3,730 research outputs found
Synchronous solutions and their stability in nonlocally coupled phase oscillators with propagation delays
We study the existence and stability of synchronous solutions in a continuum
field of non-locally coupled identical phase oscillators with
distance-dependent propagation delays. We present a comprehensive stability
diagram in the parameter space of the system. From the numerical results a
heuristic synchronization condition is suggested, and an analytic relation for
the marginal stability curve is obtained. We also provide an expression in the
form of a scaling relation that closely follows the marginal stability curve
over the complete range of the non-locality parameter.Comment: accepted in Phys. Rev. E (2010
Dirac nodal line metal for topological antiferromagnetic spintronics
Topological antiferromagnetic (AFM) spintronics is an emerging field of
research, which exploits the N\'eel vector to control the topological
electronic states and the associated spin-dependent transport properties. A
recently discovered N\'eel spin-orbit torque has been proposed to electrically
manipulate Dirac band crossings in antiferromagnets; however, a reliable AFM
material to realize these properties in practice is missing. Here, we predict
that room temperature AFM metal MnPd allows the electrical control of the
Dirac nodal line by the N\'eel spin-orbit torque. Based on first-principles
density functional theory calculations, we show that reorientation of the
N\'eel vector leads to switching between the symmetry-protected degenerate
state and the gapped state associated with the dispersive Dirac nodal line at
the Fermi energy. The calculated spin Hall conductivity strongly depends on the
N\'eel vector orientation and can be used to experimentally detect the
predicted effect using a proposed spin-orbit torque device. Our results
indicate that AFM Dirac nodal line metal MnPd represents a promising
material for topological AFM spintronics
Spin-torque switching of noncollinear antiferromagnetic antiperovskites
Antiferromagnetic (AFM) spintronics exploits the Néel vector as a state variable for novel electronic devices. Recent studies have demonstrated that the Néel vector can be switched by a spin-orbit torque. These studies however are largely limited to collinear antiferromagnets of proper magnetic space-group symmetry. There is, however, a large group of high-temperature noncollinear antiferromagnets, which are suitable for such switching. Here, we predict that spin torque can be efficiently used to switch a noncollinear AFM order in antiperovskite materials. Based on first-principles calculations and atomistic spin-dynamics modeling, we show that in antiperovskites ANMn3 (A = Ga, Ni, etc.) with the AFM Γ4g ground state, the AFM order can be switched on the picosecond timescale using a spin torque generated by a spin current. The threshold switching current density can be tuned by the ANMn3 stoichiometry engineering, changing the magnetocrystalline anisotropy. The Γ4g AFM phase supports a sizable anomalous Hall effect, which can be used to detect the spin-torque switching of the AFM order. The predicted ultrafast switching dynamics and the efficient detection of the AFM order state make noncollinear magnetic antiperovskites a promising material platform for AFM spintronics
Neuroimaging in paediatric patients with developmental delay
Background: Aim and objectives of the study were to radiologically evaluate paediatric patients with developmental delay (DD), assess the relative prevalence of abnormal brain MRI, further categorize them based on the abnormal imaging findings and structures affected. The purpose of this study is to diagnose the underlying etiology that helps in early treatment and amelioration of the condition, parental counselling regarding the outcome of the child, providing an estimate of child’s developmental potential and the recurrence risk in siblings.Methods: 135 paediatric patients of the age 3 months to 15 years with DD referred to department of radiology were investigated with MRI scans of the brain via 1.5T Siemens scanner after making the child sleep or sedated. The sequences used were: axial T1, axial T2, axial FLAIR, axial DWI, axial ADC, axial SWI, axial PHASE, sagittal T1 and coronal FLAIR. CT scan of the brain was done only when indicated on 128 slice Siemens Somatom perspective scanner. Informed consent shall be taken from patient’s parents. Clinical and demographic details of the enrolled patients were noted in the Performa. Data collected was analysed using descriptive and inferential statistics.Results: Out of 135 children with DD, 69.1% (n=92) were male and 31.9% (n=43) were female. Majority of these children belonged to 3 months to 1 year and 2 to 5 years of age group. About 81.4% (n=110) of children with DD had abnormal findings in MRI. Among children with abnormal MRI findings, 42.9% had hypoxic ischemic changes, 6.6% had congenital malformations and non-specific causes, respectively 4.4% had neurodegenerative and occlusive neurovascular conditions, respectively 3.7% had CSF disorders and neoplasms, respectively 2.9% had infection associated changes and non-traumatic intracranial bleed, respectively 2.2% had metabolic disorders and 0.7% had demyelination. Majority of cases had ventricular abnormality, followed by the corpus callosum.Conclusions: DD presents with a wide spectrum of etiologies, clinical findings and MRI features ranging from completely normal to abnormal. The present study could establish the various morphological appearances of DD on MRI and further categorize them into various subgroups be effective in diagnosis, management and prognosis determination processes
Structural studies of 1:1 quinone-hydroquinone complexes
The structures of the 1:1 quinone-hydroquinone complexes of 2-phenyl and of 2-(4'-chloro) phenylbenzoquinone have been studied by X-ray methods. A superficial study would indicate that the quinhydrones are centrosymmetric and belong to the space group P21/c. However, other evidence indicates that the true crystal structure may belong to either the space group P21 or to Pc, or that the crystal may contain regions that would coresspond to each of these two non-centrosymmetric space groups. Some possible consequences of such a structural arrangement are briefly discussed
Mass independence and asymmetry of the reaction: Multi-fragmentation as an example
We present our recent results on the fragmentation by varying the mass
asymmetry of the reaction between 0.2 and 0.7 at an incident energy of 250
MeV/nucleon. For the present study, the total mass of the system is kept
constant (ATOT = 152) and mass asymmetry of the reaction is defined by the
asymmetry parameter (? = | (AT - AP)/(AT + AP) |). The measured distributions
are shown as a function of the total charge of all projectile fragments,
Zbound. We see an interesting outcome for rise and fall in the production of
intermediate mass fragments (IMFs) for large asymmetric colliding nuclei. This
trend, however, is completely missing for large asymmetric nuclei. Therefore,
experiments are needed to verify this prediction
Multidetector computed tomographic urography for evaluation of vascular and ureteric anomalies associated with ectopic kidneys
Background: The purpose of this study is to evaluate the vascular and ureteric anomalies associated with ectopic kidneys with multidetector computed tomography (MDCT). Methods: The 40 patients with pre-diagnosed ectopic kidney undergoing MDCT urography (Contrast study) and KUB (Plain study) were included in this cross-sectional observational study. The location and number of bilateral kidneys were assessed. The number and origin of the renal arteries and renal veins were noted. Their relationship with each other and possible complications in surgical handling analysed. Data collected was analysed using descriptive and inferential statistics.Results: The renal artery originated from suprarenal aorta in 2 cases, normal origin in 10 cases, infrarenal aorta in 12 cases, aortic bifurcation in 19 cases, common iliac artery in 6 cases and iliac artery bifurcation in 2 cases. The renal vein was of normal origin in 8 cases, originated from infrarenal inferior vena cava (IVC) in 16 cases, IVC bifurcation in 14 cases, common iliac vein in 9 cases, internal iliac vein in 2 cases and external iliac vein in 1 case. There was a significant correlation between the level of ectopic kidneys (abdominal, iliac and pelvis) and level of origin of arteries (p<0.001) and veins (p<0.001). In addition, significant correlation was found between the origins of arteries and veins of ectopic kidneys (p<0.001).Conclusions: A knowledge of the possible variations in renal vasculature and ureter associated with ectopic kidneys can play a key role in preventing iatrogenic hemorrhage during surgery
Indiscriminate Data Poisoning Attacks on Pre-trained Feature Extractors
Machine learning models have achieved great success in supervised learning
tasks for end-to-end training, which requires a large amount of labeled data
that is not always feasible. Recently, many practitioners have shifted to
self-supervised learning methods that utilize cheap unlabeled data to learn a
general feature extractor via pre-training, which can be further applied to
personalized downstream tasks by simply training an additional linear layer
with limited labeled data. However, such a process may also raise concerns
regarding data poisoning attacks. For instance, indiscriminate data poisoning
attacks, which aim to decrease model utility by injecting a small number of
poisoned data into the training set, pose a security risk to machine learning
models, but have only been studied for end-to-end supervised learning. In this
paper, we extend the exploration of the threat of indiscriminate attacks on
downstream tasks that apply pre-trained feature extractors. Specifically, we
propose two types of attacks: (1) the input space attacks, where we modify
existing attacks to directly craft poisoned data in the input space. However,
due to the difficulty of optimization under constraints, we further propose (2)
the feature targeted attacks, where we mitigate the challenge with three
stages, firstly acquiring target parameters for the linear head; secondly
finding poisoned features by treating the learned feature representations as a
dataset; and thirdly inverting the poisoned features back to the input space.
Our experiments examine such attacks in popular downstream tasks of fine-tuning
on the same dataset and transfer learning that considers domain adaptation.
Empirical results reveal that transfer learning is more vulnerable to our
attacks. Additionally, input space attacks are a strong threat if no
countermeasures are posed, but are otherwise weaker than feature targeted
attacks.Comment: Accepted to SaTML 202
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