Controlling an optical beam is fundamental in optics. Recently, unique manipulation of optical wavefronts has been successfully demonstrated by metasurfaces. However, the artificially engineered nanostructures have thus far been limited to operate on light beams propagating out-of-plane. The in-plane operation is critical for on-chip photonic applications. Here, we demonstrate an anomalous negative-angle refraction of a light beam propagating along the plane, by designing a thin dielectric array of silicon nanoposts. The circularly polarized dipoles induced by the high-permittivity nanoposts at the scattering resonance significantly shape the wavefront of the light beam and bend it anomalously. The unique capability of a thin line of the nanoposts for manipulating in-plane wavefronts makes the device extremely compact.
The unidirectional alignment of graphene islands is essential to the synthesis of wafer-scale single-crystal graphene on Ge (110) surface, but the underlying mechanism is not well-understood. Here we report that the necessary co-alignment of the nucleating graphene islands on Ge (110) surface caused by the presence of step pattern; we show that on the preannealed Ge (110) textureless surface the graphene islands appear non-preferentially orientated, while on the Ge(110) surfaces with natural step pattern, all graphene islands emerge co-aligned. First-principles calculations and theoretical analysis reveal this different alignment behaviors originate from the strong chemical binding formed between the graphene island edges and the atomic steps on the Ge (110) surface, and the lattice matching at the edge-step interface dictates the alignment of graphene islands with the armchair direction of graphene along the [-110] direction of the Ge(110) substrate.
High-T-c superconductors confined to two dimension exhibit novel physical phenomena, such as superconductor-insulator transition. In the Bi2Sr2CaCu2O8+x (Bi2212) model system, despite extensive studies, it is difficult to determine the intrinsic superconducting properties at the thinness limit. Here, we report a method to fabricate high quality single-crystal Bi2212 films down to half-unit-cell thickness in the form of graphene/Bi2212 van der Waals heterostructure, in which sharp superconducting transitions are observed. The heterostructure also exhibits a nonlinear current-voltage characteristic due to the Dirac nature of the graphene band structure.
Phase-change memory based on Ti0.4Sb2Te3 material has one order of magnitude faster set speed and as low as one-fifth of the reset energy compared with the conventional Ge2Sb2Te5 based device. However, the phase-transition mechanism of the Ti0.4Sb2Te3 material remains inconclusive due to the lack of direct experimental evidence. Here we report direct atom-by-atom chemical identification of titanium-centered octahedra in crystalline Ti0.4Sb2Te3 material with a state-of-the-art atomic mapping technology. Further, by using soft X-ray absorption spectroscopy and density function theory simulations, we identify in amorphous Ti0.4Sb2Te3 the titanium atoms preferably maintain the octahedral configuration. Our work may pave the way to more thorough understanding and tailoring of the nature of the Ti-Sb-Te material, for promoting the development of dynamic random access memory-like phase-change memory as an emerging storage-class memory to reform the current memory hierarchy.
Wafer-scale single-crystalline graphene monolayers are highly sought after as an ideal platform for electronic and other applications (1-3). At present, state-of-the-art growth methods based on chemical vapour deposition allow the synthesis of one-centimeter-sized single-crystalline graphene domains similar to 12 h, by suppressing nucleation events on the growth substrate (4). Here we demonstrate an efficient strategy for achieving large-area single-crystalline graphene by letting a single nucleus evolve in a monolayer at a fast rate. By locally feeding carbon precursors to a desired position of a substrate composed of an optimized Cu-Ni alloy, we synthesized an similar to 1.5-inch-large graphene monolayer in 2.5 h. Localized feeding induces the formation of a single nucleus on the entire substrate, and the optimized alloy activates isothermal segregation mechanism that greatly expedites the growth rate (5,6).