arXiv subjects

X. Gu

Publications and source records attributed to X. Gu.

16 recordsLinked to original sources

arXiv2026

A Novel Numerical Algorithms Optimization Method with Machine Learning Frameworks: Application on Real-time Plasmas Equilibrium Reconstruction in EXL-50U Spherical Torus

This work proposes for the first time a novel optimization method for numerical algorithms, which takes advantages of machine learning frameworks PyTorch and TensorRT, leveraging their modularity, low development threshold, and automatic tuning characteristics to achieve a real-time plasmas reconstruction algorithm called PTEFIT as an application in tokamak-based controlled fusion that combines performance, flexibility, and usability. The algorithm has been deployed and routinely operated on the EXL-50U spherical tokamak, with an average inference time of only 0.268ms per time slice at $129\times 129$ resolution, and has successfully driven feedback control of the maximum radial position of plasmas and isoflux control. We believe that its design philosophy has sufficient potential to accelerate development and optimization in GPU parallel computing, and is expected to be extended to other numerical algorithms.

physics.plasm-ph↗

arXiv2025

Improving Optics Control and Measurement at RHIC

Maximizing luminosity requires precise control of the optics function at the interaction point (IP), implying that the location ($s^*$) of the beta function's minimum value ($\beta^*$) must be moved to the collision location ($s_{IP}$) as much as possible. Accurate optics measurements and reliable control of $s^*$ in both planes are therefore essential for optimal collider performance. During Relativistic Heavy Ion Collider (RHIC) operations in 2024, measurements indicate an average horizontal beta beat of approximately $20\%$ at IP8, accompanied by measurement variation in measured $s^*$ in both planes. In this paper, a sensitivity-matrix-based optics correction scheme is demonstrated to effectively steer the optics toward desired targets using power supply currents of IR quadrupoles in the 8 o'clock interaction region (IR8). In addition, a method for measuring linear optics based on the one-turn map within the interaction regions is developed and systematically compared with established optics measurement methods used in RHIC operations. A comprehensive error analysis is performed for all measurement methods considered. Through these methods, a consistent reduction of $10\%$ beta beat is achieved by moving $s^*_x$ as well as a significant improvement in the reproducibility of $s^*$ measurements in both planes. The techniques demonstrated here will be further developed to support linear optics analysis and control of the future Electron-Ion Collider project.

physics.acc-ph↗

arXiv2023

Orbital-selective charge-density wave in TaTe$_4$

TaTe$_4$, a metallic charge-density wave (CDW) material discovered decades ago, has attracted renewed attention due to its rich interesting properties such as pressure-induced superconductivity and candidate non-trivial topological phase. Here, using high-resolution angle-resolved photoemission spectroscopy and ab-initio calculation, we systematically investigate the electronic structure of TaTe$_4$. At 26 K, we observe a CDW gap as large as 290 meV, which persists up to 500 K. The CDW-modulated band structure shows a complex reconstruction that closely correlates with the lattice distortion. Inside the CDW gap, there exist highly dispersive energy bands contributing to the remnant Fermi surface and metallic behavior in the CDW state. Interestingly, our ab-initio calculation reveals that the large CDW gap mainly opens in the electronic states with out-of-plane orbital components, while the in-gap metallic states originate from in-plane orbitals, suggesting an orbital texture that couples with the CDW order. Our results shed light on the interplay between electron, lattice, and orbital in quasi-one-dimensional CDW materials.

cond-mat.str-el↗

arXiv2023

Evidence for Band Renormalizations in Strong-coupling Superconducting Alkali-fulleride Films

There has been a long-standing debate about the mechanism of the unusual superconductivity in alkali-intercalated fulleride superconductors. In this work, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of superconducting K3C60 thin films. We observe a dispersive energy band crossing the Fermi level with an occupied bandwidth of about 130 meV. The measured band structure shows prominent quasiparticle kinks and a replica band involving high-energy Jahn-Teller active Hg(8) phonon mode, reflecting strong electron-phonon coupling in the system. The electron-phonon coupling constant is estimated to be about 1.2, which dominates the quasiparticle mass renormalization. Moreover, we observe an isotropic nodeless superconducting gap beyond the mean-field estimation. Both the large electron-phonon coupling constant and large reduced superconducting gap suggest a strong-coupling superconductivity in K3C60, while the electronic correlation effect is indicated by the observation of a waterfall-like band dispersion and the small bandwidth compared with the effective Coulomb interaction. Our results not only directly visualize the crucial band structure of superconducting fulleride but also provide important insights into the mechanism of the unusual superconductivity.

cond-mat.supr-con↗

arXiv2023

Thermal hysteretic behavior and negative magnetoresistance in an unusual charge-density-wave material EuTe4

EuTe4 is a newly-discovered van der Waals material exhibiting a novel charge-density wave (CDW) with a large thermal hysteresis in the resistivity and CDW gap. In this work, we systematically study the electronic structure and transport properties of EuTe4 using high-resolution angle-resolved photoemission spectroscopy (ARPES), magnetoresistance measurements, and scanning tunneling microscopy (STM). We observe a CDW gap of about 200 meV at low temperatures that persists up to 400 K, suggesting that the CDW transition occurs at a much higher temperature. We observe a large thermal hysteretic behavior of the ARPES intensity near the Fermi level, consistent with the resistivity measurement. The hysteresis in the resistivity measurement does not change under a magnetic field up to 7 T, excluding the thermal magnetic hysteresis mechanism. Instead, the surface topography measured with STM shows surface domains with different CDW trimerization directions, which may be important for the thermal hysteretic behavior of EuTe4. Interestingly, we observe a large negative magnetoresistance at low temperatures that can be associated with the canting of magnetically ordered Eu spins. Our work shed light on the understanding of magnetic, transport, and electronic properties of EuTe4.

cond-mat.str-el↗

arXiv2023

Development of a Laser-based angle-resolved-photoemission spectrometer with sub-micrometer spatial resolution and high-efficiency spin detection

Angle-resolved photoemission spectroscopy with sub-micrometer spatial resolution ({\mu}-ARPES), has become a powerful tool for studying quantum materials. To achieve sub-micrometer or even nanometer-scale spatial resolution, it is important to focus the incident light beam (usually from the synchrotron radiation) using X-ray optics such as the zone plate or ellipsoidal capillary mirrors. Recently, we developed a laser-based {\mu}-ARPES with spin-resolution (LMS-ARPES). The 177 nm laser beam is achieved by frequency doubling a 355 nm beam using a KBBF crystal and subsequently focused using an optical lens with a focal length of about 16 mm. By characterizing the focused spot size using different methods and performing spatial-scanning photoemission measurement, we confirm the sub-micron spatial resolution of the system. Compared with the {\mu}-ARPES facilities based on synchrotron radiation, our LMS-ARPES system is not only more economical and convenient but also with higher photon flux (> 5E13 photons/s), thus enabling the high-resolution and high-statistics measurements. Moreover, the system is equipped with a two-dimensional spin detector based on exchange scattering at a surface-passivated iron film grown on a W(100) substrate. We investigate the spin structure of the prototype topological insulator Bi2Se3 and reveal a high spin-polarization rate, confirming its spin-momentum locking property. This lab-based LMS-ARPES will be a powerful research tool for studying the local fine electronic structures of different condensed matter systems, including topological quantum materials, mesoscopic materials and structures, and phase-separated materials.

cond-mat.mtrl-sci↗

arXiv2022

Crossed Luttinger Liquid Hidden in a Quasi-two-dimensional Material {\eta}-Mo4O11

Although the concept of Luttinger liquid (LL) that describes a one-dimensional (1D) interacting fermion system collapses in higher dimensions, it has been proposed to be closely related to many mysteries including the normal state of cuprate superconductor, unconventional metal, and quantum criticality. Therefore, the generalization of LL model to higher dimensions has attracted substantial research attention. Here we systematically investigate the electronic structure of a quasi-2D compound {\eta}-Mo4O11 using high-resolution angle-resolved photoemission spectroscopy and ab-initio calculation. Remarkably, we reveal a prototypical LL behavior originating from the crossing quasi-1D chain arrays hidden in the quasi-2D crystal structure. Our results suggest that {\eta}-Mo4O11 materializes the long sought-after crossed LL phase, where the orthogonal orbital components significantly reduce the coupling between intersecting quasi-1D chains and therefore maintain the essential properties of LL. Our finding not only presents a realization of 2D LL, but also provides a new angle to understand non-Fermi liquid behaviors in other 2D and 3D quantum materials.

cond-mat.str-el↗

arXiv2022

Hybridization and Correlation between f- and d-orbital electrons in a valence fluctuating compound EuNi2P2

The interaction between localized f and itinerant conduction electrons is crucial in the electronic properties of heavy fermion and valence fluctuating compounds. Using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structure of the archetypical valence fluctuating compound EuNi2P2 that hosts multiple f electrons. At low temperatures, we reveal the hybridization between Eu 4f and Ni 3d states, which contributes to the electron mass enhancement, consistent with the periodic Anderson model. With increasing temperature, interestingly, we observe opposite temperature evolution of electron spectral function above and below the Kondo coherence temperature near 110 K, which is in contrast to the monotonic valence change and beyond the expectation of the periodic Anderson model. We argue that both f-d hybridization and correlation are imperative in the electronic properties of EuNi2P2. Our results shed light on the understanding of novel properties, such as heavy fermion behaviors and valence fluctuation, of rare-earth transition-metal intermetallic compounds with multiple f electrons.

cond-mat.str-el↗

arXiv2022

Robust Kagome Electronic Structure in Topological Quantum Magnets XMn6Sn6 (X = Dy, Tb, Gd, Y)

Crystal geometry can greatly influence the emergent properties of quantum materials. As an example, the kagome lattice is an ideal platform to study the rich interplay between topology, magnetism, and electronic correlation. In this work, combining high-resolution angle-resolved photoemission spectroscopy and ab-initio calculation, we systematically investigate the electronic structure of XMn6Sn6 (X = Dy, Tb, Gd, Y) family compounds. We observe the Dirac fermion and the flat band arising from the magnetic kagome lattice of Mn atoms. Interestingly, the flat band locates in the same energy region in all compounds studied, regardless of their different magnetic ground states and 4f electronic configurations. These observations suggest a robust Mn magnetic kagome lattice across the XMn6Sn6 family, thus providing an ideal platform for the search and investigation on new emergent phenomena in magnetic topological materials.

cond-mat.str-el↗

arXiv2021

Efficient energy-preserving exponential integrators for multi-components Hamiltonian systems

In this paper, we develop a framework to construct energy-preserving methods for multi-components Hamiltonian systems, combining the exponential integrator and the partitioned averaged vector field method. This leads to numerical schemes with both advantages of long-time stability and excellent behavior for highly oscillatory or stiff problems. Compared to the existing energy-preserving exponential integrators (EP-EI) in practical implementation, our proposed methods are much efficient which can at least be computed by subsystem instead of handling a nonlinear coupling system at a time. Moreover, for most cases, such as the Klein-Gordon-Schr\"{o}dinger equations and the Klein-Gordon-Zakharov equations considered in this paper, the computational cost can be further reduced. Specifically, one part of the derived schemes is totally explicit, and the other is linearly implicit. In addition, we present rigorous proof of conserving the original energy of Hamiltonian systems, in which an alternative technique is utilized so that no additional assumptions are required, in contrast to the proof strategies used for the existing EP-EI. Numerical experiments are provided to demonstrate the significant advantages in accuracy, computational efficiency, and the ability to capture highly oscillatory solutions.

math.NA↗

arXiv2021

Band-selective Holstein polaron in Luttinger liquid material A0.3MoO3 (A = K, Rb)

(Quasi-)one-dimensional systems exhibit various fascinating properties such as Luttinger liquid behavior, Peierls transition, novel topological phases, and the accommodation of unique quasiparticles (e.g., spinon, holon, and soliton, etc.). Here we study molybdenum blue bronze A0.3MoO3 (A = K, Rb), a canonical quasi-one-dimensional charge-density-wave material, using laser-based angle-resolved photoemission spectroscopy. Our experiment suggests that the normal phase of A0.3MoO3 is a prototypical Luttinger liquid, from which the charge-density-wave emerges with decreasing temperature. Prominently, we observe strong renormalizations of band dispersions, which is recognized as the spectral function of Holstein polaron derived from band-selective electron-phonon coupling in the system. We argue that the strong electron-phonon coupling plays a dominant role in electronic properties and the charge-density-wave transition in blue bronzes. Our results not only reconcile the long-standing heavy debates on the electronic properties of blue bronzes but also provide a rare platform to study novel composite quasiparticles in Luttinger liquid materials.

cond-mat.str-el↗

arXiv2019

Default Bayesian Model Selection of Constrained Multivariate Normal Linear Models

The multivariate normal linear model is one of the most widely employed models for statistical inference in applied research. Special cases include (multivariate) t testing, (M)AN(C)OVA, (multivariate) multiple regression, and repeated measures analysis. Statistical procedures for model selection where the models may have equality and order constraints on the model parameters of interest are limited however. This paper presents a default Bayes factor for this model selection problem. The default Bayes factor is based on generalized fractional Bayes methodology where different fractions are used for different observations and where the default prior is centered on the boundary of the constrained space under investigation. First, the method is fully automatic and therefore can be applied when prior information is weak or completely unavailable. Second, using group specific fractions, the same amount of information is used from each group resulting in a minimally informative default prior having a matrix Cauchy distribution, resulting in a consistent default Bayes factor. Third, numerical computation can be done using parallelization which makes it computationally cheap. Fourth, the evidence can be updated in a relatively simple manner when observing new data. Fifth, the selection criterion can be applied relatively straightforwardly in the presence of missing data that are missing at random. Applications for the social and behavioral sciences are used for illustration.

stat.ME↗

arXiv2016

High energy Coulomb-scattered electrons for relativistic particle beam diagnostics

A new system used for monitoring energetic Coulomb-scattered electrons as the main diagnostic for accurately aligning the electron and ion beams in the new Relativistic Heavy Ion Collider (RHIC) electron lenses is described in detail. The theory of electron scattering from relativistic ions is developed and applied to the design and implementation of the system used to achieve and maintain the alignment. Commissioning with gold and 3He beams is then described as well as the successful utilization of the new system during the 2015 RHIC polarized proton run. Systematic errors of the new method are then estimated. Finally, some possible future applications of Coulomb-scattered electrons for beam diagnostics are briefly discussed.

physics.acc-ph↗

arXiv2014

Six-dimensional weak-strong simulations of head-on beam-beam compensation in RHIC

To compensate the large beam-beam tune spread and beam-beam resonance driving terms in the polarized proton operation in the Relativistic Heavy Ion Collider (RHIC), we will introduce a low-energy DC electron beam into each ring to collide head-on with the opposing proton beam. The device to provide the electron beam is called an electron lens. In this article, using a 6-D weak-strong-beam-beam interaction simulation model, we investigate the effects of head-on beam-beam compensation with electron lenses on the proton beam dynamics in the RHIC 250 GeV polarized proton operation. This article is abridged from the published article [1].

physics.acc-ph↗

arXiv2014

Status of head-on beam-beam compensation in RHIC

In polarized proton operation, the performance of the Relativistic Heavy Ion Collider (RHIC) is limited by the head-on beam-beam effect. To overcome this limitation, two electron lenses are under commissioning. We give an overview of head-on beam-beam compensation in general and in the specific design for RHIC, which is based on electron lenses. The status of installation and commissioning are presented along with plans for the future.

physics.acc-ph↗

arXiv2009

The role of the band gaps in reconstruction of polar surfaces and interfaces

Density functional theory applied to a simple ionic material, MgO, is used as a model system to clarify several aspects of electronic driven mechanism to compensate for the diverging electrostatic potential in the polar structures. We demonstrate that in the stoichiometric free standing films, the electronic reconstruction is limited by the band gap. This produces a residual electric field in the bulk of the sample which is extremely sensitive to tiny deviations in electron transfer between two surfaces of the slab. In heterostructures, the band gap is replaced by a new effective energy scale set by the band alignment of its component. This changes the reconstruction pathways so that the electronic mechanism can benefit from the smallest energy scale possible.

cond-mat.mes-hall↗