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Abstract Electron band topology is combined with intrinsic magnetic orders in MnBi 2 Te 4 , leading to novel quantum phases. Here we investigate collective spin excitations (i.e. magnons) and spin fluctuations in atomically thin MnBi 2 Te 4 flakes using Raman spectroscopy. In a two-septuple layer with non-trivial topology, magnon characteristics evolve as an external magnetic field tunes the ground state through three ordered phases: antiferromagnet, canted antiferromagnet, and ferromagnet. The Raman selection rules are determined by both the crystal symmetry and magnetic order while the magnon energy is determined by different interaction terms. Using non-interacting spin-wave theory, we extract the spin-wave gap at zero magnetic field, an anisotropy energy, and interlayer exchange in bilayers. We also find magnetic fluctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers. 

Highly efficient capacitive deionization (CDI) relies on unimpeded transport of salt ions to the electrode surface. Graphene is an ideal candidate to provide superb conditions for ion adsorption as it possesses high theoretical surface area and electrical conductivity. When ions are stored solely within the electric double layers (EDLs), a hydrophilic graphene surface with hierarchical pores can maximize the accessible surface area and promote the ion transport. In the case of synergistic ion storage via electrostatic adsorption and faradaic redox reaction, graphene can act as both the electron highway and the reciprocal spacer to provide surface-confined effects. Substantially, structural and chemical engineering towards graphene can enhance the ion removal capacity and rate, and improve the charge efficiency and ion selectivity. In this review, we keep pace with the in-depth studies of CDI technologies and recent progress on graphene-based materials for CDI. Major challenges in the rational assembly of the desired material functionalities in terms of surface area, pore structure, and hydrophilicity are addressed. As electrode materials develop, the ultimate goal is to achieve highly efficient, energy-saving, and environment-friendly CDI.