arXiv subjects

Xunwang Yan

Publications and source records attributed to Xunwang Yan.

5 recordsLinked to original sources

arXiv2026

Extracting effective scaling exponents in finite-size hyperuniform systems

Hyperuniform systems strongly suppress long-wavelength density fluctuations, which is quantitatively characterized by the small-wavenumber scaling. In finite samples, however, accurately estimating the hyperuniformity exponent {\alpha} can be challenging. The inferred value depends strongly on the range of length scales accessible in the measurement, finite-size effects, and the specific characterization method employed, whether based on Fourier-space structure factors, real-space density fluctuations, or dynamical probes such as diffusion spreadability. In particular, the structure-factor method provides the most direct estimate of {\alpha}, but is sensitive to empirical low-k fitting cutoffs. The number-variance method offers a real-space Class-like diagnosis, but contributes a numerical exponent only when the finite-size data retain Class III-like scaling information. The spreadability method provides a smoother dynamic estimate and reduces configuration-level fluctuations, but requires a physically admissible long-time fitting window. Here, we develop a practical method-aware protocol for robust estimation of the effective scaling exponent {\alpha} in finite-size hyperuniform point configurations, combining three complementary methods with distinct roles. Our protocol summarizes the method-specific estimates through a joint empirical estimator and reports the internal dispersion among the participating methods to determine the optimal estimate.

cond-mat.dis-nn↗

arXiv2025

Impact of Random Spatial Truncation and Reciprocal-Space Binning on the Detection of Hyperuniformity in Disordered Systems

We study how finite-window sampling (random spatial truncation) and reciprocal-space radial binning influence the detection of hyperuniformity in disordered systems. Using thirteen representative two-dimensional simulation systems (two stealthy hyperuniform systems with distinct constraint parameters and ; hyperuniform Gaussian pair statistics system; six hyperuniform targeted systems with distinct alpha=0.5, 0.7, 1.0, 1.3, 1.5, 3.0, random sequential addition system; Poisson points distribution system; Lennard-Jones fluid system and Yukawa fluid system) and two real biological systems (avian photoreceptor patterns and looped leaf vein networks) We find that moderate random spatial truncation (i.e., randomly extracting a smaller subwindow from the original full-field configuration) does not change qualitatively the hyperuniformity classification of the systems. Specifically, disordered hyperuniform systems retain their respective hyperuniformity classes despite a modest reduction in measured hyperuniformity exponent alpha (i.e., reduction in small-k suppression). Moreover, spatial truncation commonly induces configuration-dependent fluctuations of small-k values of S(k). We show that modest reciprocal-space radial pooling (controlled by a binning parameter m) effectively smooths such spurious wiggles without changing the hyperuniformity class. Practical guidelines for choosing m, cross-checking spectral fits with the local number variance scaling, and increasing effective sampling are provided. These results provide concrete, low-cost and effective methodology for robust spectral detection of hyperuniformity in finite and truncated datasets which abound in experimental systems.

cond-mat.dis-nn↗

arXiv2023

Ultra-high-density double-atom catalyst with spin moment as activity descriptor for oxygen reduction reaction

One of the great challenges facing atomically dispersed catalysts, including single-atom catalyst (SAC) and double-atom catalyst (DAC) is their ultra-low metal loading (typically less than 5 wt%), basically limiting the practical catalytic application, such as oxygen reduction reaction (ORR) crucial to hydrogen fuel cell and metal-air battery. Although some important progresses have been achieved on ultra-high-density (UHD) SACs, the reports on UHD-DACs with stable uniform dispersion is still lacking. Herein, based on the experimentally synthesized M2N6 motif (M = Sc-Zn), we theoretically demonstrated the existence of the UHD-DACs with the metal loading > 40 wt%, which were confirmed by systematic analysis of dynamic, thermal, mechanical, thermodynamic, and electrochemical stabilities. Furthermore, ORR activities of the UHD-DACs are comparable with or even better than those of the experimentally synthesized low-density (LD) counterparts, and the Fe2N6 and Co2N6 UHD-DACs locate at the peak of the activity volcano with ultra-low overpotentials of 0.31 and 0.33 V, respectively. Finally, spin magnetic moment of active center is found to be a catalytic descriptor for ORR on the DACs. Our work will stimulate the experimental exploration of the ultra-high-density DACs and provides the novel insight into the relationship between ORR activity of the DACs and their spin states.

cond-mat.mtrl-sci↗

arXiv2015

Geometrical and electronic structures of tripotassium-doped hydrocarbon superconductors: Density functional calculations

A systemically theoretical study has been presented to explored the crystal structures and electronic characteristics of polycyclic aromatic hydrocarbons (PAHs), such as solid phenanthrene, picene, 1,2;8,9-dibenzopentacene, and 7-phenacenes, since these PAHs exhibited the superconductivity when potassium doping into. For tripotassium-doped phenanthrene and picene, we demonstrate the K atomic positions to fit the experimental lattice parameters, and analyze the distinction between the stablest configuration and the fitted experimental one. Based on the first-principles calculations, for the first time, we predict the possible crystal configurations of pristine and tripotassium-doped 1,2;8,9-dibenzopentacene and 7-phenacenes, respectively. For these four PAHs, the electronic structures after doping are investigated in details. The results show that the electronic characters near the Fermi level are high sensitive to structure. Because of the change of the benzene rings arrangement, the 1,2;8,9-dibenzopentacene exhibits visibly different band structures from other three PAHs. In these metallic PAHs, two bands cross the Fermi level which results in the complicated multiband feature of Fermi surfaces. Fascinatingly, we find that the electronic states of potassium contribute to the Fermi surfaces especially for K-3$d$ electrons, which improves a way to understand this superconductivity. As a result, we suggest that the rigid-band picture is invalidated due to the hybridization between K atoms and PAH molecules as well as the rearrangement and distortion of PAH molecules.

cond-mat.mtrl-sci↗

arXiv2012

Electronic structure and optical properties of Graphene Monoxide

The electronic and optical properties of graphene monoxide, a new type of semiconductor materials, are first theoretically studied based on density functional theory. Electronic calculations show that the band gap is 0.952 eV which indicate that graphene monoxide is a direct band gap semiconductor. The density of states of graphene monoxide and the partial density of states for C and O are given to understand the electronic structure. In addition, we calculate the optical properties of graphene monoxide including the complex dielectric function, absorption coefficient, the complex refractive index, loss-function, reflectivity and conductivity. These results provide a physical basis for the potential applications in optoelectronic devices.

cond-mat.mtrl-sci↗