 pages, 4 figures, sumbitted to A
Quantum Antiferromagnetism in Quasicrystals
The antiferromagnetic Heisenberg model is studied on a two-dimensional
bipartite quasiperiodic lattice. The distribution of local staggered magnetic
moments is determined on finite square approximants with up to 1393 sites,
using the Stochastic Series Expansion Quantum Monte Carlo method. A non-trivial
inhomogeneous ground state is found. For a given local coordination number, the
values of the magnetic moments are spread out, reflecting the fact that no two
sites in a quasicrystal are identical. A hierarchical structure in the values
of the moments is observed which arises from the self-similarity of the
quasiperiodic lattice. Furthermore, the computed spin structure factor shows
antiferromagnetic modulations that can be measured in neutron scattering and
nuclear magnetic resonance experiments.
This generic model is a first step towards understanding magnetic
quasicrystals such as the recently discovered Zn-Mg-Ho icosahedral structure.Comment: RevTex, 4 pages with 5 figure
Bypassing Cowling's theorem in axisymmetric fluid dynamos
We present a numerical study of the magnetic field generated by an
axisymmetrically forced flow in a spherical domain. At small enough Reynolds
number, Re, the flow is axisymmetric and generates an equatorial dipole above a
critical magnetic Reynolds number Rmc . The magnetic field thus breaks
axisymmetry, in agreement with Cowling's theorem. This structure of the
magnetic field is however replaced by a dominant axial dipole when Re is larger
and allows non axisymmetric fluctuations in the flow. We show here that even in
the absence of such fluctuations, an axial dipole can also be generated, at low
Re, through a secondary bifurcation, when Rm is increased above the dynamo
threshold. The system therefore always find a way to bypass the constraint
imposed by Cowling's theorem. We understand the dynamical behaviors that result
from the interaction of equatorial and axial dipolar modes using simple model
equations for their amplitudes derived from symmetry arguments.Comment: 4 pages, 6 figure
Проектные параметры усовершенствованного резонатора для ВЧ-фотоинжекторов
Приведены расчетные характеристики резонатора для ВЧ-фотоинжекторов, работающих на частоте 1300 МГц в сочетании сильных электромагнитных полей и длинного ВЧ-импульса. Конструкция усовершенствованного резонатора использует апробированные решения, но содержит существенные изменения. В ячейки резонатора введены ВЧ-датчики. Проведена оптимизация характеристик резонатора по отдельным характеристикам. Но основное внимание уделено учету взаимного влияния процессов и поиску оптимальных решений по комплексу параметров резонатора в целом.Наведено розрахункові характеристики резонатора для ВЧ-фотоінжекторів, що працюють на частоті 1300 МГц у сполученні сильних електромагнітних полів і довгого ВЧ-імпульса. Конструкція вдосконаленого резонатора використовує апробовані рішення, але містить істотні зміни. В осередки резонатора уведені ВЧ-датчики. Проведено оптимізацію характеристик резонатора по окремих характеристиках. Але основна увага приділена обліку взаємного впливу процесів і пошуку оптимальних рішень із комплексу параметрів резонатора в цілому.During development and operation of DESY L-band RF gun cavities, desires for further improvements were formulated. The next step of development is based on the proven advantages of existing cavities, but includes very significant changes. The L-band 1.6 cell RF gun cavity is intended for operation in pulse mode with unique combination of high RF fields and long RF pulse. Each cavity cell is equipped with RF probe. The cavity design is improved to gain an advantage over existing cavities in certain parameters. But main attention paid for coupled optimization to improve the cavity parameters in total. The design ideas and expected results are described
Resonant inelastic x-ray scattering in one-dimensional copper oxides
The Cu K-edge resonant inelastic x-ray scattering (RIXS) spectrum in
one-dimensional insulating cuprates is theoretically examined by using the
exact diagonalization technique for the extended one-dimensional
Hubbard model with nearest neighbor Coulomb interaction. We find the
following characteristic features that can be detectable by RIXS experiments:
(i) The spectrum with large momentum transfer indicates the formation of
excitons, i.e., bound states of holon and doublon. (ii) The spectrum with small
momentum transfer depends on the incident photon energy. We propose that the
RIXS provides a unique opportunity to study the upper
Hubbard band in one-dimensional cuprates.Comment: 3 pages with 4 figures, minor changes, to appear in Phys.Rev.
Dynamic Exponent of t-J and t-J-W Model
Drude weight of optical conductivity is calculated at zero temperature by
exact diagonalization for the two-dimensional t-J model with the two-particle
term, . For the ordinary t-J model with =0, the scaling of the Drude
weight for small doping concentration is
obtained, which indicates anomalous dynamic exponent =4 of the Mott
transition. When is switched on, the dynamic exponent recovers its
conventional value =2. This corresponds to an incoherent-to-coherent
transition associated with the switching of the two-particle transfer.Comment: LaTeX, JPSJ-style, 4 pages, 5 eps files, to appear in J. Phys. Soc.
Jpn. vol.67, No.6 (1998
Short-term outcome after posterior versus lateral surgical approach for total hip arthroplasty - a randomized clinical trial*
<p>Abstract</p> <p>Purpose</p> <p>Currently, total hip replacement (THR) is most commonly performed via a posterior or a direct lateral approach, but the impact of the latter on the invention's outcome has yet not been quantified.</p> <p>Methods</p> <p>We compared the short-term outcome of cementless THR using the both approaches in a prospective, randomized controlled trial. 60 patients with unilateral osteoarthritis were included. Outcome assessment was performed one day before surgery and one week, four weeks, six weeks and 12 weeks after surgery, respectively, using the Harris Hip score as primary objective.</p> <p>Results</p> <p>We found no significant difference in the intraindividual Harris Hip Score improvement at the pre-and three months post-operative assessments between both treatment groups (p = 0.115). However, Harris Hip scores and most functional and psychometric secondary endpoints showed a consistent tendency of a slightly better three months result in patients implanted via the posterior approach. In contrast a significant shorter operating time of the direct lateral approach was recorded (67 minutes versus 76 minutes, p < 0.001).</p> <p>Conclusion</p> <p>In our opinion this slightly better short-term functional outcome after posterior approach is not clinical relevant. However, to make definitive conclusions all clinical relevant factors (i.e. mid- to long-term function, satisfaction, complication rates and long-term survival) have to be taken into account. <it>Level of evidence: </it>I - therapeutic</p
Foundations of the Wald Space for Phylogenetic Trees
Evolutionary relationships between species are represented by phylogenetic
trees, but these relationships are subject to uncertainty due to the random
nature of evolution. A geometry for the space of phylogenetic trees is
necessary in order to properly quantify this uncertainty during the statistical
analysis of collections of possible evolutionary trees inferred from biological
data. Recently, the wald space has been introduced: a length space for trees
which is a certain subset of the manifold of symmetric positive definite
matrices. In this work, the wald space is introduced formally and its topology
and structure is studied in detail. In particular, we show that wald space has
the topology of a disjoint union of open cubes, it is contractible, and by
careful characterization of cube boundaries, we demonstrate that wald space is
a Whitney stratified space of type (A). Imposing the metric induced by the
affine invariant metric on symmetric positive definite matrices, we prove that
wald space is a geodesic Riemann stratified space. A new numerical method is
proposed and investigated for construction of geodesics, computation of
Fr\'echet means and calculation of curvature in wald space. This work is
intended to serve as a mathematical foundation for further geometric and
statistical research on this space.Comment: 42 pages, 15 figure
Spin and Charge Texture around In-Plane Charge Centers in the CuO_2 planes
Recent experiments on La_2Cu_{1-x}Li_xO_4 show that although the doped holes
remain localized near the substitutional Li impurities, magnetic order is
rapidly suppressed. An examination of the spin texture around a bound hole in a
CuO_2 plane shows that the formation of a skyrmion is favored in a wide range
of parameters, as was previously proposed in the context of Sr doping. The spin
texture may be observable by elastic diffuse neutron scattering, and may also
have a considerable effect on NMR lineshapes.Comment: 4 pages, postscript file, hardcopy available upon request, to appear
in PR
Difference of optical conductivity between one- and two-dimensional doped nickelates
We study the optical conductivity in doped nickelates, and find the dramatic
difference of the spectrum in the gap (\alt4 eV) between one- (1D)
and two-dimensional (2D) nickelates. The difference is shown to be caused by
the dependence of hopping integral on dimensionality. The theoretical results
explain consistently the experimental data in 1D and
2D nickelates, YCaBaNiO and LaSrNiO,
respectively. The relation between the spectrum in the X-ray aborption
experiments and the optical conductivity in LaSrNiO is
discussed.Comment: RevTeX, 4 pages, 4 figure
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