71 research outputs found
Conservation-laws-preserving algorithms for spin dynamics simulations
We propose new algorithms for numerical integration of the equations of
motion for classical spin systems with fixed spatial site positions. The
algorithms are derived on the basis of a mid-point scheme in conjunction with
the multiple time staging propagation. Contrary to existing predictor-corrector
and decomposition approaches, the algorithms introduced preserve all the
integrals of motion inherent in the basic equations. As is demonstrated for a
lattice ferromagnet model, the present approach appears to be more efficient
even over the recently developed decomposition method.Comment: 13 pages, 2 figure
Formation of MnI lines in the solar atmosphere
We present a detailed NLTE analysis of 39 MnI lines in the solar spectrum.
The influence of NLTE effects on the line formation and element abundance is
investigated. Our goal is the derivation of solar log gfe values for manganese
lines, which will later be used in differential abundance analysis of
metal-poor stars. The method of spectrum synthesis is employed, which is based
on a solar model atmosphere with initially specified element abundances. A
manganese abundance of 5.47 dex is used with the theoretical line-blanketed
model atmosphere. Statistical equilibrium calculations are carried out for the
model atom, which comprises 245 and 213 levels for MnI and MnII, respectively.
Photoionization cross-sections are assumed hydrogenic. For line synthesis van
der Waals broadening is calculated according to Anstee & O'Mara's formalism. It
is shown that hyperfine structure of the Mn lines also has strong broadening
effects, and that manganese is prone to NLTE effects in the solar atmosphere.
The nature of the NLTE effects and the validity of the LTE approach are
discussed in detail. The role of photoionization and collisional interaction is
investigated.Comment: 17 pages, 27 figures, accepted for publication in A&
Adaptation to life on land at high O-2 via transition from ferredoxin-to NADH-dependent redox balance
Pyruvate : ferredoxin oxidoreductase (PFO) and iron only hydrogenase ([Fe]-HYD) are common enzymes among eukaryotic microbes that inhabit anaerobic niches. Their function is to maintain redox balance by donating electrons from food oxidation via ferredoxin (Fd) to protons, generating H2 as a waste product. Operating in series, they constitute a soluble electron transport chain of one-electron transfers between FeS clusters. They fulfil the same function—redox balance—served by two electron-transfers in the NADH- and O2-dependent respiratory chains of mitochondria. Although they possess O2-sensitive FeS clusters, PFO, Fd and [Fe]-HYD are also present among numerous algae that produce O2. The evolutionary persistence of these enzymes among eukaryotic aerobes is traditionally explained as adaptation to facultative anaerobic growth. Here, we show that algae express enzymes of anaerobic energy metabolism at ambient O2 levels (21% v/v), Chlamydomonas reinhardtii expresses them with diurnal regulation. High O2 environments arose on Earth only approximately 450 million years ago. Gene presence/absence and gene expression data indicate that during the transition to high O2 environments and terrestrialization, diverse algal lineages retained enzymes of Fd-dependent one-electron-based redox balance, while the land plant and land animal lineages underwent irreversible specialization to redox balance involving the O2-insensitive two-electron carrier NADH
The LQG -- String: Loop Quantum Gravity Quantization of String Theory I. Flat Target Space
We combine I. background independent Loop Quantum Gravity (LQG) quantization
techniques, II. the mathematically rigorous framework of Algebraic Quantum
Field Theory (AQFT) and III. the theory of integrable systems resulting in the
invariant Pohlmeyer Charges in order to set up the general representation
theory (superselection theory) for the closed bosonic quantum string on flat
target space. While we do not solve the, expectedly, rich representation theory
completely, we present a, to the best of our knowledge new, non -- trivial
solution to the representation problem. This solution exists 1. for any target
space dimension, 2. for Minkowski signature of the target space, 3. without
tachyons, 4. manifestly ghost -- free (no negative norm states), 5. without
fixing a worldsheet or target space gauge, 6. without (Virasoro) anomalies
(zero central charge), 7. while preserving manifest target space Poincar\'e
invariance and 8. without picking up UV divergences. The existence of this
stable solution is exciting because it raises the hope that among all the
solutions to the representation problem (including fermionic degrees of
freedom) we find stable, phenomenologically acceptable ones in lower
dimensional target spaces, possibly without supersymmetry, that are much
simpler than the solutions that arise via compactification of the standard Fock
representation of the string. Moreover, these new representations could solve
some of the major puzzles of string theory such as the cosmological constant
problem. The solution presented in this paper exploits the flatness of the
target space in several important ways. In a companion paper we treat the more
complicated case of curved target spaces.Comment: 46 p., LaTex2e, no figure
Part 2: CT characterisation of pancreatic neoplasm: tumour mimics
There are numerous pancreatic and peripancreatic conditions that can mimic pancreatic neoplasms. Many of these can be confidently diagnosed on computed tomography (CT), while others will require further imaging. Knowledge of these tumour mimics is important to avoid misclassification of benign conditions as malignant and to avoid unnecessary surgery. Mimics can be grouped as parenchymal, vascular, biliary and peripancreatic. These are discussed and illustrated in this review
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