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Pursuing higher-temperature superconductors under ambient pressure continues to be a prominent topic in materials discovery. Isomorphic structures like MgB2 exhibit potential for conventional BCS-type superconductivity, but their transition temperatures (Tc) have remained below 100 K based on both experimental findings and theoretical predictions. In this study, two new two-dimensional (2D) superconductors with sandwich structures,KB2C2,featuring BC layers in AA and AB stacking configurations,are designed, whose Tc can exceed 112 K,setting a new record in 2D superconductors. The analyses suggest that electrons in {\sigma}-states covalent bonds and high-frequency E phonon modes dominated by the in-plane vibrations of B/C atoms are predominately responsible for electron-phonon coupling (EPC). An exciting robust three-gap superconducting nature stems from the strong and evident three-region distribution characteristic of electronic EPC parameters {\lambda}. When biaxial tensile strain (BTS) is applied, their Tc are boosted above 153 K. The increase in Tc originates from the softening of optical E phonon modes around the {\Gamma} point and acoustic modes around the Q point, rather than an increase of electrons at the Fermi level (EF ) as observed in other similar systems. Thus, phonon plays a more beneficial role in the EPC of BTS cases, highlighting its significance as a medium in BCS superconductors. Moreover, we find KB2C2 exhibits interesting topological properties, spin antivortex, and Ising-type spin splitting. This is the first report of the coexistence of nontrivial topology and superconductivity with such a high Tc. Therefore, KB2C2 may offer promising sandwich structures to explore higher-Tc 2D superconductors, alongside present potential avenues for investigating fundamental quantum physics., Comment: 8 pages, 3 figures

We study the resonant tunneling in double quantum dots (DQD) sandwiched between surfaces of topological insulator (TI) Bi$_2$Te$_3$, which possess strong spin-orbit coupling (SOC) and $^{d}C_{3v}$ double group symmetry. Distinct from the spin-conserved case with two-dimensional electron gas (2DEG) electrodes, the conductance displays an asymmetrical double-peak Fano-type lineshape rather than Dicke-type lineshape in the zero-field cases. While a Landau-Zener-like lineshape trajectory, which is identified as a signal of competition effect, could be developed by increasing the strength of interdot hopping. Furthermore, when applying an in-plane Zeeman field, we find that the conductance lineshape crossover between Fano and Dicke type could be driven by tilting the field orientation. Moreover, the rotational symmetry of the system could also be revealed from the lineshape trajectory. Our findings will contribute to a better understanding of the resonant tunneling in the presence of electrode SOC and may be confirmed experimentally in the future., Comment: 6 pages, 4 figures

Although the metalization of semiconductor bulk LiBC has been experimentally achieved, various flaws, including the strong lattice distortion, the uncontrollability of phase transition under pressure, usually appear. In this work, based on the first-principles calculations, we propose a new way of hydrogenation to realize metalization. Using the fully anisotropic Migdal-Eliashberg theory, we investigate the superconducting behaviors in the stable monolayers LiBCH and LiCBH, in which C and B atoms are hydrogenated, respectively. Our findings indicate that the monolayers possess the high $T_{c}$ of 82.0 and 82.5 K, respectively, along with the interesting three-gap superconducting natures. The Fermi sheets showing the obvious three-region distribution characteristics and the abnormally strong electron-phonon coupling (EPC) are responsible for the high-$T_{c}$ three-gap superconductivity. Furthermore, the $T_{c}$ can be dramatically boosted up to 120.0 K under 3.5 \% tensile strain. To a great extent, the high $T_{c}$ is beyond the liquid nitrogen temperature ($77$ K), which is beneficial for the applications in future experiments. This study not only explores the superconducting properties of the monolayers LiBCH and LiCBH, but also offers practical insights into the search for high-$T_{c}$ superconductors., Comment: 4 figures. Comments are welcome

The magnetic ground state is crucial for the applications of the two-dimension magnets as it decides fundamental magnetic properties of the material, such as magnetic order, magnetic transition temperature, and low-energy excitation of the spin waves. However, the simulations for magnetism of local-electron systems are challenging due to the existence of metastable states. In this study, occupation matrix control (OMC) and density functional theory plus Hubbard $U$ calculations are applied to investigate the magnetic ground state of monolayer CeI$_{2}$. Following the predicted ferrimagnetic (FM) order, the FM ground state and the FM metastable states are identified and found to have different values of the magnetic parameters. Based on the calculated magnetic parameters of the FM ground state, the Curie temperature is estimated to be $128$ K for monolayer CeI$_{2}$. When spin-orbit coupling (SOC) is considered,the FM ground state is further confirmed to contain both off-plane and in-plane components of magnetization. SOC is shown to be essential for reasonably describing not only magnetic anisotropy but also local electronic orbital state of monolayer CeI$_{2}$., Comment: 4 figures. Comments are welcome

Design and synthesis of novel two-dimensional (2D) materials that possess robust structural stability and unusual physical properties may open up enormous opportunities for device and engineering applications. Herein we propose a 2D sumanene lattice that be regarded as a derivative of the conventional Kagome lattice. Our tight-binding analysis demonstrates sumanene lattice contains two sets of Dirac cones and two sets of flat bands near the Fermi surface, distinctively different from the Kagome lattice. Using first-principles calculations, we theoretically suggest two possible routines for realization of stable 2D sumanene monolayers (named as a phase and b phase), and a-sumanene monolayer can be experimentally synthesized with chemical vapor deposition using C21H12 as a precursor. Small binding energies on Au(111) surface signify the possibility of their peel-off after grown on the noble metal substrate. Importantly, our GW plus Bethe-Salpeter equation calculations demonstrate both monolayers have moderate band gaps (1.94 eV for a) and ultrahigh carrier mobilities (3.4*104 cm2/Vs for a). In particular, a-sumanene monolayer possesses a strong exciton binding energy of 0.73 eV, suggesting potential applications in optics.

The small atomic mass of boron indicates strong electron-phonon coupling, so it may have a brilliant performance in superconductivity. Recently, a new 2D boride sheet with ordered metal vacancies and surface terminals (Mo4/3B2-x) was realized in experiments (Science 2021, 373, 801). Here, the 2D monolayer freestanding Mo4/3B2is evidenced to be thermodynamically stable. Through electronic structure, phonon spectrum and electron-phonon coupling, monolayer Mo4/3B2 is found to be an intrinsic phonon-mediated superconductor. The superconducting transition temperature (Tc) is determined to be 4.06 K by the McMillian-Allen-Dynes formula. Remarkably, the Tc of monolayer Mo4/3B2 can be increased to 6.78 K with an appropriate biaxial tensile strain (+5%). Moreover, we predict that other transition metal replacing Mo atoms is also stable and retaining the superconductivity. Such as monolayer W4/3B2 is also a superconductor with the Tc of 2.37 K. Our research results enrich the database of 2D monolayer superconductors and boron-related formed materials science.