arXiv subjects

Zheng Gai

Publications and source records attributed to Zheng Gai.

At least 19 recordsLinked to original sources

arXiv2026

CATEKAPPA: An R Shiny Application for Design and Analysis of Consistency Tests Based on the Kappa Statistic for Categorical Responses

The kappa statistic is the most widely used measure of inter-rater agreement for categorical data. Despite its popularity, applied researchers often encounter two major hurdles: (i) determining the sample size required to achieve a desired level of agreement with given power, and (ii) computing appropriate kappa coefficients with proper interpretation. Existing R packages such as irr and kappaSize provide these functionalities but require programming skills and lack an integrated, user-friendly interface. We present CATEKAPPA, an R package that bridges this gap by combining sample size planning (via kappaSize) and agreement analysis (via irr) into a single Shiny-based web application. The package supports Cohen's kappa for two raters, Fleiss' kappa for three or more raters, and Light's kappa, and provides automatic interpretation using the Landis & Koch scale. Users can either launch an interactive graphical interface or use command-line functions for scripting. The package is freely available on CRAN.

stat.CO↗

arXiv2025

Raman Fingerprints of Phase Transitions and Ferroic Couplings in Van der Waals Material CuCrP$_2$S$_6$

CuCrP$_2$S$_6$ (CCPS), a type-II multiferroic material, exhibits unique phase transitions involving ferroelectric, antiferroelectric, and antiferromagnetic ordering. In this study, we conduct a comprehensive investigation on the intricate phase transitions and their multiferroic couplings in CCPS across a wide temperature range from 4 to 345 K through Raman spectroscopic measurements down to 5 cm$^{-1}$. We first assign the observed Raman modes with the support of theoretical calculations and angle-resolved polarized Raman measurements. We further present clear signatures of phase transitions from analyses of temperature-dependent Raman spectral parameters. Particularly, two low-frequency soft modes are observed at 36.1 cm$^{-1}$ and 70.5 cm$^{-1}$ below 145 K, indicating the antiferroelectric to quasi-antiferroelectric transition. Moreover, phonon modes hardening is observed when the temperature increases from 4 to 65 K, suggesting negative thermal expansion (NTE) and strong magnetoelastic coupling below 65 K. These findings advance the understanding of vdW multiferroic CCPS, paving the way for future design and engineering of multiferroicity in cutting-edge technologies, such as spintronics and quantum devices.

cond-mat.mtrl-sci↗

arXiv2024

Nanoscale magnetic ordering dynamics in a high Curie temperature ferromagnet

Thermally driven transitions between ferromagnetic and paramagnetic phases are characterized by critical behavior with divergent susceptibilities, long-range correlations, and spin dynamics that can span kHz to GHz scales as the material approaches the critical temperature $\mathrm{T_c}$, but it has proven technically challenging to probe the relevant length and time scales with most conventional measurement techniques. In this study, we employ scanning nitrogen-vacancy center based magnetometry and relaxometry to reveal the critical behavior of a high-$\mathrm{T_c}$ ferromagnetic oxide near its Curie temperature. Cluster analysis of the measured temperature-dependent nanoscale magnetic textures points to a 3D universality class with a correlation length that diverges near $\mathrm{T_c}$. Meanwhile, the temperature-dependent spin dynamics, measured through all optical relaxometry suggest that the phase transition is in the XY universality class. Our results capture both static and dynamic aspects of critical behavior, providing insights into universal properties that govern phase transitions in magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

Cobalt-Based Pyroxenes: A New Playground for Kitaev Physics and Ising Model Realization

Recent advances in the study of cobaltites have unveiled their potential as a promising platform for realizing Kitaev physics in honeycomb systems and the Ising model in weakly coupled chain materials. In this manuscript, we explore the magnetic properties of pyroxene SrCoGe$_2$O$_6$ using a combination of neutron scattering, {\it ab initio} methods, and linear spin-wave theory. Through careful examination of inelastic neutron scattering powder spectra, we propose a modified Kitaev model to accurately describe the twisted chains of edge-sharing octahedra surrounding Co$^{2+}$ ions. The extended Kitaev-Heisenberg model, including a significant anisotropic bond-dependent exchange term with $K/|J|=0.96$, is identified as the key descriptor of the magnetic interactions in SrCoGe$_2$O$_6$. Furthermore, our heat capacity measurements reveal an effect of an external magnetic field (approximately 13~T) which shifts the system from a fragile antiferromagnetic ordering with $T_{\mathrm{N}}=9$~K to a field-induced state. We argue that pyroxenes, particularly those modified by substituting Ge with Si and its less extended $p$ orbitals, emerge as a novel platform for the Kitaev model. This opens up possibilities for advancing our understanding of Kitaev physics.

cond-mat.str-el↗

arXiv2024

Large Bandgap Observed on the Surfaces of EuZn2As2 Single Crystals

EuM2As2 (M = Zn, Cd, In, Sn etc.) is an excellent material system for studying topological properties, which can be easily tuned by magnetism involved. Theoretical calculations predict gapped and flat bands in EuZn2As2 but gapless structure in EuCd2As2. In this work, low-temperature (77 K) cleaved EuZn2As2 crystals are studied using scanning tunneling microscopy/spectroscopy (STM/S) and density functional theory (DFT) calculations. Defects-induced local density of states (LDOS) modification with a triangular shape helps identify the surface terminations: Eu versus AsZn surface. While large bandgaps (~1.5 eV at 77 K) are observed on both pristine surfaces, the bandgap width is found to be very sensitive to local heterogeneity, such as defects and step edges, with the tendency of reduction. Combining experimental data with DFT simulations, we conclude that the modified bandgap in the heterogeneous area arises from Zn vacancies and/or substitution by As atoms. Our investigation offers important information for reevaluating the electron topology of the EuM2As2 family.

cond-mat.mtrl-sci↗

arXiv2024

Multiscale structure-property discovery via active learning in scanning tunneling microscopy

Atomic arrangements and local sub-structures fundamentally influence emergent material functionalities. The local structures are conventionally probed using spatially resolved studies and the property correlations are usually deciphered by a researcher based on sequential explorations and auxiliary information, thus limiting the throughput efficiency. Here we demonstrate a Bayesian deep learning based framework that automatically correlates material structure with its electronic properties using scanning tunneling microscopy (STM) measurements in real-time. Its predictions are used to autonomously direct exploration toward regions of the sample that optimize a given material property. This autonomous method is deployed on the low-temperature ultra-high vacuum STM to understand the structure-property relationship in a europium-based semimetal, EuZn2As2, one of the promising candidates for studying the magnetism-driven topological properties. The framework employs a sparse sampling approach to efficiently construct the scalar-property space using a minimal number of measurements, about 1 - 10 % of the data required in standard hyperspectral imaging methods. We further demonstrate a target-property-guided active learning of structures within a multiscale framework. This is implemented across length scales in a hierarchical fashion for the autonomous discovery of structural origins for an observed material property. This framework offers the choice to select and derive a suitable scalar property from the spectroscopic data to steer exploration across the sample space. Our findings reveal correlations of the electronic properties unique to surface terminations, local defect density, and point defects.

cond-mat.mtrl-sci↗

arXiv2023

Room-temperature ferromagnetism in epitaxial bilayer FeSb/SrTiO3(001) terminated with a Kagome lattice

Two-dimensional (2D) magnets exhibit unique physical properties for potential applications in spintronics. To date, most 2D ferromagnets are obtained by mechanical exfoliation of bulk materials with van der Waals interlayer interactions, and the synthesis of single or few-layer 2D ferromagnets with strong interlayer coupling remains experimentally challenging. Here, we report the epitaxial growth of 2D non-van der Waals ferromagnetic bilayer FeSb on SrTiO3(001) substrates stabilized by strong coupling to the substrate, which exhibits in-plane magnetic anisotropy and a Curie temperature above 300 K. In-situ low-temperature scanning tunneling microscopy/spectroscopy and density-functional theory calculations further reveal that a Fe Kagome layer terminates the bilayer FeSb. Our results open a new avenue for further exploring emergent quantum phenomena from the interplay of ferromagnetism and topology for application in spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Hole doping in compositionally complex correlated oxide enables tunable exchange biasing

Magnetic interfaces and the phenomena arising from them drive both the design of modern spintronics and fundamental research. Recently, it was revealed that through designing magnetic frustration in configurationally complex entropy stabilized oxides, exchange bias can occur in structurally single crystal films. This eliminates the need for complex heterostructures and nanocomposites in the design and control of magnetic response phenomena. In this work, we demonstrate through hole doping of a high entropy perovskite oxide that tuning of magnetic responses can be achieved. With detailed magnetometry, we show magnetic coupling exhibiting a variety of magnetic responses including exchange bias and antiferromagnetic spin reversal in the entropy stabilized ABO3 perovskite oxide La1-xSrx(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 family. We find that manipulation of the A-site charge state can be used to balance magnetic phase compositions and coupling responses. This allows for the creation of highly tunable exchange bias responses. In the low Sr doping regime, a spin frustrated region arising at the antiferromagnetic phase boundary is shown to directly couple to the antiferromagnetic moments of the film and emerges as the dominant mechanism, leading to a vertical shift of magnetization loops in response to field biasing. At higher concentrations, direct coupling of antiferromagnetic and ferromagnetic regions is observed. This tunability of magnetic coupling is discussed within the context of these three competing magnetic phases, revealing critical features in designing exchange bias through exploiting spin frustration and disorder in high entropy oxides.

cond-mat.str-el↗

arXiv2023

Emergence of a new band and the Lifshitz transition in kagome metal ScV$_6$Sn$_6$ with charge density wave

Topological kagome systems have been a topic of great interest in condensed matter physics due totheir unique electronic properties. The vanadium-based kagome materials are particularly intrigu-ing since they exhibit exotic phenomena such as charge density wave (CDW) and unconventionalsuperconductivity. The origin of these electronic instabilities is not fully understood, and the re-cent discovery of a charge density wave in ScV6Sn6provides a new avenue for investigation. In thiswork, we investigate the electronic structure of the novel kagome metal ScV6Sn6using angle resolvedphotoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and first-principlesdensity functional theory calculations. Our analysis reveals for the first time the temperature-dependent band changes of ScV6Sn6and identifies a new band that exhibits a strong signatureof a structure with CDW below the critical temperature. Further analysis revealed that this newband is due to the surface kagome layer of the CDW structure. In addition, a Lifshitz transition isidentified in the ARPES spectra that is related to the saddle point moving across the Fermi levelat the critical temperature for the CDW formation. This result shows the CDW behavior may alsobe related to nesting of the saddle point, similar to related materials. However, no energy gap is observed at the Fermi level and thus the CDW is not a typical Fermi surface nesting scenario. These results provide new insights into the underlying physics of the CDW in the kagome materials and could have implications for the development of materials with new functionality.

cond-mat.str-el↗

arXiv2023

Revealing intrinsic vortex-core states in Fe-based superconductors through machine-learning-driven discovery

Electronic states within superconducting vortices hold crucial information about paring mechanisms and topology. While scanning tunneling microscopy/spectroscopy(STM/S) can image the vortices, it is difficult to isolate the intrinsic electronic states from extrinsic effects like subsurface defects and disorders. We combine STM/S with unsupervised machine learning to develop a method for screening out the vortices pinned by embedded disorder in Fe-based superconductors. The approach provides an unbiased way to reveal intrinsic vortex-core states and may address puzzles on Majorana zero modes.

cond-mat.supr-con↗

arXiv2021

Competitive and Cooperative electronic states in Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ with T=Co, Ni, Cr

The electronic structure inhomogeneities in Co, Ni, and Cr doped BaFe2As2 122 single crystals are compared using scanning tunneling microscopy/spectroscopy (STM/S) at the nanoscale within three bulk property regions in the phase diagram: a pure superconducting (SC) dome region (Co-122), a coexisting SC and antiferromagnetic (AFM) region (Ni-122), and a non-SC region (Cr-122). Machine learning is utilized to categorize the various nanometer scale inhomogeneous electronic states, described here as in-gap, L-shape and S-shape states immersed into the SC matrix for Ni-and Co-doped 122, and L-shape and S-shape states into the metallic matrix for Cr-doped 122. Although the relative percentages of in-gap, L-shape and S-shape states vary in the three samples, the total volume fraction of the three electronic states is quite similar. This is coincident with the number of electrons (Ni0.04 and Co0.08) and holes (Cr0.04) doped into the 122 compound. By combining the volume fractions of the three states, the local density of states (LDOS), magnetic field dependent behavior and global properties in these three samples, the in-gap state is confirmed as a magnetic impurity state from the Co or Ni dopants. In addition, the L-shape state is identified as a spin density wave (SDW) which competes with the SC phase, and the S-shape state is found to be another form of magnetic order which constructively cooperates with the SC phase, rather than competing with it. The comparison of the vortex structures indicates that the inhomogeneous electronic states serve as pinning centers for stabilizing the vortex lattice.

cond-mat.supr-con↗

arXiv2021

Designer Magnetism in High Entropy Oxides

Disorder can have a dominating influence on correlated and quantum materials leading to novel behaviors which have no clean limit counterparts. In magnetic systems, spin and exchange disorder can provide access to quantum criticality, frustration, and spin dynamics, but broad tunability of these responses and a deeper understanding of strong limit disorder is lacking. In this work, we demonstrate that high entropy oxides present an unexplored route to designing quantum materials in which the presence of strong local compositional disorder hosted on a positionally ordered lattice can be used to generate highly tunable emergent magnetic behavior--from macroscopically ordered states to frustration-driven dynamic spin interactions. Single crystal La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 films are used as a structurally uniform model system hosting a magnetic sublattice with massive microstate disorder in the form of site-to-site spin and exchange type inhomogeneity. A classical Heisenberg model is found to be sufficient to describe how compositionally disordered systems can paradoxically host long-range magnetic uniformity and demonstrates that balancing the populating elements based on their discrete quantum parameters can be used to give continuous control over ordering types and critical temperatures. Theory-guided experiments show that composite exchange values derived from the complex mix of microstate interactions can be used to design the required compositional parameters for a desired response. These predicted materials are synthesized and found to possess an incipient quantum critical point when magnetic ordering types are designed to be in direct competition; this leads to highly controllable exchange bias sensitivity in the monolithic single crystal films previously accessible only in intentionally designed bilayer heterojunctions.

cond-mat.str-el↗

arXiv2021

Magnetic Texture in Insulating Single Crystal High Entropy Oxide Spinel Films

Magnetic insulators are important materials for a range of next generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators which can be synthesized with surface qualities that would allow these smooth interfaces and precisely tuned interfacial magnetic exchange coupling which might be applicable at room temperature. In this work, we demonstrate an example of how the configurational complexity in the magnetic insulator layer can be used to realize these properties. The entropy-assisted synthesis is used to create single crystal (Mg0.2Ni0.2Fe0.2Co0.2Cu0.2)Fe2O4 films on substrates spanning a range of strain states. These films show smooth surfaces, high resistivity, and strong magnetic responses at room temperature. Local and global magnetic measurements further demonstrate how strain can be used to manipulate magnetic texture and anisotropy. These findings provide insight into how precise magnetic responses can be designed using compositionally complex materials that may find application in next generation magnetic devices.

cond-mat.mtrl-sci↗

arXiv2021

Revealing the chemical bonding in adatoms arrays via machine learning of 3D scanning tunneling spectroscopy data

The adatom arrays on surfaces offer an ideal playground to explore the mechanisms of chemical bonding via changes in the local electronic tunneling spectra. While this information is readily available in hyperspectral scanning tunneling spectroscopy data, its analysis has been considerably impeded by a lack of suitable analytical tools. Here we develop a machine learning based workflow combining supervised feature identification in the spatial domain and un-supervised clustering in the energy domain to reveal the details of structure-dependent changes of the electronic structure in adatom arrays on the Co3Sn2S2 cleaved surface. This approach, in combination with first-principles calculations, provides insight for using artificial neural networks to detect adatoms and classifies each based on their local neighborhood comprised of other adatoms. These structurally classified adatoms are further spectrally deconvolved. The unexpected inhomogeneity of electronic structures among adatoms in similar configurations is unveiled using this method, suggesting there is not a single atomic species of adatoms, but rather multiple types of adatoms on the Co3Sn2S2 surface. This is further supported by a slight contrast difference in the images (or slight size variation) of the topography of the adatoms.

cond-mat.mtrl-sci↗

arXiv2020

Bayesian learning of adatom interactions from atomically-resolved imaging data

Atomic structures and adatom geometries of surfaces encode information about the thermodynamics and kinetics of the processes that lead to their formation, and which can be captured by a generative physical model. Here we develop a workflow based on a machine learning-based analysis of scanning tunneling microscopy images to reconstruct the atomic and adatom positions, and a Bayesian optimization procedure to minimize statistical distance between the chosen physical models and experimental observations. We optimize the parameters of a 2- and 3-parameter Ising model describing surface ordering and use the derived generative model to make predictions across the parameter space. For concentration dependence, we compare the predicted morphologies at different adatom concentrations with the dissimilar regions on the sample surfaces that serendipitously had different adatom concentrations. The proposed workflow is universal and can be used to reconstruct the thermodynamic models and associated uncertainties from the experimental observations of materials microstructures. The code used in the manuscript is available at https://github.com/saimani5/Adatom_interactions.

cond-mat.mtrl-sci↗

arXiv2020

Interlayer magnetism in Fe3-xGeTe2

Fe$_{3-x}$GeTe$_2$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in Fe$_{3-x}$GeTe$_2$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations. We conclude that the layers of Fe$_{3-x}$GeTe$_2$ are coupled ferromagnetically and that in order to capture the magnetic and electronic properties of Fe$_{3-x}$GeTe$_2$ within density functional theory, Hubbard U corrections need to be taken into account.

cond-mat.mtrl-sci↗

arXiv2020

Unusual Electrical Conductivity Driven by Localized Stoichiometry Modification at Vertical Epitaxial Interfaces

Precise control of lattice mismatch accommodation and cation interdiffusion across the interface is critical to modulate correlated functionalities in epitaxial heterostructures, particularly when the interface composition is positioned near a compositional phase transition boundary. Here we select La1-xSrxMnO3 (LSMO) as a prototypical phase transition material and establish vertical epitaxial interfaces with NiO to explore the strong interplay between strain accommodation, stoichiometry modification, and localized electron transport across the interface. It is found that localized stoichiometry modification overcomes the plaguing dead layer problem in LSMO and leads to strongly directional conductivity, as manifested by more than three orders of magnitude difference between out-of-plane to in-plane conductivity. Comprehensive structural characterization and transport measurements reveal that this emerging behavior is related to a compositional change produced by directional cation diffusion that pushes the LSMO phase transition from insulating into metallic within an ultrathin interface region. This study explores the nature of unusual electric conductivity at vertical epitaxial interfaces and establishes an effective route for engineering nanoscale electron transport for oxide electronics.

cond-mat.mtrl-sci↗

arXiv2020

Realizing gapped surface states in magnetic topological insulator MnBi$_{2-x}$Sb$_{x}$Te$_{4}$

The interplay between magnetism and non-trivial topology in magnetic topological insulators (MTI) is expected to give rise to a variety of exotic topological quantum phenomena, such as the quantum anomalous Hall (QAH) effect and the topological axion states. A key to assessing these novel properties is to tune the Fermi level in the exchange gap of the Dirac surface band. MnBi$_2$Te$_4$ possesses non-trivial band topology with intrinsic antiferromagnetic (AFM) state that can enable all of these quantum states, however, highly electron-doped nature of the MnBi$_2$Te$_4$ crystals obstructs the exhibition of the gapped topological surface states. Here, we tailor the material through Sb-substitution to reveal the gapped surface states in MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ (MBST). By shifting the Fermi level into the bulk band gap of MBST, we access the surface states and show a band gap of 50 meV at the Dirac point from quasi-particle interference (QPI) measured by scanning tunneling microscopy/spectroscopy (STM/STS). Surface-dominant conduction is confirmed below the Néel temperature through transport spectroscopy measured by multiprobe STM. The surface band gap is robust against out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. The realization of bulk-insulating MTI with the large exchange gap offers a promising platform for exploring emergent topological phenomena.

cond-mat.mtrl-sci↗