732 research outputs found
Carrier transport properties of the Group-IV ferromagnetic semiconductor Ge1-xFex with and without boron doping
We have investigated the transport and magnetic properties of group-IV
ferromagnetic semiconductor Ge1-xFex films (x = 1.0 and 2.3 %) with and without
boron doping grown by molecular beam epitaxy (MBE). In order to accurately
measure the transport properties of 100-nm-thick Ge1-xFex films, (001)-oriented
silicon-on-insulator (SOI) wafers with an ultra-thin Si body layer (~5 nm) were
used as substrates. Owing to the low Fe content, the hole concentration and
mobility in the Ge1-xFex films were exactly estimated by Hall measurements
because the anomalous Hall effect in these films was found to be negligibly
small. By boron doping, we increased the hole concentration in Ge1-xFex from
~1018 cm-3 to ~1020 cm-3 (x = 1.0%) and to ~1019 cm-3 (x = 2.3%), but no
correlation was observed between the hole concentration and magnetic
properties. This result presents a contrast to the hole-induced ferromagnetism
in III-V ferromagnetic semiconductors
VisualMRC: Machine Reading Comprehension on Document Images
Recent studies on machine reading comprehension have focused on text-level
understanding but have not yet reached the level of human understanding of the
visual layout and content of real-world documents. In this study, we introduce
a new visual machine reading comprehension dataset, named VisualMRC, wherein
given a question and a document image, a machine reads and comprehends texts in
the image to answer the question in natural language. Compared with existing
visual question answering (VQA) datasets that contain texts in images,
VisualMRC focuses more on developing natural language understanding and
generation abilities. It contains 30,000+ pairs of a question and an
abstractive answer for 10,000+ document images sourced from multiple domains of
webpages. We also introduce a new model that extends existing
sequence-to-sequence models, pre-trained with large-scale text corpora, to take
into account the visual layout and content of documents. Experiments with
VisualMRC show that this model outperformed the base sequence-to-sequence
models and a state-of-the-art VQA model. However, its performance is still
below that of humans on most automatic evaluation metrics. The dataset will
facilitate research aimed at connecting vision and language understanding.Comment: Accepted as a full paper at AAAI 2021. The first two authors have
equal contributio
Excitation polarization-independent photo-induced restoration of inversion symmetry in Td-WTe₂
Td-WTe₂ is a topologically nontrivial material and exhibits a variety of physical properties, such as giant unsaturated magnetoresistance and the unconventional thermoelectric effect, due to its topological nature. It is also known to exhibit ultrafast topological phase transitions that restore its inversion symmetry by intense terahertz and mid-infrared pulses, and these properties demonstrate the possibility of ultrafast control of devices based on topological properties. Recently, a novel photo-induced topological phase transition by using polarization-controlled infrared excitation has been proposed, which is expected to control the material topology by rearranging the atomic orbitals near the Weyl point. To examine this topological phase transition, we experimentally studied the excitation-polarization dependence of the infrared-induced phase dynamics in a thin-layer of Td-WTe₂. Time-resolved second harmonic generation (SHG) measurements showed that SHG intensity decreases after the infrared pump regardless of the polarization. Polarization-resolved infrared pump–probe measurements indicated that the polarization-selected excited state relaxes quite rapidly (i.e., within 10–40 fs). Considering these experimental results, we conclude that it is difficult to control the photo-induced phase transition through orbital-selective excitation owing to the rapid loss of carrier distribution created by polarization-selective excitation in thin-layer Td-WTe₂ under our experimental condition. These results indicate that the suppression of the electron scattering process is crucial for experimentally realizing the photo-induced phase transition based on the polarization selection rule of the materials
Head-Trajectory-Tracking Control of a Snake Robot and Its Robustness Under Actuator Failure
This brief considers the problem of trajectory tracking of a planar snake robot without a lateral constraint. The reference trajectory of the head position and the orientation of link 1 are given, and torque control is determined to reduce tracking errors. The performance of the controller was tested in a number of simulations. The robustness during actuator failure was also studied. We assumed that one of the actuators was broken and the corresponding joint became passive. Furthermore, as a more realistic situation, we considered an instance when some of the states were not readily accessible from the sensor readings and needed to be estimated by an observer. The extended Kalman filter was employed for this purpose, and the performance of the closed-loop system with the observer was also tested in simulations
Massive Protostellar Disks as a Hot Laboratory of Silicate Grain Evolution
Typical accretion disks around massive protostars are hot enough for water
ice to sublimate. We here propose to utilize the massive protostellar disks for
investigating the collisional evolution of silicate grains with no ice mantle,
which is an essential process for the formation of rocky planetesimals in
protoplanetary disks around lower-mass stars. We for the first time develop a
model of massive protostellar disks that includes the coagulation,
fragmentation, and radial drift of dust. We show that the maximum grain size in
the disks is limited by collisional fragmentation rather than by radial drift.
We derive analytic formulas that produce the radial distribution of the maximum
grain size and dust surface density in the steady state. Applying the analytic
formulas to the massive protostellar disk of GGD27-MM1, where the grain size is
constrained from a millimeter polarimetric observation, we infer that the
silicate grains in this disk fragment at collision velocities above ~ 10 m/s.
The inferred fragmentation threshold velocity is lower than the maximum grain
collision velocity in typical protoplanetary disks around low-mass stars,
implying that coagulation alone may not lead to the formation of rocky
planetesimals in those disks. With future measurements of grain sizes in
massive protostellar disks, our model will provide more robust constraints on
the sticking property of silicate grains.Comment: 17 pages, 5 figures,accepted for publication to The Astrophysical
Journa
Chromosomal integration of blaCTX-M genes in diverse Escherichia coli isolates recovered from river water in Japan
Occurrence of extended-spectrum β-lactamase (ESBL)-producing Escherichia coli (ESBLEC) in environmental waters is of great concern. However, unlike clinical ESBLEC, their genetic characteristics, in particular the genetic contexts of ESBL genes, are not well understood. In this study, we sequenced and analyzed the genomes of CTX-M-producing E. coli isolates recovered from river water to fully characterize the genetic contexts of blaCTX-M genes. Among the 14 isolates with completed genomes, blaCTX-M genes were detected on the chromosome in nine isolates. All but one chromosomal blaCTX-M genes were associated with ISEcp1 and were carried on different transposition units ranging in size from 2, 855 bp to 11, 093 bp; the exception, blaCTX-M-2, was associated with ISCR1. The remaining five isolates carried blaCTX-M genes on epidemic IncI1 plasmids of different sequence types (STs) (ST3, ST16, ST113, and ST167) (n = 4) or on an IncB/O/K/Z plasmid (n = 1). This study revealed that environmental E. coli carry blaCTX-M genes in diverse genetic contexts. Apparent high prevalence of chromosomal blaCTX-M potentially indicates that some E. coli can stably maintain blaCTX-M genes in environmental waters, though further studies are needed to confirm this
Attention as Annotation: Generating Images and Pseudo-masks for Weakly Supervised Semantic Segmentation with Diffusion
Although recent advancements in diffusion models enabled high-fidelity and
diverse image generation, training of discriminative models largely depends on
collections of massive real images and their manual annotation. Here, we
present a training method for semantic segmentation that neither relies on real
images nor manual annotation. The proposed method {\it attn2mask} utilizes
images generated by a text-to-image diffusion model in combination with its
internal text-to-image cross-attention as supervisory pseudo-masks. Since the
text-to-image generator is trained with image-caption pairs but without
pixel-wise labels, attn2mask can be regarded as a weakly supervised
segmentation method overall. Experiments show that attn2mask achieves promising
results in PASCAL VOC for not using real training data for segmentation at all,
and it is also useful to scale up segmentation to a more-class scenario, i.e.,
ImageNet segmentation. It also shows adaptation ability with LoRA-based
fine-tuning, which enables the transfer to a distant domain i.e., Cityscapes
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