## 2 Main Content of the Special Issue  
Since tetragonal Heusler compounds have many potential applications in spintronics and magnetoelectric devices, such as ultrahigh-density spintronic devices, spin transfer torque devices, and permanent magnets, they have received extensive attention in recent years [1; 2; 3; 4; 5]. In this SI, [PERSON] et al. [6] studied the magnetic and electronic structures of cubic and tetragonal types of Mn\({}_{3}\)Z (Z = Al, Ga, In, Tl, Ge, Sn, Pb) Heusler alloys. The authors used first-principles calculations to describe the impact of increasing atomic radius on the structure and properties of Heusler alloys. They investigated tetragonal distortions in relation to different volumes for Mn\({}_{3}\)Ga alloys and extended this analysis to other elements by replacing Ga with Al, In, Tl, Si, Ge, Sn, and Pb. Spintronics has many advantages over traditional electronics, such as no volatility, high data processing speed, low energy consumption, and high integration density. Therefore, spintronics, which utilizes spin instead of charge as the carrier for information transportation and processing, can be seen as one of the most promising ways to implement high-speed and low-energy electronic devices. However, in the process of developing spintronic devices, we have also encountered many bottlenecks, including spin-polarized carrier generation and injection, long-range spin-polarization transport, and spin manipulation and detection. To overcome these problems, various types of spintronic materials have been proposed, such as spin-gapless semiconductors (SGSs) [7; 8; 9; 10; 11; 12; 13], Dirac half-metals [14; 15], diluted magnetic semiconductors (DMSs) [16; 17], and bipolar magnetic semiconductors (BMSs) [18; 19; 20]. In this SI, [PERSON] et al. [21] predicted two new 1:1:1:1 quaternary Heusler alloys, ZrRhTiAl and ZrRhTiGa, and studied their mechanical, magnetic, electronic, and half-metallic properties via first principles. [PERSON] et al. [22] investigated the effect of main-group element doping on the magnetism, half-metallic property, Slater-Pauling rule, and electronic structures of the TiZrCoIn alloy. [PERSON] et al. [23] calculated the band structures, density of states, magnetic moments, and the band-gap of two quaternary Heusler half-metals, FeRhCrSi and FePdCrSi, by means of first principles. [PERSON] et al. [24] performed first-principles calculation to investigate the electronic structure of half-metallic Prussian blue analogueGaFe(CN)\({}_{6}\). They revealed its magnetic and mechanical properties. The pressure dependence of the electronic structure was also investigated in their study. In 2017, [PERSON] et al. [25] predicted a rare strain-tunable electronic band structure, which can be utilized in spintronics. Based on [PERSON] et al. 's study, [PERSON] et al. [26] demonstrated that the physical state of ScFeRhP can be tuned by uniform strain. Theoretical predictions of strain-adjustable quaternary spintronic Heusler compounds remain of high importance in the field of spintronics. Similar works can also be found in References [27; 28; 29; 30; 31; 32]. In recent years, SGSs [33] have attracted widespread attention in the field of spintronics. Thus far, nearly 100 Heusler-type SGSs have been theoretically predicted, of which Mn\({}_{2}\)CoAl, Ti\({}_{2}\)CoAl, and Ti\({}_{2}\)CoSi have been extensively studied. In this SI, [PERSON], [PERSON], and [PERSON] et al. focused on these novel materials. [PERSON] et al. [34] studied the interfacial electronic, magnetic, and spin transport properties of Mn\({}_{2}\)CoAl/Ag/Mn\({}_{2}\)CoAl current-perpendicular-to-plane spin valves (CPP-SV) based on density functional theory and non-equilibrium Green's function. [PERSON] et al. [35] conducted a comprehensive study of the electronic and magnetic properties of the Ti\({}_{2}\)CoAl/MgO (100) heterojunction with first-principles calculations. Ten potential Ti\({}_{2}\)CoAl/MgO (100) junctions are presented based on the contact between the possible atomic interfaces.