表面预处理对石墨烯上范德瓦耳斯外延生长GaN材料的影响

基于范德瓦耳斯外延生长的氮化镓/石墨烯材料异质生长界面仅靠较弱的范德瓦耳斯力束缚,具有低位错、易剥离等优势,近年来引起了人们的广泛关注.采用NH_3/H_2混合气体对石墨烯表面进行预处理,研究了不同NH_3/H_2比对石墨烯表面形貌、拉曼散射的影响,探讨了石墨烯在NH_3和H_2混合气氛下的表面预处理机制,最后在石墨烯上外延生长了1.6μm厚的GaN薄膜材料.实验结果表明:石墨烯中褶皱处的C原子更容易与气体发生刻蚀反应,刻蚀方向沿着褶皱进行;适当NH_3/H_2比的混合气体对石墨烯进行表面预处理可有效改善石墨烯上GaN材料的晶体质量.本研究提供了一种可有效提高GaN晶体质量的石墨烯表面预处理方法,可为进一步研究二维材料上高质量的GaN外延生长提供参考.     

范德堡方法在ZnO薄膜测试中的应用

近年来,随着对宽禁带半导体材料,氧化锌薄膜研究的快速发展,对其电学性质的研究也显得尤为重要。主要介绍范德堡方法在ZnO薄膜电学性质测量中的应用,并对初步测量结果作简要的讨论。     

Observation of magnetoresistance in CrI3/graphene van der Waals heterostructures

Two-dimensional ferromagnetic van der Waals (2D vdW) heterostructures have opened new avenues for creating artificial materials with unprecedented electrical and optical functions beyond the reach of isolated 2D atomic layered materials, and for manipulating spin degree of freedom at the limit of few atomic layers, which empower next-generation spintronic and memory devices. However, to date, the electronic properties of 2D ferromagnetic heterostructures still remain elusive. Here, we report an unambiguous magnetoresistance behavior in CrI₃/graphene heterostructures, with a maximum magnetoresistance ratio of 2.8%. The magnetoresistance increases with increasing magnetic field, which leads to decreasing carrier densities through Lorentz force, and decreases with the increase of the bias voltage. This work highlights the feasibilities of applying two-dimensional ferromagnetic vdW heterostructures in spintronic and memory devices.     

Electron drift velocity and mobility in graphene

We present a theoretical study of the electric transport properties of graphene-substrate systems. The drift velocity, mobility, and temperature of the electrons are self-consistently determined using the Boltzmann equilibrium equations. It is revealed that the electronic transport exhibits a distinctly nonlinear behavior. A very high mobility is achieved with the increase of the electric fields increase. The electron velocity is not completely saturated with the increase of the electric field. The temperature of the hot electrons depends quasi-linearly on the electric field. In addition, we show that the electron velocity, mobility, and electron temperature are sensitive to the electron density. These findings could be employed for the application of graphene for high-field nano-electronic devices.     

基于Chebyshev多项式逼近的随机 van der Pol系统的倍周期分岔分析

应用 Chebyshev 多项式逼近法研究了谐和激励作用下具有随机参数的随机van der Pol系统 的倍周期分岔现象.随机系统首先被转化成等价的确定性系统，然后通过数值方法求得响应 ，借此探索了随机van der Pol系统丰富的随机倍周期分岔现象.数值模拟显示随机van der Pol 系统存在与确定性系统极为相似的倍周期分岔行为，但受随机因素的影响，又有与之不 同之处.数值结果表明，Chebyshev 多项式逼近是研究非线性系统动力学问题的一种新的有 效方法. 关键词： Chebyshev 多项式 随机van der Pol 系统 倍周期分岔     

分数阶van der Pol振子的超谐共振

以含分数阶微分项的van der Pol振子为对象,研究其超谐共振时的动力学特性.首先,通过平均法得到了系统的一阶近似解,提出了超谐共振时等效线性阻尼和等效线性刚度的概念,研究了分数阶微分项的系数和阶次以等效线性阻尼和等效线性刚度的形式对系统动力学特性的影响.随后,建立了超谐共振时定常解的幅频曲线的解析表达式,得到了超谐共振周期响应的稳定性判断准则并提出等效非线性阻尼和非线性稳定性条件参数的概念.最后,通过数值仿真比较了分数阶与整数阶系统的幅频曲线,分析了分数阶微分项的系数和阶次对响应幅值、幅频曲线以及系统稳定性的影响.     

一类随机van der Pol系统的Hopf 分岔研究

研究了一类随机van der Pol 系统的Hopf分岔行为.首先根据Hilbert空间的正交展开理论,含有随机参数的van der Pol系统被约化为等价确定性系统,然后利用确定性分岔理论分析了等价系统的Hopf分岔,得出了随机van der Pol 系统的Hopf 分岔临界点,探究了随机参数对系统Hopf分岔的影响.最后利用数值模拟验证了理论分析结果. 关键词： 随机van der Pol系统 Hopf分岔 正交多项式逼近     

二维材料的新奇物理及异质结的能带调控

以石墨烯为代表的层状材料具备显著区别于三维材料的新奇物理特性。更为重要的是,原子级平整的二维材料使得科学家们可以将不同的二维材料通过堆垛或者把相同的二维材料通过堆垛加扭转构成范德瓦耳斯异质结。通过层间耦合作用,可对异质结的能带结构和物理性质进行有效调控,从而衍生出单个二维材料所不具备的新奇物性。范德瓦耳斯异质结的能带调控极大地拓宽了二维材料的科学研究和应用前景。     

二维平面和范德瓦耳斯异质结的可控制备与光电应用

自石墨烯被发现以来,二维材料因其优异的特性获得了持续且深入的探索与发展,以石墨烯、六方氮化硼、过渡金属硫化物、黑磷等为代表的二维材料相关研究层出不穷.随着二维新材料制备与应用探索的不断发展,单一材料性能的不足逐渐凸显,研究者们开始考虑采用平面拼接和层间堆垛所产生的协同效应来弥补单一材料的不足,甚至获得一些新的性能.利用二维材料晶格结构的匹配构建异质结,实现特定的功能化,或利用范德瓦耳斯力进行堆垛,将不同二维材料排列组合,从而在体系里引入新的自由度,为二维材料的性质研究和实际应用打开了新的窗口.本文从原子制造角度,介绍了二维平面和范德瓦耳斯异质结材料的可控制备和光电应用.首先简要介绍了应用于异质结制备的常见二维材料的分类及异质结的相关概念,然后从原理上分类列举了常用的表征方法,随后介绍了平面和垂直异质结的制备方法,并对其光电性质及器件应用做了简要介绍.最后,对领域内存在的问题进行了讨论,对未来发展方向做出了展望.     

Measurement of electronic structure in van der Waals ferromagnet Fe5-xGeTe2

As a van der Waals ferromagnet with high Curie temperature, Fe_5-xGeTe₂ has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe_5-xGeTe₂ crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight k_z evolution. Interestingly, across the ferromagnetic transition, we observed the merging of two split bands above the Curie temperature, suggesting the band splitting due to the exchange interaction within the itinerant Stoner model. Our results provide important insights into the electronic and magnetic properties of Fe_5-xGeTe₂ and the understanding of magnetism in a two-dimensional ferromagnetic system.     

Faraday rotations,ellipticity, and circular dichroism in magneto-optical spectrum of moiré superlattices

We study the magneto-optical conductivity of a number of van der Waals heterostructures, namely, twisted bilayer graphene, AB-AB and AB-BA stacked twisted double bilayer graphene and monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride. As the magnetic field increases, the absorption spectrum exhibits a self-similar recursive pattern reflecting the fractal nature of the energy spectrum. Whilst twisted bilayer graphene displays only weak circular dichroism, the other four structures display strong circular dichroism with monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride being particularly pronounced owing to strong inversion symmetry breaking properties of the hexagonal boron nitride layer. As the left and right circularly polarized light interact with these structures differently, plane-polarized incident light undergoes a Faraday rotation and gains an ellipticity when transmitted. The size of the respective angles is on the order of a degree.     

二维范德瓦尔斯异质结构的制备与物性研究

二维范德瓦尔斯材料(可简称二维材料)已发展成为备受瞩目的材料大家族,而由其衍生的二维范德瓦尔斯异质结构的集成、性能及应用是现今凝聚态物理和材料科学领域的研究热点之一.二维范德瓦尔斯异质结构为探索丰富多彩的物理效应和新奇的物理现象,以及构建新型的自旋电子学器件提供了灵活而广阔的平台.本文从二维材料的转移技术着手,介绍二维范德瓦尔斯异质结构的构筑、性能及应用.首先,依据湿法转移和干法转移的分类,详细介绍二维范德瓦尔斯异质结构的制备技术,内容包括转移技术的通用设备、常用转移方法的具体操作步骤、三维操纵二维材料的方法、异质界面清洁.随后介绍二维范德瓦尔斯异质结构的性能和应用,重点介绍二维磁性范德瓦尔斯异质结构,并列举在二维范德瓦尔斯磁隧道结和摩尔超晶格领域的应用.因此,二维材料转移技术的发展和优化将进一步助力二维范德瓦尔斯异质结构在基础科学研究和实际应用上取得突破性的成果.     

Amplitude control of limit cycle in a van der Pol Duffing system

This paper applies washout filter technology to amplitude control of limit cycles emerging from Hopf bifurcation of the van der Pol--Duffing system. The controlling parameters for the appearance of Hopf bifurcation are given by the Routh--Hurwitz criteria. Noticeably, numerical simulation indicates that the controllers control the amplitude of limit cycles not only of the weakly nonlinear van der Pol--Duffing system but also of the strongly nonlinear van der Pol--Duffing system. In particular, the emergence of Hopf bifurcation can be controlled by a suitable choice of controlling parameters. Gain-amplitude curves of controlled systems are also drawn.     

Electrical conductivity and resonant states of doped graphene considering next-nearest neighbor interaction

The next-nearest neighbor interaction (NNN) is included in a tight-binding Kubo formula calculation of the electronic spectrum and conductivity of doped graphene. As a result, we observe a wide variation of the behavior of the conductivity, as happens in carbon nanotubes, since the Fermi energy and the resonance peak are not shifted by the same amount when the NNN interaction is included. This finding may have a profound effect on the idea of explaining the minimal conductivity of graphene as a consequence of impurities or defects. Finally, we also estimate the mean free path and relaxation time due to resonant impurity scattering.     

二维范德瓦尔斯异质结构的制备与物性研究

二维范德瓦尔斯材料(可简称二维材料)已发展成为备受瞩目的材料大家族,而由其衍生的二维范德瓦尔斯异质结构的集成、性能及应用是现今凝聚态物理和材料科学领域的研究热点之一.二维范德瓦尔斯异质结构为探索丰富多彩的物理效应和新奇的物理现象,以及构建新型的自旋电子学器件提供了灵活而广阔的平台.本文从二维材料的转移技术着手,介绍二维范德瓦尔斯异质结构的构筑、性能及应用.首先,依据湿法转移和干法转移的分类,详细介绍二维范德瓦尔斯异质结构的制备技术,内容包括转移技术的通用设备、常用转移方法的具体操作步骤、三维操纵二维材料的方法、异质界面清洁.随后介绍二维范德瓦尔斯异质结构的性能和应用,重点介绍二维磁性范德瓦尔斯异质结构,并列举在二维范德瓦尔斯磁隧道结和摩尔超晶格领域的应用.因此,二维材料转移技术的发展和优化将进一步助力二维范德瓦尔斯异质结构在基础科学研究和实际应用上取得突破性的成果.     

Phase synchronization and synchronization frequency of two-coupled van der Pol oscillators with delayed coupling

In this paper, phase synchronization and the frequency of two synchronized van der Pol oscillators with delay coupling are studied. The dynamics of such a system are obtained using the describing function method, and the necessary conditions for phase synchronization are also achieved. Finding the vicinity of the synchronization frequency is the major advantage of the describing function method over other traditional methods. The equations obtained based on this method justify the phenomenon of the synchronization of coupled oscillators on a frequency either higher, between, or lower than the highest, in between, or lowest natural frequency of the aggregate oscillators. Several numerical examples simulate the different cases versus the various synchronization frequency delays.     

Stochastic response of van der Pol oscillator with two kinds of fractional derivatives under Gaussian white noise excitation

This paper aims to investigate the stochastic response of the van der Pol(VDP) oscillator with two kinds of fractional derivatives under Gaussian white noise excitation.First,the fractional VDP oscillator is replaced by an equivalent VDP oscillator without fractional derivative terms by using the generalized harmonic balance technique.Then,the stochastic averaging method is applied to the equivalent VDP oscillator to obtain the analytical solution.Finally,the analytical solutions are validated by numerical results from the Monte Carlo simulation of the original fractional VDP oscillator.The numerical results not only demonstrate the accuracy of the proposed approach but also show that the fractional order,the fractional coefficient and the intensity of Gaussian white noise play important roles in the responses of the fractional VDP oscillator.An interesting phenomenon we found is that the effects of the fractional order of two kinds of fractional derivative items on the fractional stochastic systems are totally contrary.     

范德瓦耳斯气体与热力学第三定律不相容

指出范德瓦耳斯气体与经典理想气体一样,也与热力学第三定律不相容．由此指明了考虑了粒子间相互作用的经典体系,仍与热力学第三定律不相容．     

Intramolecular vibrational redistribution and vibrational predissociation in 9-cyanoanthracene-Ar van der waals complexes

Processes of intramolecular vibrational redistribution and vibrational predissociation in 9-cyanoanthracene—Ar van der Waals complexes are investigated; their effect on the probabilities of nonradiative transitions is establsihed. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 184–191, March–April, 1998.     

Coupled van der Pol-Duffing oscillators: Phase dynamics and structure of synchronization tongues

Synchronization in the system of coupled non-identical non-isochronous van der Pol-Duffing oscillators with inertial and dissipative coupling is discussed. Generalized Adler’s equation is obtained and investigated in the presence of all relevant factors affecting the synchronization (non-isochronism of the oscillators, their non-identity, coupling of the dissipative and inertial types). Characteristic symmetries are revealed for the Adler’s equation responsible for equivalence of some of the factors. Numerical study of the parameters space of the initial differential equations is carried out using the method of charts of dynamic regimes in the parameter planes. Results obtained by both these approaches are compared and discussed.