arXiv subjects

Zeyu Jiang

Publications and source records attributed to Zeyu Jiang.

33 records · Page 2Linked to original sources

arXiv2024

Bond Dipole based Geometric Theory of Band Alignment

The band alignment (BA) between two materials is a fundamental property that governs the functionality and performance of electronic, as well as electrochemical, devices. However, despite decades of study, the inability to separate surface properties from those of bulk have made a deep understanding of the physics of BA illusive. Building on the theory of ideal vacuum level to separate surface from bulk [CWZ, Phys. Rev. B 103, 235202 (2021)], here we present a geometric theory for the band alignment, particularly, explaining the insensitivity of the alignment to interfacial orientation between isotropic materials. First, we adopt charge neutral polyhedron, termed Wigner-Seitz atoms (WSA), to partition the charge of atoms in a way which maintains crystal symmetry and tessellates the space. In contrast to CWZ theory, the band alignment of two materials constructed from such WSAs is independent of interface orientation. Upon electron relaxation at the interface, here we show that the interfacial charge transfer dipole can be faithfully descibed by the sum of localized point dipoles which exist between atoms at the interface (bond dipoles). For interfaces between isotropic materials, the magnitude of the bond dipole can be factored out as a multiplier, leaving only geometric factors, such as the crystal symmetry and dimension of the material, to determine band alignment, irrespective of the orientation of the interface. We considered 29 distinct interfaces and found that this bond dipole theory yields excellent agreement (RMS deviation < 30 meV) with first-principles results. Our theory can be straightforwardly applied to interface between alloys, as well as between anisotropic systems.

cond-mat.mtrl-sci↗

arXiv2024

Revisiting the Formulation of Charged Defect in Solids

Defect physics is at the heart of microelectronics. By keeping track of the reference energy in total energy calculations, we explicitly show that the "potential alignment" correction vanishes, and the classic Markov-Payne correction yields accurate results. From linear response theory, we further formulate an accurate expression for the quadrupole correction. Application to numerous defects including anisotropic material yields accurate formation energies in small supercells and the historically slow convergence of the 2+ diamond vacancy is shown to be a result of slow varying gap levels of the defect leading to a size dependent dielectric constant.

cond-mat.mtrl-sci↗

arXiv2024

MoVL:Exploring Fusion Strategies for the Domain-Adaptive Application of Pretrained Models in Medical Imaging Tasks

Medical images are often more difficult to acquire than natural images due to the specialism of the equipment and technology, which leads to less medical image datasets. So it is hard to train a strong pretrained medical vision model. How to make the best of natural pretrained vision model and adapt in medical domain still pends. For image classification, a popular method is linear probe (LP). However, LP only considers the output after feature extraction. Yet, there exists a gap between input medical images and natural pretrained vision model. We introduce visual prompting (VP) to fill in the gap, and analyze the strategies of coupling between LP and VP. We design a joint learning loss function containing categorisation loss and discrepancy loss, which describe the variance of prompted and plain images, naming this joint training strategy MoVL (Mixture of Visual Prompting and Linear Probe). We experiment on 4 medical image classification datasets, with two mainstream architectures, ResNet and CLIP. Results shows that without changing the parameters and architecture of backbone model and with less parameters, there is potential for MoVL to achieve full finetune (FF) accuracy (on four medical datasets, average 90.91% for MoVL and 91.13% for FF). On out of distribution medical dataset, our method(90.33%) can outperform FF (85.15%) with absolute 5.18 % lead.

cs.CV↗

arXiv2023

Heuristic Hyperparameter Choice for Image Anomaly Detection

Anomaly detection (AD) in images is a fundamental computer vision problem by deep learning neural network to identify images deviating significantly from normality. The deep features extracted from pretrained models have been proved to be essential for AD based on multivariate Gaussian distribution analysis. However, since models are usually pretrained on a large dataset for classification tasks such as ImageNet, they might produce lots of redundant features for AD, which increases computational cost and degrades the performance. We aim to do the dimension reduction of Negated Principal Component Analysis (NPCA) for these features. So we proposed some heuristic to choose hyperparameter of NPCA algorithm for getting as fewer components of features as possible while ensuring a good performance.

cs.CV↗

arXiv2023

Small polarom formation by electron-electron interaction

In a solid, electrons can be scattered both by phonons and other electrons. First proposed by Landau, scattering by phonons can lead to a composite entity called a polaron, in which a lattice distortion traps an itinerant electron (or hole) such that the distortion and carrier move in unison as a single particle with larger effective mass. While this is the traditional view of polarons, the rise of 2D systems, especially strongly correlated ones, open the prospect of electron scattering taking on a larger role in spontaneous carrier localization for such material systems. Here, we show that in transition metal halides, such electron-electron interactions can lead to polaron formation even in the absence of lattice distortion. This suggests an alternative direction for polaron formation, transport, and control in solids. This new mechanism of polaron formation is confirmed by first-principles calculation of 2D transition metal halides, CrI2, CoCl2 and CoBr2. These theoretical predictions are supported by scanning tunneling microscopy/spectroscopy measurements of polarons in CrI2.

cond-mat.mtrl-sci↗

arXiv2022

A Universal Description of Workfunction

At the surfaces of materials, the bulk symmetry of the charge density is broken and electron spill-out into the vacuum region creates a surface dipole. Such spill-out has been historically calculated by Lang and Kohn [Phys. Rev. B \textbf{3}, 1215 (1971)] using average electron density to sucessfully explain the workfunction in metals. However, despite its initial success, in the fifty years since it has not been extended beyond simple metals. Here we show that the degree of charge spill-out is largely controlled by the innate bulk workfunction $ϕ_I$, which is the Fermi level position of $\it bulk$ relative to the ideal vacuum. By incorporating the contribution of $ϕ_I$ to the surface dipole we show that Lang-Kohn's $\it jellium$ based approach can be broadly expanded to understand the workfunction over a wide range of metals, semiconductors, and insulators.

cond-mat.mtrl-sci↗

arXiv2022

Manipulating single excess electrons in monolayer transition metal dihalide

Polarons are entities of excess electrons dressed with local response of lattices, whose atomic-scale characterization is essential for understanding the many body physics arising from the electron-lattice entanglement, but yet difficult to achieve. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we show the visualization and manipulation of single polarons with different origin, i.e., electronic and conventional polarons, in monolayer CoCl2, that are grown on HOPG substrate via molecular beam epitaxy. Four types of polarons are identified, all inducing upward local band bending, but exhibiting distinct appearances, lattice occupations, polaronic states and local lattice distortions. First principles calculations unveil three types of polarons are stabilized by electron-electron interaction. The type-4 polaron, however, are driven by conventional lattice distortions. All the four types of polarons can be created, moved, erased, and moreover interconverted individually by the STM tip, allowing precise control of single polarons unprecedently. This finding identifies the rich category of polarons and their feasibility of manipulation in CoCl2, which can be generalized to other transition metal halides.

cond-mat.mes-hall↗

arXiv2022

1st Place Solution to the EPIC-Kitchens Action Anticipation Challenge 2022

In this report, we describe the technical details of our submission to the EPIC-Kitchens Action Anticipation Challenge 2022. In this competition, we develop the following two approaches. 1) Anticipation Time Knowledge Distillation using the soft labels learned by the teacher model as knowledge to guide the student network to learn the information of anticipation time; 2) Verb-Noun Relation Module for building the relationship between verbs and nouns. Our method achieves state-of-the-art results on the testing set of EPIC-Kitchens Action Anticipation Challenge 2022.

cs.CV↗

arXiv2021

Polarization at the Nanoscale

Modern polarization theory yields surface bound charge associated with spontaneous polarization of bulk. However, understanding polarization in nano systems also requires a proper treatment of charge transfer between surface dangling bonds. Here, we develop a real-space approach for total polarization and apply it to wurtzite semiconductors and BaTiO3 perovskite. First-principles calculations utilizing this approach not only yield spontaneous bulk polarization in agreement with Berry phase calculations, but also uncover phenomena specific to nano systems. As an example, we show surface passivation leads to a complete quenching of the piezoelectric effect, which reemerges only at larger length scale and/or spontaneous polarization.

cond-mat.mtrl-sci↗

arXiv2021

Realization of AlSb in the double layer honeycomb structure: a robust new class of two-dimensional material

Exploring new two-dimensional (2D) van der Waals (vdW) systems is at the forefront of materials physics. Here, through molecular beam epitaxy on graphene-covered SiC(0001), we report successful growth of AlSb in the double-layer honeycomb (DLHC) structure, a 2D vdW material which has no direct analogue to its 3D bulk and is predicted kinetically stable when freestanding. The structural morphology and electronic structure of the experimental 2D AlSb are characterized with spectroscopic imaging scanning tunneling microscopy and cross-sectional imaging scanning transmission electron microscopy, which compare well to the proposed DLHC structure. The 2D AlSb exhibits a bandgap of 0.93 eV versus the predicted 1.06 eV, which is substantially smaller than the 1.6 eV of bulk. We also attempt the less-stable InSb DLHC structure; however, it grows into bulk islands instead. The successful growth of a DLHC material here opens the door for the realization of a large family of novel 2D DLHC traditional semiconductors with unique excitonic, topological, and electronic properties.

cond-mat.mtrl-sci↗

arXiv2020

Vector Potential and Surface Magnetic Field in Magnetoelectric Antiferromagnetic Materials

A general formula for the average vector potential of bulk periodic systems is proposed and shown to set the boundary conditions at magnetic interfaces. For antiferromagnetic materials, the study reveals a unique relation between the macroscopic potential and the orientation-dependent magnetic quadrupole, as a result of the different crystalline and magnetic symmetries. In particular, at surfaces and interfaces of a truncated bulk without inversion and time-reversal symmetries, the average vector potential exhibits a discontinuity, which results in an interfacial magnetic field. In general, however, due to the surface and interface electronic and atomic relaxations, additional magnetization may result. For the experimentally-observed magnetoelectric antiferromagnets, in particular, our symmetry analysis suggest that the relaxation effects could well be a system response to the presence of such a potential discontinuity.

cond-mat.mtrl-sci↗

arXiv2020

Spin-Triplet Excitonic Insulator: The Case of Graphone

While various excitonic insulators have been studied in the literature, due to the perceived too-small spin splitting, spin-triplet excitonic insulator is rare. In two-dimensional systems such as a graphone, however, it is possible, as revealed by first-principles calculations coupled with Bethe-Salpeter equation. The critical temperature, given by an effective Hamiltonian, is 11.5 K. While detecting excitonic insulators is still a daunting challenge, the condensation of triplet excitons will result in spin superfluidity, which can be directly measured by a transport experiment. Nonlocal dielectric screening also leads to an unexpected phenomenon, namely, an indirect-to-direct transition crossover between single-particle band and exciton dispersion in graphone, which offers yet another test by experiment.

cond-mat.mtrl-sci↗

arXiv2019

Electronic structure of molecular beam epitaxy grown 1T$^\prime$-MoTe$_2$ film and strain effect

Atomically thin transition metal dichalcogenide films with distorted trigonal (1T$^\prime$) phase have been predicted to be candidates for realizing quantum spin Hall effect. Growth of 1T$^\prime$ film and experimental investigation of its electronic structure are critical. Here we report the electronic structure of 1T$^\prime$-MoTe$_2$ films grown by molecular beam epitaxy (MBE). Growth of the 1T$^\prime$-MoTe$_2$ film depends critically on the substrate temperature, and successful growth of the film is indicated by streaky stripes in the reflection high energy electron diffraction and sharp diffraction spots in low energy electron diffraction. Angle-resolved photoemission spectroscopy measurements reveal a metallic behavior in the as-grown film with an overlap between the conduction and valence bands. First principles calculation suggests that a suitable tensile strain along the a-axis direction is needed to induce a gap to make it an insulator. Our work not only reports the electronic structure of MBE grown 1T$^\prime$-MoTe$_2$ films, but also provides insights for strain engineering to make it possible for quantum spin Hall effect.

cond-mat.mtrl-sci↗

arXiv2019

Half Excitonic Insulator: A Single-Spin Bose-Einstein Condensate

First-principles calculations reveal an unusual electronic state (dubbed as half excitonic insulator) in monolayer 1T-MX2 (M = Co, Ni and X = Cl, Br). Its one spin channel has a many-body ground state due to excitonic instability, while the other is characterized by a conventional band insulator gap. This disparity arises from a competition between the band gap and exciton binding energy, which exhibits a spin-dependence due to different orbital occupations. Such a state can be identified by optical absorption measurements and angle-resolved photoemission spectroscopy. Our theory not only provides new insights for the study of exciton condensation in magnetic materials but also suggests that strongly-correlated materials could be fertile candidates for excitonic insulators.

cond-mat.mtrl-sci↗

arXiv2018

Realizing intrinsic excitonic insulator by decoupling exciton binding energy from the minimum band gap

Direct-gap materials hold promises for excitonic insulator. In contrast to indirect-gap materials, here the difficulty to distinguish from a Peierls charge density wave is circumvented. However, direct-gap materials still suffer from the divergence of polarizability when the band gap approaches zero, leading to diminishing exciton binding energy. We propose that one can decouple the exciton binding energy from the band gap in materials where band-edge states have the same parity. First-principles calculations of two-dimensional GaAs and experimentally mechanically exfoliated single-layer TiS 3 lend solid supports to the new principle.

cond-mat.mtrl-sci↗