Two-dimensional superconducting MoSi2N4(MoN)4n homologous compounds

Two-Dimensional MoSi2N4 Family: Progress and Perspectives Form Theory

Recently, a new two-dimensional (2D) layered MoSi2N4 has been successfully synthesized by chemical vapor deposition without knowing the 3D counterparts [ Science 2020, 369, 670-674]. The unique septuple-atomic-layer structure and diverse composition of MoSi2N4 have drawn tremendous interest in studying 2D MA2Z4 systems based on the MoSi2N4 structure. As an emerging family of 2D materials, MA2Z4 materials exhibit a wide range of properties and excellent tunability, making them highly promising for various applications. Herein, we summarize recent significant progress in property characterization of the MA2Z4 family. The electronic, magnetic, thermal transport, and superconducting properties, including their tunability through strain engineering and elemental substitution, are presented and elaborated in detail. Further perspectives and new opportunities of the emerging MA2Z4 family are presented at the end of this Perspective.

Unusually high thermal conductivity in suspended monolayer MoSi2N4

Two-dimensional semiconductors with high thermal conductivity and charge carrier mobility are of great importance for next-generation electronic and optoelectronic devices. However, constrained by the long-held Slack's criteria, the reported two-dimensional semiconductors such as monolayers of MoS2, WS2, MoSe2, WSe2 and black phosphorus suffer from much lower thermal conductivity than silicon (~142 W·m-1·K-1) because of the complex crystal structure, large average atomic mass and relatively weak chemical bonds. Despite the more complex crystal structure, the recently emerging monolayer MoSi2N4 semiconductor has been predicted to have high thermal conductivity and charge carrier mobility simultaneously. In this work, using a noncontact optothermal Raman technique, we experimentally measure a high thermal conductivity of ~173 W·m-1·K-1 at room temperature for suspended monolayer MoSi2N4 grown by chemical vapor deposition. First-principles calculations reveal that such unusually high thermal conductivity benefits from the high Debye temperature and small Grüneisen parameter of MoSi2N4, both of which are strongly dependent on the high Young's modulus induced by the outmost Si-N bilayers. Our study not only establishes monolayer MoSi2N4 as a benchmark 2D semiconductor for next-generation electronic and optoelectronic devices, but also provides an insight into the design of 2D materials for efficient heat conduction.

Tunable abundant valley Hall effect and chiral spin-valley locking in Janus monolayer VCGeN4

It is both conceptually and practically fascinating to explore fundamental research studies and practical applications of two-dimensional systems with the tunable abundant valley Hall effect. In this work, based on first-principles calculations, the tunable abundant valley Hall effect is proved to appear in Janus monolayer VCGeN4. When the magnetization is along the out-of-plane direction, VCGeN4 is an intrinsic ferromagnetic semiconductor with a valley feature. The intriguing spontaneous valley polarization exists in VCGeN4 due to the common influence of broken inversion and time-reversal symmetries, which makes it easier to realize the anomalous valley Hall effect. Furthermore, we observe that the valley-non-equilibrium quantum anomalous Hall effect is driven by external strain, which is located between two half-valley-metal states. When reversing the magnetization, the spin flipping makes the position of the edge state to change from one valley to another valley, demonstrating an intriguing behavior known as chiral spin-valley locking. Although the easy magnetic axis orientation is along the in-plane direction, we can utilize an external magnetic field to transform the magnetic axis orientation. Moreover, it is found that the valley state, electronic and magnetic properties can be well regulated by the electric field. Our works explore the mechanism of the tunable abundant valley Hall effect by applying an external strain and electric field, which provides a perfect platform to investigate the spin, valley, and topology.

Electronic and Excitonic Properties of MSi2 Z4 Monolayers

MA₂ Z₄ monolayers form a new class of hexagonal non-centrosymmetric materials hosting extraordinary spin-valley physics. While only two compounds (MoSi₂ N₄ and WSi₂ N₄ ) are recently synthesized, theory predicts interesting (opto)electronic properties of a whole new family of such two-dimensional (2D) materials. Here, the chemical trends of band gaps and spin-orbit splittings of bands in selected MSi₂ Z₄ (M = Mo, W; Z = N, P, As, Sb) compounds are studied from first-principles. Effective Bethe-Salpeter-equation-based calculations reveal high exciton binding energies. Evolution of excitonic energies under external magnetic field is predicted by providing their effective g-factors and diamagnetic coefficients, which can be directly compared to experimental values. In particular, large positive g-factors are predicted for excitons involving higher conduction bands. In view of these predictions, MSi₂ Z₄ monolayers yield a new platform to study excitons and are attractive for optoelectronic devices, also in the form of heterostructures. In addition, a spin-orbit induced bands inversion is observed in the heaviest studied compound, WSi₂ Sb₄ , a hallmark of its topological nature.