arXiv subjects

W. Xu

Publications and source records attributed to W. Xu.

At least 19 recordsLinked to original sources

arXiv2026

Quantum transport in a non-Hermitian 1D synthetic lattice: from quantum Zeno reflection to near-perfect absorption

We experimentally explore the absorption of a propagating wavepacket impinging upon a dissipative region in an engineered quantum system. We employ a 1D non-Hermitian synthetic lattice with an abrupt interface between dissipative and non-dissipative subchains, using the states of the electronic ground-state hyperfine manifold in a $^{87}$Rb Bose-Einstein condensate as sites. By tuning the dissipation rate, we observe a progression from ballistic propagation, to near-perfect absorption, to quantum Zeno reflection. Guided by numerical simulations, we identify that optimal absorption occurs when tunneling and dissipation are properly matched, and find qualitative agreement with an idealized semi-infinite model across all dissipation regimes. Our results establish synthetic lattices as a versatile Quantum simulation platform for dissipation-engineered quantum transport and highlight controlled dissipation as a resource for tailoring quantum dynamics.

cond-mat.quant-gas↗

arXiv2026

Dual AGN and Multiple SMBH Systems in the Era of SKAO

We present a radio-oriented review of current strategies for the detection and characterization of dual active galactic nuclei (DAGN) and supermassive black hole binaries (SMBHBs), emphasizing the crucial role of radio interferometry in advancing this field. We discuss how high-resolution radio imaging - particularly through very long baseline interferometry (VLBI) - provides a unique, dust-unbiased tool to identify multiple accreting SMBHs, disentangle AGN-related emission from star formation, and trace components from tens of kpc to sub-parsec scales. We summarize current observational limitations, such as insufficient sensitivity-resolution combination and area coverage. We then outline how the SKAO will overcome these constraints through its unprecedented combination of sensitivity, survey speed, imaging fidelity and angular resolution, enabling the discovery and characterization of dual and binary SMBHs from the nearby Universe to the epoch of reionization. Several science cases are presented, including radio follow-ups of optical/infrared-selected DAGN, direct blind radio selection of DAGN, studies of compact bound SMBHBs, and the link between SMBHB orbital evolution and low-frequency gravitational wave emission. We further emphasize the synergy between SKAO observations and modern and upcoming facilities such as the James Webb and Euclid space telescopes, Rubin Observatory, and gravitational wave detectors including the Laser Interferometer Space Antenna and pulsar timing arrays. These combined capabilities will allow SKAO to enable the first comprehensive radio census of dual and binary SMBH systems, bridge the gap between electromagnetic and gravitational wave observations, and provide a statistically significant view of SMBH pairing, accretion, and merger-driven feedback throughout cosmic history.

astro-ph.HE↗

arXiv2026

Inversion of CHASE H$\alpha$ Spectral Line during Solar Flares Based on RADYN Dataset via Deep Learning

Solar flares represent one of the most intense forms of solar activity. Understanding the evolution of physical parameters in the solar atmosphere during flares is key to studying flare mechanisms and improving prediction capabilities. However, directly measuring quantities such as electron number density, temperature, and plasma velocity remains difficult. Here, we introduce a novel fully connected neural network, trained on synthetic data from the Radiative Hydrodynamics Code (RADYN) simulations, to perform rapid inversion of physical parameters from H$\alpha$ spectral profiles. The spectral data were processed to align with the observational resolution of the CHASE satellite, enabling seamless application of the model to real-world observations. Results demonstrate a high degree of consistency with RADYN simulations, achieving low errors under diverse flare conditions. Furthermore, we applied the developed model to analyze CHASE observations of a class X7.1 solar flare on October 1, 2024. The results reveal reasonable spatial and temporal evolution of key parameters throughout different flare phases. This work demonstrates the potential of deep learning techniques for fast and reliable spectral inversion, providing new tools for solar flare diagnostics based on H$\alpha$ data.

astro-ph.SR↗

arXiv2025

An Adaptive cmWave/FR3 Channel Sounder for Integrated Sensing and Communication

In this paper, we present an advanced channel sounding system designed for sensing and propagation experiments in all types of cellular deployment scenarios. The system's exceptional adaptability, high resolution, and sensitivity makes it an invaluable tool for utilization in a variety of indoor and outdoor measurement campaigns. The sounder has a 2.5 ns delay resolution, 170 dB path loss measurement capability and is able to measure a {360\textdegree} power-angular delay profile of the channel in less than 0.9 ms. Additionally, the system can be easily reconfigured to measure different frequency bands by changing the RF front-end antennas. This versatile sounder is suitable for double directional channel sounding, high-speed vehicular experiments such as vehicle-to-vehicle and vehicle-to-infrastructure communications, and integrated communication and sensing experiments.

cs.IT↗

arXiv2025

A sample of 3403 galaxy clusters identified in XMM-Newton X-ray images

Currently, the number of galaxy clusters identified using galaxy data has far exceeded the number derived from intracluster medium data. In this study, we used positional information from large optical cluster catalogues to search for previously unrecognized X-ray galaxy clusters in archival XMM-Newton data. We successfully identified 1490 galaxy clusters in X-ray images for the first time. By incorporating 1913 previously known X-ray clusters, we constructed a sample of 3403 galaxy clusters observed by XMM-Newton. Our cluster mass estimates show broad consistency with previous measurements. Comparative analyses between the known and newly identified subsamples revealed that new X-ray clusters exhibit systematically higher redshifts, lower masses, and smaller X-ray-to-optical mass ratios, but show no systematic differences in dynamical properties. The newly identified X-ray clusters are a valuable addition to previous X-ray samples and are important for future statistical studies.

astro-ph.GA↗

arXiv2025

Plasmonic detection of Rashba spin-orbit coupling in monolayer transition-metal dichalcogenides

Rashba spin-orbit coupling (RSOC) induces strong momentum-dependent spin splitting and plays a crucial role in fields like spintronics and topological photonics. We here theoretically investigate the collective excitations in monolayer transition metal dichalcogenides (ML-TMDs) hosting RSOC, and conceive an approach to precisely quantify the strength of RSOC using plasmons. We determine the electron energy loss function (EELF) and plasmon dispersions for n-type ML-TMD from the dynamic dielectric function in the framework of the standard random phase approximation (RPA). In this system, both optical and acoustic plasmon modes are observed in the EELF and plasmon dispersions. Moreover, the plasmonic and spectral properties are tunable by electron density and dependent on RSOC. Crucially, we identify a minimum energy gap between the two plasmon modes to serve as a direct spectral signature of the RSOC strength. These results establish plasmons as a non-invasive, precise, and broadly tunable technique for determining RSOC in TMD van der Waals heterostructures and devices.

cond-mat.mes-hall↗

arXiv2025

Schubert defects in Lagrangian Grassmannians

In this paper, we propose a construction of GLSM defects corresponding to Schubert cycles in Lagrangian Grassmannians, following recent work of Closset-Khlaif on Schubert cycles in ordinary Grassmannians. In the case of Lagrangian Grassmannians, there are superpotential terms in both the bulk GLSM as well as on the defect itself, enforcing isotropy constraints. We check our construction by comparing the locus on which the GLSM defect is supported to mathematical descriptions, checking dimensions, and perhaps most importantly, comparing defect indices to known and expected polynomial invariants of the Schubert cycles in quantum cohomology and quantum K theory.

hep-th↗

arXiv2024

Magneto-optical conductivity of monolayer transition metal dichalcogenides in the presence of proximity-induced exchange interaction and external electrical field

We theoretically investigate the magneto-optical (MO) properties of monolayer (ML) transition metal dichalcogenides (TMDs) in the presence of external electrical and quantizing magnetic fields and of the proximity-induced exchange interaction. The corresponding Landau Level (LL) structure is studied by solving the Schr\"odinger equation and the spin polarization in ML-TMDs under the action of the magnetic field is evaluated.The impact of trigonal warping on LLs and MO absorption is examined. Furthermore, the longitudinal MO conductivity is calculated through the dynamical dielectric function under the standard random-phase approximation (RPA) with the Kubo formula. We take ML-MoS$_2$ as an example to examine the effects of proximity-induced exchange interaction, external electrical and magnetic fields on the MO conductivity induced via intra- and interband electronic transitions among the LLs. For intraband electronic transitions within the conduction or valence bands, we can observe two absorption peaks in terahertz (THz) frequency range. While the interband electronic transitions between conduction and valence LLs show a series of absorption peaks in the visible range. We find that the proximity-induced exchange interaction, the carrier density, the strengths of the external electrical and magnetic fields can effectively modulate the positions of the absorption peaks and the shapes of the MO absorption spectra. The results obtained from this study can benefit to an in-depth understanding of the MO properties of ML-TMDs which can be potentially applied for magneto-optic, spintronic and valleytronic devices working in visible to THz frequency bandwidths.

cond-mat.mes-hall↗

arXiv2024

Longitudinal and transverse mobilities of $n$-type monolayer transition metal dichalcogenides in the presence of proximity-induced interactions at low temperature

We present a detailed theoretical investigation on the electronic transport properties of $n$-type monolayer (ML) transition metal dichalcogenides (TMDs) at low temperature in the presence of proximity-induced interactions such as Rashba spin-orbit coupling (RSOC) and the exchange interaction. The electronic band structure is calculated by solving the Schr\"{o}dinger equation with a $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian, and the electric screening induced by electron-electron interaction is evaluated under a standard random phase approximation approach. In particular, the longitudinal and transverse or Hall mobilities are calculated by using a momentum-balance equation derived from a semi-classical Boltzmann equation, where the electron-impurity interaction is considered as the principal scattering center at low temperature. The obtained results show that the RSOC can induce the in-plane spin components for spin-split subbands in different valleys, while the exchange interaction can lift the energy degeneracy for electrons in different valleys. The opposite signs of Berry curvatures in the two valleys would introduce opposite directions of Lorentz force on valley electrons. As a result, the transverse currents from nondegenerate valleys can no longer be canceled out so that the transverse current or Hall mobility can be observed. Interestingly, we find that at a fixed effective Zeeman field, the lowest spin-split conduction subband in ML-TMDs can be tuned from one in the $K'$-valley to one in the $K$-valley by varying the Rashba parameter. The occupation of electrons in different valleys also varies with changing carrier density. Therefore, we can change the magnitude and direction of the Hall current by varying the Rashba parameter, effective Zeeman field, and carrier density by, e.g., the presence of a ferromagnetic substrate and/or applying a gate voltage.

cond-mat.mes-hall↗

arXiv2024

An assay-based background projection for the MAJORANA DEMONSTRATOR using Monte Carlo Uncertainty Propagation

The background index is an important quantity which is used in projecting and calculating the half-life sensitivity of neutrinoless double-beta decay ($0\nu\beta\beta$) experiments. A novel analysis framework is presented to calculate the background index using the specific activities, masses and simulated efficiencies of an experiment's components as distributions. This Bayesian framework includes a unified approach to combine specific activities from assay. Monte Carlo uncertainty propagation is used to build a background index distribution from the specific activity, mass and efficiency distributions. This analysis method is applied to the MAJORANA DEMONSTRATOR, which deployed arrays of high-purity Ge detectors enriched in $^{76}$Ge to search for $0\nu\beta\beta$. The framework projects a mean background index of $\left[8.95 \pm 0.36\right] \times 10^{-4}$cts/(keV kg yr) from $^{232}$Th and $^{238}$U in the DEMONSTRATOR's components.

nucl-ex↗

arXiv2024

Suppression of Edge Localized Modes in ITER Baseline Scenario in EAST using Edge Localized Magnetic Perturbations

We report the suppression of Type-I Edge Localized Modes (ELMs) in the EAST tokamak under ITER baseline conditions using $n = 4$ Resonant Magnetic Perturbations (RMPs), while maintaining energy confinement. Achieving RMP-ELM suppression requires a normalized plasma beta ($\beta_N$) exceeding 1.8 in a target plasma with $q_{95}\approx 3.1$ and tungsten divertors. Quasi-linear modeling shows high plasma beta enhances RMP-driven neoclassical toroidal viscosity torque, reducing field penetration thresholds. These findings demonstrate the feasibility and efficiency of high $n$ RMPs for ELM suppression in ITER.

physics.plasm-ph↗

arXiv2024

Examining LEGEND-1000 cosmogenic neutron backgrounds in Geant4 and MCNP

For next-generation neutrinoless double beta decay experiments, extremely low backgrounds are necessary. An understanding of in-situ cosmogenic backgrounds is critical to the design effort. In-situ cosmogenic backgrounds impose a depth requirement and especially impact the choice of host laboratory. Often, simulations are used to understand background effects, and these simulations can have large uncertainties. One way to characterize the systematic uncertainties is to compare unalike simulation programs. In this paper, a suite of neutron simulations with identical geometries and starting parameters have been performed with Geant4 and MCNP, using geometries relevant to the LEGEND-1000 experiment. This study is an important step in gauging the uncertainties of simulations-based estimates. To reduce project risks associated with simulation uncertainties, a novel alternative shield of methane-doped liquid argon is considered in this paper for LEGEND-1000, which could achieve large background reduction without requiring significant modification to the baseline design.

physics.ins-det↗

arXiv2024

Magnetically Driven Relativistic Jet in the High-Redshift Blazar OH~471

Context : Understanding the mechanisms that launch and shape powerful relativistic jets from supermassive black holes (SMBHs) in high-redshift active galactic nuclei (AGN) is crucial for probing the co-evolution of SMBHs and galaxies over cosmic time. Aims :We study the high-redshift ($z=3.396$) blazar OH~471 to explore the jet launching mechanism in the early Universe. Methods : Using multi-frequency radio monitoring observations and high-resolution Very Long Baseline Interferometry imaging over three decades, we study the milliarcsecond structure and long-term variability of OH~471. Results : Spectral modelling of the radio flux densities reveals a synchrotron self-absorbed spectrum indicating strong magnetic fields within the compact core. By applying the flux freezing approximation, we estimate the magnetic flux carried by the jet and find that it reaches or exceeds theoretical predictions for jets powered by black hole spin energy via the Blandford-Znajek mechanism. This implies that OH~471 was in a magnetically arrested disk (MAD) state where the magnetic flux accumulated near the horizon regulates the accretion flow, allowing efficient extraction of black hole rotational energy. Conclusions : Our study demonstrates the dominance of MAD accretion in powering the prominent radio flares and relativistic jets observed in the radio-loud AGN OH~471 and statistical studies of large samples of high-redshift AGN will shed light on the role of MAD accretion in launching and accelerating the earliest relativistic jets.

astro-ph.GA↗

arXiv2024

Optical absorption window in Na$_3$Bi based three-dimensional Dirac electronic system

We present a detailed theoretical study of the optoelectronic properties of a Na$_3$Bi-based three-dimensional Dirac electronic system (3DDES). The optical conductivity is evaluated using the energy-balance equation derived from a Boltzmann equation, where the electron Hamiltonian is taken from a simplified $\mathbf{k}\cdotp \mathbf{p}$ approach. We find that for short-wavelength irradiation, the optical absorption in Na$_3$Bi is mainly due to inter-band electronic transitions. In contrast to the universal optical conductance observed for graphene, the optical conductivity for Na$_3$Bi based 3DDES depends on the radiation frequency but not on temperature, carrier density and electronic relaxation time. In the radiation wavelength regime of about 5 $\mu m<\lambda<$ 200 $\mu m$, an optical absorption window is found. This is similar to what is observed in graphene. The position and width of the absorption window depend on the direction of the light polarization and sensitively on temperature, carrier density, and electronic relaxation time. Particularly, we demonstrate that the inter-band optical absorption channel can be switched on and off by applying the gate voltage. This implies that similar to graphene, Na$_3$Bi based 3DDES can also be applied in infrared electro-optical modulators. Our theoretical findings are helpful in gaining an in-depth understanding of the basic optoelectronic properties of recently discovered 3DDESs.

cond-mat.mtrl-sci↗

arXiv2023

Optical spectrum of $n$-type and $p$-type monolayer MoS$_2$ in the presence of proximity-induced interactions

In this paper, we examined the effects of proximity-induced interactions such as Rashba spin-orbit coupling (SOC) and effective Zeeman fields (EZFs) on the optical spectrum of $n$-type and $p$-type monolayer (ML)-MoS$_2$. The optical conductivity is evaluated using the standard Kubo formula under Random phase approximation (RPA) with including the effective electron-electron interaction. It has been found that there exists two absorption peaks in $n$-type ML-MoS$_{2}$ and two knife shaped absorptions in $p$-type ML-MoS$_{2}$ which are contributed by the inter-subband spin flip electronic transitions within conduction and valence bands at valleys $K$ and $K'$ with a lifted valley degeneracy. The optical absorptions in $n$-type and $p$-type ML-MoS$_{2}$ occur in THz and infrared radiation regimes and the position, height, and shape of them can be effectively tuned by Rashba parameter, EZFs parameters, and carrier density. The interesting theoretical predictions in this study would be helpful for the experimental observation of the optical absorption in infrared to THz bandwidths contributed by inter-subband spin flip electronic transitions in a lifted valley degeneracy monolayer transition metal dichalcogenides (ML-TMDs) system. The obtained results indicate that ML-MoS$_{2}$ with the platform of proximity interactions make it a promising infrared and THz material for optics and optoelectronics.

cond-mat.mes-hall↗

arXiv2023

Majorana Demonstrator Data Release for AI/ML Applications

The enclosed data release consists of a subset of the calibration data from the Majorana Demonstrator experiment. Each Majorana event is accompanied by raw Germanium detector waveforms, pulse shape discrimination cuts, and calibrated final energies, all shared in an HDF5 file format along with relevant metadata. This release is specifically designed to support the training and testing of Artificial Intelligence (AI) and Machine Learning (ML) algorithms upon our data. This document is structured as follows. Section I provides an overview of the dataset's content and format; Section II outlines the location of this dataset and the method for accessing it; Section III presents the NPML Machine Learning Challenge associated with this dataset; Section IV contains a disclaimer from the Majorana collaboration regarding the use of this dataset; Appendix A contains technical details of this data release. Please direct questions about the material provided within this release to liaobo77@ucsd.edu (A. Li).

cs.LG↗

arXiv2023

Quantum K theory rings of partial flag manifolds

In this paper we use three-dimensional gauged linear sigma models to make physical predictions for Whitney-type presentations of equivariant quantum K theory rings of partial flag manifolds, as quantum products of universal subbundles and various ratios, extending previous work for Grassmannians. Physically, these arise as OPEs of Wilson lines for certain Chern-Simons levels. We also include a simplified method for computing Chern-Simons levels pertinent to standard quantum K theory.

hep-th↗

arXiv2023

The First GECAM Observation Results on Terrestrial Gamma-ray Flashes and Terrestrial Electron Beams

Gravitational-wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a space-borne instrument dedicated to monitoring high-energy transients, including Terrestrial Gamma-ray Flashes (TGFs) and Terrestrial Electron Beams (TEBs). We implemented a TGF/TEB search algorithm for GECAM, with which 147 bright TGFs, 2 typical TEBs and 2 special TEB-like events are identified during an effective observation time of $\sim$9 months. We show that, with gamma-ray and charged particle detectors, GECAM can effectively identify and distinguish TGFs and TEBs, and measure their temporal and spectral properties in detail. A very high TGF-lightning association rate of $\sim$80\% is obtained between GECAM and GLD360 in east Asia region.

astro-ph.HE↗