Tunable Topological States in Stacked Chern Insulator Bilayers.

The emergence of intrinsic quantum anomalous Hall (QAH) insulators with a long-range ferromagnetic (FM) order triggers unprecedented prosperity for combining topology and magnetism in low dimensions. Built upon atom-thin Chern insulator monolayer MnBr3, we propose that the topologically nontrivial electronic states can be systematically tuned by inherent magnetic orders and external electric/optical fields in stacked Chern insulator bilayers. The FM bilayer illustrates a high-Chern-number QAH state characterized by both quantized Hall plateaus and specific magneto-optical Kerr angles. In antiferromagnetic bilayers, Berry curvature singularity induced by electrostatic fields or lasers emerges, which further leads to a novel implementation of the layer Hall effect depending on the chirality of irradiated circularly polarized light. These results demonstrate that abundant tunable topological properties can be achieved in stacked Chern insulator bilayers, thereby suggesting a universal routine to modulate d-orbital-dominated topological Dirac fermions.

Unconventional materials: the mismatch between electronic charge centers and atomic positions.

The complete band representations (BRs) have been constructed in the work of topological quantum chemistry. Each BR is expressed by either a localized orbital at a Wyckoff site in real space, or by a set of irreducible representations in momentum space. In this work, we define unconventional materials with a common feature of the mismatch between average electronic centers and atomic positions. They can be effectively diagnosed as whose occupied bands can be expressed as a sum of elementary BRs (eBRs), but not a sum of atomic-orbital-induced BRs (aBRs). The existence of an essential BR at an empty site is described by nonzero real-space invariants (RSIs). The "valence" states can be derived by the aBR decomposition, and unconventional materials are supposed to have an uncompensated total "valence" state. The high-throughput screening for unconventional materials has been performed through the first-principles calculations. We have discovered 423 unconventional compounds, including thermoelectronic materials, higher-order topological insulators, electrides, hydrogen storage materials, hydrogen evolution reaction electrocatalysts, electrodes, and superconductors. The diversity of these interesting properties and applications would be widely studied in the future.

Persona of Transition Metal Ions in Solids: A Statistical Learning on Local Structures of Transition Metal Oxides.

The local structure of a transition metal (TM) ion is a function of cation elements and valence states. More than that, in this work, by employing a trove of first-principles data of TM oxides, the local structures of TM cations are statistically analyzed to extract detailed information about cation site preference, bond length, site structural distortion, and cation magnetization. It is found that cation radius alone poorly describes the local structure of a transition metal oxide, while the statistics of coordination number as well as the TMO bond length distribution, especially that of the 3d TMs, can provide comprehensive knowledge for understanding the behavior of TM elements. Based on these statistics, the interplay of site distortion due to the Jahn-Teller effect, cation site similarity, and a new set of ionic radii are all obtained to chart the "persona" of transition metal ions in solids.

Vacancy Occupation-Driven Polymorphic Transformation in Cobalt Ditelluride for Boosted Oxygen Evolution Reaction.

Transition-metal dichalcogenides (TMDs) hold great potential as an advanced electrocatalyst for oxygen evolution reaction (OER), but to date the activity of transition metal telluride catalysts are demonstrated to be poor for this reaction. In this study, we report the activation of CoTe2 for OER by doping secondary anions into Te vacancies to trigger a structural transition from the hexagonal to the orthorhombic phase. The achieved orthorhombic CoTe2 with partial vacancies occupied by P-doping exhibits an exceptional OER catalytic activity with an overpotential of only 241 mV at 10 mA cm-2 and a robust stability more than 24 h. The combined experimental and theoretical studies suggest that the defective phase transformation is controllable and allows the synergism of vacancy, doping as well as the reconstructed crystallographic structure, ensuring more exposure of catalytic active sites, rapid charge transfer, and energetically favorable intermediates. This vacancy occupation-driven strategy of structural transformation can also be manipulated by S- and Se-doping, which may offer useful guidance for developing tellurides-based electrocatalyst for OER.

Iron-Doping-Induced Phase Transformation in Dual-Carbon-Confined Cobalt Diselenide Enabling Superior Lithium Storage.

Transition metal chalcogenides (TMCs) have been investigated as promising anodes for high-performance lithium-ion batteries, but they usually suffer from poor conductivity and large volume variation, thus leading to unsatisfactory performance. Although nanostructure engineering and hybridization with conductive materials have been proposed to address this concern, a better performance toward practical device applications is still highly desired. Herein, we report an iron-doping-induced structural phase transition from pyrite-type (cubic) to marcasite-type (orthorhombic) phases in porous carbon/rGO-coupled CoSe2. The dual-carbon-confined orthorhombic CoSe2 ( o-Fe xCo1- xSe2@NC@rGO) composites exhibit dramatically enhanced lithium storage performance (920 mAh g-1 after 1000 cycles at 1.0 A g-1) over cubic CoSe2-based composites ( c-CoSe2@NC@rGO). The combined experimental studies and density functional theory calculations reveal that this doping-induced structural phase transformation strategy could create a favorable electronic structure and ensure a rapid charge transfer. These results demonstrate that the phase-transformation engineering may provide another opportunity in the design of high-performance TMC-based electrodes.