Results 1 - 10 of 99
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[en] Resummation of the perturbative series for the amplitude of lepton pair production in nucleus-nucleus collisions is performed based on the Watson theorem and the hypothesis of infrared stability. An explicit expression for this amplitude is obtained, which is valid to within terms of the ninth order in the fine structure constant.
[en] Highlights: • The S3 state of the Mn4CaO5-cluster in photosystem II was investigated by DFT. • The S3 state structure consisting of only 2.7–2.8 Å MnMn distances was discussed. • The binding mode of the Asp170 changes from bidentate to a monodentate structure. • The present model is compared to the previous water-insertion model. The S3 state of the Mn4CaO5-cluster in photosystem II was investigated by DFT calculations and compared with EXAFS data. Considering previously proposed mechanism; a water molecule is inserted into an open coordination site of Mn upon S2 to S3 transition that becomes a substrate water, we examined if the water insertion is essential for the S3 formation, or if one cannot eliminate other possible routes that do not require a water insertion at the S3 stage. The novel S3 state structure consisting of only short 2.7–2.8 Å MnMn distances was discussed.
[en] Praseodymium (Pr) belongs to Lanthanide group; it has an unfilled f shell. Atomic Praseodymium is very rich in fine levels. Atomic structures both Pr I and Pr II have still not been completely classified. Few researchers have studied Pr fine structures theoretically. The 4f25d2 configuration of singly ionized Praseodymium (Pr II) has been investigated and term values, fine levels angular momenta and coupled wave functions arising from this configuration are being reported in this paper. To calculate the term values and wave functions, Russell-Saunders (L-S) and coefficient of fractional parentage methods have been used respectively. Spectroscopic terms for 4f25d2 configuration are 187, 457 J values of possible fine levels and 106 wave functions of given configuration have also been found. Wavefunctions could be used to calculate the energies of terms, transition probabilities, etc. (paper)
[en] Isovalent substitution of S by Se in LaOBiS2−xSex has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS2 structural instability, we have studied the local structure of LaOBiS2−xSex () using temperature dependent Bi L3-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS2 sub-lattice is largely distorted in LaOBiS2 (x = 0.0), with two in-plane Bi–S1 distances separated by ∼0.4 Å instead LaOBiSSe (x = 1.0) showing much smaller local disorder with two in-plane Bi–Se distances in the plane being separated by ∼0.2 Å. Temperature dependent study shows that the two Bi–S1 distances are characterized by different bond strength in LaOBiS2 (x = 0.0) while it is similar for the Bi–Se distances in LaOBiSSe (x = 1.0). The out of plane Bi–S2 bond is harder in LaOBiSSe indicating that the structural instability of BiS2 layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS2−xSex is an important factor to describe differing electronic and thermal transport of the two compounds. (paper)
[en] In this work, we have studied the quantum tunneling of a single spin–orbit-coupled atom held in a periodically modulated optical lattice with an impurity. As a result we find that the dynamical localization takes place globally at the collapse points of quasienergy spectrum, even when the impurity potential is far off-resonant with the driving field. Meanwhile, two types of local second-order tunneling processes appear beyond expectation between the two nearest-neighbor sites of the impurity, with the spin unchanged and with the impurity site population negligible. Though tunneling behaviors of the two types seem to be the same, they are believed to involve two distinct mechanisms: one is related to spin-independent process, while the other is to spin-dependent tunneling process. The two types of second-order processes can be identified by means of resonant tunneling with or without spin flipping by tuning the impurity potential to be in resonance with the driving field. In the Floquet picture, the system with the localized impurity develops a fine structure of avoided crossings of quasienergies near the collapse point, which is crucial to understand the so-called second-order tunneling dynamics. These results are confirmed analytically on the basis of effective three-site model and multiple-time-scale asymptotic perturbative method, and may be exploited for engineering the spin-dependent quantum transport in realistic experiments. (paper)
[en] Reconstructing four-dimensional cone-beam computed tomography (4D-CBCT) images directly from respiratory phase-sorted traditional 3D-CBCT projections can capture target motion trajectory, reduce motion artifacts, and reduce imaging dose and time. However, the limited numbers of projections in each phase after phase-sorting decreases CBCT image quality under traditional reconstruction techniques. To address this problem, we developed a simultaneous motion estimation and image reconstruction (SMEIR) algorithm, an iterative method that can reconstruct higher quality 4D-CBCT images from limited projections using an inter-phase intensity-driven motion model. However, the accuracy of the intensity-driven motion model is limited in regions with fine details whose quality is degraded due to insufficient projection number, which consequently degrades the reconstructed image quality in corresponding regions. In this study, we developed a new 4D-CBCT reconstruction algorithm by introducing biomechanical modeling into SMEIR (SMEIR-Bio) to boost the accuracy of the motion model in regions with small fine structures. The biomechanical modeling uses tetrahedral meshes to model organs of interest and solves internal organ motion using tissue elasticity parameters and mesh boundary conditions. This physics-driven approach enhances the accuracy of solved motion in the organ’s fine structures regions. This study used 11 lung patient cases to evaluate the performance of SMEIR-Bio, making both qualitative and quantitative comparisons between SMEIR-Bio, SMEIR, and the algebraic reconstruction technique with total variation regularization (ART-TV). The reconstruction results suggest that SMEIR-Bio improves the motion model’s accuracy in regions containing small fine details, which consequently enhances the accuracy and quality of the reconstructed 4D-CBCT images. (paper)
[en] Highlights: • First full geometry optimizations using the large-scale QM/MM model of OEC in PSII. • Refinements of the Mn–Mn, Ca–Mn and Mn–O distances of XRD by the QM/MM. • Confirmation of reliability of 3D global XRD structure of OEC by the QM/MM. • Proposal of a new trimer model in combination with QM/MM, XRD, XFEL and EXAFS. • Demonstration of importance of realistic QM models for theoretical modeling of OEC. Large-scale QM/MM calculations including hydrogen-bonding networks in the oxygen evolving complex (OEC) of photosystem II (PSII) were performed to elucidate the geometric structures of the CaMn4O5 cluster in the key catalytic states (Si (i = 0–3)). The optimized Mn–Mn, Ca–Mn and Mn–O distances by the large-scale QM/MM starting from the high-resolution XRD structure were consistent with those of the EXAFS experiments in the dark stable S1 state by the Berkeley and Berlin groups. The optimized geometrical parameters for other Si (i = 0, 2, 3) states were also consistent with those of EXAFS, indicating the importance of the large-scale QM/MM calculations for the PSII-OEC.
[en] The fine-structure energy levels of 1s2s and 1s2p atomic states for the He-like Ar ion immersed in dense plasmas are calculated. The ion sphere model is used to describe the plasma screening effect on the tested ion. The influences of the hard sphere confinement and plasma screening on the fine-structure energy levels are investigated respectively. The calculated results show that the confined effect of the hard sphere on the fine-structure energy levels increases with decreasing hard sphere radius, and the plasma screening effect on the fine-structure energy levels increases with the increase of free electron density. In dense plasmas, the confined effect of the hard sphere on the fine-structure energy levels can be neglected generally, compared with the contribution from free electron screening. An interesting phenomenon about the energy level crossing is found among 1s2s (1S0) and 1s2p (3P0,1) atomic states. The results reported at the present work are useful for plasma diagnostics. (paper)
[en] Precision spectroscopy of the Muonium Lamb shift and fine structure requires a robust source of 2S Muonium. To date, the beam-foil technique is the only demonstrated method for creating such a beam in vacuum. Previous experiments using this technique were statistics limited, and new measurements would benefit tremendously from the efficient 2S production at a low energy muon (<20 keV) facility. Such a source of abundant low energy μ has only become available in recent years, e.g. at the Low-Energy Muon beamline at the Paul Scherrer Institute. Using this source, we report on the successful creation of an intense, directed beam of metastable Muonium. We find that even though the theoretical Muonium fraction is maximal in the low energy range of 2–5 keV, scattering by the foil and transport characteristics of the beamline favor slightly higher μ energies of 7–10 keV. We estimate that an event detection rate of a few events per second for a future Lamb shift measurement is feasible, enabling an increase in precision by two orders of magnitude over previous determinations.