arXiv subjects

Pei Yu

Publications and source records attributed to Pei Yu.

At least 19 recordsLinked to original sources

arXiv2026

Estimation of Random-Coefficient Dynamic Panel Data Models with a Fixed T

We study dynamic linear panel data models in which the lagged outcomes and strictly exogenous covariates carry individual-specific coefficients and the time-varying errors have a flexible covariance structure. With a fixed number of time periods, we point-identify the joint distribution of the random coefficients and the structural errors under a distributional form of strict exogeneity, and propose a closed-form, multi-step estimator based on the inverse Radon transform. We establish a uniform convergence rate for the estimator of the random coefficient density, as well as uniform consistency of the estimator for the conditional density of the time-varying structural errors. Monte Carlo simulations demonstrate good finite-sample performance of the estimators.

econ.EM↗

arXiv2026

Improving Muon-Scattering Material Identification via Coarse Momentum Encoding and Unsupervised Domain Adaptation

Cosmic-ray muon scattering has shown considerable potential for detecting nuclear materials and other dense contraband, but practical deployment remains challenging. A major difficulty arises from the coupling between material properties and muon momentum, since the broad natural momentum distribution influences the scattering angle and prevents unambiguous material identification. In this work, we propose a Coarse Momentum-Aware Domain Adaptation (CMADA) method to enable precise identification of materials. Instead of relying on high-precision momentum measurements, the proposed framework adopts coarse momentum binning combined with unsupervised domain adaptation to learn transferable scattering representations. In addition, a precision review mode based on averaging repeated samplings was proposed to further enhances identification performance. The coarse momentum binning strategy improves same-domain identification accuracy from 62.15% without momentum information to 89.52% with 5-bin momentum information, and further to 93.37% (precision review mode). Furthermore, the proposed unsupervised domain adaptation framework improves the cross-domain identification accuracy from 71.71% for the source-only baseline to 89.00% without requiring target domain labels.

physics.ins-det↗

arXiv2026

U-Net Based Image Enhancement for Short-time Muon Scattering Tomography

Muon Scattering Tomography (MST) is a promising non-invasive inspection technique, yet the practical application of short-time MST is hindered by poor image quality due to limited muon flux. To address this limitation, we propose a U-Net-based framework trained on Point of Closest Approach (PoCA) images reconstructed with simulation MST data to enhance image quality. When applied to experimental MST data, the framework significantly improves image quality, increasing the Structural Similarity Index Measure (SSIM) from 0.7232 to 0.9699 and decreasing the Learned Perceptual Image Patch Similarity (LPIPS) from 0.3604 to 0.0270. These results demonstrate that our method can effectively enhance low-statistics MST images, thereby paving the way for the practical deployment of short-time MST.

eess.IV↗

arXiv2025

Projection-shifted particle-flow imaging with cosmic-ray muons

Cosmic-ray muons are natural probes for non-destructive imaging, but reaching sub-millimetre resolution with realistic exposure times has long been hindered by a fundamental limitation: the stochastic nature of multiple Coulomb scattering defies deterministic reconstruction of particle trajectories inside matter. Here we introduce Projection-shifted MUon transMission tomogrAphy (P$\mu$MA). Rather than localizing individual scattering points, P$\mu$MA records how material perturbations statistically shift the projected transmission tracks of a muon flux onto a virtual imaging plane. Near density boundaries, asymmetric projection-shift statistics naturally produce steep undershoot-overshoot profiles, sharpening edges without additional filtering. Crucially, the method operates robustly even with only two detector planes, a configuration where conventional scattering tomography cannot be applied. Cosmic-ray simulations with a lead knife-edge target yield edge widths as narrow as 1.196 mm, while monoenergetic-beam simulations reach 48 $\mu$m. Using a prototype system, we resolve 2-mm copper letters within two days---a feat unattainable by standard approaches under equivalent conditions. The projection-shift concept can be extended to accelerator- or laser-driven muon beams and to other ions, establishing a generalisable strategy for high-resolution particle-flow imaging.

physics.ins-det↗

arXiv2025

Quantum relaxometry for detecting biomolecular interactions with single NV centers

The investigation of biomolecular interactions at the single-molecule level has emerged as a pivotal research area in life science, particularly through optical, mechanical, and electrochemical approaches. Spins existing widely in biological systems, offer a unique degree of freedom for detecting such interactions. However, most previous studies have been largely confined to ensemble-level detection in the spin degree. Here, we developed a molecular interaction analysis method approaching single-molecule level based on relaxometry using the quantum sensor, nitrogen-vacancy (NV) center in diamond. Experiments utilized an optimized diamond surface functionalized with a polyethylenimine nanogel layer, achieving $\sim$10 nm average protein distance and mitigating interfacial steric hindrance. Then we measured the strong interaction between streptavidin and spin-labeled biotin complexes, as well as the weak interaction between bovine serum albumin and biotin complexes, at both the micrometer scale and nanoscale. For the micrometer-scale measurements using ensemble NV centers, we re-examined the often-neglected fast relaxation component and proposed a relaxation rate evaluation method, substantially enhancing the measurement sensitivity. Furthermore, we achieved nanoscale detection approaching single-molecule level using single NV centers. This methodology holds promise for applications in molecular screening, identification and kinetic studies at the single-molecule level, offering critical insights into molecular function and activity mechanisms.

quant-ph↗

arXiv2025

Determining codimension of Bogdanov-Takens and Bautin bifurcations via simplest normal form computation

In solving real-world problems, determining the codimension of Bogdanov-Takens (BT) and Bautin (generalized Hopf) bifurcations can be very challenging, even for simple two-dimensional dynamical systems. This difficulty becomes particularly evident when the number of system parameters exceeds the codimension of the bifurcations. Such challenges are closely linked to analyzing complex dynamics, such as the bifurcation of multiple limit cycles and homoclinic/heteroclinic bifurcations. In this paper, we use two population systems to demonstrate a systematic approach for determining the conditions that define the codimension of BT and Bautin bifurcations.

math.DS↗

arXiv2025

A simple criterion for identifying slow-fast relaxation oscillations

Simple conditions have been developed in [Zhang, Wahl and Yu, SIAM Rev. 2014; Yu and Wang, Math. Biosci. Eng. 2019], which are used to identify the existence of slow-fast relaxation oscillations that appear in differential systems, where the geometric singular perturbation theory is not applicable. In this study, we present seven models which fail to satisfy these conditions, yet still show recurrence phenomena, which motivate us to generalize these conditions to establish new simple hypothetical conditions for identifying slow-fast relaxation oscillations and providing arguments towards the development of a mathematical theory.

math.DS↗

arXiv2025

Improving Muon Scattering Tomography Performance With A Muon Momentum Measurement Scheme

Muon imaging, especially muon scattering tomography (MST), has recently garnered significant attention. MST measures the magnitude of muon scattering angles inside an object, which depends not only on the material properties but also on the muon momentum. Due to the difficulty of simultaneous measurement of momentum, it was neglected and taken as a constant in multiple MST reconstruction algorithms. Recently, an experimental measurement scheme has emerged that is feasible in engineering, but it requires many layers of detectors to approach the true momentum. From this, we proposed both an algorithm to incorporating momentum into MST, and a scheme to determine the thresholds of Cherenkov detectors. This novel scheme, termed the "equi-percentage scheme", sets momentum thresholds for Cherenkov detector layers based on cosmic muon momentum distribution. Results showed our approach delivers noticeable enhancement in reconstructed image quality even with only two detector layers, reaching near-saturation performance with four layers. This study proves that momentum measurement significantly enhances short-duration MST, and that substantial improvement can be achieved with relatively coarse momentum measurement using 2-4 layers of Cherenkov detectors.

physics.ins-det↗

arXiv2025

Room-temperature hybrid 2D-3D quantum spin system for enhanced magnetic sensing and many-body dynamics

Advances in hybrid quantum systems and their precise control are pivotal for developing advanced quantum technologies. Two-dimensional (2D) materials with optically accessible spin defects have emerged as a promising platform for building integrated quantum spin systems due to their exceptional flexibility and scalability. However, experimentally realizing such systems and demonstrating their superiority remains challenging. Here, we present a hybrid spin system operating under ambient conditions, integrating boron vacancy (V_B^-) spins in 2D hexagonal boron nitride flakes with a single nitrogen vacancy (NV) center in 3D single-crystal diamonds. This combined system achieves full controllability and exhibits enhanced performance for nanoscale magnetic sensing, including an improved dynamic range. Moreover, we investigate the rich many-body spin dynamics within the hybrid system, which enables us to estimate the concentration of V_B^- spins. This work provides a critical foundation for advancing the development of 2D-3D integrated quantum spin systems.

quant-ph↗

arXiv2025

Oxygen-vacancy quantum spin defects in silicon carbide

Optically addressable spin defects in wide-bandgap semiconductors are promising building blocks for quantum sensing and quantum networks. Establishing their atomic structure is essential for understanding functionality and enabling controlled engineering. In 4H-SiC, the PL5 and PL6 centers have long been recognized for their exceptional charge stability and room-temperature optically detected magnetic resonance (ODMR) performance, but their structural origin has remained elusive for over a decade. Here, we provide direct evidence for their oxygen-vacancy (${\rm O_C V_{Si}}$) origins through a combined chemical and isotopic control strategy. Under oxygen ion implantation, we observe over tenfold enhancement in the yield of PL5 and PL6 compared to nitrogen ion implantation. Furthermore, implantation with $^{17}{\rm O}$ ions produces PL5 and PL6 defects that exhibit a characteristic six-fold $^{17}{\rm O}$ hyperfine splitting in their ODMR spectra. These results affirm PL6 as the ${\rm O_C V_{Si}}$ defect in the $hh$ configuration. For PL5, the oxygen-related evidence, together with \textit{ab initio} calculations and additional measurements of the zero-field splitting and hyperfine structure, establishes it as the ${\rm O_C V_{Si}}$ defect in the $kh$ configuration. This unambiguous structural identification, achieved through materials-level chemical control, provides the microscopic foundation for deterministic engineering of these defects, paving the way for scalable photonic devices and high-sensitivity ensemble quantum sensors based on oxygen-vacancy centers.

cond-mat.mtrl-sci↗

arXiv2025

Transfer learning empowers material Z classification with muon tomography

Cosmic-ray muon sources exhibit distinct scattering angle distributions when interacting with materials of different atomic numbers (Z values), facilitating the identification of various Z-class materials, particularly those radioactive high-Z nuclear elements. Most of the traditional identification methods are based on complex muon event reconstruction and trajectory fitting processes. Supervised machine learning methods offer some improvement but rely heavily on prior knowledge of target materials, significantly limiting their practical applicability in detecting concealed materials. For the first time, transfer learning is introduced into the field of muon tomography in this work. We propose two lightweight neural network models for fine-tuning and adversarial transfer learning, utilizing muon tomography data of bare materials to predict the Z-class of coated materials. By employing the inverse cumulative distribution function method, more accurate scattering angle distributions could be obtained from limited data, leading to an improvement by nearly 4\% in prediction accuracy compared with the traditional random sampling based training. When applied to coated materials with limited labeled or even unlabeled muon tomography data, the proposed method achieves an overall prediction accuracy exceeding 96\%, with high-Z materials reaching nearly 99\%. Simulation results indicate that transfer learning improves prediction accuracy by approximately 10\% compared to direct prediction without transfer. This study demonstrates the effectiveness of transfer learning in overcoming the physical challenges associated with limited labeled/unlabeled data, highlights the promising potential of transfer learning in the field of muon tomography.

physics.ins-det↗

arXiv2025

The Feasibility Study of the GeV-Energy Muon Source Based on HIAF

Generating a mono-energetic, high-energy muon beam using accelerator facilities can be very attractive for many purposes, for example, improving muon tomography currently limited by the low flux and wide energy spread of cosmic ray muons, and searching for muon related new physics beyond the Standard Model. One potential accelerator facility is the High Intensity Heavy-Ion Accelerator Facility (HIAF), which is currently under construction in Huizhou City, China. Considering the projectile energy and beamline length, a high-intensity and GeV-energy muon flux could be produced and delivered by the High Energy Fragment Separator beamline of the HIAF facility. In this paper, the flux intensity and purity of muon beam based on HIAF are discussed in detail. For the $\mu^+$ beam, the highest muon yield reaches $8.2 \times 10^6 ~ \mu$/s with the purity of approximately $2\%$ at a momentum of 3.5 GeV/c; meanwhile, for the $\mu^-$ beam, the maximum muon yield is 4.2 $\times 10^6 ~ \mu$/s with the purity of around $20\%$ at a momentum of 1.5 GeV/c. The results also indicate that, for muon beams with an energy of several GeV, by applying a suitable purification strategy, we can get a muon beam with a purity of 100\% and an intensity of the order of $10^5 ~ \mu$/s.

physics.acc-ph↗

arXiv2024

Discovery of an anomalous non-evaporating sub-nanometre water layer in open environment

Water exhibits complex behaviors as a result of hydrogen bonding, and low-dimensional confined water plays a key role in material science, geology, and biology science. Conventional techniques like STM, TEM, and AFM enable atomic-scale observations but face limitations under ambient conditions and surface topographies. NV center magnetic resonance technology provides an opportunity to overcome these limitations, offering non-contact atomic-scale measurements with chemical resolution capability. In this study, a nanoscale layer dissection method was developed utilizing NV center technology to analyze water layers with diverse physicochemical properties. It unveiled the presence of a non-evaporating sub-nanometer water layer on a diamond surface under ambient conditions. This layer demonstrated impervious to atmospheric water vapor and exhibited unique electronic transport mediated via hydrogen bonding. These findings provide new perspectives and a platform for studying the structure and behavior of low-dimensional water, as well as the surface properties influenced by adsorbed water under native conditions.

cond-mat.mtrl-sci↗

arXiv2024

Exploring the lifetime frontier with a beam-dump experiment at CiADS

We propose a beam-dump experiment (BDE) at the upcoming facility of China initiative Accelerator Driven System (CiADS), called CiADS-BDE, in order to search for long-lived particles (LLPs) predicted in various beyond-the-Standard-Model (BSM) theories. The experiment is to be located in the forward direction of the incoming low-energy proton beam at CiADS, leveraging the strong forward boost of the produced particles at the beam dump in general. The space between the dump and the detector is largely available, allowing for installation of shielding and veto materials and hence low levels of background events. We elaborate on the detector setup, and choose dark photon as a benchmark model for sensitivity study. We restrict ourselves to the signature of an electron-positron pair and perform detailed background estimates. We find that with 5 years' operation, unique, currently unexcluded parts of the parameter space for $\mathcal{O}(100)$~MeV dark-photon masses and $\mathcal{O}(10^{-9}\text{--}10^{-8})$ kinetic mixing can be probed at the CiADS-BDE. Furthermore, considering that there is no need to set up a proton beam specifically for this experiment and that the detector system requires minimal instrumentation, the experiment is supposed to be relatively cost-effective. Therefore, we intend this work to promote studies on the sensitivity reach of the proposed experiment to additional LLP scenarios, and in the end, the realization of the experiment. Incidentally, we study the sensitivity of the same BDE setups at the High Intensity Heavy-ion Accelerator Facility (HIAF), presently in operation near the CiADS program site, and conclude that HIAF-BDE could probe new parameter regions, too.

hep-ph↗

arXiv2024

New Constraints on Exotic Spin-Spin-Velocity-Dependent Interactions with Solid-State Quantum Sensors

We report new experimental results on exotic spin-spin-velocity-dependent interactions between electron spins. We designed an elaborate setup that is equipped with two nitrogen-vacancy (NV) ensembles in diamonds. One of the NV ensembles serves as the spin source, while the other functions as the spin sensor. By coherently manipulating the quantum states of two NV ensembles and their relative velocity at the micrometer scale, we are able to scrutinize exotic spin-spin-velocity-dependent interactions at short force ranges. For a T-violating interaction, $V_6$, new limits on the corresponding coupling coefficient, $f_6$, have been established for the force range shorter than 1 cm. For a P,T-violating interaction, $V_{14}$, new constraints on the corresponding coupling coefficient, $f_{14}$, have been obtained for the force range shorter than 1 km.

hep-ex↗

arXiv2024

Bi-center conditions and bifurcation of limit cycles in a class of $Z_2$-equivariant cubic switching systems with two nilpotent points

In this paper, we generalize the Poincar\'e-Lyapunov method for systems with linear type centers to study nilpotent centers in switching polynomial systems and use it to investigate the bi-center problem of planar $Z_2$-equivariant cubic switching systems associated with two symmetric nilpotent singular points. With a properly designed perturbation, 6 explicit bi-center conditions for such polynomial systems are derived. Then, based on the $6$ center conditions, by using Bogdanov-Takens bifurcation theory with general perturbations, we prove that there exist at least $20$ small-amplitude limit cycles around the nilpotent bi-center for a class of $Z_2$-equivariant cubic switching systems. This is a new lower bound of cyclicity for such cubic polynomial systems, increased from $12$ to $20$.

math.DS↗

arXiv2024

Correlated sensing with a solid-state quantum multi-sensor system for atomic-scale structural analysis

Developing superior quantum sensing strategies ranging from ultra-high precision measurement to complex structural analysis is at the heart of quantum technologies. While strategies using quantum resources, such as entanglement among sensors, to enhance the sensing precision have been abundantly demonstrated, the signal correlation among quantum sensors is rarely exploited. Here we develop a novel sensing paradigm exploiting the signal correlation among multiple quantum sensors to resolve overlapping signals from multiple targets that individual sensors can't resolve and complex structural construction struggles with. With three nitrogen-vacancy centers as a quantum electrometer system, we demonstrate this multi-sensor paradigm by resolving individual defects' fluctuating electric fields from ensemble signals. We image the three-dimensional distribution of 16 dark electronic point-defects in diamond with accuracy approaching 1.7 nm via a GPS-like localization method. Furthermore, we obtain the real-time charge dynamics of individual point defects and visualize how the dynamics induce the well-known optical spectral diffusion. The multi-sensor paradigm extends the quantum sensing toolbox and offers new possibilities for structural analysis.

quant-ph↗

arXiv2023

Nilpotent center conditions in cubic switching polynomial Li\'enard systems by higher-order analysis

The aim of this paper is to investigate two classical problems related to nilpotent center conditions and bifurcation of limit cycles in switching polynomial systems. Due to the difficulty in calculating the Lyapunov constants of switching polynomial systems at non-elementary singular points, it is extremely difficult to use the existing Poincar\'e-Lyapunov method to study these two problems. In this paper, we develop a higher-order Poincar\'e-Lyapunov method to consider the nilpotent center problem in switching polynomial systems, with particular attention focused on cubic switching Li\'enard systems. With proper perturbations, explicit center conditions are derived for switching Li\'enard systems at a nilpotent center, which is characterized as global. Moreover, with Bogdanov-Takens bifurcation theory, the existence of five limit cycles around the nilpotent center is proved for a class of switching Li\'enard systems, which is a new lower bound of cyclicity for such polynomial systems around a nilpotent center.

math.DS↗