arXiv subjects

Wei Ning

Publications and source records attributed to Wei Ning.

At least 37 records · Page 2Linked to original sources

arXiv2020

Recent advancements in the study of intrinsic magnetic topological insulators and magnetic Weyl semimetals

The studies of topological insulators and topological semimetals have been at frontiers of condensed matter physics and material science. Both classes of materials are characterized by robust surface states created by the topology of the bulk band structures and exhibit exotic transport properties. When magnetism is present in topological materials and breaks the time-reversal symmetry, more exotic quantum phenomena can be generated, e.g. quantum anomalous Hall effect, axion insulator, large intrinsic anomalous Hall effect, etc. In this research update, we briefly summarize the recent research progresses in magnetic topological materials, including intrinsic magnetic topological insulators and magnetic Weyl semimetals.

cond-mat.mtrl-sci↗

arXiv2020

Three-dimensional topological semimetal phase in layered TaNiTe5 probed by de Haas-van Alphen effect

Layered three-dimensional (3D) topological semimetals have attracted intensively attention due to the exotic phenomena and abundantly tunable properties. Here we report the experimental evidence for the 3D topological semimetal phase in layered material TaNiTe5 single crystals through quantum oscillations. Strong quantum oscillations have been observed with diamagnetism background in TaNiTe5. By analyzing the de Haas-van Alphen oscillations, multi-periodic oscillations were extracted, in content with magnetotransport measurements. Moreover, nontrivial "π" Berry phase with 3D Fermi surface is identified, indicating the topologically nontrivial feature in TaNiTe5. Additionally, we demonstrated the thin-layer of TaNiTe5 crystals is highly feasible by the mechanical exfoliation, which offers a platform to explore exotic properties in low dimensional topological semimetal and paves the way for potential applications in nanodevices.

cond-mat.str-el↗

arXiv2020

Robust utility maximization under model uncertainty via a penalization approach

This paper addresses the problem of utility maximization under uncertain parameters. In contrast with the classical approach, where the parameters of the model evolve freely within a given range, we constrain them via a penalty function. We show that this robust optimization process can be interpreted as a two-player zero-sum stochastic differential game. We prove that the value function satisfies the Dynamic Programming Principle and that it is the unique viscosity solution of an associated Hamilton-Jacobi-Bellman-Isaacs equation. We test this robust algorithm on real market data. The results show that robust portfolios generally have higher expected utilities and are more stable under strong market downturns. To solve for the value function, we derive an analytical solution in the logarithmic utility case and obtain accurate numerical approximations in the general case by three methods: finite difference method, Monte Carlo simulation, and Generative Adversarial Networks.

math.OC↗

arXiv2019

Giant room temperature anomalous Hall effect and magnetically tuned topology in the ferromagnetic Weyl semimetal Co2MnAl

Weyl semimetals (WSM) have been extensively studied due to their exotic properties such as topological surface states and anomalous transport phenomena. Their band structure topology is usually predetermined by material parameters and can hardly be manipulated once the material is formed. Their unique transport properties appear usually at very low temperature, which sets challenges for practical device applications. In this work, we demonstrate a way to modify the band topology via a weak magnetic field in a ferromagnetic topological semimetal, Co2MnAl, at room temperature. We observe a tunable, giant anomalous Hall effect, which is induced by the transition between Weyl points and nodal rings as rotating the magnetization axis. The anomalous Hall conductivity is as large as that of a 3D quantum anomalous Hall effect (QAHE), with the Hall angle reaching a record value (21%) at the room temperature among magnetic conductors. Furthermore, we propose a material recipe to generate the giant anomalous Hall effect by gaping nodal rings without requiring the existence of Weyl points. Our work reveals an ideal intrinsically magnetic platform to explore the interplay between magnetic dynamics and topological physics for the development of a new generation of spintronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Personalized Attraction Enhanced Sponsored Search with Multi-task Learning

We study a novel problem of sponsored search (SS) for E-Commerce platforms: how we can attract query users to click product advertisements (ads) by presenting them features of products that attract them. This not only benefits merchants and the platform, but also improves user experience. The problem is challenging due to the following reasons: (1) We need to carefully manipulate the ad content without affecting user search experience. (2) It is difficult to obtain users' explicit feedback of their preference in product features. (3) Nowadays, a great portion of the search traffic in E-Commerce platforms is from their mobile apps (e.g., nearly 90% in Taobao). The situation would get worse in the mobile setting due to limited space. We are focused on the mobile setting and propose to manipulate ad titles by adding a few selling point keywords (SPs) to attract query users. We model it as a personalized attractive SP prediction problem and carry out both large-scale offline evaluation and online A/B tests in Taobao. The contributions include: (1) We explore various exhibition schemes of SPs. (2) We propose a surrogate of user explicit feedback for SP preference. (3) We also explore multi-task learning and various additional features to boost the performance. A variant of our best model has already been deployed in Taobao, leading to a 2% increase in revenue per thousand impressions and an opt-out rate of merchants less than 4%.

cs.IR↗

arXiv2019

IntentGC: a Scalable Graph Convolution Framework Fusing Heterogeneous Information for Recommendation

The remarkable progress of network embedding has led to state-of-the-art algorithms in recommendation. However, the sparsity of user-item interactions (i.e., explicit preferences) on websites remains a big challenge for predicting users' behaviors. Although research efforts have been made in utilizing some auxiliary information (e.g., social relations between users) to solve the problem, the existing rich heterogeneous auxiliary relationships are still not fully exploited. Moreover, previous works relied on linearly combined regularizers and suffered parameter tuning. In this work, we collect abundant relationships from common user behaviors and item information, and propose a novel framework named IntentGC to leverage both explicit preferences and heterogeneous relationships by graph convolutional networks. In addition to the capability of modeling heterogeneity, IntentGC can learn the importance of different relationships automatically by the neural model in a nonlinear sense. To apply IntentGC to web-scale applications, we design a faster graph convolutional model named IntentNet by avoiding unnecessary feature interactions. Empirical experiments on two large-scale real-world datasets and online A/B tests in Alibaba demonstrate the superiority of our method over state-of-the-art algorithms.

cs.IR↗

arXiv2018

Thermal tuning of the carrier density in Dirac semimetal Cd3As2 nanoplates

The tunable carrier density plays a key role in the investigation of novel transport properties in three-dimensional topological semimetals. Here we demonstrate that the carrier density as well as the mobility of Dirac semimetal Cd3As2 nanoplates can be effectively tuned by the in-situ thermal treatment at 350 K for one hour, both showing a non-monotonic evolution with the thermal cycling treatments. The upward shift of Fermi level relative to the Dirac nodes blurs the surface Fermi-arc states, accompanying with an anomalous phase shift of oscillations of bulk states due to the change of topology of electrons. Meanwhile, the peaks of oscillations of bulk longitudinal magnetoresistivity shift at high fields due to their coupling to the oscillations of the surface Fermi-arc states. Our work provides a thermal control knob for manipulations of the quantum states through the carrier density in Dirac semimetal Cd3As2 at high temperature.

cond-mat.mes-hall↗

arXiv2018

de Haas-van Alphen study on three-dimensional topological semimetal pyrite PtBi2

We present the systematic de Haas-van Alphen (dHvA) quantum oscillations studies on the recently discovered topological Dirac semimetal pyrite PtBi2 single crystals. Remarkable dHvA oscillations were observed at field as low as 1.5 T. From the analyses of dHvA oscillations, we have extracted high quantum mobility, light effective mass and phase shift factor for Dirac fermions in pyrite PtBi2. From the angular dependence of dHvA oscillations, we have mapped out the topology of the Fermi surface and identified additional oscillation frequencies which were not probed by SdH oscillations.

cond-mat.mtrl-sci↗

arXiv2017

Empirical Likelihood for Change Point Detection in Autoregressive Models

Change point analysis has become an important research topic in many fields of applications. Several research work has been carried out to detect changes and its locations in time series data. In this paper, a nonparametric method based on the empirical likelihood is proposed to detect the structural changes of the parameters in autoregressive (AR) models . Under certain conditions, the asymptotic null distribution of the empirical likelihood ratio test statistic is proved to be the extreme value distribution. Further, the consistency of the test statistic has been proved. Simulations have been carried out to show that the power of the proposed test statistic is significant. The proposed method is applied to real world data set to further illustrate the testing procedure.

stat.ME↗

arXiv2017

Recognition of Fermi-arc states through the magnetoresistance quantum oscillations in Dirac semimetal Cd3As2 nanoplates

The disjointed Fermi-arcs in Weyl semimetals can intertwine with chiral bulk modes and participate in unusual closed magnetic orbits in the presence of a vertical magnetic field. Here we carry out the quantum oscillation study of such unusual Weyl magnetic orbits in Dirac semimetal Cd3As2, a close cousin of Weyl semimetals. We find that extra two-dimensional (2D) quantum oscillations emerge at high field, which superimpose on 3D bulk background and can be attributed to the Weyl magnetic orbits, when the thickness of nanoplates is smaller than the mean free path of the electrons. Further evidence of the 2D quantum oscillations from the Weyl magnetic orbits is provided by the nonlocal detection, which demonstrates an alternative way to study the quantum transport properties of Fermi-arcs under magnetic field.

cond-mat.mes-hall↗

arXiv2017

Probing the Chiral Anomaly by Planar Hall Effect in Three-dimensional Dirac Semimetal Cd3As2 Nanoplates

Searching for exotic transport properties in new topological state of matters is an active topic. One of the most fascinating achievements is the chiral anomaly in recently discovered Weyl semimetals (WSMs), which is manifested as a negative longitudinal magnetoresistance (LMR) in the presence of a magnetic field B parallel to an electric field E. Another predicted key effect closely related to the chiral anomaly is the planar Hall effect (PHE), which has not been identified in WSMs so far. Here we carried out the planar Hall measurements on Cd3As2 nanoplates, and found that, accompanied by the large negative LMR, a PHE with non-zero transverse voltage can be developed while tilting the in-plane magnetic field B away from the electric field E. Further experiments reveal that both the PHE and the negative LMR can be suppressed synchronously by increasing the temperature, but still visible at room temperature, indicating the same origin of these two effects. The observation of PHE in Cd3As2 nanoplates gives another transport evidence for the chiral anomaly and provides a deep insight into the chiral charge pumping in Weyl Fermions system.

cond-mat.mes-hall↗

arXiv2017

Field induced topological phase transition from a three-dimensional Weyl semimetal to a two-dimensional massive Dirac metal in ZrTe5

Symmetry protected Dirac semimetals can be transformed into Weyl semimetals by breaking the protecting symmetry, leading to many exotic quantum phenomena such as chiral anomaly and anomalous Hall effect. Here we show that, due to the large Zeeman g factor and small band width along b-axis in Dirac semimetal ZrTe5, a magnetic field of about 8 T along b-axis direction may annihilate the Weyl points and open up a two-dimensional (2D) Dirac mass gap, when the Zeeman splitting exceeds the band width along b-axis. This is manifested by a sharp drop of magnetoresistance (MR) above 8 T, which is probably due to additional carriers induced by the orbital splitting of the zeroth Landau level associated with the 2D Dirac point, which is descendant of the original Weyl points. Further evidence of the additional carriers is provided by the Hall effect and different anisotropic magnetoresistance (AMR) in low and high field regions. Our experiment reveals a probable topological quantum phase transition of field induced Weyl points annihilation in Dirac semimetal ZrTe5 and gives an alternative explanation for the drop of MR at high field.

cond-mat.mtrl-sci↗

arXiv2017

Extremely large magnetoresistance in a topological semimetal candidate pyrite PtBi2

While pyrite-type PtBi2 with face-centered cubic structure has been predicted to be a three-dimensional (3D) Dirac semimetal, experimental study on its physical properties remains absent. Here we report the angular-dependent magnetoresistance (MR) measurements of PtBi2 single-crystal under high magnetic fields. We observed extreme large unsaturated magnetoresistance (XMR) up to 11.2 million percent at T = 1.8 K in a magnetic field of 33 T, which surpasses the previously reported Dirac materials, such as WTe2, LaSb and NbP. The crystals exhibit an ultrahigh mobility and significant Shubnikov-de Hass (SdH) quantum oscillations with nontrivial Berry's phase. Analysis of Hall resistivity indicates that the XMR can be ascribed to the nearly compensated electron and hole. Our experimental results associated with the ab initio calculations suggest that pyrite PtBi2 is a topological semimetal candidate which might provide a platform for exploring topological materials with XMR in noble metal alloys.

cond-mat.mtrl-sci↗

arXiv2016

Thickness-tuned transition of band topology in ZrTe5 nanosheets

We report thickness-tuned electrical transport and Hall resistivity in highly anisotropic three-dimensional Dirac semimetal ZrTe5 nanosheets. We find that when the thickness of the nanosheet is blow about 40 nm, the system takes a clear transition from topological semimetal with two bands carriers to a single band with conventional hole carriers. The resistivity peak temperature T* decreases systematically with decreasing thickness down to about 40 nm, then shifts up with the further decrease of the thickness. Analysis of the data below 40 nm indicates that the hole carriers completely dominate the transport in the entire temperature range, regardless of the temperature being below or above T*. By further tracking the carrier density, we find that the Fermi level shifts consecutively downward from conduction band to the valence band as decreasing the thickness. Our experiments unambiguously reveal a highly thickness-tuned transition of band topology in ZrTe5 nanosheets.

cond-mat.mes-hall↗

arXiv2016

Superconductivity and Charge Density Wave in ZrTe$_{3-x}$Se$_{x}$

Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe$_{3}$ when the long range CDW order is gradually suppressed. Superconducting critical temperature $T_c(x)$ in ZrTe$_{3-x}$Se$_x$ (${0\leq}x\leq0.1$) increases up to 4 K plateau for $0.04$$\leq$$x$$\leq$$0.07$. Further increase in Se content results in diminishing $T_{c}$ and filametary superconductivity. The CDW modes from Raman spectra are observed in $x$ = 0.04 and 0.1 crystals, where signature of ZrTe$_{3}$ CDW order in resistivity vanishes. The electronic-scattering for high $T_{c}$ crystals is dominated by local CDW fluctuations at high temperures, the resistivity is linear up to highest measured $T=300K$ and contributes to substantial in-plane anisotropy.

cond-mat.supr-con↗

arXiv2016

Weak localization effect in topological insulator micro flakes grown on insulating ferrimagnet BaFe12O19

Many exotic physics anticipated in topological insulators require a gap to be opened for their topologica surface states by breaking time reversal symmetry. The gap opening has been achieved by doping magnetic impurities, which however inevitably create extra carriers and disorder that undermine the electronic transport. In contrast, the proximity to a ferromagnetic/ferrimagnetic insulator may improve the device quality, thus promises a better way to open the gap while minimizing the side-effects. Here, we grow thin single-crystal Sb1.9Bi0.1Te3 micro flakes on insulating ferrimagnet BaFe12O19 by using the van der Waals epitaxy technique. The micro flakes show a negative magnetoresistance in weak perpendicular fields below 50 K, which can be quenched by increasing temperature. The signature implies the weak localization effect as its origin, which is absent in intrinsic topological insulators, unless a surface state gap is opened. The surface state gap is estimated to be 10 meV by using the theory of the gap-induced weak localization effect. These results indicate that the magnetic proximity effect may open the gap for the topological surface attached to BaM insulating ferrimagnet. This heterostructure may pave the way for the realization of new physical effects as well as the potential applications of spintronics devices.

cond-mat.mes-hall↗

arXiv2016

Adjusted Empirical Likelihood for Long-memory Time Series Models

Empirical likelihood method has been applied to short-memory time series models by Monti (1997) through the Whittle's estimation method. Yau (2012) extended this idea to long-memory time series models. Asymptotic distributions of the empirical likelihood ratio statistic for short and long-memory time series have been derived to construct confidence regions for the corresponding model parameters. However, computing profile empirical likelihood function involving constrained maximization does not always have a solution which leads to several drawbacks. In this paper, we propose an adjusted empirical likelihood procedure to modify the one proposed by Yau (2012) for autoregressive fractionally integrated moving average (ARFIMA) model. It guarantees the existence of a solution to the required maximization problem as well as maintains same asymptotic properties obtained by Yau (2012). Simulations have been carried out to illustrate that the adjusted empirical likelihood method for different long-time series models provides better confidence regions and coverage probabilities than the unadjusted ones, especially for small sample sizes.

stat.ME↗

arXiv2016

Transport evidence for the three-dimensional Dirac semimetal phase in ZrTe5

Topological Dirac semimetal is a newly discovered class of materials and has attracted intense attentions. This material can be viewed as a three-dimensional (3D) analogue of graphene and has linear energy dispersion in bulk, leading to a range of exotic transport properties. Here we report direct quantum transport evidence of 3D Dirac semimetal phase of layered material ZrTe5 by angular dependent magnetoresistance measurements under high magnetic fields up to 31 Tesla. We observed very clear negative longitudinal magnetoresistance induced by chiral anomaly under the condition of the magnetic field aligned only along the current direction. Pronounced Shubnikov-de Hass (SdH) quantum oscillations in both longitudinal magnetoresistance and transverse Hall resistance were observed, revealing anisotropic light cyclotron masses and high mobility of the system. In particular, a nontrivial π-Berry phase in the SdH gives clear evidence for 3D Dirac semimetal phase. Furthermore, we observed clear Landau Level splitting under high magnetic field, suggesting possible splitting of Dirac point into Weyl points due to broken time reversal symmetry. Our results indicate that ZrTe5 is an ideal platform to study 3D massless Dirac and Weyl fermions in a layered compound.

cond-mat.mes-hall↗