1,932 research outputs found
Surface segregation and the Al problem in GaAs quantum wells
Low-defect two-dimensional electron systems (2DESs) are essential for studies
of fragile many-body interactions that only emerge in nearly-ideal systems. As
a result, numerous efforts have been made to improve the quality of
modulation-doped AlGaAs/GaAs quantum wells (QWs), with an emphasis
on purifying the source material of the QW itself or achieving better vacuum in
the deposition chamber. However, this approach overlooks another crucial
component that comprises such QWs, the AlGaAs barrier. Here we show
that having a clean Al source and hence a clean barrier is instrumental to
obtain a high-quality GaAs 2DES in a QW. We observe that the mobility of the
2DES in GaAs QWs declines as the thickness or Al content of the
AlGaAs barrier beneath the QW is increased, which we attribute to
the surface segregation of Oxygen atoms that originate from the Al source. This
conjecture is supported by the improved mobility in the GaAs QWs as the Al cell
is cleaned out by baking
Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films
We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial
thin films, resulting in a giant flexoelectric effect. Using grazing-incidence
in-plane X-ray diffraction, we measured strain gradients in the films, which
were 6 or 7 orders of magnitude larger than typical values reported for bulk
oxides. The combination of transmission electron microscopy, electrical
measurements, and electrostatic calculations showed that flexoelectricity
provides a means of tuning the physical properties of ferroelectric epitaxial
thin films, such as domain configurations and hysteresis curves.Comment: Accepted by Phys. Rev. Let
Intra- and inter-hemispheric effective connectivity in the human somatosensory cortex during pressure stimulation
Background: Slow-adapting type I (SA-I) afferents deliver sensory signals to the somatosensory cortex during low-frequency (or static) mechanical stimulation. It has been reported that the somatosensory projection from SA-I afferents is effective and reliable for object grasping and manipulation. Despite a large number of neuroimaging studies on cortical activation responding to tactile stimuli mediated by SA-I afferents, how sensory information of such tactile stimuli flows over the somatosensory cortex remains poorly understood. In this study, we investigated tactile information processing of pressure stimuli between the primary (SI) and secondary (SII) somatosensory cortices by measuring effective connectivity using dynamic causal modeling (DCM). We applied pressure stimuli for 3 s to the right index fingertip of healthy participants and acquired functional magnetic resonance imaging (fMRI) data using a 3T MRI system. Results: DCM analysis revealed intra-hemispheric effective connectivity between the contralateral SI (cSI) and SII (cSII) characterized by both parallel (signal inputs to both cSI and cSII) and serial (signal transmission from cSI to cSII) pathways during pressure stimulation. DCM analysis also revealed inter-hemispheric effective connectivity among cSI, cSII, and the ipsilateral SII (iSII) characterized by serial (from cSI to cSII) and SII-level (from cSII to iSII) pathways during pressure stimulation. Conclusions: Our results support a hierarchical somatosensory network that underlies processing of low-frequency tactile information. The network consists of parallel inputs to both cSI and cSII (intra-hemispheric), followed by serial pathways from cSI to cSII (intra-hemispheric) and from cSII to iSII (inter-hemispheric). Importantly, our results suggest that both serial and parallel processing take place in tactile information processing of static mechanical stimuli as well as highlighting the contribution of callosal transfer to bilateral neuronal interactions in SII.open1
Dynamic Response of Wigner Crystals
The Wigner crystal, an ordered array of electrons, is one of the very first
proposed many-body phases stabilized by the electron-electron interaction. This
electron solid phase has been reported in ultra-clean two-dimensional electron
systems at extremely low temperatures, where the Coulomb interaction dominants
over the kinetic energy, disorder potential and thermal fluctuation. We closely
examine this quantum phase with capacitance measurements where the device
length-scale is comparable with the crystal's correlation length. The
extraordinarily high performance of our technique makes it possible to
quantitatively study the dynamic response of the Wigner crystal within the
single crystal regime. Our result will greatly boost the study of this
inscrutable electron solid
Moxifloxacin: Clinically compatible contrast agent for multiphoton imaging
Multiphoton microscopy (MPM) is a nonlinear fluorescence microscopic technique widely used for cellular imaging of thick tissues and live animals in biological studies. However, MPM application to human tissues is limited by weak endogenous fluorescence in tissue and cytotoxicity of exogenous probes. Herein, we describe the applications of moxifloxacin, an FDA-approved antibiotic, as a cell-labeling agent for MPM. Moxifloxacin has bright intrinsic multiphoton fluorescence, good tissue penetration and high intracellular concentration. MPM with moxifloxacin was demonstrated in various cell lines, and animal tissues of cornea, skin, small intestine and bladder. Clinical application is promising since imaging based on moxifloxacin labeling could be 10 times faster than imaging based on endogenous fluorescence.1152sciescopu
Solving Continual Combinatorial Selection via Deep Reinforcement Learning
We consider the Markov Decision Process (MDP) of selecting a subset of items
at each step, termed the Select-MDP (S-MDP). The large state and action spaces
of S-MDPs make them intractable to solve with typical reinforcement learning
(RL) algorithms especially when the number of items is huge. In this paper, we
present a deep RL algorithm to solve this issue by adopting the following key
ideas. First, we convert the original S-MDP into an Iterative Select-MDP
(IS-MDP), which is equivalent to the S-MDP in terms of optimal actions. IS-MDP
decomposes a joint action of selecting K items simultaneously into K iterative
selections resulting in the decrease of actions at the expense of an
exponential increase of states. Second, we overcome this state space explo-sion
by exploiting a special symmetry in IS-MDPs with novel weight shared
Q-networks, which prov-ably maintain sufficient expressive power. Various
experiments demonstrate that our approach works well even when the item space
is large and that it scales to environments with item spaces different from
those used in training.Comment: Accepted to IJCAI 2019,14 pages,8 figure
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