173 research outputs found
An observation of the meson in the system in the Pion- Interaction at Momentum of 29 GeV
The charge-exchange reaction ,
, is studied with
the upgraded VES facility (U-70, Protvino) in the interaction of a 29 GeV pion
beam with a beryllium target. The distribution over the invariant mass of the
system shows a near-threshold signal. A partial wave analysis
reveals that the scalar state () dominates in this mass region.
The observed signal can be described with a contribution of the known
resonance . Using OPE approximation for the reaction the product of branching fractions is found to be:
Spin-texture inversion in the giant Rashba semiconductor BiTeI
Semiconductors with strong spin-orbit interaction as the underlying mechanism for the generation of spin-polarized electrons are showing potential for applications in spintronic devices. Unveiling the full spin texture in momentum space for such materials and its relation to the microscopic structure of the electronic wave functions is experimentally challenging and yet essential for exploiting spin-orbit effects for spin manipulation. Here we employ a state-of-the-art photoelectron momentum microscope with a multichannel spin filter to directly image the spin texture of the layered polar semiconductor BiTeI within the full two-dimensional momentum plane. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the valence and conduction band electrons in BiTeI have spin textures of opposite chirality and of pronounced orbital dependence beyond the standard Rashba model, the latter giving rise to strong optical selection-rule effects on the photoelectron spin polarization. These observations open avenues for spin-texture manipulation by atomic-layer and charge carrier control in polar semiconductors.This work was supported by DFG (through SFB 1170 'ToCoTronics') and through FOR1162 (P3). We acknowledge the support by the Basque Departamento de Educacion, UPV/EHU (Grant Number IT-756-13), Spanish Ministerio de Economia y Competitividad (MINECO Grant Number FIS2013-48286-C2-2-P), Tomsk State University Academic D.I. Mendeleev Fund Program in 2015 (Research Grant Number 8.1.05.2015), the Russian Foundation for Basic Research (Grant Numbers 15-02-01797 and 15-02-589 02717). Partial support by the Saint Petersburg State University (Grant Number 15.61.202.2015) is also acknowledged
Design of Materials for Solid Oxide Fuel Cells, Permselective Membranes, and Catalysts for Biofuel Transformation into Syngas and Hydrogen Based on Fundamental Studies of Their Real Structure, Transport Properties, and Surface Reactivity
Advances in design of materials for solid oxide fuel cells, oxygen and hydrogen separation membranes, and catalysts for biofuel conversion into syngas and hydrogen are reviewed. Application of new efficient techniques of material synthesis and characterization of their atomic-scale structure, transport properties, and reactivity allowed to develop new types of efficient cathodes and anodes for solid oxide fuel cells, asymmetric supported oxygen, and hydrogen separation membranes with high permeability and structured catalysts with nanocomposite-active components demonstrating high performance and stability to coking in steam/autothermal reforming of biofuels. © 2021 Elsevier B.V.This work was supported by the АААА-А21-121011390007-7 budget project of the Boreskov Institute of catalysis. A.A.Y. gratefully acknowledges financial support within the project CICECO — Aveiro Institute of Materials ( UIDB/50011/2020 and UIDP/50011/2020 ) financed by national funds through the FCT/MCTES and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement
Advanced surface treatments for medium-velocity superconducting RF cavities for high accelerating gradient continuous-wave operation
Nitrogen-doping and furnace-baking are advanced high-Q0 recipes developed for
1.3 GHz TESLA-type cavities. These treatments will significantly benefit the
high-Q0 linear accelerator community if they can be successfully adapted to
different cavity styles and frequencies. Strong frequency- and geometry-
dependence of these recipes makes the technology transfer amongst different
cavity styles and frequencies far from straightforward, and requires rigorous
study. Upcoming high-Q0 continuous-wave linear accelerator projects, such as
the proposed Michigan State University Facility for Rare Isotope Beam Energy
Upgrade, and the underway Fermilab's Proton Improvement Plan-II, could benefit
enormously from adapting these techniques to their beta_opt = 0.6 ~650 MHz
5-cell elliptical superconducting rf cavities, operating at an accelerating
gradient of around ~17 MV/m. This is the first investigation of the adaptation
of nitrogen doping and medium temperature furnace baking to prototype 644 MHz
beta_opt = 0.65 cavities, with the aim of demonstrating the high-Q0 potential
of these recipes in these novel cavities for future optimization as part of the
FRIB400 project R&D. We find that nitrogen-doping delivers superior Q0, despite
the sub-GHz operating frequency of these cavities, but is sensitive to the
post-doping electropolishing removal step and experiences elevated residual
resistance. Medium temperature furnace baking delivers reasonable performance
with decreased residual resistance compared to the nitrogen doped cavity, but
may require further recipe refinement. The gradient requirement for the FRIB400
upgrade project is comfortably achieved by both recipes.Comment: 16 pages, 5 figure
Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities
A model for the onset of the reduction in SRF cavity quality factor, the
so-called Q-drop, at high accelerating electric fields is presented. Breakdown
of the surface barrier against magnetic flux penetration at the cavity equator
is considered to be the critical event that determines the onset of Q-drop. The
worst case of triangular grooves with low field of first flux penetration Hp,
as analyzed previously by Buzdin and Daumens, [1998 Physica C 294: 257], was
adapted. This approach incorporates both the geometry of the groove and local
contamination via the Ginzburg-Landau parameter kappa, so the proposed model
allows new comparisons of one effect in relation to the other. The model
predicts equivalent reduction of Hp when either roughness or contamination were
varied alone, so smooth but dirty surfaces limit cavity performance about as
much as rough but clean surfaces do. When in combination, contamination
exacerbates the negative effects of roughness and vice-versa. To test the model
with actual data, coupons were prepared by buffered chemical polishing and
electropolishing, and stylus profilometry was used to obtain distributions of
angles. From these data, curves for surface resistance generated by simple flux
flow as a function of magnetic field were generated by integrating over the
distribution of angles for reasonable values of kappa. This showed that
combined effects of roughness and contamination indeed reduce the Q-drop onset
field by ~30%, and that that contamination contributes to Q-drop as much as
roughness. The latter point may be overlooked by SRF cavity research, since
access to the cavity interior by spectroscopy tools is very difficult, whereas
optical images have become commonplace. The model was extended to fit cavity
test data, which indicated that reduction of the superconducting gap by
contaminants may also play a role in Q-drop.Comment: 15 pages with 7 figure
Decadal changes of the Western Arabian sea ecosystem
Historical data from oceanographic expeditions and remotely sensed data on outgoing longwave radiation, temperature, wind speed and ocean color in the western Arabian Sea (1950–2010) were used to investigate decadal trends in the physical and biochemical properties of the upper 300 m. 72 % of the 29,043 vertical profiles retrieved originated from USA and UK expeditions. Increasing outgoing longwave radiation, surface air temperatures and sea surface temperature were identified on decadal timescales. These were well correlated with decreasing wind speeds associated with a reduced Siberian High atmospheric anomaly. Shoaling of the oxycline and nitracline was observed as well as acidification of the upper 300 m. These physical and chemical changes were accompanied by declining chlorophyll-a concentrations, vertical macrofaunal habitat compression, declining sardine landings and an increase of fish kill incidents along the Omani coast
Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator MnBi2Te4A
Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator MnBi2Te4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9-35 K), light polarizations and photon energies. We have distinguished both large (60-70 meV) and reduced (< 20 meV) gaps at the DP in the ARPES dispersions, which remain open above the Neel temperature (T-N = 24.5 K). We propose that the gap above T-N remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the "large gap" sample and apparently significantly reduced effective magnetic moment for the "reduced gap" sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.The authors acknowledge support by the Saint Petersburg State University (Grant No. 51126254), Russian Science Foundation (Grant No. 18-12-00062 in part of the photoemission measurements and Grant No. 18-12-00169 in part of the electronic band structure calculations) and by Russian Foundation of Basic Researches (Grants Nos. 18-52-06009 and 20-32-70179) and Science Development Foundation under the President of the Republic of Azerbaijan (Grant No. EI F-BGM-4-RFTF1/2017-21/04/1-M-02). A. Kimura was financially supported by KAKENHI (Grants No. 17H06138, No. 17H06152, and No. 18H03683). S.V.E. and E.V.C. acknowledge support by the Fundamental Research Program of the State Academies of Sciences (line of research III.23.2.9). The authors kindly acknowledge the HiSOR staff and A. Harasawa at ISSP for technical support and help with the experiment. The ARPES measurements at HiSOR were performed with the approval of the Proposal Assessing Committee (Proposal Numbers: 18BG027 and 19AG048). XAS and XMCD measurements were performed at BL23SU of SPring-8 (Proposal Nos. 2018A3842 and 2018B3842) under the Shared Use Program of JAEA Facilities (Proposal Nos. 2018A-E25 and 2018B-E24) with the approval of Nanotechnology Platform project supported by MEXT, Japan (Proposal Nos. A-18-AE-0020 and A-18-AE-0042). M. M. Otrokov acknowledges the support by Spanish Ministerio de Ciencia e Innovacion (Grant no. PID2019-103910GB-I00). K. Yaji was financially supported by KAKENHI (Grants No. 18K03484)
Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator MnBiTe
Modification of the gap at the Dirac point (DP) in antiferromagnetic (AFM)
axion topological insulator MnBiTe and its electronic and spin
structure has been studied by angle- and spin-resolved photoemission
spectroscopy (ARPES) under laser excitation with variation of temperature
(9-35~K), light polarization and photon energy. We have distinguished both a
large (62-67~meV) and a reduced (15-18~meV) gap at the DP in the ARPES
dispersions, which remains open above the N\'eel temperature
(~K). We propose that the gap above remains
open due to short-range magnetic field generated by chiral spin fluctuations.
Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a
surface ferromagnetic ordering for large-gap sample and significantly reduced
effective magnetic moment for the reduced-gap sample. These effects can be
associated with a shift of the topological DC state towards the second Mn layer
due to structural defects and mechanical disturbance, where it is influenced by
a compensated effect of opposite magnetic moments
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