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Motivated by the potential of atomically-thin magnets towards tunable high-frequency magnonics, we detail the spin-wave dispersion of bilayer CrI$_3$. We demonstrate that the magnonic behavior of the bilayer strongly depends on its stacking configuration and the interlayer magnetic ordering, where a topological bandgap opens in the dispersion caused by the Dzyaloshinskii-Moriya and Kitaev interactions, classifying bilayer CrI$_3$ as a topological magnon insulator. We further reveal that both size and topology of the bandgap in a CrI$_3$ bilayer with an antiferromagnetic interlayer ordering are tunable by an external magnetic field.

Monolayer chromium trihalides, the archetypal two dimensional (2D) magnetic materials, are readily suggested as a promising platform for high frequency magnonics. Here we detail the spin wave properties of monolayer CrBr$_3$ and CrI$_3$, using spin dynamics simulations parametrized from the first principles. We reveal that spin wave dispersion can be tuned in a broad range of frequencies by strain, paving the way towards flexo magnonic applications. We further show that ever present halide vacancies in these monolayers host sufficiently strong Dzyaloshinskii Moriya interaction to scatter spin waves, which promotes design of spin-wave guides by defect engineering. Finally we discuss the spectra of spin-waves propagating across a moir\'e periodic modulation of magnetic parameters in a van der Waals heterobilayer, and show that the nanoscale moir\'e periodicities in such samples are ideal for realization of a magnonic crystal in the terahertz frequency range. Recalling the additional tunability of magnetic 2D materials by electronic gating, our results situate these systems among the front-runners for prospective high frequency magnonic applications.

Clays and micas are receiving attention as materials that, in their atomically thin form, could allow for novel proton conductive, ion selective, osmotic power generation, or solvent filtration membranes. The interest arises from the possibility of controlling their properties by exchanging ions in the crystal lattice. However, the ion exchange process itself remains largely unexplored in atomically thin materials. Here we use atomic-resolution scanning transmission electron microscopy to study the dynamics of the process and reveal the binding sites of individual ions in atomically thin and artificially restacked clays and micas. Imaging ion exchange after different exposure time and for different crystal thicknesses, we find that the ion diffusion constant, D, for the interlayer space of atomically thin samples is up to 10^4 times larger than in bulk crystals and approaches its value in free water. Surprisingly, samples where no bulk exchange is expected display fast exchange if the mica layers are twisted and restacked; but in this case, the exchanged ions arrange in islands controlled by the moir\'e superlattice dimensions. We attribute the fast ion diffusion to enhanced interlayer expandability resulting from weaker interlayer binding forces in both atomically thin and restacked materials. Finally, we demonstrate images of individual surface cations for these materials, which had remained elusive in previous studies. This work provides atomic scale insights into ion diffusion in highly confined spaces and suggests strategies to design novel exfoliated clays membranes., Comment: 11 pages, 4 figures, plus 35 pages supplementary information

After the discovery of magnetism in monolayer CrI3, the magnetic properties of different 2D materials from the chromium-trihalide family are intuitively assumed to be similar, yielding magnetic anisotropy from the spin-orbit coupling on halide ligands. Here we reveal significant differences between the CrI3 and CrBr3 magnetic monolayers in their magnetic anisotropy, resulting Curie temperature, hysteresis in external magnetic field, and evolution of magnetism with strain, all predominantly attributed to distinctly different interplay of atomic contributions to spin-orbit coupling in two materials.

Dzyaloshinskii-Moriya interaction is at heart of chiral magnetism and causes emergence of rich non-collinear and unique topological spin textures in magnetic materials, including cycloids, helices, skyrmions and other. Here we show that strong intrinsic DMI lives in recently discovered van der Waals magnetic two-dimensional materials, due to the sizeable spin-orbit coupling on the non-magnetic ions. In a perfect crystal, this intrinsic DMI remains hidden, but is released with any break of point-inversion symmetry between magnetic ions, unavoidable at the sample edges, at ever present structural defects, with any buckling of the material, or with non-uniform strain on an uneven substrate. We demonstrate such release of latent chirality on an archetypal magnetic monolayer - CrI3, and discuss the plethora of realizable DMI patterns, their control by nanoengineering and tuning by external electric field, thereby opening novel routes in 2D magnetoelectronics., Comment: Preprint, 22 pages, 6 figures, submitted to Nanoletters

Recent years have seen a tremendous rise of two-dimensional (2D) magnetic materials, several of which verified experimentally. However, most of the theoretical predictions to date rely on ab-initio methods, at zero temperature and fluctuations-free, while one certainly expects detrimental quantum fluctuations at finite temperatures. Here we present the solution of the quantum Heisenberg model for honeycomb/hexagonal lattices with anisotropic exchange interaction up to third nearest neighbors and in an applied field in arbitrary direction, that answers the question whether long-range magnetization can indeed survive in the ultrathin limit of materials, up to which temperature, and what the characteristic excitation (magnon) frequencies are, all essential to envisaged applications of magnetic 2D materials. We find that long-range magnetic order persists at finite temperature for materials with overall easy-axis anisotropy. We validate the calculations on the examples of monolayer CrI3, CrBr3 and MnSe2. Moreover, we provide an easy-to-use tool to calculate Curie temperatures of new 2D computational materials., Comment: 14 pages, 5 figures. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 117, 052401 (2020) and may be found at https://doi.org/10.1063/5.0015619

Defect-free graphene is impermeable to gases and liquids but highly permeable to thermal protons. Atomic-scale defects such as vacancies, grain boundaries and Stone-Wales defects are predicted to enhance graphene's proton permeability and may even allow small ions through, whereas larger species such as gas molecules should remain blocked. These expectations have so far remained untested in experiment. Here we show that atomically thin carbon films with a high density of atomic-scale defects continue blocking all molecular transport, but their proton permeability becomes ~1,000 times higher than that of defect-free graphene. Lithium ions can also permeate through such disordered graphene. The enhanced proton and ion permeability is attributed to a high density of 8-carbon-atom rings. The latter pose approximately twice lower energy barriers for incoming protons compared to the 6-atom rings of graphene and a relatively low barrier of ~0.6 eV for Li ions. Our findings suggest that disordered graphene could be of interest as membranes and protective barriers in various Li-ion and hydrogen technologies.

Motivated by the recent synthesis of two-dimensional $\alpha$-Fe$_2$O$_3$ [Balan $et$ $al.$ Nat. Nanotech. 13, 602 (2018)], we analyze the structural, vibrational, electronic and magnetic properties of single- and few-layer $\alpha$-Fe$_2$O$_3$ compared to bulk, by $ab-initio$ and Monte-Carlo simulations. We reveal how monolayer $\alpha$-Fe$_2$O$_3$ (hematene) can be distinguished from the few-layer structures, and how they all differ from bulk through observable Raman spectra. The optical spectra exhibit gradual shift of the prominent peak to higher energy, as well as additional features at lower energy when $\alpha$-Fe$_2$O$_3$ is thinned down to a monolayer. Both optical and electronic properties have strong spin asymmetry, meaning that lower-energy optical and electronic activities are allowed for the single-spin state. Finally, our considerations of magnetic properties reveal that 2D hematite has anti-ferromagnetic ground state for all thicknesses, but the critical temperature for Morin transition increases with decreasing sample thickness. On all accounts, the link to available experimental data is made, and further measurements are prompted.

The recent development in the field of 2D magnetic materials urges reliable theoretical methodology for determination of magnetic properties. Among the available methods, ab initio four-state energy mapping based on Density Functional Theory stands out as a powerful technique to calculate the magnetic exchange interaction in the Heisenberg spin model. Although the required formulae were explained in earlier works, the considered Hamiltonian in those studies always corresponded to the specific case that J-matrix is anti-symmetric, which may be misleading in other cases. Therefore, using the most general form of the Heisenberg spin Hamiltonian, we here derive the generic formulae. With a proper choice of four different magnetic states, a single formula governs all elements of the exchange interaction matrix for any considered pair of spin sites., Comment: 8 pages, 1 figure

Monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons. For thicker two-dimensional (2D) materials, proton conductivity diminishes exponentially so that, for example, monolayer MoS2 that is just three atoms thick is completely impermeable to protons. This seemed to suggest that only one-atom-thick crystals could be used as proton conducting membranes. Here we show that few-layer micas that are rather thick on the atomic scale become excellent proton conductors if native cations are ion-exchanged for protons. Their areal conductivity exceeds that of graphene and hBN by one-two orders of magnitude. Importantly, ion-exchanged 2D micas exhibit this high conductivity inside the infamous gap for proton-conducting materials, which extends from 100 C to 500 C. Areal conductivity of proton-exchanged monolayer micas can reach above 100 S cm-2 at 500 C, well above the current requirements for the industry roadmap. We attribute the fast proton permeation to 5 A-wide tubular channels that perforate micas' crystal structure which, after ion exchange, contain only hydroxyl groups inside. Our work indicates that there could be other 2D crystals with similar nm-scale channels, which could help close the materials gap in proton-conducting applications.

By performing density functional theory-based ab-initio calculations, Raman active phonon modes of novel single-layer two-dimensional (2D) materials and the effect of in-plane biaxial strain on the peak frequencies and corresponding activities of the Raman active modes are calculated. Our findings confirm the Raman spectrum of the unstrained 2D crystals and provide expected variations in the Raman active modes of the crystals under in-plane biaxial strain. The results are summarized as follows; (i) frequencies of the phonon modes soften (harden) under applied tensile (compressive) strains, (ii) the response of the Raman activities to applied strain for the in-plane and out-of-plane vibrational modes have opposite trends, thus, the built-in strains in the materials can be monitored by tracking the relative activities of those modes, (iii) in particular, the A-peak in single-layer Si and Ge disappear under a critical tensile strain, (iv) especially in mono and di- atomic single-layers, the shift of the peak frequencies is stronger indication of the strain rather than the change in Raman activities, (v) Raman active modes of single-layer ReX 2 (X=S, Se) are almost irresponsive to the applied strain. Strain-induced modifications in the Raman spectrum of 2D materials in terms of the peak positions and the relative Raman activities of the modes could be a convenient tool for characterization.

Motivated by recent studies that reported the successful synthesis of monolayer Mg(OH)$_{2}$ [Suslu \textit{et al.}, Sci. Rep. \textbf{6}, 20525 (2016)] and hexagonal (\textit{h}-)AlN [Tsipas \textit{et al}., Appl. Phys. Lett. \textbf{103}, 251605 (2013)], we investigate structural, electronic, and optical properties of vertically stacked $h$-AlN and Mg(OH)$_{2}$, through \textit{ab initio} density-functional theory (DFT), many-body quasi-particle calculations within the GW approximation, and the Bethe-Salpeter equation (BSE). It is obtained that the bilayer heterostructure prefers the $AB^{\prime}$ stacking having direct band gap at the $\Gamma$ with Type-II band alignment in which the valance band maximum and conduction band minimum originate from different layer. Regarding the optical properties, the imaginary part of the dielectric function of the individual layers and hetero-bilayer are investigated. The hetero-bilayer possesses excitonic peaks which appear only after the construction of the hetero-bilayer. The lowest three exciton peaks are detailedly analyzed by means of band decomposed charge density and the oscillator strength. Furthermore, the wave function calculation shows that the first peak of the hetero-bilayer originates from spatially indirect exciton where the electron and hole localized at $h$-AlN and Mg(OH)$_{2}$, respectively, which is important for the light harvesting applications., Comment: Accepted by Physical Review B

Successful isolation of graphene from graphite opened a new era for material science and con- densed matter physics. Due to this remarkable achievement, there has been an immense interest to synthesize new two dimensional materials and to investigate their novel physical properties. Silicene, form of Si atoms arranged in a buckled honeycomb geometry, has been successfully synthesized and emerged as a promising material for nanoscale device applications. However, the major obstacle for using silicene in electronic applications is the lack of a band gap similar to the case of graphene. Therefore, tuning the electronic properties of silicene by using chemical functionalization methods such as hydrogenation, halogenation or oxidation has been a focus of interest in silicene research. In this paper, we review the recent studies on the structural, electronic, optical and mechanical proper- ties of silicene-derivative structures. Since these derivatives have various band gap energies, they are promising candidates for the next generation of electronic and optoelectronic device applications.

By performing density functional theory-based calculations, we investigate how hydrogen atom interacts with the surfaces of monolayer PbI2 and how one and two side hydrogenation modify its structural, electronic, and magnetic properties. Firstly, it was shown that T-phase of single layer PbI2 is energetically favorable than the H-phase. It is found that hydrogenation of its surfaces is possible through the adsorption of each hydrogen on iodine sites. While H atoms do not form a particular bonding-type at low concentration, by increasing the number of hydrogenated I-sites well-ordered hydrogen patterns are formed on PbI2 matrix. In addition, we found that for one-side hydrogenation, the structure forms a (2x1) Jahn-Teller type distorted structure and band gap is dramatically reduced compared to hydrogen-free single layer PbI2 . Moreover, in the case of full-hydrogenation, the structure also possesses another (2x2) reconstruction with reduction in the band gap. Easy-tunable electronic and structural properties of single layer PbI2 by hydrogenation reveal its potential use in nanoscale semiconducting device applications., Comment: accepted by RCS Advances