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      Unique gap structure and symmetry of the charge density wave in single-layer VSe\(_2\)

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          Abstract

          Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-two-dimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe\(_2\), which shows a (\(\sqrt7 \times \sqrt3\)) CDW in contrast to the (4 \(\times\) 4) CDW for the layers in bulk VSe\(_2\). Angle-resolved photoemission spectroscopy (ARPES) from the single layer shows a sizable (\(\sqrt7 \times \sqrt3\)) CDW gap of \(\sim\)100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicted by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.

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          Layer-dependent Ferromagnetism in a van der Waals Crystal down to the Monolayer Limit

          Since the celebrated discovery of graphene, the family of two-dimensional (2D) materials has grown to encompass a broad range of electronic properties. Recent additions include spin-valley coupled semiconductors, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semi-metals with edge transport. Despite this progress, there is still no 2D crystal with intrinsic magnetism, which would be useful for many technologies such as sensing, information, and data storage. Theoretically, magnetic order is prohibited in the 2D isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem. However, magnetic anisotropy removes this restriction and enables, for instance, the occurrence of 2D Ising ferromagnetism. Here, we use magneto-optical Kerr effect (MOKE) microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 K is only slightly lower than the 61 K of the bulk crystal, consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase transition, showcasing the hallmark thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, while in trilayer the interlayer ferromagnetism observed in the bulk crystal is restored. Our work creates opportunities for studying magnetism by harnessing the unique features of atomically-thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering for novel interface phenomena.
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            Evidence of the existence of magnetism in pristine VX₂ monolayers (X = S, Se) and their strain-induced tunable magnetic properties.

            First-principles calculations are performed to study the electronic and magnetic properties of VX(2) monolayers (X = S, Se). Our results unveil that VX(2) monolayers exhibit exciting ferromagnetic behavior, offering evidence of the existence of magnetic behavior in pristine 2D monolayers. Furthermore, interestingly, both the magnetic moments and strength of magnetic coupling increase rapidly with increasing isotropic strain from -5% to 5% for VX(2) monolayers. It is proposed that the strain-dependent magnetic moment is related to the strong ionic-covalent bonds, while both the ferromagnetism and the variation in strength of magnetic coupling with strain arise from the combined effects of both through-bond and through-space interactions. These findings suggest a new route to facilitate the design of nanoelectronic devices for complementing graphene.
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              Strong room-temperature ferromagnetism in VSe2 monolayers on van der Waals substrates

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                Author and article information

                Journal
                14 November 2018
                Article
                10.1103/PhysRevLett.121.196402
                1811.05690

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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                Phys. Rev. Lett. 121, 196402 (2018)
                cond-mat.mes-hall cond-mat.mtrl-sci

                Condensed matter, Nanophysics

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