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Juan Li

Publications and source records attributed to Juan Li.

At least 55 records · Page 3Linked to original sources
arXiv2025

$^{14}$N$/^{15}$N abundance ratio toward massive star-forming regions with different Galactic distances

The abundance ratio of $^{14}$N$/^{15}$N is, in principle, a powerful tool for tracing stellar nucleosynthesis. This work aims to measure and analyze ($^{14}$N/$^{15}$N)$\times$($^{13}$C/$^{12}$C) and $^{14}$N$/^{15}$N abundance ratios in massive star-forming regions across a range of galactocentric distances to provide constraints on galactic chemical evolution (GCE) models. We present H$^{13}$CN and HC$^{15}$N J=2-1 results toward 51massive star-forming regions obtained with the Institut de Radioastronomie Millimétrique (IRAM) 30 meter telescope.\ We used these results to derive ($^{14}$N/$^{15}$N)$\times$($^{13}$C/$^{12}$C) abundance ratios as well as $^{14}$N$/^{15}$N ratios using the double isotope method. We find an overall decreasing trend in the ($^{14}$N/$^{15}$N)$\times$($^{13}$C/$^{12}$C) abundance ratio and an increasing trend in the $^{14}$N$/^{15}$N ratio with increasing galactocentric distance ($D_{\rm GC}$), which provides a good constraint for the GCE model based on high signal to noise ratio measurements. While the predicted ($^{14}$N/$^{15}$N)$\times$($^{13}$C/$^{12}$C) ratios between 6 and 12 kpc determined using current GCE models are consistent with our observational results, the ratios from models for $D_{\rm GC}$ less than 6 kpc are significantly higher than the observational results, which indicates GCE models for $^{14}$N/$^{15}$N and/or $^{13}$C/$^{12}$C ratios need to be updated for at least this range.

astro-ph.GA↗
arXiv2024

3 mm Spectroscopic Observations of Massive Star-Forming Regions with IRAM 30-m

Broadband spectroscopic observations with high sensitivity provide an unbiased way to detect emissions of molecules in space. We present deep observations from ~ 105.8 GHz to 113.6 GHz toward 50 Galactic massive star-forming regions using IRAM 30-m millimeter telescope, with noise levels ranging from 6 to 29 at frequency channel spacing of 195 kHz, which corresponds to ~ 0.54 km/s at 110 GHz. Totally, 27 molecular species have been identified, of which 16 are complex organic molecules. The related parameters, such as peak temperature, integrated intensity, and line width of the identified molecular lines were obtained. The line widths of the chemically related molecules show strong positive correlations, suggesting they likely originate from similar gases within star-forming regions. This work highlights the fundamental properties of the detected molecular lines and offers a valuable dataset for further studies on the astrochemical evolution of molecules in massive star-forming cores.

astro-ph.SR↗
arXiv2024

The deuterium fractionation of NH$_3$ in massive star-forming regions

Deuteration is sensitive to environmental conditions in star-forming regions. To investigate NH$_2$D chemistry, we compared the spatial distribution of ortho-NH$_2$D $1_{11}^s-1_{01}^a$, NH$_3$(1,1) and NH$_3$(2,2) in 12 late-stage massive star-forming regions. By averaging several pixels along the spatial slices of ortho-NH$_2$D $1_{11}^s-1_{01}^a$, we obtained the deuterium fractionation of NH$_3$. In seven targets, the deuterium fractionation of NH$_3$ shows a decreasing trend with increasing rotational temperature. This trend is less clear in the remaining five sources, likely due to limited spatial resolution. However, when considering all 12 sources together, the anticorrelation between NH$_3$ deuterium fractionation and rotational temperature becomes less significant, suggesting that other physical parameters may influence the fractionation. Additionally, we found that the region of highest deuterium fractionation of NH$_3$ is offset from the NH$_3$ peak in each source, likely because the temperature is higher near the NH$_3$ peaks and NH$_2$D may be depleted from the gas phase as the molecular cloud core evolves, as well as the increased release of CO from grains into the gas phase.

astro-ph.GA↗
arXiv2024

A Stochastic Porous Media Schr{ö}dinger Equation: Feynman-type Motivation, Well-Posedness and Control Interpretation

This paper's aim is threefold. First, using Feynman's path approach to the derivation of theclassical Schr{ö}dinger's equation in [6] and by introducing a slight path (or wave) dependency ofthe action, we derive a new class of equations of Schr{ö}dinger type where the driving operatoris no longer the Laplace one but rather of complex porous media-type. Second, using suitableconcepts of monotonicity in the complex setting and on appropriate functional spaces, we showthe existence and uniqueness of the solution to this type of equation. In the formulation of ourequation, we adjoin possible measurement absolute errors translating in an additive Brownianperturbation and interactions between different waves translating in a mean-field (or McKean-Vlasov) dependency of drift coefficient. Finally, using Fitzpatrick's characterization of maximalmonotone operators (cf. [7]), we propose a Br{é}zis-Ekeland type characterization of the solutionof the deterministic equation via a control problem. This is envisaged as a possible way toovercome strict monotonicity requirements in the complex setting.

math.AP↗
arXiv2024

In-architecture X-ray assisted C-Br dissociation for on-surface fabrication of diamondoid chains

The fabrication of well-defined, low-dimensional diamondoid-based materials is a promising approach for tailoring diamond properties such as superconductivity. On-surface self-assembly of halogenated diamondoids under ultrahigh vacuum conditions represents an effective strategy in this direction, enabling reactivity exploration and on-surface synthesis approaches. Here we demonstrate through scanning probe microscopy, time-of-flight mass spectrometry and photoelectron spectroscopy, that self-assembled layers of dibromodiamantanes on gold can be debrominated at atomic wavelengths (Al K$α$ at 8.87 $\mathring{A}$ and Mg K$α$ at 9.89 $\mathring{A}$) and low temperatures without affecting their well-defined arrangement. The resulting 'in-architecture' debromination enables the fabrication of diamantane chains from self-assembled precursors in close proximity, which is otherwise inaccessible through annealing on metal surfaces. Our work introduces a novel approach for the fabrication of nanodiamond chains, with significant implications for in-architecture and layer-by-layer synthesis.

cond-mat.mtrl-sci↗
arXiv2024

Rapid Automatic Multiple Moving Objects Detection Method Based on Feature Extraction from Images with Non-sidereal Tracking

Optically observing and monitoring moving objects, both natural and artificial, is important to human space security. Non-sidereal tracking can improve the system's limiting magnitude for moving objects, which benefits the surveillance. However, images with non-sidereal tracking include complex background, as well as objects with different brightness and moving mode, posing a significant challenge for accurate multi-object detection in such images, especially in wide field of view (WFOV) telescope images. To achieve a higher detection precision in a higher speed, we proposed a novel object detection method, which combines the source feature extraction and the neural network. First, our method extracts object features from optical images such as centroid, shape, and flux. Then it conducts a naive labeling based on those features to distinguish moving objects from stars. After balancing the labeled data, we employ it to train a neural network aimed at creating a classification model for point-like and streak-like objects. Ultimately, based on the neural network model's classification outcomes, moving objects whose motion modes consistent with the tracked objects are detected via track association, while objects with different motion modes are detected using morphological statistics. The validation, based on the space objects images captured in target tracking mode with the 1-meter telescope at Nanshan, Xinjiang Astronomical Observatory, demonstrates that our method achieves 94.72% detection accuracy with merely 5.02% false alarm rate, and a processing time of 0.66s per frame. Consequently, our method can rapidly and accurately detect objects with different motion modes from wide-field images with non-sidereal tracking.

astro-ph.IM↗
arXiv2024

SynthDoc: Bilingual Documents Synthesis for Visual Document Understanding

This paper introduces SynthDoc, a novel synthetic document generation pipeline designed to enhance Visual Document Understanding (VDU) by generating high-quality, diverse datasets that include text, images, tables, and charts. Addressing the challenges of data acquisition and the limitations of existing datasets, SynthDoc leverages publicly available corpora and advanced rendering tools to create a comprehensive and versatile dataset. Our experiments, conducted using the Donut model, demonstrate that models trained with SynthDoc's data achieve superior performance in pre-training read tasks and maintain robustness in downstream tasks, despite language inconsistencies. The release of a benchmark dataset comprising 5,000 image-text pairs not only showcases the pipeline's capabilities but also provides a valuable resource for the VDU community to advance research and development in document image recognition. This work significantly contributes to the field by offering a scalable solution to data scarcity and by validating the efficacy of end-to-end models in parsing complex, real-world documents.

cs.CV↗
arXiv2024

Optimal control problems with generalized mean-field dynamics and viscosity solution to Master Bellman equation

We study an optimal control problem of generalized mean-field dynamics with open-loop controls, where the coefficients depend not only on the state processes and controls, but also on the joint law of them. The value function $V$ defined in a conventional way, but it does not satisfy the Dynamic Programming Principle (DPP for short). For this reason we introduce subtly a novel value function $\vartheta$, which is closely related to the original value function $V$, such that, a description of $\vartheta$, as a solution of a partial differential equation (PDE), also characterizes $V$. We establish the DPP for $\vartheta$. By using an intrinsic notion of viscosity solutions, initially introduced in Burzoni, Ignazio, Reppen and Soner [8] and specifically tailored to our framework, we show that the value function $\vartheta$ is a viscosity solution to a Master Bellman equation on a subset of Wasserstein space of probability measures. The uniqueness of viscosity solution is proved for coefficients which depend on the time and the joint law of the control process and the controlled process. Our approach is inspired by Buckdahn, Li, Peng and Rainer [7], and leads to a generalization of the mean-field PDE in [7] to a Master Bellman equation in the case of controls.

math.OC↗
arXiv2024

Reliable Academic Conference Question Answering: A Study Based on Large Language Model

As the development of academic conferences fosters global scholarly communication, researchers consistently need to obtain accurate and up-to-date information about academic conferences. Since the information is scattered, using an intelligent question-answering system to efficiently handle researchers' queries and ensure awareness of the latest advancements is necessary. Recently, Large Language Models (LLMs) have demonstrated impressive capabilities in question answering, and have been enhanced by retrieving external knowledge to deal with outdated knowledge. However, these methods fail to work due to the lack of the latest conference knowledge. To address this challenge, we develop the ConferenceQA dataset, consisting of seven diverse academic conferences. Specifically, for each conference, we first organize academic conference data in a tree-structured format through a semi-automated method. Then we annotate question-answer pairs and classify the pairs into four different types to better distinguish their difficulty. With the constructed dataset, we further propose a novel method STAR (STructure-Aware Retrieval) to improve the question-answering abilities of LLMs, leveraging inherent structural information during the retrieval process. Experimental results on the ConferenceQA dataset show the effectiveness of our retrieval method. The dataset and code are available at https://github.com/zjukg/ConferenceQA.

cs.CL↗
arXiv2024

Topological Woodward-Hoffmann classification for cycloadditions in polycyclic aromatic azomethine ylides

The study of cycloaddition mechanisms is central to the fabrication of extended sp2 carbon nanostructures. Reaction modeling in this context has focused mostly on putative, energetically preferred, exothermic products with limited consideration for symmetry allowed or forbidden mechanistic effects. Here, we introduce a scheme for classifying symmetry-forbidden reaction coordinates in Woodward-Hoffmann correlation diagrams. Topological classifiers grant access to the study of reaction pathways and correlation diagrams in the same footing, for the purpose of elucidating mechanisms and products of polycyclic aromatic azomethine ylide (PAMY) cycloadditions with pentacene-yielding polycyclic aromatic hydrocarbons with an isoindole core in the solid-state and on surfaces as characterized by mass spectrometry and scanning tunneling microscopy, respectively. By means of a tight-binding reaction model and density functional theory (DFT) we find topologically-allowed pathways if a product is endothermic, and topologically-forbidden if a product is exothermic. Our work unveils topological classification as a crucial element for reaction modeling for nanographene engineering, and highlights its fundamental role in the design of cycloadditions in on-surface and solid-state chemical reactions, while underscoring that exothermic pathways can be topologically-forbidden.

physics.chem-ph↗
arXiv2024

Thermal spin-crossover and temperature-dependent zero-field splitting in magnetic nanographene chains

Nanographene-based magnetism at interfaces offers an avenue to designer quantum materials towards novel phases of matter and atomic-scale applications. Key to spintronics applications at the nanoscale is bistable spin-crossover which however remains to be demonstrated in nanographenes. Here we show that antiaromatic 1,4-disubstituted pyrazine-embedded nanographene derivatives, which promote magnetism through oxidation to a non-aromatic radical are prototypical models for the study of carbon-based thermal spin-crossover. Scanning tunneling spectroscopy studies reveal symmetric spin excitation signals which evolve at Tc to a zero-energy peak, and are assigned to the transition of a S = 3/2 high-spin to a S = 1/2 low-spin state by density functional theory. At temperatures below and close to the spin-crossover Tc, the high-spin S= 3/2 excitations evidence pronouncedly different temperature-dependent excitation energies corresponding to a zero-field splitting in the Hubbard-Kanamori Hamiltonian. The discovery of thermal spin crossover and temperature-dependent zero-field splitting in carbon nanomaterials promises to accelerate quantum information, spintronics and thermometry at the atomic scale.

physics.chem-ph↗
arXiv2024

Dense Outflowing Molecular Gas in Massive Star-forming Regions

Dense outflowing gas, traced by transitions of molecules with large dipole moment, is important for understanding mass loss and feedback of massive star formation. HCN 3-2 and HCO$^+$ 3-2 are good tracers of dense outflowing molecular gas, which are closely related to active star formation. In this study, we present on-the-fly (OTF) mapping observations of HCN 3-2 and HCO$^+$ 3-2 toward a sample of 33 massive star-forming regions using the 10-m Submillimeter Telescope (SMT). With the spatial distribution of line wings of HCO$^+$ 3-2 and HCN 3-2, outflows are detected in 25 sources, resulting in a detection rate of 76$\%$. The optically thin H$^{13}$CN and H$^{13}$CO$^+$ 3-2 lines are used to identify line wings as outflows and estimate core mass. The mass $M_{out}$, momentum $P_{out}$, kinetic energy $E_{K}$, force $F_{out}$ and mass loss rate $\dot M_{out}$ of outflow and core mass, are obtained for each source. A sublinear tight correlation is found between the mass of dense molecular outflow and core mass, with an index of $\sim$ 0.8 and a correlation coefficient of 0.88.

astro-ph.GA↗
arXiv2024

Comparison theorems for mean-field BSDEs whose generators depend on the law of the solution $(Y,Z)$

For general mean-field backward stochastic differential equations (BSDEs) it is well-known that we usually do not have the comparison theorem if the coefficients depend on the law of $Z$-component of the solution process $(Y, Z)$. A natural question is whether general mean-field BSDEs whose coefficients depend on the law of $Z$ have the comparison theorem for some cases. In this paper we establish the comparison theorems for one-dimensional mean-field BSDEs whose coefficients also depend on the joint law of the solution process $(Y,Z)$. With the help of Malliavin calculus and a BMO martingale argument, we obtain two comparison theorems for different cases and a strong comparison result. In particular, in this framework, we compare not only the first component $Y$ of the solution $(Y,Z)$ for such mean-field BSDEs, but also the second component $Z$.

math.PR↗
arXiv2024

Stochastic porous media equation with Robin boundary conditions, gravity-driven infiltration and multiplicative noise

We aim at studying a novel mathematical model associated to a physical phenomenon of infiltration in an homogeneous porous medium. The particularities of our system are connected to the presence of a gravitational acceleration term proportional to the level of saturation, and of a Brownian multiplicative perturbation. Furthermore, the boundary conditions intervene in a Robin manner with the distinction of the behavior along the inflow and outflow respectively. We provide qualitative results of well-posedness, the investigation being conducted through a functional approach.

math.AP↗
arXiv2024

A Global Stochastic Maximum Principle for Mean-Field Forward-Backward Stochastic Control Systems with Quadratic Generators

Our paper is devoted to the study of Peng's stochastic maximum principle (SMP) for a stochastic control problem composed of a controlled forward stochastic differential equation (SDE) as dynamics and a controlled backward SDE which defines the cost functional. Our studies combine the difficulties which come, on one hand, from the fact that the coefficients of both the SDE and the backward SDE are of mean-field type (i.e., they do not only depend on the control process and the solution processes but also on their law), and on the other hand, from the fact that the coefficient of the BSDE is of quadratic growth in $Z$. Our SMP is novel, it extends in a by far non trivial way existing results on SMP.

math.OC↗
arXiv2024

Assessing cognitive function among older adults using machine learning and wearable device data: a feasibility study

Timely implementation of interventions to slow cognitive decline among older adults requires accurate monitoring to detect changes in cognitive function. Data gathered using wearable devices that can continuously monitor factors known to be associated with cognition could be used to train machine learning models and develop wearable-based cognitive monitoring systems. Using data from over 2,400 older adults in the National Health and Nutrition Examination Survey (NHANES) we developed prediction models to differentiate older adults with normal cognition from those with poor cognition based on outcomes from three cognitive tests measuring different domains of cognitive function. During repeated cross-validation, CatBoost, XGBoost, and Random Forest models performed best when predicting cognition based on processing speed, working memory, and attention (median AUCs >0.82) compared to immediate and delayed recall (median AUCs >0.72) and categorical verbal fluency (median AUC >0.68). Activity and sleep parameters were also more strongly associated with processing speed, working memory, and attention compared to other cognitive subdomains. Our work provides proof of concept that wearable-based cognitive monitoring systems may be a viable alternative to traditional methods for monitoring processing speeds, working memory, and attention. We further identified novel metrics that could be targets in future causal studies seeking to better understand how sleep and activity parameters influence cognitive function among older adults.

eess.SP↗