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Two-dimensional (2D) Dirac states with linear band dispersion have attracted enormous interest since the discovery of graphene. However, to date, 2D Dirac semimetals are still very rare due to the fact that 2D Dirac states are generally fragile against perturbations such as spin-orbit couplings. Nonsymmorphic crystal symmetries can enforce the formation of Dirac nodes, providing a new route to establishing symmetry-protected Dirac states in 2D materials. Here we report the symmetry-protected Dirac states in nonsymmorphic alpha-antimonene. The antimonene was synthesized by the method of molecular beam epitaxy. Two Dirac cones with large anisotropy were observed by angle-resolved photoemission spectroscopy. The Dirac state in alpha-antimonene is of spin-orbit type in contrast to the spinless Dirac states in graphene. The result extends the 'graphene' physics into a new family of 2D materials where spin-orbit coupling is present., Comment: 4 figures

Selenium and tellurium are among the few elements that form $AB{\mathrm{O}}_{3}$ perovskite structures with a four valent ion in the $A$ site. This leads to highly distorted structures and unusual magnetic behavior. Here we investigate the Co and Ni selenite and tellurite compounds, ${\mathrm{CoSeO}}_{3}, {\mathrm{CoTeO}}_{3}, {\mathrm{NiSeO}}_{3}$, and ${\mathrm{NiTeO}}_{3}$, using first principles calculations. We find an interplay of crystal field and Jahn-Teller distortions that underpin the electronic and magnetic properties. While all compounds are predicted to show an insulating G-type antiferromagnetic ground state, there is a considerable difference in the anisotropy of the exchange interactions between the Ni and Co compounds. This is related to the Jahn-Teller distortion. Finally, we observe that these four compounds show characteristics generally associated with Mott insulators, even when described at the level of standard density functional theory. These are then dense bulk insulators that are correctly described as insulators in a standard band structure picture but that satisfy the common experimental criteria to classify them as Mott-type insulators.