Context. Solar active region (AR) 12673 in 2017 September produced the two largest flares in Solar Cycle 24: the X9.3 flare on September 6 and the X8.2 flare on September 10. Aims. We attempt to ...investigate the evolutions of the two large flares and their associated complex magnetic system in detail. Methods. Combining observations from the Solar Dynamics Observatory and results of nonlinear force-free field (NLFFF) modeling, we identify various magnetic structures in the AR core region and examine the evolution of these structures during the flares. Results. Aided by the NLFFF modeling, we identify a double-decker flux rope configuration above the polarity inversion line (PIL) in the AR core region. The north ends of these two flux ropes were rooted in a negative- polarity magnetic patch, which began to move along the PIL and rotate anticlockwise before the X9.3 flare on September 6. The strong shearing motion and rotation contributed to the destabilization of the two magnetic flux ropes, of which the upper one subsequently erupted upward due to the kink-instability. Then another two sets of twisted loop bundles beside these ropes were disturbed and successively erupted within five minutes like a chain reaction. Similarly, multiple ejecta components were detected as consecutively erupting during the X8.2 flare occurring in the same AR on September 10. We examine the evolution of the AR magnetic fields from September 3 to 6 and find that five dipoles emerged successively at the east of the main sunspot. The interactions between these dipoles took place continuously, accompanied by magnetic flux cancellations and strong shearing motions. Conclusions. In AR 12673, significant flux emergence and successive interactions between the different emerging dipoles resulted in a complex magnetic system, accompanied by the formations of multiple flux ropes and twisted loop bundles. We propose that the eruptions of a multi-flux-rope system resulted in the two largest flares in Solar Cycle 24.
In superconductors, electrons are paired and condensed into the ground state. An impurity can break the electron pairs into quasiparticles with energy states inside the superconducting gap. The ...characteristics of such in-gap states reflect accordingly the properties of the superconducting ground state. A zero-energy in-gap state is particularly noteworthy, because it can be the consequence of non-trivial pairing symmetry or topology. Here we use scanning tunnelling microscopy/spectroscopy to demonstrate that an isotropic zero-energy bound state with a decay length of ∼10 Å emerges at each interstitial iron impurity in superconducting Fe(Te,Se). More noticeably, this zero-energy bound state is robust against a magnetic field up to 8 T, as well as perturbations by neighbouring impurities. Such a spectroscopic feature has no natural explanation in terms of impurity states in superconductors with s-wave symmetry, but bears all the characteristics of the Majorana bound state proposed for topological superconductors, indicating that the superconducting state and the scattering mechanism of the interstitial iron impurities in Fe(Te,Se) are highly unconventional.
The enhancement of the functional properties of materials at reduced dimensions is crucial for continuous advancements in nanoelectronic applications. Here, we report that the scale reduction leads ...to the emergence of an important functional property, ferroelectricity, challenging the long-standing notion that ferroelectricity is inevitably suppressed at the scale of a few nanometers. A combination of theoretical calculations, electrical measurements, and structural analyses provides evidence of room-temperature ferroelectricity in strain-free epitaxial nanometer-thick films of otherwise nonferroelectric strontium titanate (SrTiO3). We show that electrically induced alignment of naturally existing polar nanoregions is responsible for the appearance of a stable net ferroelectric polarization in these films. This finding can be useful for the development of low-dimensional material systems with enhanced functional properties relevant to emerging nanoelectronic devices.
This paper presents a comprehensive study on the influences of parasitic elements on the MOSFET switching performance. A circuit-level analytical model that takes MOSFET parasitic capacitances and ...inductances, circuit stray inductances, and reverse current of the freewheeling diode into consideration is given to evaluate the MOSFET switching characteristics. The equations derived for emulating MOSFET switching transients are assessed graphically, which, compared to results obtained merely from simulation or parametric study, can offer better insight into where the changes in switching performance lie when the parasitic elements are varied. The analysis has been successfully substantiated by the experimental results of a 400 V, 6 A test bench. A discussion on the physical meanings behind these parasitic effect phenomena is included. Knowledge about the effects of parasitic elements on the switching behavior serves as an important basis for the design guidelines of fast switching power converters.
The Korea Invisible Mass Search (KIMS) collaboration has developed low-background NaI(Tl) crystals that are suitable for the direct detection of WIMP dark matter. Building on experience accumulated ...during the KIMS-CsI programs, the KIMS-NaI experiment will consist of a 200 kg NaI(Tl) crystal array surrounded by layers of shielding structures and will be operated at the Yangyang underground laboratory. The goal is to provide an unambiguous test of the DAMA/LIBRA annual modulation signature. Measurements of six prototype crystals show progress in the reduction of internal contamination from radioisotopes. Based on our understanding of these measurements, we expect to achieve a background level in the final detector configuration that is less than 1 count/day/keV/kg for recoil energies around 2 keV. The annual modulation sensitivity for the KIMS-NaI experiment shows that an unambiguous 7
σ
test of the DAMA/LIBRA signature would be possible with a 600 kg year exposure with this system.
We present experimental studies on ion acceleration using an 800-nm circularly polarized laser pulse with a peak intensity of 6.9×10^{19} W/cm^{2} interacting with an overdense plasma that is ...produced by a laser prepulse ionizing an initially ultrathin plastic foil. The proton spectra exhibit spectral peaks at energies up to 9 MeV with energy spreads of 30% and fluxes as high as 3×10^{12} protons/MeV/sr. Two-dimensional particle-in-cell simulations reveal that collisionless shocks are efficiently launched by circularly polarized lasers in exploded plasmas, resulting in the acceleration of quasimonoenergetic proton beams. Furthermore, this scheme predicts the generation of quasimonoenergetic proton beams with peak energies of approximately 150 MeV using current laser technology, representing a significant step toward applications such as proton therapy.
Organic aerosol (OA) concentrations are simulated over the
Beijing–Tianjin–Hebei (BTH) region from 9 to 26 January 2014 using the
Weather Research and Forecasting model coupled with chemistry ...(WRF-CHEM),
with the goal of examining the impact of heterogeneous HONO sources on
SOA formation and SOA formation from different pathways during wintertime
haze days. The model generally shows good performance with respect to simulating air
pollutants and organic aerosols against measurements in BTH. Model results
show that heterogeneous HONO sources substantially enhance near-surface
SOA formation, increasing the regional average near-surface SOA concentration by
about 46.3 % during the episode. Oxidation and partitioning of primary
organic aerosols treated as semi-volatile dominate SOA formation,
contributing 58.9 % of the near-surface SOA mass in BTH. Irreversible
uptake of glyoxal and methylglyoxal on aerosol surfaces constitutes the
second most important SOA formation pathway during the episode, with the SOA
contribution increasing from 8.5 % under non-haze conditions to 30.2 %
under haze conditions. Additionally, direct emissions of glyoxal and
methylglyoxal from residential sources contribute about 25.5 % of
the total SOA mass on average in BTH. Our study highlights the importance of
heterogeneous HONO sources and primary residential emissions of glyoxal and
methylglyoxal to SOA formation over the BTH region in winter.
Multiferroics, where (anti-) ferromagnetic, ferroelectric and ferroelastic order parameters coexist, enable manipulation of magnetic ordering by an electric field through switching of the electric ...polarization. It has been shown that realization of magnetoelectric coupling in a single-phase multiferroic such as BiFeO3 requires ferroelastic (71 , 109 ) rather than ferroelectric (180 ) domain switching. However, the control of such ferroelastic switching in a single-phase system has been a significant challenge as elastic interactions tend to destabilize small switched volumes, resulting in subsequent ferroelastic back-switching at zero electric field, and thus the disappearance of non-volatile information storage. Guided by our phase-field simulations, here we report an approach to stabilize ferroelastic switching by eliminating the stress-induced instability responsible for back-switching using isolated monodomain BiFeO3 islands. This work demonstrates a critical step to control and use non-volatile magnetoelectric coupling at the nanoscale. Beyond magnetoelectric coupling, it provides a framework for exploring a route to control multiple order parameters coupled to ferroelastic order in other low-symmetry materials.
Organic aerosol (OA) represents a large fraction of submicron aerosols in the
megacity of Beijing, yet long-term characterization of its sources and
variations is very limited. Here we present an ...analysis of in situ
measurements of OA in submicrometer particles with an aerosol chemical
speciation monitor (ACSM) for 2 years from July 2011 to May 2013. The sources
of OA are analyzed with a multilinear engine (ME-2) by constraining three
primary OA factors including fossil-fuel-related OA (FFOA), cooking OA (COA),
and biomass burning OA (BBOA). Two secondary OAs (SOA), representing a less
oxidized oxygenated OA (LO-OOA) and a more oxidized (MO-OOA), are identified
during all seasons. The monthly average concentration OA varied from 13.6 to
46.7 µg m−3 with a strong seasonal pattern that is usually
highest in winter and lowest in summer. FFOA and BBOA show similarly
pronounced seasonal variations with much higher concentrations and
contributions in winter due to enhanced coal combustion and biomass burning
emissions. The contribution of COA to OA, however, is relatively stable
(10–15 %) across different seasons, yet presents significantly higher
values at low relative humidity levels (RH < 30 %),
highlighting the important role of COA during clean periods. The two SOA
factors present very different seasonal variations. The pronounced
enhancement of LO-OOA concentrations in winter indicates that emissions from
combustion-related primary emissions could be a considerable source of SOA
under low-temperature (T) conditions. Comparatively, MO-OOA shows high
concentrations consistently at high RH levels across different T levels,
and the contribution of MO-OOA to OA is different seasonally with lower
values occurring more in winter (30–34 %) than other seasons
(47–64 %). Overall, SOA (= LO-OOA + MO-OOA) dominates OA
composition during all seasons by contributing 52–64 % of the total OA
mass in the heating season and 65–75 % in non-heating seasons. The
variations in OA composition as a function of OA mass loading further
illustrate the dominant role of SOA in OA across different mass loading
scenarios during all seasons. However, we also observed a large increase in
FFOA associated with a corresponding decrease in MO-OOA during periods with
high OA mass loadings in the heating season, illustrating an enhanced role of
coal combustion emissions during highly polluted episodes. Potential source
contribution function analysis further shows that the transport from the
regions located to the south and southwest of Beijing within ∼ 250 km
can contribute substantially to high FFOA and BBOA concentrations in the
heating season.
PDF
