Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. Recently, the van der Waals magnet, chromium triiodide (CrI 3 ), emerged as the first layered antiferromagnetic insulator in its few-layer form 1 , opening up ample opportunities for novel device functionalities 2-7 . Here, we discovered an emergent nonreciprocal second order nonlinear optical effect in bilayer CrI 3 . The observed second harmonic generation (SHG) is giant: several orders of magnitude larger than known magnetization induced SHG 8-11 and comparable to SHG in the best 2D nonlinear optical materials studied so far 12-15 (e.g. MoS 2 ). We showed that while the parent lattice of bilayer CrI 3 is centrosymmetric and thus does not contribute to the SHG signal, the observed nonreciprocal SHG originates purely from the layered antiferromagnetic order, which breaks both spatial inversion and time reversal symmetries. Furthermore, polarization-resolved measurements revealed the underlying C 2h symmetry, and thus monoclinic stacking order in CrI 3 bilayers, providing crucial structural information for the microscopic origin of layered antiferromagnetism 16-20 . Our results highlight SHG as a highly sensitive probe that can reveal subtle magnetic order and open novel nonlinear and nonreciprocal optical device possibilities based on 2D magnets.
Bottom-up synthesized graphene nanoribbons and graphene nanoribbon heterostructures have promising electronic properties for high-performance field-effect transistors and ultra-low power devices such as tunneling field-effect transistors. However, the short length and wide band gap of these graphene nanoribbons have prevented the fabrication of devices with the desired performance and switching behavior. Here, by fabricating short channel (L
ch ~ 20 nm) devices with a thin, high-κ gate dielectric and a 9-atom wide (0.95 nm) armchair graphene nanoribbon as the channel material, we demonstrate field-effect transistors with high on-current (I
on > 1 μA at V
d = −1 V) and high I
on
/I
off ~ 105 at room temperature. We find that the performance of these devices is limited by tunneling through the Schottky barrier at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high-performance short-channel field-effect transistors with bottom-up synthesized armchair graphene nanoribbons.
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