Spin-Valley Half-Metal as a Prospective Material for Spin Valleytronics

A novel 2D intrinsic metal-free ferromagnetic semiconductor Si3C8 monolayer

Metal-free magnets, a special kind of ferromagnetic (FM) material, have evolved into an important branch of magnetic materials for spintronic applications. We herein propose a silicon carbide (Si3C8) monolayer and investigate its geometric, electronic, and magnetic properties by using first-principles calculations. The thermal and dynamical stability of the Si3C8 monolayer was confirmed by ab initio molecular dynamics and phonon dispersion simulations. Our results show that the Si3C8 monolayer is a FM semiconductor with a band gap of 1.76 eV in the spin-down channel and a Curie temperature of 22 K. We demonstrate that the intrinsic magnetism of the Si3C8 monolayer is derived from pz orbitals of C atoms via superexchange interactions. Furthermore, the half-metallic state in the FM Si3C8 monolayer can be induced by electron doping. Our work not only illustrates that carrier doping could manipulate the magnetic states of the FM Si3C8 monolayer but also provides an idea to design two-dimensional metal-free magnetic materials for spintronic applications.

An intrinsic room-temperature half-metallic ferromagnet in a metal-free PN2 monolayer

In spintronics, the embodiment of abundance availability, long spin relaxation time, complete spin-polarization and high Curie temperature (T_C) in intrinsic metal-free half-metallic ferromagnets (MFHMFs) are highly desirable and challenging. In this work, employing density functional theory, we first propose a dynamically, thermally, and mechanically stable two-dimensional (2D) intrinsic MFHMF, i.e. a MoS₂-like PN₂ monolayer, which possesses not only completely spin-polarized half-metallicity, but also an above-room-temperature T_C (385 K). The half-metallic gap is calculated to be 1.70 eV, which can effectively prevent the spin-flip transition caused by thermal agitation. The mechanism of magnetism in the PN₂ monolayer is mainly derived from the p electron direct exchange interaction that separates from usual d-state magnetic materials. Moreover, the robustness of the ferromagnetism and half-metallicity is observed against an external strain and carrier (electron or hole) doping. Surprisingly, electron doping can effectively increase the Curie temperature of the PN₂ monolayer. The proposed research work provides an insight that PN₂ can be a promising candidate for realistic room-temperature metal-free spintronic applications.

Spatiotemporal characteristics of spin transport in compensated metals with electron-hole exchange interaction

We develop a theory describing spatiotemporal behavior of spin transport in two-band metals by postulating a spin-exchange interaction between electrons and holes. Starting with the semiclassical Boltzmann equation, we derive a system of coupled diffusion equations and solve them analytically under steady-state conditions. The solutions reveal two types of electron-hole coupled-spin transport modes: a dissipative mode and a nondissipative mode with an infinite spin diffusion length. The two modes are the manifestations of two types of spin coupling channels. Besides the exchange interaction, we incorporate into our derivation the relaxation caused by the spin-orbit interaction to show how it affects the spin transport characteristics of the two modes.

Ti-fraction-induced electronic and magnetic transformations in titanium oxide films

Titanium dioxide has been widely used in modern industrial applications, especially as an effective photocatalyst. Recently, freestanding TiO₂ films with a markedly reduced bandgap of ∼1.8 eV have been synthesized, indicating that the dimension has a considerable influence on the bulk band gap (>∼3 eV) and enhances the adsorption range of visible light. Titanium oxide compounds have various stoichiometries and versatile properties. Therefore, it is very necessary to explore the electronic properties and functionalities of other titanium oxide films with different stoichiometries. Here, we combined structure searches with first-principle calculations to explore candidate Ti-O films with different stoichiometries. In addition to the experimentally synthesized TiO₂ film, the structure searches identified three new energetically and dynamically stable Ti-O films with stoichiometries of Ti₃O₅, Ti₃O₂, and Ti₂O. Calculations show that the Ti-O films undergo several interesting electronic transformations as the Ti fraction increases, namely, from a wide-gap semiconductor (TiO₂, 3.2 eV) to a narrow-gap semiconductor (Ti₃O₅, 1.80 eV) and then to metals (Ti₃O₂ and Ti₂O) due to the abundance of unpaired Ti_d electrons. In addition to the electronic transformations, we observed nonmagnetic (TiO₂) to ferromagnetic (Ti₃O₅, Ti₃O₂, and Ti₂O) transformations. Notably, the Ti₃O₅ film possesses both narrow-gap semiconductive and ferromagnetic properties, with a large magnetic moment of 2.0 µ_B per unit cell; therefore, this film has high potential for use in applications such as spintronic devices. The results highlight metal fraction-induced electronic and magnetic transformations in transition metal oxide films and provide an alternative route for the design of new, functional thin-film materials.

Externally Controlled Magnetism and Band Gap in Twisted Bilayer Graphene

We theoretically study the effects of electron-electron interaction in twisted bilayer graphene in a transverse dc electric field. When the twist angle is not very small, the electronic spectrum of the bilayer consists of four Dirac cones inherited from each graphene layer. An applied bias voltage leads to the appearance of two holelike and two electronlike Fermi surface sheets with perfect nesting among electron and hole components. Such a band structure is unstable with respect to the exciton band-gap opening due to the screened Coulomb interaction. The exciton order parameter is accompanied by spin-density-wave order. The gap depends on the twist angle and can be varied by a bias voltage. This result correlates well with recent transport measurements [J.-B. Liu et al., Sci. Rep. 5, 15285 (2015)SRCEC32045-232210.1038/srep15285]. Our proposal allows the coexistence of (i) an externally controlled semiconducting gap and (ii) a nontrivial multicomponent magnetic order. This is interesting for both fundamental research and applications.