Altermagnetism can achieve spin-split bands in collinear symmetry-compensated antiferromagnets. Here, we predict altermagnetic order in Janus monolayer Cr2SO with eliminated inversion symmetry, which can realize the combination of piezoelectricity and altermagnetism in a two-dimensional (2D) material, namely, 2D piezoelectric altermagnetism. It is found that Cr2SO is an altermagnetic semiconductor, and the spin-split bands of both valence and conduction bands are near the Fermi level. The Cr2SO has large out-of-plane piezoelectricity (|d31| = 0.97 pm/V), which is highly desirable for ultrathin piezoelectric device application. Due to spin-valley locking, both spin and valley can be polarized by simply breaking the corresponding crystal symmetry with uniaxial strain. Our findings provide a platform to integrate spin, piezoelectricity, and valley in a single material, which is useful for multi-functional device applications.

Recently, a two-dimensional (2D) heterostructure has been widely investigated as a photocatalyst to decompose water using the extraordinary type-II band structure. In this work, the MoTe₂/PtS₂ van der Waals heterostructure (vdWH) is constructed with different stacking structures. Based on density functional calculations, the stacking-dependent electronic characteristic is explored, so that the MoTe₂/PtS₂ vdWH possesses type-I and type-II band structures for the light-emitting device and photocatalyst, respectively, with decent stacking configurations. The band alignment of the MoTe₂/PtS₂ vdWH is also addressed to obtain suitable band edge positions for water-splitting at pH 0. Furthermore, the potential drop is investigated, resulting from charge transfer between the MoTe₂ and PtS₂, which is another critical promotion to prevent the recombination of the photogenerated charges. Additionally, the MoTe₂/PtS₂ vdWH also demonstrates a novel and excellent optical absorption capacity in the visible wavelength range. Our work suggests a theoretical guide to designing and tuning the 2D heterostructure using photocatalytic and photovoltaic devices.

Recently, two-dimensional (2D) magnets have drawn substantial attention from researchers for their fascinating properties and great application potential in the fields of biomedicine, data storage, signal transfer, and energy conversion. However, the low Curie/Néel temperature of 2D magnets hinders their application. In this Perspective, we present some physical insights into enhancing the magnetic stability of 2D magnets. First, the microscope theoretical model of 2D magnets is introduced. Then, we review and analyze several effective and commonly used methods for enhancing the magnetic stability of 2D magnets. Finally, we present the perspective and summary. This Perspective presents the advanced understanding of magnetic stability in 2D materials, which can provide new opportunities for further advancement in a wide variety of applications.

A biphenylene network is a novel 2D allotropy of carbon with periodic 4-6-8 rings, which was synthesized successfully in 2021. In recent years, although the mechanical properties and thermal transport received a lot of research attention, how to open the Dirac cone in the band structure of a biphenylene network is still a confused question. In this work, we utilized uniaxial and biaxial lattice strains to manipulate the electronic properties and phonon frequencies of biphenylene, and we found an indirect band gap under 10% biaxial strain through the first-principles calculations. This indirect band gap is caused by the competition between the band-edge state A and the Dirac cone for the conduction band minimum (CBM). Additionally, the lightest carrier’s effective mass in biphenylene is 0.184 m₀ for electrons along x (Γ→X) direction, while the effective mass for holes shows a remarkable anisotropy, suggesting the holes in the tensile biphenylene monolayer are confined within a one-dimensional chain along x direction. For phonon dispersion, we discovered that the Raman-active ${\mathrm{A}}_{\mathrm{g}}^3$ phonon mode shows a robust single phonon mode character under both compressive and tensile strain, but its frequency is sensitive to lattice strain, suggesting the lattice strain in biphenylene can be identified by Raman spectroscopy

In this work, we explore the impacts of charge doping on the magnetism of a Cr₂Ge₂Te₆ monolayer using first-principles calculations. Our results reveal that doping with 0.3 electrons per unit cell can enhance the ferromagnetic exchange constant in a Cr₂Ge₂Te₆ monolayer from 6.874 meV to 10.202 meV, which is accompanied by an increase in the Curie temperature from ~85 K to ~123 K. The enhanced ratio of the Curie temperature is up to 44.96%, even higher than that caused by surface functionalization on monolayer Cr₂Ge₂Te₆, manifesting the effectiveness of charge doping by improving the magnetic stability of 2D magnets. This remarkable enhancement in the ferromagnetic exchange constant and Curie temperature can be attributed to the increase in the magnetic moment on the Te atom, enlarged Cr-Te-Cr bond angle, reduced Cr-Te distance, and the significant increase in super-exchange coupling between Cr and Te atoms. These results demonstrate that charge doping is a promising route to improve the magnetic stability of 2D magnets, which is beneficial to overcome the obstacles in the application of 2D magnets in spintronics.