Robust half-metallicity in transition metal tribromide nanowires

Controllable dimensionality conversion between 1D and 2D CrCl3 magnetic nanostructures

The fabrication of one-dimensional (1D) magnetic systems on solid surfaces, although of high fundamental interest, has yet to be achieved for a crossover between two-dimensional (2D) magnetic layers and their associated 1D spin chain systems. In this study, we report the fabrication of 1D single-unit-cell-width CrCl₃ atomic wires and their stacked few-wire arrays on the surface of a van der Waals (vdW) superconductor NbSe₂. Scanning tunneling microscopy/spectroscopy and first-principles calculations jointly revealed that the single wire shows an antiferromagnetic large-bandgap semiconducting state in an unexplored structure different from the well-known 2D CrCl₃ phase. Competition among the total energies and nanostructure-substrate interfacial interactions of these two phases result in the appearance of the 1D phase. This phase was transformable to the 2D phase either prior to or after the growth for in situ or ex situ manipulations, in which the electronic interactions at the vdW interface play a nontrivial role that could regulate the dimensionality conversion and structural transformation between the 1D-2D CrCl₃ phases.

Nanowires exfoliated from one-dimensional van der Waals transition metal trihalides and quadrihalides

The exfoliation of van der Waals (vdW) materials has been widely used to fabricate two-dimensional (2D) materials. However, the exfoliation of vdW materials to isolate atomically thin nanowires (NWs) is an emerging research topic. In this letter, we identify a large class of transition metal trihalides (TMX₃), which have one-dimensional (1D) vdW structures, i.e., they comprise columns of face-sharing TMX₆ octahedral chains, whereas the chains are bound by weak vdW forces. Our calculations show that the single-chain and multiple-chain NWs constructed from these 1D vdW structures are stable. The calculated binding energies of the NWs are relatively small, suggesting that it is possible to exfoliate NWs from the 1D vdW materials. We further identify several 1D vdW transition metal quadrihalides (TMX₄) that are candidates for exfoliation. This work opens a paradigm for exfoliating NWs from 1D vdW materials.

A high performance N-doped graphene nanoribbon based spintronic device applicable with a wide range of adatoms

Designing and fabricating nanosize spintronic devices is a crucial task to develop information technology of the future. However, most of the introduced spin filters suffer from several limitations including difficulty in manipulating the spin current, incapability in utilizing a wide range of dopants to provide magnetism, or obstacles in their experimental realization. Here, by employing first principles calculations, we introduce a structurally simple and functionally efficient spin filter device composed of a zigzag graphene nanoribbon (ZGNR) with an embedded nitrogenated divacancy. We show that the proposed system, possessing a robust ferromagnetic (FM) ordering, exhibits perfect half metallic behavior in the absence of frequently used transition metals (TMs). Our calculations also show that the suggested system is compatible with a wide range of adatoms including basic metals, metalloids, and TMs. It means that besides d electron magnetism originating from TMs, p electrons of incorporated elements of the main group can also cause half metallicity in the electronic structure of the introduced system. Our system exploiting the robustness of doping-induced FM ordering would be beneficial for promising multifunctional spin filter devices.

Enhanced robustness of half-metallicity in VBr3 nanowires by strains and transition metal doping

The study of half-metallic behavior for transition metal tribromide nanowires is of great significance to the basic research and application in spintronics. We have theoretically investigated the spin transport properties of half-metallic VBr3 nanowires sandwiched between Au(100) electrodes. For the VBr3 nanowire with a length of 24.75 Å, the spin-filter efficiency (SFE) achieves 100% when the applied bias is less than 0.4 V. The robustness of half-metallic behavior in VBr3 nanowires is investigated by strains and transition metal doping. The bias range of VBr3 nanowires exhibiting perfect SFE is extended by tensile strains, while it becomes narrower under compressive strains. For VBr3 nanowires doped with Co and Cr, the bias range with ideal SFE is significantly broadened at a low doping concentration. In particular, the VBr3 nanowire doped with 1 Cr atom exhibits perfect SFE in an extremely wide bias range of 0-1.0 V. Our results show that the robustness of half-metallicity of VBr3 nanowires can be improved by tensile strains or certain doping, which provides promising ways for further design of high-performance spintronic devices.

Giant Rashba splitting in one-dimensional atomic tellurium chains

The search for a one-dimensional (1D) system with purely 1D bands and strong Rashba spin splitting is essential for the realization of Majorana fermions and spin transport but presents a fundamental challenge to date. Herein, using first-principles calculations, we demonstrated that atomic Tellurium (Te) chains exhibit purely 1D bands and giant Rashba spin splitting, and their splitting parameters depend strongly on strain and structure distortion. This phenomenon stems from the helical structure of atomic Te chains, which can not only sustain significant strain but also realize the synergy of orbital angular momentum and in-chain potential gradient in enhancing spin splitting. The structure distortion of stretched helical Te chains is critical to execute this synergy, generating a large Rashba spin splitting among the known systems. Our findings proposed a potential 1D giant Rashba splitting system for exploring spintronics and Majorana fermions, and provide routes for engineering spin splitting in other materials.

One-dimensional transition metal dihalide nanowires as robust bipolar magnetic semiconductors

One-dimensional (1D) materials with robust ferromagnetic ground states are difficult to achieve but provide a significant platform for potential spintronic device applications in future. Herein, a new family of 1D transition metal dihalide (TMCl2; where TM = Cu, Co, Cr) nanowires are proposed by using first-principles calculations. Their dynamic stability is ensured by Born-Oppenheimer molecular dynamics simulations. The electronic structures demonstrate that both CoCl2 and CuCl2 nanowires are promising bipolar magnetic semiconductors (BMSs) and can be converted into 1D half-metal materials by a small amount of carrier doping. The CrCl2 nanowire is an antiferromagnetic semiconductor (AFS). The formation of a BMS is attributed to the superexchange coupling between the Co/Cu atoms through the 3p orbitals in the Cl atoms. By using Monte Carlo simulations, we found that the CoCl2 nanowire has a Curie point of 6 K, while the CuCl2 nanowire has a corresponding Curie point of 14 K. Our results allow us to put forward a strategy to realize 1D BMSs and to design low-dimensional AF spintronic devices.

Two-dimensional transition-metal halide CoBr3 with spin-polarized Dirac cone

Recently, the discovery of two-dimensional transition-metal materials with non-trivial magnetic and electronic properties has spurred huge interest in investigating their applications in nanotechnology. Here, we report that the monolayer of CoBr₃ possesses a quantum anomalous Hall insulating phase generated on the basis of first-principles calculations. We find that the CoBr₃ monolayer is an intrinsic two-dimensional ferromagnetic material with a Curie temperature T_c = 264 K predicted from Monte Carlo simulations. The phonon spectra analysis indicates that the CoBr₃ monolayer is dynamically stable. Taking into account spin-orbit coupling, this makes the electronic structure of the CoBr₃ monolayer topologically non-trivial with a global band gap of 8.7 meV. The anomalous Hall conductivity calculation shows a Chern number C = 2, meaning the presence of a two edge state in nanoribbons of finite width. These findings not only add an experimentally feasible member to the quantum anomalous Hall insulator family, but also pave the way for highly promising application potentials in nanoelectronics and spintronics.

Intrinsic ferromagnetism and topological properties in two-dimensional rhenium halides

The realization of robust intrinsic ferromagnetism in two-dimensional (2D) materials in conjunction with the intriguing quantum anomalous Hall (QAH) effect has provided a fertile ground for novel physics and for the next-generation spintronic and topological devices. On the basis of density functional theory (DFT), we predict that layered 5d transition-metal heavier halides (TMHs), such as ReX3 (X = Br, I), show intrinsic ferromagnetism with high spin polarization and high Curie temperatures. The outstanding dynamic and thermodynamic stability ensures their experimental feasibility. The strong spin-orbit coupling (SOC) of Re makes the electronic structure of the ReI3 monolayer topologically nontrivial with a large Chern number (C = -4). DFT+U calculations reveal that the 2D system undergoes a nontrivial to trivial transition with increasing on-site Hubbard Coulomb interaction U through the emergence of a Dirac cone. This transition is corroborated by the emergence of chiral edge states and the anomalous Hall conductivity. These findings not only demonstrate room-temperature ferromagnetism in atomically thin 5d TMHs, but also pave the way for the potential realization of the QAH effect with high Chern numbers in pristine 2D layers.