arXiv subjects

Yufan Li

Publications and source records attributed to Yufan Li.

35 records · Page 2Linked to original sources

arXiv2022

Unequivocal Determination of Spin-Triplet Superconductivity Using Composite Rings

Phase-sensitive measurements on a composite ring made of a superconductor of interest connected by a known singlet $s$-wave superconductor can unambiguously determine its pairing symmetry. In composite rings with epitaxial $β-$Bi$_2$Pd and $s$-wave Nb, we have observed half-integer quantum flux when Nb is connected to the opposite crystalline ends of $β-$Bi$_2$Pd and integer-quantum flux when Nb is connected to the same crystalline ends of $β-$Bi$_2$Pd. These findings provide unequivocal evidence of odd-parity pairing state of the triplet superconductor $β-$Bi$_2$Pd.

cond-mat.supr-con↗

arXiv2022

Automatic Identification of Coal and Rock/Gangue Based on DenseNet and Gaussian Process

To improve the purity of coal and prevent damage to the coal mining machine, it is necessary to identify coal and rock in underground coal mines. At the same time, the mined coal needs to be purified to remove rock and gangue. These two procedures are manually operated by workers in most coal mines. The realization of automatic identification and purification is not only conducive to the automation of coal mines, but also ensures the safety of workers. We discuss the possibility of using image-based methods to distinguish them. In order to find a solution that can be used in both scenarios, a model that forwards image feature extracted by DenseNet to Gaussian process is proposed, which is trained on images taken on surface and achieves high accuracy on images taken underground. This indicates our method is powerful in few-shot learning such as identification of coal and rock/gangue and might be beneficial for realizing automation in coal mines.

cs.CV↗

arXiv2022

Modeling Temporal-Modal Entity Graph for Procedural Multimodal Machine Comprehension

Procedural Multimodal Documents (PMDs) organize textual instructions and corresponding images step by step. Comprehending PMDs and inducing their representations for the downstream reasoning tasks is designated as Procedural MultiModal Machine Comprehension (M3C). In this study, we approach Procedural M3C at a fine-grained level (compared with existing explorations at a document or sentence level), that is, entity. With delicate consideration, we model entity both in its temporal and cross-modal relation and propose a novel Temporal-Modal Entity Graph (TMEG). Specifically, graph structure is formulated to capture textual and visual entities and trace their temporal-modal evolution. In addition, a graph aggregation module is introduced to conduct graph encoding and reasoning. Comprehensive experiments across three Procedural M3C tasks are conducted on a traditional dataset RecipeQA and our new dataset CraftQA, which can better evaluate the generalization of TMEG.

cs.CL↗

arXiv2022

Multi-scale Sparse Representation-Based Shadow Inpainting for Retinal OCT Images

Inpainting shadowed regions cast by superficial blood vessels in retinal optical coherence tomography (OCT) images is critical for accurate and robust machine analysis and clinical diagnosis. Traditional sequence-based approaches such as propagating neighboring information to gradually fill in the missing regions are cost-effective. But they generate less satisfactory outcomes when dealing with larger missing regions and texture-rich structures. Emerging deep learning-based methods such as encoder-decoder networks have shown promising results in natural image inpainting tasks. However, they typically need a long computational time for network training in addition to the high demand on the size of datasets, which makes it difficult to be applied on often small medical datasets. To address these challenges, we propose a novel multi-scale shadow inpainting framework for OCT images by synergically applying sparse representation and deep learning: sparse representation is used to extract features from a small amount of training images for further inpainting and to regularize the image after the multi-scale image fusion, while convolutional neural network (CNN) is employed to enhance the image quality. During the image inpainting, we divide preprocessed input images into different branches based on the shadow width to harvest complementary information from different scales. Finally, a sparse representation-based regularizing module is designed to refine the generated contents after multi-scale feature aggregation. Experiments are conducted to compare our proposal versus both traditional and deep learning-based techniques on synthetic and real-world shadows. Results demonstrate that our proposed method achieves favorable image inpainting in terms of visual quality and quantitative metrics, especially when wide shadows are presented.

cs.CV↗

arXiv2021

Culture-inspired Multi-modal Color Palette Generation and Colorization: A Chinese Youth Subculture Case

Color is an essential component of graphic design, acting not only as a visual factor but also carrying cultural implications. However, existing research on algorithmic color palette generation and colorization largely ignores the cultural aspect. In this paper, we contribute to this line of research by first constructing a unique color dataset inspired by a specific culture, i.e., Chinese Youth Subculture (CYS), which is an vibrant and trending cultural group especially for the Gen Z population. We show that the colors used in CYS have special aesthetic and semantic characteristics that are different from generic color theory. We then develop an interactive multi-modal generative framework to create CYS-styled color palettes, which can be used to put a CYS twist on images using our automatic colorization model. Our framework is illustrated via a demo system designed with the human-in-the-loop principle that constantly provides feedback to our algorithms. User studies are also conducted to evaluate our generation results.

cs.CV↗

arXiv2020

A polariton electric field sensor

We experimentally demonstrate a dipolar polariton based electric field sensor. We tune and optimize the sensitivity of the sensor by varying the dipole moment of polaritons. We show polariton interactions play an important role in determining the conditions for optimal electric field sensing, and achieve a sensitivity of 0.12 V-m$^{-1}$-Hz$^{-0.5}$. Finally we apply the sensor to illustrate that excitation of polaritons modify the electric field in a spatial region much larger than the optical excitation spot.

cond-mat.mes-hall↗

arXiv2020

Fractional Little-Parks effect observed in a topological superconductor

In superconductors, the condensation of Cooper pairs gives rise to fluxoid quantization in discrete units of $Φ_0 = hc / 2e$. The denominator of $2e$ is the signature of electron pairing, which is evidenced by a number of macroscopic quantum phenomena, such as the Little-Parks effect and the Josephson effect, where the critical temperature or the critical current oscillates in the period of $Φ_0$. Here we report the observation of fractional Little-Parks effect in mesoscopic rings of epitaxial $β$-Bi$_2$Pd, a topological superconductor. Besides $Φ_0$, novel Little-Parks oscillation periodicities of $2Φ_0$, $3Φ_0$ and $4Φ_0$ are also observed, implying quasiparticles with effective charges being a fraction of a Cooper pair. We show that the fractional Little-Parks effect may be closely related to the fractional Josephson effect, which is a key signature of chiral Majorana edge states.

cond-mat.supr-con↗

arXiv2020

Spin-triplet pairing state evidenced by half-quantum flux in a noncentrosymmetric superconductor

A prime category of superconducting materials in which to look for spin-triplet pairing and topological superconductivity are superconductors without inversion symmetry. It is predicted that the breaking of parity symmetry gives rise to an admixture of spin-singlet / spin-triplet pairing states; a triplet pairing component, being substantial, seems all but guaranteed. However, the experimental confirmation of pair mixing in any particular material remains elusive. In this work, we perform phase-sensitive experiment to examine the pairing state of noncentrosymmetric superconductor $α-$BiPd. The Little-Parks effect observed in mesoscopic polycrystalline $α-$BiPd ring devices reveals the presence of half-integer magnetic flux quantization, which provides a decisive evidence for the spin-triplet pairing state. We find mixed half-quantum fluxes and integer-quantum fluxes, consistent with the scenario of singlet-triplet pair mixing.

cond-mat.supr-con↗

arXiv2019

A Divergent Random Walk on Stairs

We consider a state-dependent, time-dependent, discrete random walks $X_t^{\{a_n\}}$ defined on natural numbers $\mathbb{N}$ (bent to a "stair" in $\mathbb{N}^2$) where the random walk depends on input of a positive deterministic sequence $\{a_n\}$. This walk has the peculiar property that if we set $a_n$ to be $+\infty$ for all $n$, it converges to a stationary distribution $π(\cdot)$; but if $a_n$ is uniformly bounded (over all $n$) by any upper bound $a \in (0,\infty)$, this walk diverges to infinity with probability 1. It is thus interesting to consider the intermediate case where $a_n<\infty$ for all $n$ but $a_n$ eventually tends to $+\infty$. (Latuszynski et al., 2013) first defined this walk and conjectured that a particular choice of sequence $\{a_n\}$ exists such that (i) $a_n \to \infty$ and, (ii) $P(X_t^{\{a_n\}} \to \infty )=1$. They managed to construct a sequence $\{a_n\}$ that satisfies (i) and $P(X_t^{\{a_n\}}\to \infty)>0$, which is weaker than (ii). In this paper, we obtain a stronger result: for any $σ<1$, there exists a choice of $\{a_n\}$ so that $P(X_t\to \infty)\ge σ$. Our result does not apply when $σ=1$, the original conjecture remains open. We record our method here for technical interests.

math.ST↗

arXiv2019

Improve Orthogonal GARCH with Hidden Markov Model

Orthogonal Generalized Autoregressive Conditional Heteroskedasticity model (OGARCH) is widely used in finance industry to produce volatility and correlation forecasts. We show that the classic OGARCH model, nevertheless, tends to be too slow in reflecting sudden changes in market condition due to excessive persistence of the integral univariate GARCH processes. To obtain more flexibility to accommodate abrupt market changes, e.g. financial crisis, we extend classic OGARCH model by incorporating a two-state Markov regime-switching GARCH process. This novel construction allows us to capture recurrent systemic regime shifts. Empirical results show that this generalization resolves the problem of excessive persistency effectively and greatly enhances OGARCH's ability to adapt to sudden market breaks while preserving OGARCH's most attractive features such as dimension reduction and multi-step ahead forecasting. By constructing a global minimum variance portfolio (GMVP), we are able to demonstrate significant outperformance of the extended model over the classic OGARCH model and the commonly used Exponentially Weighted Moving Average (EWMA) model. In addition, we show that the extended model is superior to OGARCH and EWMA in terms of predictive accuracy.

stat.ME↗

arXiv2018

Anomalous Hall Effect in Thin Film of the Weyl Antiferromagnet Mn$_3$Sn

The Weyl antiferromagnet Mn$_3$Sn has recently attracted significant attention as it exhibits various useful functions such as large anomalous Hall effect that are normally absent in antiferromagnets. Here we report the thin film fabrication of the single phase of Mn$_3$Sn and the observation of the large anomalous Hall effect at room temperature despite its vanishingly small magnetization. Our work on the high-quality thin film growth of the Weyl antiferromagnet paves the path for developing the antiferromagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2018

Observation of Half-Quantum Flux in Unconventional Superconductor $β$-Bi$_2$Pd

We report the observation of half-integer magnetic flux quantization in mesoscopic rings of superconducting $β$-Bi$_2$Pd thin films. The half-quantum fluxoid manifests itself as a $π$ phase shift in the quantum oscillation of the critical temperature. This result verifies unconventional superconductivity of $β$-Bi$_2$Pd, in accord with the expectation of a topological superconductor. We also discuss the strong indication that $β$-Bi$_2$Pd is a spin-triplet superconductor.

cond-mat.supr-con↗

arXiv2018

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties as well as for various applications including magneto-optical recording. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of $M \sim$0.002 $μ_{\rm B}$/Mn, the non-collinear AF metal Mn$_3$Sn exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Neel state may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.

cond-mat.mtrl-sci↗

arXiv2016

Topological Charge Analysis of Ultrafast Single Skyrmion Creation

Magnetic skyrmions are topologically non-trivial spin textures of potential interest for future information storage applications, and for such purposes, the control and understanding of single skyrmion creation is required. A scheme is analyzed to create single Néel-type and Bloch-type skyrmions in helimagnetic thin films utilizing the dynamical excitations induced by the Oersted field and the spin transfer torque given by a vertically injected spin-polarized current. A topological charge analysis using a lattice version of the topological charge provides insight into the locally triggered transition from a trivial to a non-trivial topological spin texture of the Néel or Bloch type skyrmion. The topological protection of the magnetic skyrmion is determined by the symmetric Heisenberg exchange energy. The critical switching current density is $\sim10^{7}\thinspace\textrm{A/cm}^{2}$, which decreases with the easy-plane type uniaxial anisotropy and thermal fluctuations. In-plane spin polarization of the injected current performs better than out-of-plane polarization, and it provides ultrafast switching times (within 100 ps) and reliable switching outcomes.

cond-mat.mes-hall↗

arXiv2014

Evidence of the side jump mechanism in the anomalous Hall effect in paramagnets

Persistent confusion has existed between the intrinsic (Berry curvature) and the side jump mechanisms of anomalous Hall effect (AHE) in ferromagnets. We provide unambiguous identification of the side jump mechanism, in addition to the skew scattering contribution in epitaxial paramagnetic Ni$_{34}$Cu$_{66}$ thin films, in which the intrinsic contribution is by definition excluded. Furthermore, the temperature dependence of the AHE further reveals that the side jump mechanism is dominated by the elastic scattering.

cond-mat.mtrl-sci↗

arXiv2013

A Splitting Augmented Lagrangian Method for Low Multilinear-Rank Tensor Recovery

This paper studies a recovery task of finding a low multilinear-rank tensor that fulfills some linear constraints in the general settings, which has many applications in computer vision and graphics. This problem is named as the low multilinear-rank tensor recovery problem. The variable splitting technique and convex relaxation technique are used to transform this problem into a tractable constrained optimization problem. Considering the favorable structure of the problem, we develop a splitting augmented Lagrangian method to solve the resulting problem. The proposed algorithm is easily implemented and its convergence can be proved under some conditions. Some preliminary numerical results on randomly generated and real completion problems show that the proposed algorithm is very effective and robust for tackling the low multilinear-rank tensor completion problem.

math.OC↗

arXiv2012

Robust formation of skyrmions and topological Hall effect in epitaxial thin films of MnSi

Magneto-transport properties have been investigated for epitaxial thin films of B20-type MnSi grown on Si(111) substrates. Both Lorentz transmission electron microscopy (TEM) images and topological Hall effect (THE) clearly point to the robust formation of skyrmions over a wide temperature-magnetic field region. New features distinct from those of bulk MnSi are observed for epitaxial MnSi films: a shorter (nearly half) period of the spin helix and skyrmions, and an opposite sign of THE. These observations suggest versatile features of skyrmion-induced THE beyond the current understanding.

cond-mat.mtrl-sci↗