Vertical ferroelectric switching by in-plane sliding of two-dimensional bilayer WTe2, leading to the spin texture and spin-FET design with strain tunability.
Combining the particle-swarm optimization method with first-principles calculations, we explore a new category of two-dimensional (2D) monolayers composed of solely the element selenium. Three stable structures are screened from outputs of crystal search computations, namely T-Se (1T-MoS 2 -like), C-Se (tiled 1D helical chain), and S-Se (square structure). Phonon calculations, as well as formation energy calculations have been performed to confirm the stability of the three phases. The electronic structure calculations show that both T-Se and C-Se are indirect-band-gap semiconductors, with gap values of 1.11 eV and 2.64 eV respectively when using the hybrid HSE06 functional. In particular, C-Se has a centrosymmetry-breaking structure which provides a spontaneous in-plane ferroelectric polarization of about 2.68 × 10 −10 C m −1 per layer. Interestingly, S-Se has a Dirac cone that can open up a band gap of 0.11 eV if spin-orbit coupling is included. The tilted Dirac cone of S-Se shows anisotropic band dispersion as characterized with different Fermi velocities of 1.26 × 10 6 and 0.24 × 10 6 m s −1 around the Dirac point. Our works enrich the family of 2D materials of selenium allotropes and show that their versatile properties could give rise to potential application in various fields.
Organic-inorganic multiferroics are promising for the next generation of electronic devices. To date, dozens of organic-inorganic multiferroics have been reported; however, most of them show magnetic Curie temperature much lower than room temperature, which drastically hampers their application. Here, by performing first-principle calculations and building effective model Hamiltonians, we reveal a molecular orbital-mediated magnetic coupling mechanism in twodimensional Cr(pyz)2 (pyz=pyrazine), and the role that the valence state of the molecule plays in determining the magnetic coupling type between metal ions. Based on these, we demonstrate that a two-dimensional organic-inorganic room-temperature multiferroic, Cr(h-fpyz)2 (h-fpyz= halffluoropyrazine), can be rationally designed by introducing ferroelectricity in Cr(pyz)2 while keeping the valence state of the molecule unchanged. Our work not only reveals the origin of magnetic coupling in 2D organic-inorganic systems, but also provides a way to design room temperature multiferroic materials rationally.
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