Negative Differential Friction Predicted in 2D Ferroelectric In2 Se3 Commensurate Contacts.

At the macroscopic scale, the friction force (f) is found to increase with the normal load (N), according to the classic law of Da Vinci-Amontons, namely, f = µN, with a positive definite friction coefficient (µ). Here, first-principles calculations are employed to predict that, the static force f, measured by the corrugation in the sliding potential energy barrier, is lowered upon increasing the normal load applied on one layer of the recently discovered ferroelectric In2 Se3 over another commensurate layer of In2 Se3 . That is, a negative differential friction coefficient µ can be realized, which thus simultaneously breaking the classic Da Vinci-Amontons law. Such a striking and counterintuitive observation can be rationalized by the delicate interplay of the interfacial van der Waals repulsive interactions and the electrostatic energy reduction due to the enhancement of the intralayer SeIn ionic bonding via charge redistribution under load. The present findings are expected to play an instrumental role in design of high-performance solid lubricants and mechanical-electronic nanodevices.

Strain induced spin-splitting and half-metallicity in antiferromagnetic bilayer silicene under bending.

Searching for half-metals in low dimensional materials is not only of scientific importance, but also has important implications for the realization of spintronic devices on a small scale. In this work, we show theoretically that simple bending can induce spin-splitting in bilayer silicene. For bilayer silicene with Bernal stacking, the monolayer has a long range ferromagnetic spin order and between the two monolayers, the spin orders are opposite, giving rise to an antiferromagnetic configuration for the ground state of the bilayer silicene. Under bending, the antiferromagnetic spin order is retained but the energetic degeneracy of opposite spin states is lifted. Due to the unusual deformation potentials of the conduction band minimum (CBM) and valence band maximum (VBM) as revealed by density-functional theory calculations and density-functional tight-binding calculations, this spin-splitting is nearly proportional to the degree of bending deformation. Consequently, the spin-splitting can be significant and the desired half-metallic state may emerge when the bending increases, which has been verified by direct simulation of the bent bilayer silicene using the generalized Bloch theorem. Our results hint that bilayer silicene may be an excellent candidate for half-metallicity.

Formation of Bloch Flat Bands in Polar Twisted Bilayers without Magic Angles.

The existence of Bloch flat bands of electrons provides a facile pathway to obtain exotic quantum phases owing to strong correlation. Despite the established magic angle mechanism for twisted bilayer graphene, understanding of the emergence of flat bands in twisted bilayers of two-dimensional polar crystals remains elusive. Here, we show that due to the polarity between constituent elements in the monolayer, the formation of complete flat bands in twisted bilayers is triggered as long as the twist angle is less than a certain critical value. Using the twisted bilayer of hexagonal boron nitride (hBN) as an example, our simulations using the density-functional tight-binding method reveal that the flat band originates from the stacking-induced decoupling of the highest occupied (lowest unoccupied) states, which predominantly reside in the regions of the moiré superlattice where the anion (cation) atoms in both layers are overlaid. Our findings have important implications for the future search for and study of flat bands in polar materials.

Unconventional deformation potential and half-metallicity in zigzag nanoribbons of 2D-Xenes.

Realization of half-metallicity (HM) in low dimensional materials is a fundamental challenge for nano spintronics and a critical component for developing alternative generations of information technology. Using first-principles calculations, we reveal an unconventional deformation potential for zigzag nanoribbons (NRs) of 2D-Xenes. Both the conduction band minimum (CBM) and valence band maximum (VBM) of the edge states have negative deformation potentials. This unique property, combined with the localization and spin-polarization of the edge states, enables us to induce spin-splitting and HM using an inhomogeneous strain pattern, such as simple in-plane bending. Indeed, our calculation using the generalized Bloch theorem reveals the predicted HM in bent zigzag silicene NRs. Furthermore, the magnetic stability of the long range magnetic order for the spin-polarized edge states is maintained well against the bending deformation. These aspects indicate that it is a promising approach to realize HM in low dimensions with the zigzag 2D-Xene NRs.

Twist-driven separation of p-type and n-type dopants in single-crystalline nanowires.

The distribution of dopants significantly influences the properties of semiconductors, yet effective modulation and separation of p-type and n-type dopants in homogeneous materials remain challenging, especially for nanostructures. Employing a bond orbital model with supportive atomistic simulations, we show that axial twisting can substantially modulate the radial distribution of dopants in Si nanowires (NWs) such that dopants of smaller sizes than the host atom prefer atomic sites near the NW core, while dopants of larger sizes are prone to staying adjacent to the NW surface. We attribute such distinct behaviors to the twist-induced inhomogeneous shear strain in NW. With this, our investigation on codoping pairs further reveals that with proper choices of codoping pairs, e.g. B and Sb, n-type and p-type dopants can be well separated along the NW radial dimension. Our findings suggest that twisting may lead to realizations of p-n junction configuration and modulation doping in single-crystalline NWs.