arXiv subjects

R. Yang

Publications and source records attributed to R. Yang.

At least 19 recordsLinked to original sources

arXiv2026

Camera Calibration Using Inaccurate and Asynchronous Discrete GPS Trajectory from Drones

This paper considers a stationary camera calibration problem, which estimates the camera orientation angles yaw, pitch and roll, using a drone trajectory recorded by a GPS. There are three challenges in using a GPS trajectory as ground truth for camera calibration. One, the altitude of GPS data is inaccurate with an unknown bias. Two, the GPS receiver and camera are not time synchronized, and there is an unknown time offset between the two systems. Three, the GPS trajectory is time-discrete and accurate interpolation is needed. This is actually an estimation problem since velocity is also needed. To address the first two challenges, we formulate the problem as a parameter estimation problem to estimate a vector consisting of the GPS altitude bias and time offset in addition to the camera yaw, pitch and roll biases. We then develop a special maximum likelihood estimator using the Iterated Least Squares algorithm which can work with a non-synchronized time-discrete GPS trajectory for the third challenge. Since the camera measurement errors are usually small, this requires a high calibration accuracy so that the residual bias error following the calibration should not be significant compared to the measurement error standard deviation. The calibration accuracy depends highly on the drone trajectory. This paper also recommends an appropriate drone trajectory which can yield a good calibration accuracy, namely, 14\% of the measurement error standard deviation. Simulation tests are conducted to demonstrate the algorithm performance. The estimation results meet the Cramer-Rao Lower Bound (CRLB) since the Normalized Estimation Error Squared w.r.t.\ the CRLB is statistically acceptable.

eess.SY↗

arXiv2026

C-SWIM: A Coupled Space Weather Impact Model for Satellite Fleet Vulnerability and Economic Loss Under a 1-in-100-Year Solar Energetic Particle Event

Modern economies depend critically on satellite infrastructure, yet the aggregate economic consequences of extreme solar energetic particle (SEP) events have not been rigorously assessed. This study develops an integrated framework linking SEP hazard characterization, dynamic geomagnetic cutoff rigidity modeling, radiation dose transport, and fleet-wide failure probability estimation to macroeconomic impact analysis. Using extreme-value analysis of 160 SEP events over 27.4 years (1996-2025), failure probability is estimated for ~10,650 US operational satellites under orbital regime-dependent shielding assumptions. The assessment reveals that ~100 satellites (1.0%) are at Critical risk, concentrated in high-altitude low Earth orbit and highly elliptical orbit, while medium Earth orbit and geosynchronous orbit satellites fall in the Negligible class (P_fail < 10^-9) under the assumed radiation-hardened components and shielding. The expected capital loss across the ~$254B fleet totals ~$5.2B. Three failure scenarios, expanding from Critical satellites only (P_fail > 10^-2), to Critical and Elevated (P_fail > 10^-3), and to all satellites with non-negligible risk (P_fail > 10^-6), yield daily economic impacts of ~$70M, ~$270M, and ~$1.3B, respectively. Earth observation suffers up to 95.6% capacity loss in the worst case, while military services experience 16.1-20.4% disruption across scenarios. Results are first-order estimates: hardware failure counts are conservative because only total ionizing dose is modeled, and daily economic impacts represent upper bounds because operator response and recovery are not included.

physics.space-ph↗

arXiv2026

A Comparative Multi-Hazard Risk Assessment of the US High-Voltage Transmission Network

Modern economies depend critically on high-voltage power transmission networks. Yet this infrastructure is routinely disrupted by natural hazards ranging from earthquakes and floods to tornadoes and geomagnetic storms. Risk assessments have historically addressed hazards in isolation, leaving no common basis for comparing economic impacts across the full hazard portfolio. This study addresses this gap by developing an integrated framework linking hazard characterization, fragility modeling, and macroeconomic impact propagation. The framework is applied consistently across nine primary hazards and one compound freezing rain and wind gust hazard. Using national hazard datasets and a US high-voltage transmission network of over 13,000 line segments and 10,000 substations, we derive failure probabilities, expected damage, affected population, and downstream economic output losses. Among individual hazards, tropical cyclone wind produces the largest expected daily damage at $137 M/day, followed by lightning at $87 M/day, earthquake at $47 M/day, flood at $46 M/day, tornado at $42 M/day, and landslide at $34 M/day. Downstream economic output losses are largest for tornado at $4.93 B/day, followed by flood at $3.59 B/day and earthquake at $3.02 B/day. A 250-year geomagnetic storm produces $2.07 B/day, placing space weather within the range of major terrestrial hazards. The compound freezing rain and wind gust scenario produces the largest stress-test disruption, affecting 237.4 M people and yielding a modeled downstream output loss of $85.16 B/day. These results should be interpreted as first-order bounding estimates, with the compound scenario representing an upper-bound stress test. Overall, the framework establishes a consistent baseline for prioritizing investments in transmission network resilience.

econ.EM↗

arXiv2024

Optical evidence of the band reconstruction during the charge-density wave transition in annealed Kagome magnet FeGe

In Kagome magnet FeGe, the coexistence of electron correlation, charge-density wave (CDW), and magnetism renders it ideal to study their interactions. Here, we combined the optical spectroscopy and the first-principles calculations to investigate the band structures of FeGe annealed at different temperatures. Our observations reveal that the sample annealed at 320C experienced dramatic change in optical conductivity following the CDW transition. Specifically, a substantial portion of the spectral weight (SW) in the low-energy region ( < 0.4 eV) was redistributed to the high-energy region (0.8 - 1.5 eV), suggesting a reconstruction of the band structure. The sample annealed at 560 C did not exhibit a CDW transition, but its SW transfer occurred progressively from 300 to 5 K. We noticed that: i) after the CDW transition, the sample annealed at 320 C showed similar tendency of SW transfer to that of the 560 C annealed sample; ii) the high-energy SW of both materials displayed a temperature dependence consistent with the magnetic roperties. Combining the first-principles calculations, we attribute the SW transfer to the band reconstruction triggered by the distortion of Ge1 atoms induced either by annealing at 560C or by the CDW transitions. This lattice distortion affects the energies of Fe 3d orbitals. Under the influence of Hund's rule coupling, the magnetic moment of Fe atoms is enhanced. Our findings elucidate the interactions among charge, lattice, and spin in FeGe, offering pivotal insights to modulate properties of this Kagome magnet.

cond-mat.mtrl-sci↗

arXiv2024

Inherent circular dichroism of phonons in magnetic Weyl semimetal Co3Sn2S2

We investigated the infrared-active phonons in ferromagnetic Weyl semimetal Co3Sn3S3 using optical spectroscopy. Below the Curie temperature (T~175~K), we observed asymmetric Fano lineshapes of phonons peaks in the optical conductivities, reflecting the presence of electron-phonon coupling (EPC). Additionally, the detected phonon signals by the polar Kerr rotation and the ellipticity spectroscopy indicate the circular dichroism (CD) of phonons. We attribute the CD of phonons to their distinct couplings with charge excitations on the tilted Weyl nodal rings in two circularly polarized channels. Our findings provide experimental evidence that, without external fields, phonons can also become circularly polarized by coupling with the electronic topology. Since the magnetic exchange splitting gradually shifts the topological bands in Co3Sn2S2, the CD of phonons exhibits significant temperature dependence, hinting at a promising approach for manipulation.

cond-mat.mtrl-sci↗

arXiv2023

ForMAX -- a beamline for multiscale and multimodal structural characterization of hierarchical materials

The ForMAX beamline at the MAX IV Laboratory provides multiscale and multimodal structural characterization of hierarchical materials in the nm to mm range by combining small- and wide-angle x-ray scattering with full-field microtomography. The modular design of the beamline is optimized for easy switching between different experimental modalities. The beamline has a special focus on the development of novel, fibrous materials from forest resources, but it is also well suited for studies within, e.g., food science and biomedical research.

physics.ins-det↗

arXiv2023

Surface skyrmions and dual topological Hall effect in antiferromagnetic topological insulator EuCd$_2$As$_2$

In this work, we synthesized single crystal of EuCd$_2$As$_2$, which exhibits A-type antiferromagnetic (AFM) order with in-plane spin orientation below $T_N$ = 9.5~K.Optical spectroscopy and transport measurements suggest its topological insulator (TI) nature with an insulating gap around 0.1eV. Remarkably, a dual topological Hall resistivity that exhibits same magnitude but opposite signs in the positive to negative and negative to positive magnetic field hysteresis branches emerges below 20~K. With magnetic force microscopy (MFM) images and numerical simulations, we attribute the dual topological Hall effect to the N\'{e}el-type skyrmions stabilized by the interactions between topological surface states and magnetism, and the sign reversal in different hysteresis branches indicates potential coexistence of skyrmions and antiskyrmions. Our work uncovers a unique two-dimensional (2D) magnetism on the surface of intrinsic AFM TI, providing a promising platform for novel topological quantum states and AFM spintronic applications.

cond-mat.supr-con↗

arXiv2023

Charge-density wave transition in magnetic topological semimetal EuAl$_4$

The interplay among topology, charge-density wave (CDW), and magnetism can give rise to a plethora of exotic quantum phenomena. Recently, a group of magnetic topological semimetals with tetragonal lattices and CDW order were found to exhibit anomalous magnetic instability, helical spin ordering, and the presence of skyrmions. However, the underlying mechanism responsible for these observations remains unclear. Here, we conducted a comprehensive investigation into the impact of CDW on the topological and magnetic properties of EuAl$_4$ using optical spectroscopy and the first-principles calculations. Through optical spectroscopy, we observed a partial gap (60~meV) on the Fermi surface and an enhanced mid-infrared absorption around 0.4~eV after the CDW transition. Magneto-optical spectroscopy and the first-principles calculations proved that, by affecting the band structure, the CDW order frustrates the antiferromagnetic interactions but strengthened the ferromagnetic ones, which can destabilize the magnetism. With lower symmetry in the CDW ordered state, carriers from the Weyl bands will mediate the anisotropic magnetic interactions promoting the formation of chiral spin textures. Conversely, without the CDW order, the counterpart EuGa$_4$ shows robust collinear antiferromagnetic order. Our findings uncover the pivotal role played by CDW order in arousing intricate magnetism in topological materials and provide valuable insights into controlling topological and magnetic properties through the manipulation of CDW orders.

cond-mat.supr-con↗

arXiv2023

Impact of disorder in the charge-density-wave state of Pd-intercalated ErTe$_3$ revealed by the electrodynamic response

It is a general notion that disorder, introduced by either chemical substitution or intercalation as well as by electron-irradiation, is detrimental to the realisation of long-range charge-density-wave (CDW) order. We study the disorder-induced suppression of the in-plane CDW orders in the two-dimensional Pd-intercalated ErTe$_3$ compositions, by exploring the real part of the optical conductivity with light polarised along the in-plane $a$ and $c$ axes. Our findings reveal an anisotropic charge dynamics with respect to both incommensurate unidirectional CDW phases of ErTe$_3$, occurring within the $ac$-plane. The anisotropic optical response gets substantially washed out with Pd-intercalation, hand-in-hand with the suppression of both CDW orders. The spectral weight analysis though advances the scenario, for which the CDW phases evolve from a (partially) depleted Fermi surface already above their critical onset temperatures. We therefore argue that the long-range CDW orders of ErTe$_3$ tend to be progressively dwarfed by Pd-intercalation, which favours the presence of short-range CDW segments for both crystallographic directions persisting in a broad temperature ($T$) interval up to the normal state, and being suggestive of precursor effects of the CDW orders as well as possibly coexisting with superconductivity at low $T$.

cond-mat.str-el↗

arXiv2023

Optical fingerprints of the electronic band reconstruction in van der Waals magnetic materials

We report a broadband study of the charge dynamics in the van der Waals (vdW) magnetic materials 2H-$M_x$TaS$_2$ ($M$ = Mn and Co), which span the onset of both long-range antiferromagnetic (AFM) and ferromagnetic (FM) order, depending on the intercalation $M$ and its concentration $x$. We discover a spectral weight ($SW$) shift from high to low energy scales for FM compositions, while reversely $SW$ is removed from low towards high spectral energies for AFM compounds. This maps the related reconstruction of the electronic band structure along the crossover from the FM to AFM order, which restores an occupation balance in the density of states between spin majority and minority bands of the intercalated 3$d$ elements.

cond-mat.mtrl-sci↗

arXiv2022

Optimizing for periodicity: a model-independent approach to flux crosstalk calibration for superconducting circuits

Flux tunability is an important engineering resource for superconducting circuits. Large-scale quantum computers based on flux-tunable superconducting circuits face the problem of flux crosstalk, which needs to be accurately calibrated to realize high-fidelity quantum operations. Typical calibration methods either assume that circuit elements can be effectively decoupled and simple models can be applied, or require a large amount of data. Such methods become ineffective as the system size increases and circuit interactions become stronger. Here we propose a new method for calibrating flux crosstalk, which is independent of the underlying circuit model. Using the fundamental property that superconducting circuits respond periodically to external fluxes, crosstalk calibration of N flux channels can be treated as N independent optimization problems, with the objective functions being the periodicity of a measured signal depending on the compensation parameters. We demonstrate this method on a small-scale quantum annealing circuit based on superconducting flux qubits, achieving comparable accuracy with previous methods. We also show that the objective function usually has a nearly convex landscape, allowing efficient optimization.

quant-ph↗

arXiv2021

Investigations on Cmc2$_1$-Si$_2$P$_2$X structures and physical properties by first-principles calculations

The new structures, Cmc2$_1$-Si$_2$P$_2$X (X=S, Se, Te, and Po), are predicted, and their mechanical, electronic and optical properties are investigated with the density functional theory, by first principles calculations. The elastic constants of the four compounds are calculated by the stress-strain method. The calculations of the elastic stability criteria and phonon dispersion spectra imply that they are mechanically and dynamically stable at zero pressure. The mechanical parameters, such as shear moduli $G$, bulk moduli $B$, Young's moduli $E$ and Poisson's ratios $v$ are evaluated by the Voigt-Reuss-Hill approach. The Cmc2$_1$-Si$_2$P$_2$X has the largest hardness due to the largest Young's modulus in the four compounds, and it is a covalent crystal. The anisotropies of their mechanical properties are also analyzed. The band structures and densities of states, which are calculated by using HSE06, show that Cmc2$_1$-Si$_2$P$_2$X compounds are indirect bandgap semiconductors, and the values of the band gaps decrease with increasing atomic number from S, Se, Te, to Po. In addition, the longitudinal sound velocity and transverse sound velocity for Cmc2$_1$-Si$_2$P$_2$X have been investigated. The dielectric constant, electron energy loss, refractive index, reflectivity, absorption and conductivity are analyzed to gain the optical properties of Si$_2$P$_2$X.

cond-mat.mtrl-sci↗

arXiv2021

Beam halo from Touschek scattering in the KEK Accelerator Test Facility

Beam halo is one of the crucial issues limiting the machine performance and causing radioactivation in high-intensity accelerators. A clear picture of beam-halo formation is of great importance for successful suppression of the undesired beam loss. We present numerical and experimental studies of transverse and longitudinal halos in the KEK Accelerator Test Facility. The fair accordance between predictions and observations in various conditions indicates that the Touschek scattering is the dominant mechanism forming the horizontal and momentum halos.

physics.acc-ph↗

arXiv2021

Real-time Electron Solvation Induced by Bursts of Laser-accelerated Protons in Liquid Water

Understanding the mechanisms of proton energy deposition in matter and subsequent damage formation is fundamental to radiation science. Here we exploit the picosecond (10^-12 s) resolution of laser-driven accelerators to track ultra-fast solvation dynamics for electrons due to proton radiolysis in liquid water (H2O). Comparing these results with modelling that assumes initial conditions similar to those found in photolysis reveals that solvation time due to protons is extended by > 20 ps. Supported by magneto-hydrodynamic theory this indicates a highly dynamic phase in the immediate aftermath of the proton interaction that is not accounted for in current models.

physics.plasm-ph↗

arXiv2020

Can amplified spontaneous emission produce intense laser guide stars for adaptive optics?

Adaptive optics (AO) is a key technology for ground-based optical and infrared astronomy, providing high angular resolution and sensitivity. AO systems employing laser guide stars (LGS) can achieve high sky coverage, but their performance is limited by LGS return flux. We examine the potential of two new approaches that might produce high-intensity atmospheric laser beacons. Amplified spontaneous emission could potentially boost the intensity of beacons produced by conventional resonant excitation of atomic or molecular species in the upper atmosphere. This requires the production of a population inversion in an electronic transition that is optically-thick to stimulated emission. Potential excitation mechanisms include continuous wave pumping, pulsed excitation and plasma generation. Alternatively, a high-power femtosecond pulsed laser could produce a white-light supercontinuum high in the atmosphere. The broad-band emission from such a source could also facilitate the sensing of the tilt component of atmospheric turbulence.

astro-ph.IM↗

arXiv2020

The Hund's Superconductor Li(Fe,Co)As

We combine transport, angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy to investigate several low energy manifestations of the Hund coupling in a canonical FeSC family Li(Fe,Co)As. We determine the doping dependence of the coherent-incoherent crossover temperature and the quasi-particle effective mass enhancement in the normal state. Our tunneling spectroscopy result in the superconducting state supports the idea that superconductivity emerging from Hund's metal state displays a universal maximal superconducting gap vs transition temperature (2$\Delta_{max}/k_{B}T_{c}$) value, which is independent of doping level and $T_{c}$.

cond-mat.supr-con↗

arXiv2020

Direct Observation of Coherent Longitudinal and Shear Acoustic Phonons in TaAs Using Ultrafast X-ray Diffraction

Using femtosecond time-resolved X-ray diffraction, we investigated optically excited coherent acoustic phonons in the Weyl semimetal TaAs. The low symmetry of the (112) surface probed in our experiment enables the simultaneous excitation of longitudinal and shear acoustic modes, whose dispersion closely matches our simulations. We observed an asymmetry in the spectral lineshape of the longitudinal mode that is notably absent from the shear mode, suggesting a time-dependent frequency chirp that is likely driven by photoinduced carrier diffusion. We argue on the basis of symmetry that these acoustic deformations can transiently alter the electronic structure near the Weyl points and support this with model calculations. Our study underscores the benefit of using off-axis crystal orientations when optically exciting acoustic deformations in topological semimetals, allowing one to transiently change their crystal and electronic structures.

cond-mat.mtrl-sci↗

arXiv2020

Electronic correlations in the van der Waals ferromagnet Fe$_3$GeTe$_2$ revealed by its charge dynamics

The layered van der Waals ferromagnetic Fe$_3$GeTe$_2$ harbours an unconventional interplay between topology and magnetism, leading to a large anomalous Hall conductivity at low temperatures. Here, we investigate the temperature dependence of its charge dynamics and reveal that upon entering the ferromagnetic state at $T_C \sim 200$ K and further lowering the temperature there is the onset of a gradual spectral weight reshuffling from the mid-infrared range towards far- as well as near-infrared frequencies. This two-fold spectral weight transfer indicates the important role of the Hund's coupling as primary source for electronic correlations and signals an incoherent-coherent crossover at low temperatures. Our findings also convey the electronic environment, based on nodal-line topological states, favouring the large anomalous Hall conductivity.

cond-mat.str-el↗