| Metadata | Details |
|---|
| Publication Date | 2020-03-18 |
| Journal | Physical review. B./Physical review. B |
| Authors | Wei Jiang, Huaqing Huang, Feng Liu, JianâPing Wang, Tony Low |
| Institutions | University of Minnesota, University of Utah |
| Citations | 29 |
| Analysis | Full AI Review Included |
- Novel Topological Model: Discovery of a novel two eg-orbital (dz2, dx2-y2) tight-binding model on the diamond lattice (eg-diamond model), serving as a 3D analogue of the 2D px,y-graphene model.
- Topological Feature: The eg-diamond model inherently yields a 3D Nodal Cage (3D-NC) structure, protected by the coexistence of eg-orbital and diamond crystal sublattice degeneracies.
- Material Realization: The model is realized in the well-known 4-2 spinel compound, VMg2O4, confirmed via first-principles calculations (DFT) and Wannier function fitting.
- Ideal Half-Metallicity: VMg2O4 is an ideal ferromagnetic half-metal, featuring one metallic spin channel and one insulating spin channel with a large spin gap (up to 6.62 eV).
- Magnetic Weyl Semimetal (MWSM): Inclusion of Spin-Orbit Coupling (SOC) breaks the 3D-NC degeneracy, transforming VMg2O4 into a magnetic Weyl semimetal state characterized by 18 pairs of Weyl points and topological Fermi arcs.
- Integration Potential: VMg2O4 exhibits excellent lattice matching (< 0.4%) with the widely used spintronics oxide MgO, facilitating high-quality thin film growth and device integration.
- Enhanced Functionality: Strain engineering is theoretically shown to enhance the intrinsic anomalous Hall conductivity (ÏHA) by at least 2-fold.
| Parameter | Value | Unit | Context |
|---|
| Spin Gap (PBE) | 4.36 | eV | Band gap in the insulating spin-down channel (PBE level). |
| Spin Gap (HSE) | 6.62 | eV | Band gap in the insulating spin-down channel (HSE hybrid functional). |
| Magnetic Moment | 2 | ”B/UC | Ferromagnetic state moment per unit cell, primarily V cation contribution. |
| FM/AFM Energy Difference | 0.43 | eV | Energy difference between Ferromagnetic (FM) and Antiferromagnetic (AFM) states (PBE level). |
| Nodal Cage Energy Oscillation | ± 20 | meV | Energy dispersion of the nodal line on the (110) surface. |
| Intrinsic Anomalous Hall Conductivity (Peak) | ~100 | Ω-1 cm-1 | Calculated peak value near the Fermi level in VMg2O4. |
| Lattice Mismatch with MgO | < 0.4 | % | Mismatch for both (001) and (111) planes, critical for heteroepitaxy. |
| Weyl Points | 18 | pairs | Number of Weyl degenerate points confirmed in the MWSM state. |
| Crystal Structure | Fd-3m | Space Group | Same group symmetry as the diamond structure (Spinel compound). |
- Tight-Binding (TB) Model Development: A novel four-band Hamiltonian was constructed using two eg-orbitals (dz2 and dx2-y2) on the diamond lattice, incorporating both nearest-neighbor (NN) and next-nearest-neighbor (NNN) hopping interactions.
- First-Principles Calculations (DFT): Density Functional Theory (DFT) calculations, primarily using the PBE functional (and hybrid functionals like HSE for verification), were performed to determine the electronic and magnetic properties of VMg2O4.
- Wannier Function Fitting: Maximally Localized Wannier Functions (MLWFs) were calculated using the Wannier90 package to fit the DFT band structure, confirming the orbital character (dz2 and dx2-y2) and validating the eg-diamond model.
- Nodal Cage Confirmation: 3D band structures and 2D Fermi surfaces were calculated across high-symmetry k-planes ((111) and (110)) to confirm the existence and shape of the 3D Nodal Cage (3D-NC).
- Magnetic Weyl Semimetal Analysis: Non-collinear calculations including Spin-Orbit Coupling (SOC) were performed to confirm the transition from the 3D-NC to the MWSM state.
- Topological Invariants Calculation: The topological properties were confirmed by calculating the surface states and Fermi arcs (using Greenâs function method) and integrating the Berry curvature to confirm the Berry phase (±Ï) around the 18 pairs of Weyl points.
- Strain Engineering Simulation: Theoretical calculations were performed to model the effect of compressive strain on the band dispersion, demonstrating a method to enhance the intrinsic anomalous Hall conductivity (ÏHA).
- Spintronics and Quantum Computing: The discovery of a âcleanâ MWSM state in a well-known oxide family (spinels) opens a door for utilizing topological properties in next-generation spintronic and quantum devices.
- Magnetic Tunneling Junctions (MTJ): VMg2O4âs ideal half-metallicity and large spin gap make it a promising candidate for use as a highly efficient spin filtering layer in MTJ devices (e.g., VMg2O4/MgO/VMg2O4 stacks).
- Anomalous Hall Sensors: The material exhibits a relatively large intrinsic anomalous Hall conductivity (ÏHA), which can be further enhanced by strain, making it suitable for high-sensitivity Hall effect sensors and memory applications.
- Spin Filters: Related spinel compounds (like CrMg2O4) show multiple band gaps in different spin channels, offering potential for highly selective spin filtering applications.
- Heteroepitaxial Devices: The small lattice mismatch (< 0.4%) with MgO, a widely used industrial oxide, simplifies the integration and high-quality growth of VMg2O4 thin films for practical device fabrication.
- Low-Energy Magnetic Devices: The materialâs strong ferromagnetic features and potential for low switching energy suggest applications in energy-efficient magnetic memory and logic.
View Original Abstract
Diamond-structure materials have been extensively studied for decades, which\nform the foundation for most semiconductors and their modern day electronic\ndevices. Here, we discover a e$g$-orbital ($d{z^2}$,$d_{x^2-y^2}$ ) model\nwithin the diamond lattice (e$_g$-diamond model) that hosts novel topological\nstates. Specifically, the e$_g$-diamond model yields a 3D nodal cage (3D-NC),\nwhich is characterized by a $d$-$d$ band inversion protected by two types of\ndegenerate states (i.e., e$_g$-orbital and diamond-sublattice degeneracies). We\ndemonstrate materials realization of this model in the well-known spinel\ncompounds (AB$_2$X$_4$), where the tetrahedron-site cations (A) form the\ndiamond sub-lattice. An ideal half metal with one metallic spin channel formed\nby well-isolated and half-filled e$_g$-diamond bands, accompanied by a large\nspin gap (4.36 eV) is discovered in one 4-2 spinel compound (VMg$_2$O$_4$),\nwhich becomes a magnetic Weyl semimetal when spin-orbit coupling effect is\nfurther considered. Our discovery greatly enriches the physics of diamond\nstructure and spinel compounds, opening a door to their application in\nspintronics.\n