arXiv subjects

Xia Zhang

Publications and source records attributed to Xia Zhang.

At least 19 recordsLinked to original sources

arXiv2026

ALMA Observations of Acetone in Hot Cores

Acetone (CH3COCH3) is a ubiquitous interstellar molecule, and serves as an important tracer of hot core chemistry. We conducted a line survey of acetone and its precursor acetaldehyde (CH3CHO) towards 60 hot cores by using the ALMA 3 mm lines observations. We calculated the rotational temperatures and column densities of acetone using the XCLASS software. Acetone was detected in 15 hot cores with rotational temperatures ranging from 89 to 176 K. Its column densities range from (0.9-24)x 10^16 cm^-2. The spatial distributions of acetone exhibit similarities with those of acetaldehyde. The emissions of acetone are concentrated toward the hot core regions and generally exhibit a compact spatial distribution, whereas the emission of acetaldehyde shows a more extended spatial profile. Combined with previous studies, we found a moderately positive correlation between the column densities and rotational temperatures of acetone for the high-mass hot cores (r = 0.59). We also found a strong positive correlation between the column densities of acetone and acetaldehyde (r = 0.82), indicating a chemical relationship between them. By comparing these observational results with the three-phase model results, we found that the models overpredict the ratio of acetone to methanol relative to the observational data. This discrepancy suggests that current chemical networks may inadequately account for acetone destruction pathways or potential missing physical conditions in the model. Therefore, our large sample observations can provide constraints on chemical models and reinforce the role of acetone as a tracer of complex organic chemistry in warm, dense regions.

astro-ph.GA↗

arXiv2025

Odd clique minors and chromatic bounds of {3$K_1$, paraglider}-free graphs

A paraglider, house, 4-wheel, is the graph that consists of a cycle $C_4$ plus an additional vertex adjacent to three vertices, two adjacent vertices, all the vertices of the $C_4$, respectively. For a graph $G$, let $\chi(G)$, $\omega(G)$ denote the chromatic number, the clique number of $G$, respectively. Gerards and Seymour from 1995 conjectured that every graph $G$ has an odd $K_{\chi(G)}$ minor. In this paper, based on the description of graph structure, it is shown that every graph $G$ with independence number two satisfies the conjecture if one of the following is true: $\chi(G) \leq 2\omega(G)$ when $n $ is even, $\chi(G) \leq 9\omega(G)/5$ when $n$ is odd, $G$ is a quasi-line graph, $G$ is $H$-free for some induced subgraph $H$ of paraglider, house or $W_4$. Moreover, we derive an optimal linear $\chi$-binding function for {3$K_1$, paraglider}-free graph $G$ that $\chi(G)\leq \max\{\omega(G)+3, 2\omega(G)-2\}$, which improves the previous result, $\chi(G)\leq 2\omega(G)$, due to Choudum, Karthick and Shalu in 2008.

math.CO↗

arXiv2025

Odd clique minors in graphs with independence number two

A $K_t$-expansion consists of $t$ vertex-disjoint trees, every two of which are joined by an edge. We call such an expansion odd if its vertices can be two-colored so that the edges of the trees are bichromatic but the edges between trees are monochromatic. A graph contains an odd $K_t$ minor or an odd clique minor of order $t$ if it contains an odd $K_t$-expansion. Gerards and Seymour from 1995 conjectured that every graph $G$ contains an odd $K_{\chi(G)}$ minor, where $\chi(G)$ denotes the chromatic number of $G$. This conjecture is referred to as ``Odd Hadwiger's Conjecture". Let $\alpha(G)$ denote the independence number of a graph $G$. In this paper we investigate the Odd Hadwiger's Conjecture for graphs $G$ with $\alpha(G)\le2$. We first observe that a graph $G$ on $n$ vertices with $\alpha(G)\le2$ contains an odd $K_{\chi(G)}$ minor if and only if $G$ contains an odd clique minor of order $\lceil n/2\rceil$. We then prove that every graph $G$ on $n$ vertices with $\alpha(G)\le 2$ contains an odd clique minor of order $\lceil n/2\rceil$ if $G$ contains a clique of order $n/4$ when $n$ is even and $(n+3)/4$ when $n$ is odd, or $G$ does not contain $H$ as an induced subgraph, where $\alpha(H)\le 2$ and $H$ is an induced subgraph of $K_1 + P_4$, $K_2+(K_1\cup K_3)$, $K_1+(K_1\cup K_4)$, $K_7^-$, $K_7$, or the kite graph.

math.CO↗

arXiv2025

Leveraging Semantic Type Dependencies for Clinical Named Entity Recognition

Previous work on clinical relation extraction from free-text sentences leveraged information about semantic types from clinical knowledge bases as a part of entity representations. In this paper, we exploit additional evidence by also making use of domain-specific semantic type dependencies. We encode the relation between a span of tokens matching a Unified Medical Language System (UMLS) concept and other tokens in the sentence. We implement our method and compare against different named entity recognition (NER) architectures (i.e., BiLSTM-CRF and BiLSTM-GCN-CRF) using different pre-trained clinical embeddings (i.e., BERT, BioBERT, UMLSBert). Our experimental results on clinical datasets show that in some cases NER effectiveness can be significantly improved by making use of domain-specific semantic type dependencies. Our work is also the first study generating a matrix encoding to make use of more than three dependencies in one pass for the NER task.

cs.CL↗

arXiv2025

$C$-existence families, $C$-semigroups and their associated abstract Cauchy problems in complete random normed modules

In this paper, we first introduce the notion of a (mild) $C$-existence family in complete random normed modules, then we prove that a (mild) $C$-existence family can guarantee the existence of the (mild) solutions of the associated abstract Cauchy problem in the random setting. Second, we investigate several important properties peculiar to locally almost surely bounded $C$-semigroups in complete random normed modules, which are not involved in the classical theory of $C$-semigroups. Finally, based on the above work, some relations among $C$-existence families, $C$-semigroups and their associated abstract Cauchy problems in complete random normed modules are established, which extend and improve some known results. Besides, an application to a type of stochastic differential equations is also given.

math.FA↗

arXiv2025

A Laplace transform approach to $C$-semigroups on a $\mathcal{T}_{\varepsilon, \lambda}$-complete random normed module

In this paper, we first introduce the notion of the Laplace transform for an abstract-valued function from $[0, \infty)$ to a $\mathcal{T}_{\varepsilon, \lambda}$-complete random normed module $S$. Then, combining respective advantages of the $(\varepsilon, \lambda)$-topology and the locally $L^0$-convex topology on $S$, we prove the differentiability, Post-Widder inversion formula and uniqueness of such a Laplace transform. Second, based on the above work, we establish the Hille-Yosida theorem for an exponentially bounded $C$-semigroup on $S$, considering both the dense and nondense cases of the range of $C$, respectively, which extends and improves several important results. Finally, we also apply such a Laplace transform to abstract Cauchy problems in the random setting.

math.FA↗

arXiv2025

A dual representation theorem on the conditional Orlicz space generated from a random normed module

In this paper, we first introduce the notion of a random Orlicz function, and further present the conditional Orlicz space generated from a random normed module. Second, we prove the denseness of the Orlicz heart of a random normed module $E$ in $E$ with respect to the $(\varepsilon, \lambda)$-topology. Finally, based on the above work, we establish a dual representation theorem on the conditional Orlicz space generated from a random normed module, which extends and improves some known results.

math.FA↗

arXiv2024

Friedrich-Wintgen bound states in the continuum in dimerized dielectric metasurfaces

Bound states in the continuum (BIC) are trapped eigenmodes with infinite $Q$ factors that are confined in the system. In this work, we propose a simple design for engineering a Friedrich-Wintgen BIC through the interference between a symmetry protected BIC and a surface lattice mode in a dimerized dielectric metasurface. The meta-atoms are comprised a symmetric double bar dimer. Using incident angle tuning an avoided crossing between the symmetry protected BIC and surface lattice mode is observed. At a specific detuning, the lower energy resonance vanishes, resulting in the formation of a Friedrich-Wintgen BIC. Investigations using coupled mode theory elucidate the role of the damping rate and coupling strength in the formation of the Friedrich-Wintgen BIC in the dimerized bar metasurface. By tuning the spacing between the two bars, Friedrich-Wintgen BIC can be engineered with controlled energy and damping rate. It is shown that the damping rate of the coupled modes can be considerably suppressed in this system. We envision that these results will not only enhance the understanding of strongly coupled interactions in metasurfaces but also indicate new paths for actively and passively controlling metasurface resonators for photonic applications.

physics.optics↗

arXiv2024

The Radon-Nikod$\acute{Y}$m property of $\mathbb{L}$-Banach spaces and the dual representation theorem of $\mathbb{L}$-Bochner function spaces

In this paper, we first introduce $\mathbb{L}$-$μ$-measurable functions and $\mathbb{L}$-Bochner integrable functions on a finite measure space $(S,\mathcal{F},μ),$ and give an $\mathbb{L}$-valued analogue of the canonical $L^{p}(Ω,\mathcal{F},μ).$ Then we investigate the completeness of such an $\mathbb{L}$-valued analogue and propose the Radon-Nikod$\acute{y}$m property of $\mathbb{L}$-Banach spaces. Meanwhile, an example constructed in this paper shows that there do exist an $\mathbb{L}$-Banach space which fails to possess the Radon-Nikod$\acute{y}$m property. Finally, based on above work, we establish the dual representation theorem of $\mathbb{L}$-Bochner integrable function spaces, which extends and improves the corresponding classical result.

math.FA↗

arXiv2024

Chemical models of interstellar glycine and adenine precursor aminoacetonitrile (NH2CH2CN)

Aminoacetonitrile (AAN), also known as glycinenitrile, has been suggested as a possible precursor of glycine and adenine in the interstellar medium. Here we present the chemical modeling of AAN and its isomers in hot cores using the three-phase chemical model NAUTILUS with the addition of over 300 chemical reactions of the three AAN isomers and related species. Our models predicted a peak gas phase abundance of AAN reaching the order of 10-8, which is consistent with observation towards Sgr B2(N). Regarding the reaction pathways of AAN and its isomers, we found that AAN is primarily formed via free radical reactions on grain surfaces during the early evolutionary stages. Subsequently, it is thermally desorbed into the gas phase as the temperature rises and is then destroyed by positive ions and radicals in gas phase. The isomers of AAN are formed through the hydrogenation reaction of CH3NCN on the grain surface and via electron recombination reactions of ion C2H5N2+ in gas phase. We speculate that there is a possibility for NCCN and AAN to react with each other, eventually leading to the formation of adenine in hot cores. However, further investigation is required to understand the efficiency of grain surfaces in adenine formation, through theoretical calculations or laboratory experiments in future research.

astro-ph.GA↗

arXiv2024

Investigating Sulfur Chemistry in the HD 163296 disk

Sulfur chemistry in the formation process of low-mass stars and planets remains poorly understood. The protoplanetary disks (PPDs) are the birthplace of planets and its distinctive environment provides an intriguing platform for investigating models of sulfur chemistry. We analyzed the ALMA observations of CS 7-6 transitions in the HD 163296 disk and perform astrochemical modeling to explore its sulfur chemistry. We simulated the distribution of sulfur-containing molecules and compared it with observationally deduced fractional column densities. We have found that the simulated column density of CS is consistent with the observationally deduced fractional column densities, while the simulated column density of C$_2$S is lower than the observationally deduced upper limits on column densities. This results indicate that we have a good understanding of the chemical properties of CS and C$_2$S in the disk. We also investigated the influence of the C/O ratio on sulfur-containing molecules and found that the column densities of SO, SO$_2$, and H$_2$S near the central star are dependent on the C/O ratio. Additionally, we found that the $N$[CS]/$N$[SO] ratio can serve as a promising indicator of the disk's C/O ratio in the HD 163296. Overall, the disk of HD 163296 provides a favorable environment for the detection of sulfur-containing molecules.

astro-ph.EP↗

arXiv2023

Tentative detection of cyanoformamide NCCONH2 in space

The peptide-like molecules, cyanoformamide (NCCONH2), is the cyano (CN) derivative of formamide (NH2CHO). It is known to play a role in the synthesis of nucleic acid precursors under prebiotic conditions. In this paper, we present a tentative detection of NCCONH2 in the interstellar medium (ISM) with the Atacama Large Millimeter/submillimeter Array (ALMA) archive data. Ten unblended lines of NCCONH2 were seen around 3sigma noise levels toward Sagittarius B2(N1E), a position that is slightly offset from the continuum peak. The column density of NCCONH2 was estimated to be 2.4\times 10^15 cm ^-2, and the fractional abundance of NCCONH2 toward Sgr B2(N1E) was 6.9\times10^-10. The abundance ratio between NCCONH2 and NH2CHO is estimated to be ~0.01. We also searched for other peptide-like molecules toward Sgr B2(N1E). The abundances of NH2CHO, CH3NCO and CH3NHCHO toward Sgr B2(N1E) were about one tenth of those toward Sgr B2(N1S), while the abundances of CH3CONH2 was only one twentieth of that toward Sgr B2(N1S).

astro-ph.GA↗

arXiv2023

Supercavity Modes in Stacked Identical Mie-resonant Metasurfaces

Modes with a high-$Q$ factor are crucial for photonic metadevices with advanced functionalities. In sharp contrast to recent techniques which generate a supercavity mode by bound states in the continuum via symmetry breaking, we reveal a general and new route, by stacking two parallel and identical Mie-resonant metasurfaces with an air separation. The supercavity mode can be designed by the established theoretical model by overlapping Mie resonant modes with tailor-made Fabry-Pérot modes. The simplified system, with free-space field concentration which can exist in plane as well as out of plane of the metasurfaces, provides for ease of integration with added matter, creating exciting different opportunities for the study of fundamental light-matter coupling. This work deepens our understanding of light manipulation using metasurfaces. It paves a different and general route for generating supercavity modes which can be easily engineered and controlled for different applications of all dielectric metaoptics.

physics.optics↗

arXiv2023

Coverage Dependent H$_2$ Desorption Energy: a Quantitative Explanation Based on Encounter Desorption Mechanism

Recent experiments show that the desorption energy of H$_2$ on a diamond-like carbon (DLC) surface depends on the H$_2$ coverage of the surface. We aim to quantitatively explain the coverage dependent H$_2$ desorption energy measured by the experiments. We derive a math formula to calculate an effective H$_2$ desorption energy based on the encounter desorption mechanism. The effective H$_2$ desorption energy depends on two key parameters, the desorption energy of H$_2$ on H$_2$ substrate and the ratio of H$_2$ diffusion barrier to its desorption energy. The calculated effective H$_2$ desorption energy qualitatively agrees with the coverage dependent H$_2$ desorption energy measured by the experiments if the values of these two parameters in literature are used in the calculations. We argue that the difference between the effective H$_2$ desorption energy and the experimental results is due to the lacking of knowledge about these two parameters. So, we recalculate these two parameters based on experimental data. Good agreement between theoretical and experimental results can be achieved if these two updated parameters are used in the calculations.

astro-ph.GA↗

arXiv2023

Improved tests of genuine entanglement for multiqudits

We give an improved criterion of genuine multipartite entanglement for an important class of multipartite quantum states using generalized Bloch representations of the density matrices. The practical criterion is designed based on the Weyl operators and can be used for detecting genuine multipartite entanglement in higher dimensional systems. The test is shown to be significantly stronger than some of the most recent criteria.

quant-ph↗

arXiv2023

Real time spectrogram inversion on mobile phone

We present two methods of real time magnitude spectrogram inversion: streaming Griffin Lim(GL) and streaming MelGAN. We demonstrate the impact of looking ahead on perceptual quality of MelGAN. As little as one hop size (12.5ms) of lookahead is able to significantly improve perceptual quality in comparison to its causal version. We compare streaming GL with the streaming MelGAN and show different trade-offs in terms of perceptual quality, on-device latency, algorithmic delay, memory footprint and noise sensitivity. For fair quality assessment of the GL approach, we use input log magnitude spectrogram without mel transformation. We evaluate presented real time spectrogram inversion approaches on clean, noisy and atypical speech. We specified conditions when streaming GL has comparable quality with MelGAN: noisy audio and no mel transformation. Streaming GL is 2.4x faster than real time on the ARM CPU of a Pixel4 and it uses 4.5x times less memory than MelGAN.

eess.AS↗

arXiv2023

Streaming Parrotron for on-device speech-to-speech conversion

We present a fully on-device streaming Speech2Speech conversion model that normalizes a given input speech directly to synthesized output speech. Deploying such a model on mobile devices pose significant challenges in terms of memory footprint and computation requirements. We present a streaming-based approach to produce an acceptable delay, with minimal loss in speech conversion quality, when compared to a reference state of the art non-streaming approach. Our method consists of first streaming the encoder in real time while the speaker is speaking. Then, as soon as the speaker stops speaking, we run the spectrogram decoder in streaming mode along the side of a streaming vocoder to generate output speech. To achieve an acceptable delay-quality trade-off, we propose a novel hybrid approach for look-ahead in the encoder which combines a look-ahead feature stacker with a look-ahead self-attention. We show that our streaming approach is almost 2x faster than real time on the Pixel4 CPU.

eess.AS↗

arXiv2023

Chemical models of adenine precursors cyanamide and carbodiimide in the interstellar medium

Cyanamide (NH2CN) and its isomer, carbodiimide (HNCNH), may form adenine in the interstellar medium (ISM) via a series of reactions. Therefore, they are considered key prebiotic molecules in the study of the origin of life. We used the three-phase NAUTILUS chemical code, which includes the gas, the dust surface, and the icy mantle, to investigate the formation and destruction of cyanamide and carbodiimide. We added over 200 new chemical reactions of the two isomers and related species, and established a relatively complete network. We applied cold core, hot corino/core and shock models to simulate the different physical environments, and found that the two isomers are mainly produced by the free radical reactions on grain surfaces. Our simulated results suggest that cyanamide and carbodiimide molecules come from surface chemistry at early evolutionary stages. Then they are released back to the gas phase, either by thermal process (in hot cores, hot corinos) or shock-induced desorption (in shock regions).We speculate that it is an inefficient route to form a tautomer of adenine by starting from molecules cyanoacetylene (C3NH), cyanamide and carbodiimide in ISM.

astro-ph.GA↗