1,881 research outputs found
Pinned Low Energy Electronic Excitation in Metal Exchanged Vanadium Oxide Nanoscrolls
We measured the optical properties of mixed valent vanadium oxide nanoscrolls
and their metal exchanged derivatives in order to investigate the charge
dynamics in these compounds. In contrast to the prediction of a metallic state
for the metal exchanged derivatives within a rigid band model, we find that the
injected charges in Mn exchanged vanadium oxide nanoscrolls are pinned.
A low-energy electronic excitation associated with the pinned carriers appears
in the far infrared and persists at low temperature, suggesting that the
nanoscrolls are weak metals in their bulk form, dominated by inhomogeneous
charge disproportionation and Madelung energy effects.Comment: 4 figure
Photoassociation spectra and the validity of the dipole approximation for weakly bound dimers
Photoassociation (PA) of ultracold metastable helium to the 2s2p manifold is
theoretically investigated using a non-perturbative close-coupled treatment in
which the laser coupling is evaluated without assuming the dipole
approximation. The results are compared with our previous study [Cocks and
Whittingham, Phys. Rev. A 80, 023417 (2009)] that makes use of the dipole
approximation. The approximation is found to strongly affect the PA spectra
because the photoassociated levels are weakly bound, and a similar impact is
predicted to occur in other systems of a weakly bound nature. The inclusion or
not of the approximation does not affect the resonance positions or widths,
however significant differences are observed in the background of the spectra
and the maximum laser intensity at which resonances are discernable. Couplings
not satisfying the dipole selection rule |J-1| <= J' <= |J+1| do not lead to
observable resonances.Comment: 5 pages, 2 figures; Minor textual revision
Symmetry improvement of 3PI effective actions for O(N) scalar field theory
[Abridged] n-Particle Irreducible Effective Actions (PIEA) are a powerful
tool for extracting non-perturbative and non-equilibrium physics from quantum
field theories. Unfortunately, practical truncations of PIEA can
unphysically violate symmetries. Pilaftsis and Teresi (PT) addressed this by
introducing a "symmetry improvement" scheme in the context of the 2PIEA for an
O(2) scalar theory, ensuring that the Goldstone boson is massless in the broken
symmetry phase [A. Pilaftsis and D. Teresi, Nuc.Phys. B 874, 2 (2013), pp.
594--619]. We extend this by introducing a symmetry improved 3PIEA for O(N)
theories, for which the basic variables are the 1-, 2- and 3-point correlation
functions. This requires the imposition of a Ward identity involving the
3-point function. The method leads to an infinity of physically distinct
schemes, though an analogue of d'Alembert's principle is used to single out a
unique scheme. The standard equivalence hierarchy of PIEA no longer holds
with symmetry improvement and we investigate the difference between the
symmetry improved 3PIEA and 2PIEA. We present renormalized equations of motion
and counter-terms for 2 and 3 loop truncations of the effective action, leaving
their numerical solution to future work. We solve the Hartree-Fock
approximation and find that our method achieves a middle ground between the
unimproved 2PIEA and PT methods. The phase transition predicted by our method
is weakly first order and the Goldstone theorem is satisfied. We also show
that, in contrast to PT, the symmetry improved 3PIEA at 2 loops does not
predict the correct Higgs decay rate, but does at 3 loops. These results
suggest that symmetry improvement should not be applied to PIEA truncated to
loops. We also show that symmetry improvement is compatible with the
Coleman-Mermin-Wagner theorem, a check on the consistency of the formalism.Comment: 27 pages, 15 figures, 2 supplemental Mathematica notebooks. REVTeX
4.1 with amsmath. Updated with minor corrections. Accepted for publication in
Phys. Rev.
Factorization fits to charmless strangeless B decays
We present fits to charmless strangeless hadronic B decay data for mean
branching ratios and CP-violating asymmetries using the QCD factorization model
of Beneke et al. Apart from one CP-violating parameter, the model gives a very
good representation of 26 measured data. We find the CKM angle alpha = (93.5
+/- 8.4 -1.3) degrees and to be quite stable to plausible "charming penguin"
corrections.Comment: 4 pages, LaTeX, Minor changes to text, references adde
Ultracold collisions in tight harmonic traps: Quantum defect model and application to metastable helium atoms
We analyze a system of two colliding ultracold atoms under strong harmonic
confinement from the viewpoint of quantum defect theory and formulate a
generalized self-consistent method for determining the allowed energies. We
also present two highly efficient computational methods for determining the
bound state energies and eigenfunctions of such systems. The perturbed harmonic
oscillator problem is characterized by a long asymptotic region beyond the
effective range of the interatomic potential. The first method, which is based
on quantum defect theory and is an adaptation of a technique developed by one
of the authors (GP) for highly excited states in a modified Coulomb potential,
is very efficient for integrating through this outer region. The second method
is a direct numerical solution of the radial Schr\"{o}dinger equation using a
discrete variable representation of the kinetic energy operator and a scaled
radial coordinate grid. The methods are applied to the case of trapped
spin-polarized metastable helium atoms. The calculated eigenvalues agree very
closely for the two methods, and with those computed self-consistently using
the generalized self-consistent method.Comment: 11 pages,REVTEX, text substantially revised, title modifie
Agriculturally Improved and Semi-Natural Permanent Grasslands Provide Complementary Ecosystem Services in Swedish Boreal Landscapes
\ua9 2024 by the authors. Permanent grasslands cover more than a third of European agricultural land and are important for a number of ecosystem services. Permanent grasslands used for agriculture are broadly separated into agriculturally improved and semi-natural grasslands. High cultural and natural values linked to semi-natural grasslands are well documented. However, in boreal and hemi-boreal agricultural landscapes, less information is available about the areal coverage of improved permanent grasslands and their role for ecosystem service provision and biodiversity. In Sweden, grasslands are administratively separated into semi-natural (i.e., land that cannot be ploughed) or arable (i.e., improved temporary or permanent grassland on land that can be ploughed). We used data from a large-scale environmental monitoring program to show that improved permanent grassland (i.e., permanent grasslands on arable fields) may be a previously unrecognised large area of the agricultural land use in Sweden. We show that improved permanent grasslands together with semi-natural grasslands are both comparable but also complementary providers of a range of ecosystem services (plant species richness, plant resources for pollinators and forage amount for livestock production). However, as expected, semi-natural grasslands with the highest-level AESs (special values) show high species richness values for vascular plants, plants indicating traditional semi-natural management conditions and red-listed species. Improved permanent grasslands on arable fields are likely an underestimated but integral part of the agricultural economy and ecological function in boreal landscapes that together with high nature value semi-natural grasslands provide a broad range of ecosystem services
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