Engineering atomic-scale native point defects has become an attractive strategy to improve the performance of thermoelectric materials. Here, we theoretically predict that Ag-Mg antisite defects as shallow acceptors can be more stable than other intrinsic defects under Mg-poor‒Ag/Sb-rich conditions. Under more Mg-rich conditions, Ag vacancy dominates the intrinsic defects. The p-type conduction behavior of experimentally synthesized α-MgAgSb mainly comes from Ag vacancies and Ag antisites (Ag on Mg sites), which act as shallow acceptors. Ag-Mg antisite defects significantly increase the thermoelectric performance of α-MgAgSb by increasing the number of band valleys near the Fermi level. For Li-doped α-MgAgSb, under more Mg-rich conditions, Li will substitute on Ag sites rather than on Mg sites and may achieve high thermoelectric performance. A secondary valence band is revealed in α-MgAgSb with 14 conducting carrier pockets.

Two-dimensional (2D) van der Waals (vdW) materials provide the versatile playground to stack two or more vdW layers for creation of superior materials with desired properties. Here we theoretically adopt a twisted stack-engineering of two LaBr2 monolayers to break space inversion symmetry for ferroelectricity and ultimately multiferroism. The enhancement and reversal of electric polarization are accompanied with the transition from interlayer ferromagnetic and antiferromagnetic orderings, demonstrating an effective magnetoelectric coupling effect with a mechanism dissimilar to that of the conventional multiferroics. Magnetization dynamics simulations show that such magnetic phase transition can excite topologically protected bimeron, and the skyrmion Hall effect can be suppressed by bilayer-bimeron stabilized in both ferromagnetic and antiferromagnetic configurations. Moreover, in the small-angle twisted moiré superlattice, the uniform polarization will evolve into a staggered domain structure, accompanied with the appearance of bimeron, which forms a significant discrepancy with the non-twisted stack-engineered multiferroic LaBr2 bilayer. This work provides a strategy for 2D multiferroic materials by twisted stack engineering of magnetic single layers.

Magnetic ZnO, one of the most important diluted magnetic semiconductors (DMS), has attracted great scientific interest because of its possible technological applications in optomagnetic devices. Magnetism in this material is usually delicately tuned by the doping level, dislocations, and local structures. The rational control of magnetism in ZnO is a highly attractive approach for practical applications. Here, the tuning effect of biaxial strain on the d(0) magnetism of native imperfect ZnO is demonstrated through first-principles calculations. Our calculation results show that strain conditions have little effect on the defect formation energy of Zn and O vacancies in ZnO, but they do affect the magnetism significantly. For a cation vacancy, increasing the compressive strain will obviously decrease its magnetic moment, while tensile strain cannot change the moment, which remains constant at 2 μB. For a singly charged anion vacancy, however, the dependence of the magnetic moment on strain is opposite to that of the Zn vacancy. Furthermore, the ferromagnetic state is always present, irrespective of the strain type, for ZnO with two zinc vacancies, 2VZns. A large tensile strain is favorable for improving the Curie temperature and realizing room temperature ferromagnetism for ZnO-based native semiconductors. For ZnO with two singly charged oxygen vacancies, 2Vs, no ferromagnetic ordering can be observed. Our work points the way to the rational design of materials beyond ZnO with novel non-intrinsic functionality by simply tuning the strain in a thin film form.