二维范德瓦尔斯材料

1959年Richard Feynman在其著名的演讲中曾经提出一个问题：当选取最合适的单层来搭建层状结构时,我们可以实现什么？随着近年来对各种家族的单层材料的研究,我们正愈发接近问题的答案,并有望实现Feynman的梦想。     

范德华尔斯材料在转角光学中的研究进展

极化激元是光与不同极化子相互作用形成的半光半物质的准粒子,可用于亚波长尺度的光场调控,在光学成像、非线性效应增强及新型超构材料设计等领域扮演着举足重轻的角色.近年来,随着人们对转角范德华尔斯材料体系的制备工艺和物性研究的不断深入,其中许多新奇的极化激元现象也被揭示.本文综述了近年来转角范德华尔斯材料在光学领域的研究进展...     

从二维材料到范德瓦尔斯异质结

二维材料领域经过了十三年的蓬勃发展,涌现出一大批新材料和新技术。文章将介绍二维材料领域的发展历史,一系列具有代表性的二维材料的重要性质,以及由其衍生的范德瓦尔斯异质结的相关研究工作。     

Magnetic two-dimensional van der Waals materials forspintronic devices

Magnetic two-dimensional (2D) van der Waals (vdWs) materials and their heterostructures attract increasing attention in the spintronics community due to their various degrees of freedom such as spin, charge, and energy valley, which may stimulate potential applications in the field of low-power and high-speed spintronic devices in the future. This review begins with introducing the long-range magnetic order in 2D vdWs materials and the recent progress of tunning their properties by electrostatic doping and stress. Next, the proximity-effect, current-induced magnetization switching, and the related spintronic devices (such as magnetic tunnel junctions and spin valves) based on magnetic 2D vdWs materials are presented. Finally, the development trend of magnetic 2D vdWs materials is discussed. This review provides comprehensive understandings for the development of novel spintronic applications based on magnetic 2D vdWs materials.     

van der Waals revisited

The van-der-Waals version of the second virial coefficient is not far from being exact if the model parameters are appropriately chosen. It is shown how the van-der-Waals resemblance originates from the interplay of thermal averaging and superposition of scattering phase shift contributions. The derivation of the two parameters from the quantum virial coefficient reveals a fermion-boson symmetry in non-ideal quantum gases. Numerical details are worked out for the Helium quantum gases.     

Six-body van der Waals interactions

We report on the six-body van der Waals interactions within Rydberg atoms. Specifically, we focus on the octahedron case. The results are compared with previous calculations for two to five bodies' interactions. This research is useful for crystal structure in condensed matter physics, such as p-type doping in Silicon or other types of semiconductors. This research is also useful for studying big molecules in chemistry, chemical engineering, and other fields.     

Nonuniform van der Waals theory

The liquid-vapor interface of a confined fluid at the condensation phase transition is studied in a combined hydrostatic/mean-field limit of classical statistical mechanics. Rigorous and numerical results are presented. The limit accounts for strongly repulsive short-range forces in terms of local thermodynamics. Weak attractive longer-range ones, like gravitational or van der Waals forces, contribute a self-consistent mean potential. Although the limit is fluctuationfree, the interface is not a sharp Gibbs interface, but its structure is resolved over the range of the attractive potential. For a fluid of hard balls with –r ^–6 interactions the traditional condensation phase transition with critical point is exhibited in the grand ensemble: A vapor state coexists with a liquid state. Both states are quasiuniform well inside the container, but wall-induced inhomogeneities show up close to the boundary of the container. The condensation phase transition of the grand ensemble bridges a region of negative total compressibility in the canonical ensemble which contains canonically stable proper liquid-vapor interface solutions. Embedded in this region is a new, strictly canonical phase transition between a quasiuniform vapor state and a small droplet with extended vapor atmosphere. This canonical transition, in turn, bridges a region of negative total specific heat in the microanonical ensemble. That region contains subcooled vapor states as well as superheated very small droplets which are microcanonically stable.     

Review of Raman spectroscopy of two-dimensional magnetic van der Waals materials

Ultrathin van der Waals (vdW) magnets provide a possibility to access magnetic ordering in the two-dimensional (2D) limit, which are expected to be applied in the spintronic devices. Raman spectroscopy is a powerful characterization method to investigate the spin-related properties in 2D vdW magnets, including magnon and spin-lattice interaction, which are hardly accessible by other optical methods. In this paper, the recent progress of various magnetic properties in 2D vdW magnets studied by Raman spectroscopy is reviewed, including the magnetic transition, spin-wave, spin-lattice interaction, symmetry tuning induced by spin ordering, and nonreciprocal magneto-phonon Raman scattering.     

Static van der Waals interactions in electrolytes

Using a field-theoretic formalism, we calculate the static contribution to the van der Waals interaction between two dielectric semi-infinite half-spaces in the presence of mobile salt ions. The ions can be located in the slab, in one, or in both half-spaces. We include an excess polarizability of the salt ions, i.e., each (spherical) ion has a dielectric constant which in general is different from the surrounding medium. This leads to a modification of the effective dielectric constant of the medium hosting the ions. This shift can be large for high salt concentrations and therefore influences the Hamaker constant decisively. Salt ions in the slab screen the static van der Waals interaction, as was shown by Davies and Ninham. The salt-modified van der Waals interaction also contains salt-confinement and salt-correlation effects. This is clearly demonstrated by the fact that the interaction is non-zero even in the case when the dielectric constant is homogeneous throughout the system, in which case salt correlations are solely responsible for the interaction. If the salt ions are in one or both of the two half-spaces (and no ions in the slab), the van der Waals interaction is not screened but the effective Hamaker constant approaches a universal value for large slab thickness which is different from the value in the absence of salt ions and which is independent of the salt concentration and of the effective electrolyte dielectric constant. If both half-spaces contain salt, the asymptotic value of the Hamaker constant for large separation between the half-spaces is the one obtained for the interaction between two metallic half-spaces through an arbitrary dielectric medium, which is given by A≃ - 1.20. As is explicitly demonstrated, ion enrichment or depletion at the interfaces due to image-charge effects is already included on the one-loop level and therefore does not lead to a change of the screened van der Waals interaction as given by Davies and Ninham. For positive and negative ions with different valencies or different excess polarizabilities, we obtain different adsorbed surface excesses of positive and negative ions, leading to a non-vanishing surface potential, which is computed explicitly. All of these results are obtained on the linear one-loop level. For a special case we extend the calculation of the dispersion interaction to the two-loop level. We find the corrections to the one-loop results to be quite large for high salt concentrations or multivalent ions. Received 17 February 2000     

双分子反应中的范德华作用

本文综述了近年来双分子反应中范德华作用的理论和实验研究进展.对于直接的活化反应和形成络合物的反应中范德华力对动力学的显著影响深化了人们的认识的研究工作进行了回顾,并进一步讨论了涉及更多原子的反应以及在低温和超低温条件下的反应.无论是对于从头算还是势能面拟合而言,准确描述势能面的范德华区域的结构以及长程势仍是当前挑战性的工作.此外,本文还对最近提出的"范德华鞍"的概念作了阐明,该概念可能具有一般的意义.     

Oriented dynamics in van der Waals complexes

The effects of relative orientation on collision and reaction dynamics can be examined by characterizing the unimolecular decay of van der Waals complexes. Most commonly, decomposition is initiated by exciting one of the monomers within a complex, and the relative orientation is defined by the zero-point motions for the intermolecular degrees of freedom. However, if simultaneous excitation of the intermolecular degrees of freedom is achieved, a considerable range of starting configurations may be accessed. We are currently studying I₂-Rg and CN-H₂/D₂ complexes with the goal of using these systems to examine oriented dynamics. For I₂(B)-Ne we have observed excited intermolecular vibrations and the effects of these motions on the predissociation dynamics. The correlation between structure and dynamics suggested by these results can be understood using classical mechanics, but the implications of this model are at variance with accepted ideas about the topology of I₂(B)-Rg potential surfaces. Spectroscopic and theoretical studies of CN-H₂/D₂ indicate that this pre-reactive complex may be used to examine steric effects in the H₂+CN→H+HCN reaction. The complex was characterized using the A-X and B-X electronic transitions. As is often the case for weakly bound systems, the insights provided by high-level theoretical calculations were essential for interpretation of the spectra. The properties of excited intermolecular vibrations of CN-H₂ were predicted as a prelude to studies of the intra-cluster reaction dynamics.     

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。     

Spin–phonon coupling in van der Waals antiferromagnet VOCl

We report magnetization and Raman spectroscopy study on single crystals of VOCl, a van der Waals antiferromagnetic material. Magnetization measurement confirms an antiferromagnetic transition at 79 K and a magnetic easy axis along crystallographic a direction. The temperature-dependent Raman spectrum reveals five peaks at 30 K. Below the Neel temperature TN, the Raman-active modes 247 cm~(-1) and 404 cm~(-1) remarkably deviate from the standard Boltzmann function,which is ascribed to the strong magnetoelastic coupling between spins and phonons. We further observe an anomaly in 383 cm~(-1) mode at around 150 K. This coincides with the broad maximum in VOCl's magnetic susceptibility, suggesting a development of short-ranged magnetic order at this temperature.     

Non-Analyticity and the van der Waals Limit

We study the analyticity properties of the free energy f _γ (m) of the Kac model at points of first order phase transition, in the van der Waals limit γ↘0. We show that there exists an inverse temperature β ₀ and γ ₀>0 such that for all β≥β ₀ and for all γ∈(0,γ ₀), f _γ (m) has no analytic continuation along the path m↘m ^* (m ^* denotes spontaneous magnetization). The proof consists in studying high order derivatives of the pressure p _γ (h), which is related to the free energy f _γ (m) by a Legendre transform.     

Estimation of van der Waals dipole-quadrupole interactions

A formula has been derived based on a variational approach for the van der Waals dipole-quadrupole interaction coefficient between two atoms or ions. The coefficients for the various ion pairs in the alkali halides have been estimated on the basis of this formula. The results agree quite well with those estimated by Mayer (J. Chem. Phys.1, 270 (1933)) using a perturbation approach. The present formula is shown to have a practical advantage over the perturbation formula.     

MBPT studies of van der Waals molecules

The many-body perturbation theory is employed for the calculation of the interaction potential for the F^- … He system in the framework of the supermolecule method. A particular attention is paid to the choice of the basis set functions for the two subsystems and the related basis set superposition effects. It has been found that the main features of the interaction potential are recovered in the SCF approximation. The SCF potential has its minimum at the distance R = 6·4 a ₀ with the interaction energy of 53 cm^-1. The complete fourth-order MBPT method gives the potential minimum position and depth equal to 6·5 a ₀ and 64 cm^-1, respectively. The basis set superposition effects estimated by using the counterpoise technique are negligibly small for the SCF interaction potential, while at the correlated level their magnitude is comparable to the value of the total correlation contribution to the interaction energy. The basis set superposition effect in calculations of the electron correlation contribution to weak intermolecular interactions is found to be the major factor limiting the reliability of the corresponding theoretical data.     

Strong pressure-energy correlations in van der Waals liquids

Strong correlations between equilibrium fluctuations of the configurational parts of pressure and energy are found in computer simulations of the Lennard-Jones liquid and other simple liquids, but not for hydrogen-bonding liquids such as methanol and water. The correlations that are present also in the crystal and glass phases reflect an effective inverse power-law repulsive potential dominating fluctuations, even at zero and slightly negative pressure. In experimental data for supercritical argon, the correlations are found to be approximately 96%. Consequences for viscous liquid dynamics are discussed.