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Using data from the Chandra X-Ray Observatory, we revisited the reverse shock in the supernova remnant (SNR) Cassiopeia A.Based on the spectroscopic of a series of annuli in the northwest (NW) and southeast (SE), we get the radial profiles of the S/Si K-alpha line flux ratio and Fe K-alpha line centroid energy. They both show monotonic increase, confirming that the Si- and Fe-rich ejecta are heated by the reverse shock.The abrupt change of the S and Si line flux ratio is clearly observed in Cassiopeia A, leading to the determination of the reverse shock location (~1.71+-0.16 arcmin and ~1.35+-0.18 arcmin in the NW and SE, with respect to the central source). By comparing the radial profiles of S and Si line flux, we find that the reverse shock is moving outward in the frame of the observer, and the velocities are ~3950+-210 km/s and ~2900+-260 km/s in the NW and SE, respectively. In contrast, the velocities become ~1150 km/s (NW) and ~1300 km/s (SE) in the ejecta frame. Our measured reverse shock velocities are quite consistent with those obtained from the X-ray and/or optical images. It therefore supplies a crosscheck of the accuracy for the two available methods to measure the reverse shock velocity in SNRs. Both the location and the velocity of the reverse shock show apparent asymmetry, suggesting that the asymmetric explosion of the progenitor plays a key role in the interaction between the reverse shock and the ejecta, ultimately shaping complex features observed in SNRs., Comment: 14 pages, 6 figures, 2 tables, accepted for publication in ApJ

In interstellar environment, fullerene species readily react with large molecules (e.g., PAHs and their derivatives) in the gas phase, which may be the formation route of carbon dust grains in space. In this work, the gas-phase ion-molecule collision reaction between fullerene cations (Cn+, n=32, 34, ..., 60) and functionalized PAH molecules (9-hydroxyfluorene, C13H10O) are investigated both experimentally and theoretically. The experimental results show that fullerene/9-hydroxyfluorene cluster cations are efficiently formed, leading to a series of large fullerene/9-hydroxyfluorene cluster cations (e.g., [(C13H10O)C60]+, [(C13H10O)3C58+, and [(C26H18O)(C13H10O)2C48]+). The binding energies and optimized structures of typical fullerene/9-hydroxyfluorene cluster cations were calculated. The bonding ability plays a decisive role in the cluster formation processes. The reaction surfaces, modes and combination reaction sites can result in different binding energies, which represent the relative chemical reactivity. Therefore, the geometry and composition of fullerene/9-hydroxyfluorene cluster cations are complicated. In addition, there is an enhanced chemical reactivity for smaller fullerene cations, which is mainly attributed to the newly formed deformed carbon rings (e.g., 7 C-ring). As part of the coevolution network of interstellar fullerene chemistry, our results suggest that ion-molecule collision reactions contribute to the formation of various fullerene/9-hydroxyfluorene cluster cations in the ISM, providing insights into different chemical reactivity caused by oxygenated functional groups (e.g., hydroxyl, OH, or ether, C-O-C) on the cluster formations., Comment: 13 pages, 11 figure, accepted for publication in MNRAS

In-context Learning (ICL) has emerged as a powerful capability alongside the development of scaled-up large language models (LLMs). By instructing LLMs using few-shot demonstrative examples, ICL enables them to perform a wide range of tasks without updating millions of parameters. However, the precise contributions of demonstrations towards improving end-task performance have not been thoroughly investigated in recent analytical studies. In this paper, we empirically decompose the overall performance of ICL into three dimensions, label space, format, and discrimination, and we evaluate four general-purpose LLMs across a diverse range of tasks. Counter-intuitively, we find that the demonstrations have a marginal impact on provoking discriminative knowledge of language models. However, ICL exhibits significant efficacy in regulating the label space and format, which helps LLMs respond to desired label words. We then demonstrate that this ability functions similar to detailed instructions for LLMs to follow. We additionally provide an in-depth analysis of the mechanism of retrieval helping with ICL. Our findings demonstrate that retrieving the semantically similar examples notably boosts the model's discriminative capability. However, we also observe a trade-off in selecting good in-context examples regarding label diversity., Comment: 39 pages, 8 figures. Accepted by Conference On Language Modeling (COLM) 2024

The two-dimensional (2D) multiferroic materials have widespread of application prospects in facilitating the integration and miniaturization of nanodevices. However, it is rarely coupling between the magnetic, ferroelectric, and ferrovalley in one 2D material. Here, we propose a mechanism for manipulating magnetism, ferroelectric, and valley polarization by interlayer sliding in 2D bilayer material. Monolayer GdI2 exhibits a ferromagnetic semiconductor with the valley polarization up to 155.5 meV. More interestingly, the magnetism and valley polarization of bilayer GdI2 can be strongly coupled by sliding ferroelectricity, appearing these tunable and reversible. In addition, we uncover the microscopic mechanism of magnetic phase transition by spin Hamiltonian and electron hopping between layers. Our findings offer a new direction for investigating 2D multiferroic in the implication for next-generation electronic, valleytronic, and spintronic devices., Comment: 21 pages, 5 figures Accepted Nano Lett.(2024)

Single-atom catalysts are considered as a promising method for efficient energy conversion, owing to their advantages of high atom utilization and low catalyst cost. However, finding a stable two-dimensional structure and high hydrogen evolution reaction (HER) performance is a current research hotspot. Herein, based on the first-principles calculations, we identify the HER properties of six catalysts (TM@MoSi2N4, TM = Sc, Ti, V, Fe, Co, and Ni) comprising transition metal atoms anchored on MoSi2N4 monolayer. The results show that the spin-polarized states appear around the Fermi level after anchoring TM atoms. Therefore, the energy level of the first available unoccupied state for accommodating hydrogen drops, regulating the bonding strength of hydrogen. Thus, the single transition metal atom activates the active site of the MoSi2N4 inert base plane, becoming a quite suitable site for the HER. Based on {\Delta}GH*, the exchange current density and volcano diagram of the corresponding catalytic system were also calculated. Among them, V@MoSi2N4 ({\Delta}GH* = -0.07 eV) and Ni@MoSi2N4 ({\Delta}GH* = 0.06 eV) systems show efficient the HER property. Our study confirms that the transition metal atom anchoring is an effective means to improve the performance of electrocatalysis, and TM@MoSi2N4 has practical application potential as a high efficiency HER electrocatalyst., Comment: 20 Pages, 6 figures

The valley-related multiple topological phase transitions attracted significant attention due to their providing significant opportunities for fundamental research and practical applications. However, unfortunately, to date there is no real material that can realize valley-related multiple topological phase transitions. Here, through first-principles calculations and model analysis, we investigate the structural, magnetic, electronic, and topological properties of VSiXN$_4$ (X = C, Si, Ge, Sn, Pb) monolayers. VSiXN$_4$ monolayers are stable and intrinsically ferrovalley materials. Intriguingly, we found that the built-in electric field and strain can induce valley-related multiple topological phase transitions in the materials from valley semiconductor to valley-half-semimetal, to valley quantum anomalous Hall insulator, to valley-half-semimetal, and to valley semiconductor (or to valley-metal). The nature of topological phase transition is the built-in electric field and strain induce band inversion between the d$_{xy}$/d$_{x2-y2}$ and d$_{z2}$ at obritals at K and K' valleys. Our findings not only reveal the mechanism of multiple topological phase transitions, but also provides an ideal platform for the multi-field manipulating the spin, valley, and topological physics. It will open new perspectives for spintronic, valleytronic, and topological nanoelectronic applications based on these materials., Comment: 9 pages, 8 figures, Accepted Physical Review Materials (2023). arXiv admin note: text overlap with arXiv:2204.07488

Efficient and selective CO2 electroreduction into value-added chemicals and fuels emerged as a significant approach for CO2 conversion, however, it relies on catalysts with controllable product selectivity and reaction paths. In this work, by means of first-principles calculations, we identify five catalysts (TM@MoSi2N4, TM = Sc, Ti, Fe, Co and Ni) comprising transition-metal atoms anchored on a MoSi2N4 monolayer, whose catalytic performance can be controlled by adjusting the d-band center and occupation of supported metal atoms. During CO2 reduction, the single metal atoms function as the active sites activates the MoSi2N4 inert basal-plane, and as-designed electrocatalysts exhibit excellent activity in CO2 reduction. Interestingly, HCOOH is the preferred product of CO2 reduction on the Co@MoSi2N4 catalyst with a rate-determining barrier of 0.89 eV, while the other four catalysts prefer to reduce CO2 to CH4 with a rate-determining barrier of 0.81-1.24 eV. Moreover, MoSi2N4 is an extremely-air-stable material, which will facilitate its application in various environments. Our findings provide a promising candidate with high activity, catalysts for renewable energy technologies, and selectivity for experimental work., Comment: 17 Pages, 5 figures