Exploring High Transition Temperature Superconductivity in a Freestanding or SrTiO_{3}-Supported CoSb Monolayer

Metallic CoSb and Janus Co2AsSb monolayers as promising anode materials for metal-ion batteries

Structural symmetry breaking plays a pivotal role in fine-tuning the properties of nano-layered materials. Here, based on the first-principles approaches we propose a Janus monolayer of metallic CoSb by breaking the out-of-plane structural symmetry. Specifically, within the CoSb monolayer by replacing the top-layer 'Sb' with 'As' atoms entirely, the Janus Co2AsSb monolayer can be formed, whose structure is confirmed via structural optimization and ab initio molecular dynamics simulations. Notably, the Janus Co2AsSb monolayer demonstrates stability at an elevated temperature of 1200 K, surpassing the stability of the CoSb monolayer, which remains stable only up to 900 K. We propose that both the CoSb and Janus Co2AsSb monolayers could serve as capable anode materials for power-driven metal-ion batteries, owing to their substantial theoretical capacity and robust binding strength. The theoretical specific capacities for Li/Na reach up to 1038.28/1186.60 mA h g-1 for CoSb, while Janus Co2AsSb demonstrates a marked improvement in electrochemical storage capacity of 3578.69/2215.38 mA h g-1 for Li/Na, representing a significant leap forward in this domain. The symmetry-breaking effect upgrades the CoSb monolayer, as a more viable contender for power-driven metal-ion batteries. Furthermore, electronic structure calculations indicate a notable charge transfer that augments the metallic nature, which would boost electrical conductivity. These simulations demonstrate that the CoSb and Janus Co2AsSb monolayers have immense potential for application in the design of metal-ion battery technologies.

Coulomb blockade and Coulomb staircases in CoBi nanoislands on SrTiO3(001)

We successfully fabricated two-dimensional metallic CoBi nanoislands on SrTiO3(001) substrate by molecular beam epitaxy, and systematically investigated their electronic structures by scanning tunneling microscopy and spectroscopyin situat 4.2 K. Coulomb blockade and Coulomb staircases with discrete and well-separated levels are observed for the individual nanoisland, which is attributed to single-electron tunneling via two tunnel junction barriers. They are in excellent agreement with the simulations based on orthodox theory. Furthermore, we demonstrated that the Coulomb blockade becomes weaker with increasing temperature and almost disappears at ∼22 K in our variable temperature experiment, and its full-width at half-maximum of dI/dVpeaks with temperature is ∼6 mV. Our results provide a new platform for designing single-electron transistors that have potential applications in future microelectronics.

Database Construction of Two-Dimensional Charged Building Blocks for Functional-Oriented Material Design

Databases for charge-neutral two-dimensional (2D) building blocks (BBs), i.e., 2D materials, have been built for years due to their applications in nanoelectronics. Though lots of solids are constructed from charged 2DBBs, a database for them is still missing. Here, we identify 1028 charged 2DBBs from Materials Project database using a topological-scaling algorithm. These BBs host versatile functionalities including superconductivity, magnetism, and topological properties. We construct layered materials by assembling these BBs considering valence state and lattice mismatch and predict 353 stable layered materials by high-throughput density functional theory calculations. These materials can not only inherit their functionalities but also show enhanced/emergent properties compared with their parent materials: CaAlSiF displays superconducting transition temperature higher than NaAlSi; Na₂CuIO₆ shows bipolar ferromagnetic semiconductivity and anomalous valley Hall effect that are absent in KCuIO₆; LaRhGeO possesses nontrivial band topology. This database expands the design space of functional materials for fundamental research and potential applications.

Coexistence of Superconductivity and Nontrivial Band Topology in Monolayered Cobalt Pnictides on SrTiO3

As an intrinsically layered material, FeSe has been extensively explored for potentially revealing the underlying mechanisms of high transition temperature (high-T_c) superconductivity and realizing topological superconductivity and Majorana zero modes. Here we use first-principles approaches to identify that the cobalt pnictides of CoX (X = As, Sb, Bi), none of which is a layered material in bulk form. Nevertheless, all can be stabilized as monolayered systems either in freestanding form or supported on the SrTiO₃(001) substrate. We further show that each of the cobalt pnictides may potentially harbor high-T_c superconductivity beyond the Cu- and Fe-based superconducting families, and the underlying mechanism is inherently tied to their isovalency nature with the FeSe monolayer. Most strikingly, each of the monolayered CoX's on SrTiO₃ is shown to be topologically nontrivial, and our findings provide promising new platforms for realizing topological superconductors in the two-dimensional limit.

Realistic flat-band model based on degenerate p-orbitals in two-dimensional ionic materials

Though several theoretical models have been proposed to design electronic flat-bands, the definite experimental realization in two-dimensional atomic crystal is still lacking. Here we propose a novel and realistic flat-band model based on threefold degenerate p-orbitals in two-dimensional ionic materials. Our theoretical analysis and first-principles calculations show that the proposed flat-band can be realized in 1T layered materials of alkali-metal chalogenides and metal-carbon group compounds. Some of the former are theoretically predicted to be stable as layered materials (e.g., K₂S), and some of the latter have been experimentally fabricated in previous works (e.g., Gd₂CCl₂). More interestingly, the flat-band is partially filled in the heterostructure of a K₂S monolayer and graphene layers. The spin polarized nearly flat-band can be realized in the ferromagnetic state of a Gd₂CCl₂ monolayer, which has been fabricated in experiments. Our theoretical model together with the material predictions provide a realistic platform for the study of flat-bands and related exotic quantum phases.