We combine theory, calculation and experiment techniques to investigate frontier science problems in condensed matter physics and possible practical applications.
In theory, we develop first-principles calculation methods. We design software packages related to Berry phase calculations to study topological materials, abnormal Hall transport, quantum magnetism, superconducting and various exotic quantum phenomena as well as their potential applications in novel energy materials.
In experiment, our main researches focus on the frontier of condensed state physics and their possible applications, including growth, characterization and tuning of topological materials, abnormal quantum hall effect, quantum magnetic properties and various exotic quantum phenomena; Novel 2D Dirac materials and its application in photoelectric detection; novel photovoltaic and photocatalytic materials and devices. Preparation of high quality single crystals by vapor phase transport or cosolvent. Preparation of high quality single crystals by vapor phase transport or cosolvent method. Preparation of high quality thin films by chemical vapor deposition or molecular beam epitaxy. We employ low temperature physical property measurement instrument and femtosecond optical system to characterize the electrical, optical, photoelectric and magnetic properties prepared sample.
Theoretical and experimental researches include the following five directions:
We investigate the anomalous Hall effect, spin Hall effect, orbital Hall effect, planar Hall effect, thermoelectric effect, magneto-optical effect, magnetic resistance, magnetic torque, and other topological quantum properties. Especially we focus on the electronic structures, Berry phase, and quantum properties in the spin-orbital systems. Based on this, we develop the high performance parallel computing package, e.g., for the computations of anomalous Hall effect and Z2 topological invariants.
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We elucidate the intrinsic mechanism of anomalous Hall effect (AHE) and point out the importance of the Berry phase based intrinsic part in AHE. In collaboration with experiments, we propose a method to decompose AHE into intrinsic part and extrinsic part, and by quantitative calculations. We explain the linear relation between the intrinsic conductivity and the magnetization in AHE. Internationally, we are the first to develop first-principles based methods to calculate transport quantities such as anomalous thermoelectric coefficient and spin Hall conductivity; we explain quantitatively and predict related experiments.
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We studied the spin-orbit coupling (SOC) effect in graphene in detail for the first time, and pointed out that quantum spin Hall effect (QSHE) could not be realized in graphene under experimental accessible conditions. Quantum anomalous Hall effect (QAHE) was predicted to be feasible by adsorbing iron atoms on graphene or by putting graphene on ferromagnetic insulators. For the first time, we pointed out that silicene, germanene, and stanene are 2D topological insulators (TI), and predicted realizable QSHE, valley-polarized QAHE, and topological superconductors in these materials, which has triggered an upsurge in related fields. We proposed an excellent QSH insulator Bi4Br4, and BiX/SbX (X = H, F, Cl and Br) monolayers with a record-breaking bulk band gap. We also predicted that there might be a large number of 3D TIs in the families of Half-Heusler and chalcopyrite materials, which has been partially confirmed by experiments. Recently, we predicted that weak TIs and composite Weyl semimetals can be realized in beta-Bi4X4 (X=Br, I).
Kane and Mele proposed that a small gap can open on the two Dirac points of graphene due to SOC, which at the same time makes the system a spin Hall insulator with quantized spin Hall conductance. We provided a careful calculation on the spin-orbit gap of graphene, which leads to the same mass term for the relativistic Dirac fermions in the continuum limit, but with a much smaller magnitude of the gap 10−3 meV. The physical reason for the minuteness of the spin-orbit gap can also be understood from the tight-binding model as deriving from the lattice C3 symmetry, which leads to the vanishing of the leading-order contributions. Such a small gap is consistent with the experimental observation of semimetallic behavior of graphene. It shows that the proposed quantum spin Hall effect in graphene cannot be observed until temperatures as low as T<<10−2 K.
The extrinsic Rashba SOC from breaking the mirror symmetry of the graphene plane tends to destroy this effect. Rashba SOC can be very large for graphene grown on a substrate. For example, a 225 meV of Rashba spin splitting has been observed for graphene grown on Ni. We expect that Rashba SOC should also be present for graphene on insulating substrate, or with dilute adsorbates. We show that although Rashba SOC is detrimental to the QSHE, it helps to realize another important topological phenomenon: the QAHE. We find that a nontrivial bulk gap in graphene can be produced in the presence of both Rashba SOC and exchange field, where we predict a quantum anomalous Hall conductance of quantized as 2e2/h. This is followed up with a more concrete example of graphene sheet with Fe atoms adsorbed on top. Our first- principles calculations show a bulk gap as large as～5.5 meV that can be opened at the Dirac points, producing the same topological effect.
We find theoretically a new quantum state of matter—the valley-polarized QAH states in silicene. In the presence of Rashba spin-orbit coupling and an exchange field, silicene hosts a quantum anomalous Hall state with Chern number C= 2. We show that through tuning the Rashba spin-orbit coupling, a topological phase transition results in a valley-polarized quantum anomalous Hall state, i.e., a quantum state that exhibits the electronic properties of both the quantum valley Hall state (valley Chern number Cv = 3) and QAH state with C= −1. This finding provides a platform for designing dissipationless valleytronics in a more robust manner.
Starting from symmetry considerations and the tight-binding method in combination with first-principles calculations, we systematically derive the low-energy effective Hamiltonian involving SOC for silicene. This Hamiltonian is very general because it applies not only to silicene itself but also to the low-buckled counterparts of graphene for other group-IVA elements Ge and Sn, as well as to graphene when the structure returns to the planar geometry. The effective Hamitonian is the analog to the graphene quantum QSHE Hamiltonian. As in the graphene model, the effective SOC in low-buckled geometry opens a gap at the Dirac points and establishes the QSHE. The effective SOC actually contains the first order in the atomic intrinsic SOC strength ξ0, while this leading-order contribution of SOC vanishes in the planar structure. Therefore, silicene, as well as the low-buckled counterparts of graphene for the other group-IVA elements Ge and Sn, has a much larger gap opened by the effective SOC at the Dirac points than graphene, due to the low-buckled geometry and larger atomic intrinsic SOC strength. Further, the more buckled the structure is, the greater the gap will be. Therefore, the QSHE can be observed in low-buckled Si, Ge, and Sn systems in an experimentally accessible temperature regime. In addition, the Rashba SOC in silicene is intrinsic due to its own low-buckled geometry, which vanishes at the Dirac point K, while it has a nonzero value with deviation of k from the K point. Therefore, the QSHE in silicene is robust against the intrinsic Rashba SOC.
On the basis of first-principles calculations, we predicted a group of 2D TI BiX/SbX (X = H, F, Cl and Br) monolayers with extraordinarily large bulk gaps from 0.32 eV to a record value of 1.08 eV. These giant-gaps are entirely due to the result of the strong spin-orbit interaction related to the px and py orbitals of the Bi/Sb atoms around the two valleys K and K′ of the honeycomb lattice, which is significantly different from that consisting of the pz orbital as in graphene/silicene. The topological characteristic of BiX/SbX monolayers is confirmed by the calculated nontrivial Z2 index and an explicit construction of the low-energy effective Hamiltonian in these systems. We demonstrate that the honeycomb structures of BiX monolayers remain stable even at 600 K. Owing to these features, the giant-gap TIs BiX/SbX monolayers are an ideal platform to realize many exotic phenomena and fabricate new quantum devices operating at RT. Furthermore, biased BiX/SbX monolayers become a quantum valley Hall insulator, exhibiting valley-selective circular dichroism. Using the tight-binding method in combination with first-principles calculations, we systematically derive a low-energy effective Hilbert subspace and Hamiltonian with spin-orbit coupling for these 2D Tis with a record bulk band gap.
QSH insulators have gapless topological edge states inside the bulk band gap, which can serve as dissipationless spin current channels. The major challenge currently is to find suitable materials for this topological state. Here, we predict a new large-gap QSH insulator with bulk direct band gap of ∼0.18 eV, in single-layer Bi4Br4, which could be exfoliated from its three-dimensional bulk material due to the weakly bonded layered structure. The band gap of single-layer Bi4Br4 is tunable via strain engineering, and the QSH phase is robust against external strain. Moreover, because this material consists of special one-dimensional molecular chain as its basic building block, the single layer Bi4Br4 could be torn to ribbons with clean and atomically sharp edges. These nanoribbons, which have single-Dirac-cone edge states crossing the bulk band gap, are ideal wires for dissipationless transport. Our work thus provides a new promising material for experimental studies and practical applications of the QSH effect.
We theoretically investigate the possibility of establishing ferromagnetism in the topological insulator Bi2Se3 via magnetic doping of 3d transition metal elements. The formation energies, charge states, band structures, and magnetic properties of doped Bi2Se3 are studied using first-principles calculations within density functional theory. Our results show that Bi substitutional sites are energetically more favorable than interstitial sites for single impurities. Detailed electronic structure analysis reveals that Cr and Fe doped materials are still insulating in the bulk but the intrinsic band gap of Bi2Se3 is substantially reduced due to the strong hybridization between the d states of the dopants and the p states of the neighboring Se atoms. The calculated magnetic coupling suggests that Cr doped Bi2Se3 is possible to be both ferromagnetic and insulating, while Fe doped Bi2Se3 tends to be weakly antiferromagnetic.
Using first-principles calculations within density functional theory, we explore the feasibility of converting ternary half-Heusler compounds into a new class of 3D TI. We demonstrate that the electronic structure of unstrained LaPtBi as a prototype system exhibits a distinct band-inversion feature. The 3DTI phase is realized by applying a uniaxial strain along the  direction, which opens a band gap while preserving the inverted band order. A definitive proof of the strained LaPtBi as a 3DTI is provided by directly calculating the topological Z2 invariants in systems without inversion symmetry. We discuss the implications of the present study to other half-Heusler compounds as 3DTI, which, together with the magnetic and superconducting properties of these materials, may provide a rich platform for novel quantum phenomena.
Using first-principles calculations within density functional theory, we investigate the band topology of ternary chalcopyrites of composition I-III-VI2 and II-IV-V2. By exploiting adiabatic continuity of their band structures to the binary 3D-HgTe, combined with direct evaluation of the Z2 topological invariant, we show that a large number of chalcopyrites can realize the topological insulating phase in their native states. The ability to host room-temperature ferromagnetism in the same chalcopyrite family makes them appealing candidates for novel spintronics devices.
While strong topological insulators (STIs) were experimentally realized soon after they were theoretically predicted, a weak topological insulator (WTI) has yet to be unambiguously confirmed. A major obstacle is the lack of distinct natural cleavage surfaces to test the surface selective hallmark of a WTI. With a new scheme, we discover that beta-Bi4X4 (X 1⁄4 Br, I), dynamically stable or synthesized before, can be a prototype WTI with two natural cleavage surfaces, where two anisotropic Dirac cones stabilize and annihilate, respectively. We further find four surface-state Lifshitz transitions under charge doping and two bulk topological phase transitions under uniaxial strain. Near the WTI-STI transition, there emerges a novel Weyl semimetal phase, in which the Fermi arcs generically appear at both cleavage surfaces whereas the Fermi circle only appears at one selected surface.
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We perform multi-scale rational material design by means of combination of first-principles calculations with Monte Carlo simulation. Our target material is mainly novel energy materials related to renewable energy, photoelectric and photocatalytic applications. We also carry out cooperative research with relevant experimental groups. Recent representative progress is as follows:
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In our lab, the main research focus on the frontier research of condensed state physics, including the growth, characterization and tuning of topological materials, anomalous quantum hall effect and quantum magnetic properties; the 2D dirac materials and its application in the photo-electronic detector; the novel photovoltaic and photocatalyst materials and devices. The details are shown as following:
(a) Growth and characterization of topological insulator: Preparation single crystal with flux method or vapor transport method; analyses of electronic properties with PPMS; Studying the surface properties with STM; Investigation of optical properties with Femtosecond pump probe technology.
(b) Two-dimensional topological materials and Dirac materials: Synthesizing 2D materials via the molecular beam epitaxy in ultrahigh vacuum; Investigating their structural and electronic properties by low-temperature scanning tunneling microscopy; Tuning their quantum properties by foreign atoms or organic molecules; Studying their optic, electronic and magnetic properties by PPMS and laser spectroscopy.
(c) Novel Photovoltaic and photocatalyst materials and devices: Preparation of thin films with evaporation or ALD; Investigation of electronic and optical properties and studying the carrier transport properties under the light; designing the high performance devices.
(d) Optical and optoelectronic properties of topological semimetal and their hetrostructures： Studying the optical and optoelectronic properties of topological semimetals and their hetrostructures using ultrafast pump probe spectroscopy, scanning photocurrent mapping, and time resolved photocurrent spectroscopy. Especially the carrier relaxation and transport processes in topological Dirac and Weyl semimetals after optical excitation. Studying the hot carriers’ properties in topological semimetal.