Faraday rotations,ellipticity, and circular dichroism in magneto-optical spectrum of moire 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.     

Rotational Spectrum, Dynamics, and Bond Energy of the Oxirane···Krypton van der Waals Complex

From the free jet millimeter-wave spectra of four isotopomers of the weakly bonded oxirane···Kr complex, information on the equilibrium conformation, dynamics, and dissociation energy has been deduced. A C_s symmetry is found for the complex, with Kr lying in the σ_v plane of symmetry of oxirane. The equilibrium distance of Kr with respect to the center of mass of bare oxirane is 3.67 Å, with Kr tilted 13.6° from the perpendicular to the center of mass of the ring toward the oxygen atom. The dissociation energy is estimated, from the centrifugal distortion constant D_J, to be ca. 3.1 kJ/mol.     

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.     

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.     

Long range dispersion interactions of the low lying states of Mg with H,He, Ne,Ar, Kr and Xe

The dispersion coefficients for the van der Waals interactions for the low lying states of magnesium (3s² ¹S ^e , 3s3p ³P ^o , 3s3p¹P ^o , 3s4s ³S ^e , 3s4s ¹S ^e , 3s3d ³D ^e and 3s3d ¹D ^e ) interacting with H, He, Ne, Ar, Kr and Xe are determined using sum rules of reduced matrix elements over pseudo-state representations of the atomic excitation spectra. The sets of transition matrix elements were taken from previous investigations of the van der Waals interactions of the Mg₂ dimer and an investigation of the van der Waals interaction for alkali atoms interacting with rare gases.     

新型机械解理方法在二维材料研究中的应用

自从石墨烯被发现以来,机械解理技术已经成为制备高质量二维材料的重要方法之一,在二维材料本征物性的研究方面展现出了独特的优势.然而传统机械解理方法存在明显的不足,如制备效率低、样品尺寸小等,阻碍了二维材料领域的研究进展.近些年我们在机械解理技术方面取得了一系列的突破,独立发展了一套具有普适性的新型机械解理方法.这种新型机械解理方法的核心在于通过改变解理过程中的多个参数,增强层状材料与基底之间的范德瓦耳斯相互作用,从而提高单层样品的产率和面积.本文着重以石墨烯为例,介绍了该技术的过程和机理.相比于传统机械解理方法,石墨烯的尺寸从微米量级提高到毫米量级,面积提高了十万倍以上,产率大于95%,同时石墨烯依然保持着非常高的质量.这种新型机械解理方法具有良好的普适性,目前已经在包括MoS₂,WSe₂,MoTe₂,Bi2212等几十种材料体系中得到了毫米量级以上的高质量单层样品.更重要的是,在解理过程中,通过调控不同的参数,可以在层状材料中实现一些特殊结构的制备,如气泡、褶皱结构等,为研究这些特殊材料体系提供了重要的物质保障.未来机械解理技术还有很多值得深入研究的科学问题,该技术的突破将会极大地推动二维材料领域的研究进展.     

TATB晶体声子谱及比热容的第一性原理研究

利用第一性原理并结合vd W-DF2范德瓦耳斯力校正研究了TATB(C6H6O6N6)晶体声子谱及比热容.采用冷冻声子法计算了TATB晶体声子谱和声子态密度,发现在2.3 THz附近TATB声子态密度最大,证实了太赫兹光谱实验观察到的2.22 THz附近的强吸收峰.基于声子态密度研究了振动模式对比热容的贡献,分析结果表明,常温下0—27.5 THz频段振动模式贡献了比热容的93.7%.同时比较了升温过程中振动模式对比热容的贡献,指出TATB热分解的引发键是C—NO_2键断裂的可能性更大.     

Millimeter-wave spectrum of Ne–CO: new measurements

The b-type rotational transitions of the van der Waals complex, Ne–CO have been measured using the intracavity OROTRON jet spectrometer in the frequency range of 80–115 GHz. The high sensitivity of this technique enabled us to detect all three Ne isotopic modifications of the complex, ²⁰Ne–CO, ²²Ne–CO, and ²¹Ne–CO in natural abundance. The observed and assigned transitions belong to the Q-branch of the K = 1–0 subband and include also R (0) and P (2) lines. The newly obtained data were analysed together with previously observed millimeter-wave b-type and microwave a-type rotational transitions.     

The microwave spectrum and structure of the argon trifluoroacetonitrile complex

The rotational spectrum of argon trifluoroacetonitrile complex has been studied by pulsed-nozzle, Fourier transform microwave spectroscopy. Both a-type and b-type transitions have been observed. The rotational constants are A = 3053.0903(2) MHz, B = 1039.9570(2) MHz, and C = 895.5788(1) MHz. The ¹⁴N nuclear quadrupole hyperfine components of the rotational transitions have been resolved, the ¹⁴N nuclear quadrupole coupling constants are χ_aa = 1.746(1) MHz, and χ_bb − χ_cc = −6.426(2) MHz. The complex is T-shaped, with the argon atom located 3.73 Å from the center of mass of the trifluoroacetonitrile molecule.     

溶胶-凝胶法制备Fe-Al2O3巨磁旋光薄膜

用溶胶-凝胶法制备了Fe-Al2O3铁磁金属-非磁绝缘体基体薄膜。实验结果表明,当Fe与Al2O3的质量比为1∶1,热处理温度为420℃时,所制备的薄膜具有最大的磁致旋光(Faraday)效应,测得的费尔德(Verdet)常数V=(6.8×104)°/(T.cm)。通过分析,得出了Fe-Al2O3薄膜巨磁Faraday旋光效应主要是由光、磁场与薄膜相互作用产生剧烈塞曼(Zeeman)分裂引起的。对影响薄膜Faraday旋光效应的各种主要因素进行了讨论。     

Reversible doping polarity and ultrahigh carrier density in two-dimensional van der Waals ferroelectric heterostructures

Van der Waals semiconductor heterostructures (VSHs) composed of two or more two-dimensional (2D) materials with different band gaps exhibit huge potential for exploiting high-performance multifunctional devices. The application of 2D VSHs in atomically thin devices highly depends on the control of their carrier type and density. Herein, on the basis of comprehensive first-principles calculations, we report a new strategy to manipulate the doping polarity and carrier density in a class of 2D VSHs consisting of atomically thin transition metal dichalcogenides (TMDs) and α-In₂X₃ (X = S, Se) ferroelectrics via switchable polarization field. Our calculated results indicate that the band bending of In₂X₃ layer driven by the FE polarization can be utilized for engineering the band alignment and doping polarity of TMD/In₂X₃ VSHs, which enables us to control their carrier density and type of the VSHs by the orientation and magnitude of local FE polarization field. Inspired by these findings, we demonstrate that doping-free p−n junctions achieved in MoTe₂/In₂Se₃ VSHs exhibit high carrier density (10¹³−10¹⁴ cm⁻²), and the inversion of the VHSs from n−p junctions to p−i−n junctions has been realized by the polarization switching from upward to downward states. This work provides a nonvolatile and nondestructive doping strategy for obtaining programmable p−n van der Waals (vdW) junctions and opens the possibilities for self-powered and multifunctional device applications.     

Direct measurements of conductivity and mobility in millimeter-sized single-crystalline graphene via van der Pauw geometry

We report the direct measurements of conductivity and mobility in millimeter-sized single-crystalline graphene on SiO_2/Si via van der Pauw geometry by using a home-designed four-probe scanning tunneling microscope(4P-STM). The gate-tunable conductivity and mobility are extracted from standard van der Pauw resistance measurements where the four STM probes contact the four peripheries of hexagonal graphene flakes, respectively. The high homogeneity of transport properties of the single-crystalline graphene flake is confirmed by comparing the extracted conductivities and mobilities from three setups with different geometry factors. Our studies provide a reliable solution for directly evaluating the entire electrical properties of graphene in a non-invasive way and could be extended to characterizing other two-dimensional materials.     

Magnetic van der Waals materials: Synthesis,structure, magnetism,and their potential applications

As the family of magnetic materials is rapidly growing, two-dimensional (2D) van der Waals (vdW) magnets have attracted increasing attention as a platform to explore fundamental physical problems of magnetism and their potential applications. This paper reviews the recent progress on emergent vdW magnetic compounds and their potential applications in devices. First, we summarize the current vdW magnetic materials and their synthetic methods. Then, we focus on their structure and the modulation of magnetic properties by analyzing the representative vdW magnetic materials with different magnetic structures. In addition, we pay attention to the heterostructures of vdW magnetic materials, which are expected to produce revolutionary applications of magnetism-related devices. To motivate the researchers in this area, we finally provide the challenges and outlook on 2D vdW magnetism.     

Internal rotation effects in the pulsed jet rotational spectrum of the trifluoromethane-carbon dioxide dimer

The rotational spectrum of the normal isotopic species of the HCF₃-CO₂ weakly bound complex has been measured by Fourier-transform microwave (FTMW) spectroscopy. All transitions are split into A and E states by internal rotation of the trifluoromethane subunit. A global fit of these states gives rotational constants that are consistent with a structure predicted by an MP2/6-311++G(2d,2p) ab initio calculation in which the axes of the monomers are coplanar, with the hydrogen atom of the trifluoromethane angled toward one of the oxygen atoms of the CO₂. Measured dipole moment components (μ_a = 0.431(6) D, μ_b = 0, μ_c = 1.436(6) D, μ_total = 1.499(6) D) confirm the ab initio prediction of an ac plane of symmetry; however, the very near-prolate nature of the complex (κ = −0.997), combined with the relatively high barrier to internal rotation (∼30 cm⁻¹) leads to asymmetry splittings and internal rotation splittings of similar magnitude, resulting in the observation of dipole forbidden b-type E-state transitions in addition to the expected a- and c-type lines. Although this effect has been observed previously in several monomer spectra, this appears to be one of few examples for a weakly bound complex.     

Infrared diode laser spectroscopy of O2–N2O van der Waals complex in the ν1 symmetric stretch region of N2O

The rovibrational spectrum of O₂–N₂O van der Waals complex is measured in the v₁ symmetric stretch region of N₂O monomer using a tunable diode laser spectrometer. The complex is generated by a slit-pulsed supersonic expansion with gas mixtures of O₂, N₂O, and He. Both a- and b-type transitions are observed. The effective Hamiltonian for an open-shell complex consisting of a diatomic molecule in a ³Σ electronic state and a closed-shell partner is used to analyze the observed spectrum. Molecular constants in the vibrationally excited state are determined accurately. The band-origin of the spectrum is determined to be 1284.7504(25) cm^-1, red-shifted from that of the N₂O monomer by ～ 0.1529 cm^-1.