Professor Zhongqin Yang

1. Research Field

Computational Condensed Matter Physics
● Theoretical studies of topological electronic states and effects of spin-orbit coupling
● Quantum charge and spin transport in nano-junctions

Based on density functional theory, Wannier function methods, and topological quantum chemistry theory, topological behaviors of novel materials are studied, including Berry curvature, Chern number, edge states, and Hall conductivity etc. Some nontrivial topological states, such as quantum anomalous Hall effects (QAHE), valley Hall states (VHS), Dirac and Weyl semimetals, and higher-order topological states etc., are predicted and comprehended. We also study the effects of spin-orbit coupling (SOC) in spintronics, including spin polarization, spin Hall Effect, and spin current. A numerical method was developed to deal with spin transport of SOC sample with boundaries and interfaces. For quantum transport in nanostructures, the problems studied contain I-V characteristics, local heating, current-induced forces, and shot noise in quantum-point contacts. The electronic transport properties in currently hot materials of graphene are studied.

2. Research Progress

• Realization of Electron Correlation–Induced High-Temperature Quantum Anomalous Hall Effect in Hund Metals (2024)

Many-body effects have long been a central focus and challenge in condensed matter physics, serving as one of the key driving forces for emergent phenomena. In this work, we investigate the coupling mechanism between many-body effects arising from Hund’s interaction and topological electronic states, and for the first time propose that Hund metals provide an excellent material platform for realizing topologically nontrivial electronic states. Our study identifies the van der Waals material MgFeP as a Hund metal, which simultaneously manifests as a high-temperature quantum anomalous Hall (QAH) insulator. Furthermore, the van der Waals heterostructure MgFeP/LiOH also exhibits a high-temperature QAH insulating state. The orbital-projected densities of states of monolayer MgFeP characterize the orbital-selective Mott phase unique to Hund metals: localized dxz/yz and dx²–y² orbitals open a Mott gap between the occupied and unoccupied states, while the itinerant dxy and dz² orbitals cross the Fermi level, forming a spin-polarized half-metallic channel. The magnetic ground state of MgFeP is out-of-plane ferromagnetic, with a Curie temperature exceeding 1500 K. Based on correlation-enhanced spin–orbit coupling (SOC) analysis, combined with in-plane biaxial strain or out-of-plane compression, we shift the phosphorus p orbitals closer to the dx²–y² orbital to increase their Coulomb repulsion energy. Through this orbital-selective effect, the nontrivial band gap of the system is enlarged from 56 meV to 137 meV. These results demonstrate that many-body correlations, under suitable conditions, can significantly optimize the topological electronic properties. The discovered material can serve as a fundamental building block for designing van der Waals homostructures or heterostructures of topological materials, providing new theoretical insights for realizing high-temperature QAH states.

The work was published in the international core journalNano Letters  (Yao et al., Nano Lett. 24, 1563 (2024)), with Qingzhao Yao as the first author.

• Strain-Tuned Topological Phase Transitions from Ferrovalley Insulator to Semivalley Metal to Chern Insulator (2021)

Two-dimensional group-IV and group-V elemental materials, such as graphene, silicene, plumbene, and bismuthene, can form hexagonal lattices with the valley degrees of freedom, giving rise to novel valleytronic effects. In this study, we construct monolayer MBr₂ (M = Ru, Os) materials with hexagonal lattices. Monolayer MBr₂ exhibits a sandwich structure similar to that of monolayer 1H-MoS₂ and belongs to the D₃_h point group. To investigate its dynamical and thermodynamic stability, we calculated the phonon spectra and performed ab initio molecular dynamics simulations. The absence of soft modes in the phonon spectra indicates dynamical stability, and molecular dynamics simulations further confirm thermodynamic stability.

The ground state of monolayer MBr₂ is ferromagnetic. Due to the lack of spatial inversion symmetry, the degeneracy at the K⁺ and K⁻ valleys is lifted by spin–orbit coupling (SOC). We define the energy difference ∆c = E(c__K⁺) – E(c__K⁻) between the conduction-band minima at K⁺ and K⁻ as the valley polarization strength. Our results show that MBr₂ (M = Ru, Os) is an intrinsic ferrovalley material with a giant valley polarization of up to 530 meV. Strain engineering is a widely used approach in condensed matter physics. We simulated the band evolution of monolayer RuBr₂ under biaxial strain. At compressive strains of 1.6% and 2.8%, one valley closes while the other remains open, leading to a half valley metallic state with 100% valley polarization. In this regime, incident circularly polarized light would be completely reflected by the closed valley. Remarkably, between these two critical transition points, the system enters a quantum anomalous Hall (QAH) state, i.e., a Chern insulator. The QAH effect here is distinguished from the conventional QAH by the emergence of exotic boundary states with chirality–spin–valley locking. Similar effects are found in OsBr₂, where stronger SOC reduces the critical strain needed for the half valley transition. Thus, both RuBr₂ and OsBr₂ undergo multiple topological phase transitions readily under strain engineering. A two-band k·p model is further constructed to elucidate the mechanism of these transitions. This work provides theoretical guidance for the realization of microelectronic and optoelectronic devices with full spin polarization, full valley polarization, and nontrivial topological properties.

The work was published in the international core journal Physical Review B (Huan et al., Phys. Rev. B 104, 165427 (2021)), with Hao Huan as the first author, and has been cited about 100 times.

• Explaining Why Plumbene Differs from Other Group-IV Monolayers with Topologically Trivial Electronic States (2019)

Low-buckled plumbene differs from its group-IV counterparts such as graphene-stannene, which is not a quantum spin Hall (QSH) insulator; however, no explanation for this result has been reported. Our study reveals that the band structure of plumbene is indeed distinct from other group-IV monolayers. In addition to the linear Dirac bands formed at the K points of the Brillouin zone at the Fermi level, parabolic doubly degenerate px and py bands—determined by the crystal symmetry—also emerge at the Γ point. For such a unique band structure, it is necessary to construct a theoretical model for a deeper investigation of the underlying mechanism.

Taking the spin-polarized s, px, py, and pz orbitals of Pb as the basis, we constructed a tight-binding Hamiltonian for the system in the framework of second quantization, including three terms: hopping, spin–orbit coupling (SOC), and magnetic exchange. The hopping term only considers nearest-neighbor transitions, and the integrals are expressed in terms of the Slater–Koster parameters. The parameters of the model were obtained by fitting to the density functional theory (DFT) band structure. Based on this model, we further calculated the spin Chern number using the Kubo formula, thereby enabling an in-depth study of the electronic states of the system. Using the established tight-binding model in combination with first-principles calculations, we studied the special band dispersion of low-buckled plumbene. Without considering SOC, two types of dispersions appear near the Fermi level: Dirac-like dispersions near the K/K′ points, similar to silicene, and non-Dirac dispersions near the Γ point. Calculations of the edge states confirmed that the system is indeed topologically trivial.

Within the tight-binding model, by applying an exchange field of M = 5.5 eV, the spin-up and spin-down bands are split. The Berry curvatures at the gap positions were calculated separately for the spin-up and spin-down subspaces, yielding Chern numbers of +2 and –2, respectively. We found that since both the Γ and K/K′ points contribute Berry curvatures of the same sign, the Chern number in each spin subspace is even. This coupling effect between the Γ and K/K′ points can thus be termed a “constructive coupling” effect. However, this “constructive coupling” ultimately results in Z₂ = 0, explaining the topologically trivial nature of plumbene. If this “constructive coupling” effect is broken, for instance by saturating with acetylene groups, a topological phase transition from trivial to nontrivial can be induced in plumbene. Furthermore, we predict that introducing spin polarization into plumbene would enable the emergence of the quantum anomalous Hall effect.

This work was published in Physical Review B in 2019 (Li et al., Phys. Rev. B 99, 195402 (2019)), with Yue Li as the first author.

• Realization of High-Chern-Number Quantum Anomalous Hall States in Graphene/Magnetic Substrate Heterostructures (2018, 2015)

We have realized the quantum anomalous Hall (QAH) state in graphene/CrI₃ and graphene/CrGeTe₃ heterostructures, and proposed that graphene–magnetic substrate heterostructures constitute a highly promising material platform for designing topologically nontrivial states. Recently, two-dimensional monolayer ferromagnetic insulators CrI₃ and CrGeTe₃ have been successfully synthesized experimentally. Using first-principles calculations, we investigated the electronic states of such van der Waals (vdW) heterostructures, which are relatively easy to fabricate experimentally.

For the graphene/CrI₃ system, when the vdW gap is compressed to about 3.3–2.4 Å, the system acquires a Chern insulating state, corresponding to an applied external pressure of about 1.4–18.3 GPa. Remarkably, even though the interaction between the two layers is purely vdW, a very strong magnetization (about 150 meV) is induced in graphene by the CrI₃ substrate. To understand this mechanism, we employed a low-energy effective model. We also studied the work functions, contact types, and band alignments of the Gr/CrI₃ heterostructure. Our results demonstrate that the Gr/CrI₃ heterostructure is a promising candidate for observing the quantum anomalous Hall effect at relatively high temperatures (up to 45 K) in experiments. Similar effects are also found in the graphene/CrGeTe₃ heterostructure. We also discovered that multilayer stacking of alternating graphene and magnetic layers can yield Chern insulators with high Chern numbers.

The results were published in Physical Review B in 2018 and 2015 (Zhang et al., Phys. Rev. B 97, 085401 (2018), cited about 200 times; Zhang et al., Phys. Rev. B 92, 165418 (2015), cited about 140 times), with Jiayong Zhang as the first author of both papers.