Photoexcitation in solids brings about transitions of electrons/holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous ...photocurrent due to the geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells.
Rubble-Pile Asteroid Itokawa as Observed by Hayabusa Fujiwara, A; Kawaguchi, J; Yeomans, D.K ...
Science (American Association for the Advancement of Science),
06/2006, Volume:
312, Issue:
5778
Journal Article
Peer reviewed
During the interval from September through early December 2005, the Hayabusa spacecraft was in close proximity to near-Earth asteroid 25143 Itokawa, and a variety of data were taken on its shape, ...mass, and surface topography as well as its mineralogic and elemental abundances. The asteroid's orthogonal axes are 535, 294, and 209 meters, the mass is 3.51 x 10¹⁰ kilograms, and the estimated bulk density is 1.9 ± 0.13 grams per cubic centimeter. The correspondence between the smooth areas on the surface (Muses Sea and Sagamihara) and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting. Itokawa is considered to be a rubble-pile body because of its low bulk density, high porosity, boulder-rich appearance, and shape. The existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re-agglomeration into a rubble-pile object.
Background. Increasing attention has been paid to simulation and gaming (S&G) as an active learning technique within Japanese universities; however, the more that active learning sessions are ...conducted, the fewer rooms there are available for these activities. This shortage sometimes results in having to conduct S&G in a traditional lecture room; as such, this study explores the influence of room condition on participants.
Intervention. During each gaming session, the author played the role of facilitator and made herself as invisible as possible. In the debriefing sessions, the author delivered a lecture and organized the debriefing in the role of a teacher.
Aims and Methods. The two research aims were as follows: first, to understand the influence of room conditions on participants; and second, to understand the influence of content and question order in debriefing forms on participants. Data collected from debriefing forms regarding two gaming sessions were analyzed.
Results. The degree of satisfaction in HACONORI and the degree of usefulness in BLOCK WORK were influenced by room condition. This study found that debriefing forms influenced participants less than room condition.
Discussion. These study results reflect the powerful effect of room condition, which varies due to individual game attributes. The effect of debriefing forms was found to be less potent than the effect of room condition, although the order of questions should be taken into consideration.
Suggestions and Limitations. A large room with movable desks and movable chairs is recommended for S&G.
Conclusion. The factor of room condition had a powerful effect on the degree of satisfaction for HACONORI and the degree of usefulness for BLOCK WORK. The factor of debriefing forms was less potent than the factor of room condition; as such, S&G facilitators should be sensitive to room conditions.
Objective
To describe clinical characteristics of pregnant and postpartum women with severe COVID‐19 in Brazil and to examine risk factors for mortality.
Design
Cross‐sectional study based on ...secondary surveillance database analysis.
Setting
Nationwide Brazil.
Population or sample
978 Brazilian pregnant and postpartum women notified as COVID‐19 Acute Respiratory Distress Syndrome (ARDS) cases with complete outcome (death or cure) up to 18 June 2020.
Methods
Data was ed from the Brazilian ARDS Surveillance System (ARDS‐SS) database. All eligible cases were included. Data on demographics, clinical characteristics, intensive care resources use and outcomes were collected. Risk factors for mortality were examined by multivariate logistic regression.
Main outcome measures
Case fatality rate.
Results
We identified 124 maternal deaths, corresponding to a case fatality rate among COVID‐19 ARDS cases in the obstetric population of 12.7%. At least one comorbidity was present in 48.4% of fatal cases compared with 24.9% in survival cases. Among women who died, 58.9% were admitted to ICU, 53.2% had invasive ventilation and 29.0% had no respiratory support. The multivariate logistic regression showed that the main risk factors for maternal death by COVID‐19 were being postpartum at onset of ARDS, obesity, diabetes and cardiovascular disease, whereas white ethnicity had a protective effect.
Conclusions
Negative outcomes of COVID‐19 in this population are affected by clinical characteristics but social determinants of health also seem to play a role. It is urgent to reinforce containment measures targeting the obstetric population and ensure high quality care throughout pregnancy and the postpartum period.
Tweetable
A total of 124 COVID‐19 maternal deaths were identified in Brazil. Symptoms onset at postpartum and comorbidities are risk factors.
Tweetable
A total of 124 COVID‐19 maternal deaths were identified in Brazil. Symptoms onset at postpartum and comorbidities are risk factors.
Shift current is a steady-state photocurrent generated in non-centrosymmetric single crystals and has been considered to be one of the major origins of the bulk photovoltaic effect. The mechanism of ...this effect is the transfer of photogenerated charges by the shift of the wave functions, and its amplitude is closely related to the polarization of the electronic origin. Here, we report the photovoltaic effect in an organic molecular crystal tetrathiafulvalene-p-chloranil with a large ferroelectric polarization mostly induced by the intermolecular charge transfer. We observe a fairly large zero-bias photocurrent with visible-light irradiation and switching of the current direction by the reversal of the polarization. Furthermore, we reveal that the travel distance of photocarriers exceeds 200 μm. These results unveil distinct features of the shift current and the potential application of ferroelectric organic molecular compounds for novel optoelectric devices.The bulk photovoltaics refers to an effect whereby electrons move directionally in non-centrosymmetric crystals upon light radiation. Here, Nakamura et al. observe this effect in a ferroelectric organic charge-transfer complex, which shows large diffusion distance of photogenerated electrons over 200 µm.
In the Earth's magnetotail, magnetic reconnection releases stored magnetic energy and drives magnetospheric convection. The rate at which magnetic flux is transferred from the reconnection inflow to ...outflow regions is determined by the reconnection electric field Er, which is often referred to as the unnormalized reconnection rate. To better quantify the efficiency of reconnection, this electric field Er is often normalized by the characteristic Alfvén speed and the reconnecting magnetic field. This parameter is generally called the normalized or dimensionless reconnection rate R. In this paper, we employ a two‐dimensional fully kinetic simulation to model a magnetotail reconnection event with weak geomagnetic activity (<200 nT of the AE index) observed by the Magnetospheric Multiscale (MMS) mission on 11 July 2017. We obtain R and Er from direct measurements in the diffusion region and indirect measurements of the rate at the separatrix using a recently proposed remote sensing technique. The measured normalized rate for this MMS event is R ∼0.15–0.2, consistent with theoretical and simulation models of fast collisionless reconnection. This corresponds to an unnormalized rate of Er ∼2–3 mV/m. Based on quantitative consistencies between the simulation and the MMS observations, we conclude that our estimates of the reconnection rates are reasonably accurate. Given that past studies have found Er of the order ∼10 mV/m during strong geomagnetic substorms, these results indicate that the local Er in magnetotail reconnection may be an important parameter controlling the amplitude of geomagnetic disturbances.
Key Points
Reliable reconnection rates are obtained based on virtual observations in a fully kinetic simulation of an MMS tail reconnection event
The normalized rates obtained from the simulation and MMS data are 0.15–0.2, indicating the occurrence of fast reconnection
The observed unnormalized rate is 2–3 mV/m, while higher rates were observed in other events with stronger geomagnetic activities
Magnetic reconnection is believed to be the main driver to transport solar wind into the Earth's magnetosphere when the magnetopause features a large magnetic shear. However, even when the magnetic ...shear is too small for spontaneous reconnection, the Kelvin-Helmholtz instability driven by a super-Alfvénic velocity shear is expected to facilitate the transport. Although previous kinetic simulations have demonstrated that the non-linear vortex flows from the Kelvin-Helmholtz instability gives rise to vortex-induced reconnection and resulting plasma transport, the system sizes of these simulations were too small to allow the reconnection to evolve much beyond the electron scale as recently observed by the Magnetospheric Multiscale (MMS) spacecraft. Here, based on a large-scale kinetic simulation and its comparison with MMS observations, we show for the first time that ion-scale jets from vortex-induced reconnection rapidly decay through self-generated turbulence, leading to a mass transfer rate nearly one order higher than previous expectations for the Kelvin-Helmholtz instability.
Abstract
Coalescence of multiple magnetic islands is recognized as an effective process to energize particles during magnetic reconnection, while its energy conversion process still remains unclear. ...Here, a two-dimensional fully kinetic simulation of multiple island coalescence with a small reconnection guide field is studied. In the analysis of energy conversion within a magnetic island, the dot product of
V
e
·
j
×
B
=
w
1
is a useful quantity to compare with
j
·
E
=
w
2
, since the average work done by the Lorentz force on the circulating particles is negligible in the island and
w
2
−
w
1
=
j
·
E
+
V
e
×
B
=
j
·
E
′
=
w
3
. A bipolar pattern of
w
1
is found at a secondary island when the electrons are in circular motion inside the island. Significant energy dynamo (
w
3
< 0) resulting from
j
∥
E
∥
is found at the secondary island, which has not been reported before, where the parallel electric field
E
∥
is highly correlated with
w
3
. Moreover, significant energy dissipation (
w
3
> 0) due to
j
⊥
·
E
⊥
′
is seen in the merging region between two coalescing islands. Both types of energy conversions are accompanied by enhancements in
j
∥
and the parallel electron temperature
T
e
∥
. Three ion-scale magnetic islands (FR1, FR2, and FR3) observed by the Magnetospheric Multiscale spacecraft are compared favorably with the simulated signatures of energy dynamo and dissipation of an evolving secondary island. In particular, FR1 displayed a similar energy dynamo signature as that simulated in an early stage of the secondary island. FR2 and FR3 showed a dominant
j
⊥
·
E
⊥
′
energy conversion similar to that obtained in a later stage of the secondary island.
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