Amorphous magnetic semiconductors with Curie temperatures above room temperature

Recently, amorphous magnetic semiconductors as a new family of magnetic semiconductors have been developed by oxidizing ferromagnetic amorphous metals/alloys. Intriguingly, tuning the relative atomic ratios of Co and Fe in a Co-Fe-Ta-B-O system leads to the formation of an intrinsic magnetic semiconductor. Starting from high Curie-temperature amorphous ferromagnets, these amorphous magnetic semiconductors show Curie temperatures well above room temperature. Among them, one typical example is a p-type Co28.6Fe12.4Ta4.3B8.7O46 magnetic semiconductor, which has an optical bandgap of ~2.4 eV, roomtemperature saturation magnetization of ~433 emu/cm3, and the Curie temperature above 600 K. The amorphous Co28.6Fe12.4Ta4.3B8.7O46 magnetic semiconductor can be integrated with n-type Si to form p-n heterojunctions with a threshold voltage of ~1.6 V, validating its p-type semiconducting character. Furthermore, the demonstration of electric field control of its room-temperature ferromagnetism reflects the interplay between the electricity and ferromagnetism in this material. It is suggested that the carrier density, ferromagnetism and conduction type of an intrinsic magnetic semiconductor are controllable by means of an electric field effect. These findings may pave a new way to realize magnetic semiconductor-based spintronic devices that work at room temperature.

Review: Nanostructured TiO_2 for Enhanced Photocatalytic Property by Glancing Angle Deposition Method

The glancing angle deposition( GLAD) method is a kind of physical vapor deposition and many kinds of self-standing nanostructured materials can be prepared. Here we report GLAD method to form selfstanding TiO_2 nanostructures on specific substrates,which makes photocatalysts easier to recycle. And smart designs of doping,decorating band gap matched materials can also make traditional photocatalysts take use of more visible light. In this review,we present some previous work fabricating nanostructured TiO_2 composites for enhanced photocatalytic performance by these methods.

Review:Ferromagnetism in Undoped ZrO2 Thin Films

Diluted magnetic oxides have evolved into a popular branch of materials science during the last decade. In the first few years,people attributed the ferromagnetism to the magnetic dopants. However,the observation of ferromagnetism in undoped HfO_2 thin films made it more controversial and promoted extensive research on the ferromagnetism in various undoped oxides. Both of the experimental works and theoretical studies have shown that intrinsic defects in oxide nanomaterials play a crucial role in the origin of such an unexpected ferromagnetism,in spite of some contradicting views which kind of defects is predominant. In the past several years,we have conducted systematic and thorough research on the room temperature ferromagnetism in undoped ZrO_2 thin films,and clarify some physics behind it. We firstly prepared undoped ZrO_2 thin films by different ways,such as Pulsed electron beam deposition,magnetron sputtering,and electron beam evaporation,and successfully obtained ZrO_2 thin films with different crystalline structure,in particular a pure high-temperature stabilized one,by adjusting some preparation parameters during the deposition process or post-annealing treatment. A phase-dependent ferromagnetism was then confirmed to exist in such ZrO_2 thin films. Further,we conducted exhaustive defect analysis and characterization by X-ray photoelectron spectroscopy,photoluminescence spectra,and electron paramagnetic resonance,respectively,and found the oxygen vacancy,specifically the single ionized oxygen vacancy( V_O^+),has a remarkable influence on the enhancement of ferromagnetism. Herein,we will review the work in detail on the phase-dependent and oxygen vacancy-enhanced room temperature ferromagnetism in undoped ZrO_2 thin films.

Thickness dependence of the crystallization and phase transition in ZrO_(2)thin films

Fluorite-structure binary oxides(e.g.,HfO_(2)and ZrO_(2))have attracted increasing interest for a broad range of applications including thermal barrier coatings,high-k dielectrics,and novel ferroelectrics.A crystalline structure plays a crucial role in determining physical and chemical properties.Structure evolution of ZrO_(2)thin films,particularly down to the nanometer scale,has not been thoroughly studied.In this work,we carried out systematic annealing analysis on the ZrO_(2)thin films.Through in-situ high-temperature X-ray diffraction(XRD)characterizations,a thickness dependence of crystallization and phase transition is observed.Irrespective of the thickness(10-300 nm),the as-prepared amorphous ZrO_(2)thin films are preferentially crystallized into a tetragonal(t)structure(high-temperature phase),which can be preserved down to room temperature(RT)upon anncaling at the corresponding crystallization temperaturc(T).When anncaling at temperaturcs higher than Tc,the transition from t to monoclinic(m;RT phase)will occur,and the quantity of the transition strongly depends on the film thickness.Our work expands the basic understanding of the phase transition in the ZrO_(2)thin films,and offers a path to the selective control over the phase structure for novel functionalities.

Rapid Recognition of Isomers of Monochlorobiphenyls at Trace Levels by Surface-Enhanced Raman Scattering Using Ag Nanorods as a Substrate

Isomers and homologues of organic pollutants are hard to distinguish-especially in trace amounts-due to the similarities in their physical and chemical properties. We report here that by identifying the Raman characteristics of isomers of monochlorobiphenyls, these compounds can be recognized, even at trace levels, by using the surface-enhance Raman scattering method with silver nanorods as a substrate. When dissolved in acetone, 2-, 3-, and 4-chlorobiphenyls were detected at a concentration of 10^(-8) mol/L, at which their characteristic Raman peaks were visible. This study may provide a fast, simple, and sensitive method for the detection and recognition of organic pollutants such as polychlorinated biphenyls.

Realignment of Slanted Fe Nanorods on Silicon Substrates by a Strong Magnetic Field

Slanted Fe nanorods prepared by glancing angle deposition on silicon substrates exhibited easy magnetization along their growth axis. By using a thin gold film on a silicon substrate as a buffer layer, slanted Fe nanorods can be realigned towards the substrate surface normal by a strong magnetic field. After realignment, the Fe nanorods retained the easy magnetization axis along their growth axis. The effects of the realignment by the strong magnetic field on the properties of the slanted Fe nanorods were also investigated. This study provides a possible way to fabricate magnetic nanostructures for perpendicular recording applications.

Wettability manipulation nanorod arrays of magnetic transition metal by X-ray irradiation

Wettability manipulation of glancing angle deposited Fe/Co/Ni nanorod arrays was realized by X-ray irradiation in ultra-high vacuum chamber. Reversible transition was also purchased by alternating ethanol immersion and X-ray irradiation. Alkyl group adsorption-desorption mechanism and corresponding morphology depen- dence of wettability manipulation were revealed.

An extended sparsemax-linearmoving model with application to high-frequency financial data

There continues to be unfading interest in developing parametric max-stable processes for modelling tail dependencies and clustered extremes in time series data.However,this comes with some difficulties mainly due to the lack of models that fit data directly without transforming the data and the barriers in estimating a significant number of parameters in the existing models.In thiswork,we study the use of the sparsemaxima ofmovingmaxima(M3)process.After introducing random effects and hidden Fréchet type shocks into the process,we get an extended maxlinear model.The extended model then enables us to model cases of tail dependence or independence depending on parameter values.We present some unique properties including mirroring the dependence structure in real data,dealing with the undesirable signature patterns found in most parametricmax-stable processes,and being directly applicable to real data.ABayesian inference approach is developed for the proposed model,and it is applied to simulated and real data.

On studying extreme values and systematic risks with nonlinear time series models and tail dependence measures

This review paper discusses advances of statistical inference in modeling extreme observations from multiple sources and heterogeneous populations.The paper starts briefly reviewing classical univariate/multivariate extreme value theory,tail equivalence,and tail(in)dependence.New extreme value theory for heterogeneous populations is then introduced.Time series models for maxima and extreme observations are the focus of the review.These models naturally form a new system with similar structures.They can be used as alternatives to the widely used ARMA models and GARCH models.Applications of these time series models can be in many fields.The paper discusses two important applications:systematic risks and extreme co-movements/large scale contagions.

New extreme value theory for maxima of maxima

Although advanced statistical models have been proposed to fit complex data better,the advances of science and technology have generated more complex data,e.g.,Big Data,in which existing probability theory and statistical models find their limitations.This work establishes probability foundations for studying extreme values of data generated from a mixture process with the mixture pattern depending on the sample length and data generating sources.In particular,we show that the limit distribution,termed as the accelerated max-stable distribution,of the maxima of maxima of sequences of random variables with the above mixture pattern is a product of three types of extreme value distributions.As a result,our theoretical results are more general than the classical extreme value theory and can be applicable to research problems related to Big Data.Examples are provided to give intuitions of the new distribution family.We also establish mixing conditions for a sequence of random variables to have the limit distributions.The results for the associated independent sequence and the maxima over arbitrary intervals are also developed.We use simulations to demonstrate the advantages of our newly established maxima of maxima extreme value theory.

Extreme vocabulary learning

Regarding extreme value theory,the unseen novelclasses in the openset recognition can be seen as the extremevalues of training classes.Following this idea,we introducethe margin and coverage distribution to model the trainingclasses.A novel visual-semantic embedding framework-extreme vocabulary learning(EVoL)is proposed;the EVoL embeds the visual features into semantic space in a probabilisticway.Notably,we adopt the vast open vocabulary in the semantic space to help further constraint the margin and coverage of training classes.The learned embedding can directlybe used to solve supervised learning,zero-shot learning,andopen set recognition simultaneously.Experiments on twobenchmark datasets demonstrate the effectiveness of the proposed framework against conventional ways.

Rejoinder of“On studying extreme values and systematic risks with nonlinear time series models and tail dependence measures”

such broader and thoughtful discussions in probability theory,theoretical statistics,estimation methods,and applications.The discussants have made many excellent points.I appreciate the discussants’interest in the reviewed contents and much broader theoretical and methodological topics related to extreme value study.

Mechanically robust antireflective coatings

Mechanical strength is an essential parameter that influences and limits the lifetime performance of antireflective （AR） coatings in optical devices. Speciflcally, amphiphobic AR coatings with reduced reflectance are of great significance as they considerably enlarge the range of fundamental applications. Herein, we describe the design and fabrication of amphiphobic AR coatings with reduced reflectance and enhanced mechanical resilience. Introducing a thin polytetrafluoroethylene （PTFE） layer on top of the bilayer SiOz coating via vapor deposition method makes it highly liquid repellent. We achieved reduced reflectance （〈 1%） over the entire visible wavelength range, as well as tunability according to the desired wavelength region. The fabricated film showed better thermal stability （up to 300℃） with stable AR efficiency, when an ultrathin dense coat of Al2O3 was deposited via atomic layer deposition （ALD） on the polymer-based bilayer SiO2 antireflective coating （P-BSAR）. The experimental results prove that the omnidirectional AR coating in this study exhibits multifunctional properties and should be suitable for the production of protective optical equipment and biocompatible polymer films for the displays of portable electronic devices.

Defects-Driven Ferromagnetism in Undoped Dilute Magnetic Oxides:A Review

In the past several decades, dilute magnetic semiconductors, particularly the dilute magnetic oxides have evolved into an important branch of materials science due to their potential application in spintronic devices combining of properties of semiconductors and ferromagnets. In spite of a major effort devoted to the mechanism of ferromagnetism with a high Curie temperature in these materials, it still remains the most controversial research topic, especially given the unexpected do ferromagnetism in a series of undoped wide-band-gap oxides films or nanostructures. Recently, an abundance of research has shown the critical role of various defects in the origin and control of spontaneous magnetic ordering, but contradicting views from intertwined theoretical calculations and experiments require more in-depth systematic research. In our previous work, considerable efforts have been focused on two major oxides, i.e. ZnO and Zr02. This review will present a summary of current experimental status of this defect-driven ferromag- netism in dilute magnetic oxides （DMOs）.