Crystal Thermal Transport in Altermagnetic RuO_{2}.

We demonstrate the emergence of a pronounced thermal transport in the recently discovered class of magnetic materials-altermagnets. From symmetry arguments and first-principles calculations performed for the showcase altermagnet, RuO_{2}, we uncover that crystal Nernst and crystal thermal Hall effects in this material are very large and strongly anisotropic with respect to the Néel vector. We find the large crystal thermal transport to originate from three sources of Berry's curvature in momentum space: the Weyl fermions due to crossings between well-separated bands, the strong spin-flip pseudonodal surfaces, and the weak spin-flip ladder transitions, defined by transitions among very weakly spin-split states of similar dispersion crossing the Fermi surface. Moreover, we reveal that the anomalous thermal and electrical transport coefficients in RuO_{2} are linked by an extended Wiedemann-Franz law in a temperature range much wider than expected for conventional magnets. Our results suggest that altermagnets may assume a leading role in realizing concepts in spin caloritronics not achievable with ferromagnets or antiferromagnets.

Spin-Chirality-Driven Quantum Anomalous and Quantum Topological Hall Effects in Chiral Magnets.

The quantum anomalous Hall effect (QAHE) is a highly researched topic in condensed matter physics due to its ability to enable dissipationless transport. Previous studies have mainly focused on the ferromagnetic QAHE, which arises from the combination of collinear ferromagnetism and two-dimensional (2D) Z2 topological insulator phases. In our study, we demonstrate the emergence of the spin-chirality-driven QAHE and the quantum topological Hall effect (QTHE) by sandwiching a 2D Z2 topological insulator between two chiral kagome antiferromagnetic single-layers synthesized experimentally. The QAHE is surprisingly realized with fully compensated noncollinear antiferromagnetism in contrast to conventional collinear ferromagnetism. The Chern number can be regulated periodically with the interplay between vector- and scalar-spin chiralities, and the QAHE emerges even without spin-orbit coupling, indicating the rare QTHE. Our findings open a new avenue for realizing antiferromagnetic quantum spintronics based on the unconventional mechanisms from chiral spin textures.

Disorder- and Topology-Enhanced Fully Spin-Polarized Currents in Nodal Chain Spin-Gapless Semimetals.

Recently discovered high-quality nodal chain spin-gapless semimetals MF_{3} (M=Pd, Mn) feature an ultraclean nodal chain in the spin up channel residing right at the Fermi level and displaying a large spin gap leading to a 100% spin polarization of transport properties. Here, we investigate both intrinsic and extrinsic contributions to anomalous and spin transport in this class of materials. The dominant intrinsic origin is found to originate entirely from the gapped nodal chains without the entanglement of any other trivial bands. The side-jump mechanism is predicted to be negligibly small, but intrinsic skew scattering enhances the intrinsic Hall and Nernst signals significantly, leading to large values of respective conductivities. Our findings open a new material platform for exploring strong anomalous and spin transport properties in magnetic topological semimetals.

Constructing van der Waals heterostructures by dry-transfer assembly for novel optoelectronic device.

Since the first successful exfoliation of graphene, the superior physical and chemical properties of two-dimensional (2D) materials, such as atomic thickness, strong in-plane bonding energy and weak inter-layer van der Waals (vdW) force have attracted wide attention. Meanwhile, there is a surge of interest in novel physics which is absent in bulk materials. Thus, vertical stacking of 2D materials could be critical to discover such physics and develop novel optoelectronic applications. Although vdW heterostructures have been grown by chemical vapor deposition, the available choices of materials for stacking is limited and the device yield is yet to be improved. Another approach to build vdW heterostructure relies on wet/dry transfer techniques like stacking Lego bricks. Although previous reviews have surveyed various wet transfer techniques, novel dry transfer techniques have been recently been demonstrated, featuring clean and sharp interfaces, which also gets rid of contamination, wrinkles, bubbles formed during wet transfer. This review summarizes the optimized dry transfer methods, which paves the way towards high-quality 2D material heterostructures with optimized interfaces. Such transfer techniques also lead to new physical phenomena while enable novel optoelectronic applications on artificial vdW heterostructures, which are discussed in the last part of this review.

Weyl Monoloop Semi-Half-Metal and Tunable Anomalous Hall Effect.

Nodal monoloop, enjoying the cleanest scenario with a single loop, is recognized as the basic building block of intricate linked loops including chains, nets, and knots. Here, we explore the interplay of magnetic ordering and band topology in one system by introducing a brand-new quantum state, referred to as Weyl monoloop semi-half-metal, which is characterized by a single loop at the Fermi level stemming from the same spin channel. Such a nodal line Fermion, yielding 100% spin polarization, is protected by mirror (Mz) symmetry. As a prominent example, a realistic rutile-type metal fluorides LiV2F6 achieves the hitherto unmaterialized state, featuring fully spin-polarized ultraflat surface states. More interestingly, LiV2F6 has a "soft" ferromagnetic property, which is one of the desired systems to control the anomalous Hall effect by rotating the magnetization direction. Our findings offer a promising candidate for exploring the topology and magnetism with intriguing effects.

Discovery of Real-Space Topological Ferroelectricity in Metallic Transition Metal Phosphides.

Ferroelectric metals-with coexisting ferroelectricity and structural asymmetry-challenge traditional perceptions because free electrons screen electrostatic forces between ions, the driving force of breaking the spatial inversion symmetry. Despite ferroelectric metals having been unveiled one after another, topologically switchable polar objects with metallicity have never been identified so far. Here, the discovery of real-space topological ferroelectricity in metallic and non-centrosymmetric Ni2 P is reported. Protected by the rotation-inversion symmetry operation, it is found that the balanced polarity of alternately stacked polyhedra couples intimately with elemental valence states, which are verified using quantitative electron energy-loss spectroscopy. First-principles calculations reveal that an applied in-plane compressive strain creates a tunable bilinear double-well potential and reverses the polyhedral polarity on a unit-cell scale. The dual roles of nickel cations, including polar displacement inside polyhedral cages and a 3D bonding network, facilitate the coexistence of topological polarity with metallicity. In addition, the switchable in-plane polyhedral polarity gives rise to a spin-orbit-coupling-induced spin texture with large momentum-dependent spin splitting. These findings point out a new direction for exploring valence-polarity-spin correlative interactions via topological ferroelectricity in metallic systems with structural asymmetry.

Topological magneto-optical effects and their quantization in noncoplanar antiferromagnets.

Reflecting the fundamental interactions of polarized light with magnetic matter, magneto-optical effects are well known since more than a century. The emergence of these phenomena is commonly attributed to the interplay between exchange splitting and spin-orbit coupling in the electronic structure of magnets. Using theoretical arguments, we demonstrate that topological magneto-optical effects can arise in noncoplanar antiferromagnets due to the finite scalar spin chirality, without any reference to exchange splitting or spin-orbit coupling. We propose spectral integrals of certain magneto-optical quantities that uncover the unique topological nature of the discovered effect. We also find that the Kerr and Faraday rotation angles can be quantized in insulating topological antiferromagnets in the low-frequency limit, owing to nontrivial global properties that manifest in quantum topological magneto-optical effects. Although the predicted topological and quantum topological magneto-optical effects are fundamentally distinct from conventional light-matter interactions, they can be measured by readily available experimental techniques.

Fully Spin-Polarized Nodal Loop Semimetals in Alkaline Metal Monochalcogenide Monolayers.

Topological semimetals in ferromagnetic materials have attracted an enormous amount of attention due to potential applications in spintronics. Using first-principles density functional theory together with an effective lattice model, here we present a new family of topological semimetals with a fully spin-polarized nodal loop in alkaline metal monochalcogenide MX (M = Li, Na, K, Rb, or Cs; X = S, Se, or Te) monolayers. The half-metallic ferromagnetism can be established in MX monolayers, in which one nodal loop formed by two crossing bands with the same spin components is found at the Fermi energy. This nodal loop half-metal survives even when considering the spin-orbit coupling owing to the symmetry protection provided by the Mz mirror plane. The quantum anomalous Hall state and Weyl-like semimetal in this system can be also achieved by rotating the spin from the out-of-plane to the in-plane direction. The MX monolayers hosting rich topological phases thus offer an excellent platform for realizing advanced spintronic concepts.

Large magneto-optical effects in hole-doped blue phosphorene and gray arsenene.

Blue phosphorene (BP) and gray arsenene (GA), consisting of phosphorus and arsenic atoms in two-dimensional (2D) low-buckled honeycomb lattices, respectively, have received great interest because of their excellent electronic and optoelectronic performances. Here, using first-principles density functional theory, we investigate magneto-optical (MO) Kerr and Faraday effects in BP and GA under hole doping. Ferromagnetic ground states are found in hole-doped monolayer and bilayer BP and GA due to the Stoner electronic instability, which originates from the van Hove singularity of the density of states at the valence band edge. The Kerr and Faraday effects strongly depend on the doping concentration and therefore are electrically controllable by adjusting the number of holes via the gate voltage. The influences of the thin film thickness, spin-polarized direction, and the substrate on the MO effects are further studied. We find that the MO effects are weakened remarkably as the thin film thickness increases and can be negligible more than three single-layers; the MO effects are much more prominent when spin polarization is along the out-of-plane direction and will decrease more than one order of magnitude on turning the spin in the crystal plane; the insulating substrates with small refractive indices are favorable to generate large MO effects and appropriate compressive strains applied on BP and GA due to lattice mismatch with substrates are further beneficial. The MO effects in GA are generally larger than those in BP because the strength of spin-orbit coupling in the arsenic atom is larger than that in the phosphorus atom. Monolayer GA possesses the largest Kerr and Faraday rotation angles, which are comparable to or even larger than those of well-known MO materials such as 3d-transition-metal multilayers and compounds. Our results indicate that BP and GA are a promising material platform for MO device applications.

Nonmetallization and band inversion in beryllium dicarbide at high pressure.

Carbides have attracted much attention owing to their interesting physical and chemical properties. Here, we systematically investigated global energetically stable structures of BeC2 in the pressure range of 0-100 GPa using a first-principles structural search. A transition from the ambient-pressure α-phase to the high-pressure ß-phase was theoretically predicted. Chemical bonding analysis revealed that the predicted phase transition is associated with the transformation from sp(2) to sp(3) C-C hybridization. The electrical conductivity of the high-pressure phase changed from a metal (α-phase) to a narrow bandgap semiconductor (ß-phase), and the ß-phase had an inverted band structure with positive pressure dependence. Interestingly, the ß-phase was a topological insulator with the metallic surface states protected by the time-reversal symmetry of the crystal. The results indicate that pressure modulates the electronic band structure of BeC2, which is an important finding for fundamental physics and for a wide range of potential applications in electronic devices.

Large-gap quantum spin Hall insulator in single layer bismuth monobromide Bi4Br4.

Quantum spin Hall (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.

Engineering topological surface states and giant Rashba spin splitting in BiTeI/Bi2Te3 heterostructures.

The search for strongly inversion asymmetric topological insulators is an active research field because these materials possess distinct properties compared with the inversion symmetric ones. In particular, it is desirable to realize a large Rashba spin-splitting (RSS) in such materials, which combined with the topological surface states (TSS) could lead to useful spintronics applications. In this report, based on first principles calculations, we predict that the heterostructure of BiTeI/Bi2Te3 is a strong topological insulator with a giant RSS. The coexistence of TSS and RSS in the current system is native and stable. More importantly, we find that both the Z2 invariants and the Rashba energy can be controlled by engineering the layer geometries of the heterostructure, and the Rashba energy can be made even larger than that of bulk BiTeI. Our work opens a new route for designing topological spintronics devices based on inversion asymmetric heterostructures.

Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides.

We show that inversion symmetry breaking together with spin-orbit coupling leads to coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides, making possible controls of spin and valley in these 2D materials. The spin-valley coupling at the valence-band edges suppresses spin and valley relaxation, as flip of each index alone is forbidden by the valley-contrasting spin splitting. Valley Hall and spin Hall effects coexist in both electron-doped and hole-doped systems. Optical interband transitions have frequency-dependent polarization selection rules which allow selective photoexcitation of carriers with various combination of valley and spin indices. Photoinduced spin Hall and valley Hall effects can generate long lived spin and valley accumulations on sample boundaries. The physics discussed here provides a route towards the integration of valleytronics and spintronics in multivalley materials with strong spin-orbit coupling and inversion symmetry breaking.

Quantum spin Hall effect in silicene and two-dimensional germanium.

We investigate the spin-orbit opened energy gap and the band topology in recently synthesized silicene as well as two-dimensional low-buckled honeycomb structures of germanium using first-principles calculations. We demonstrate that silicene with topologically nontrivial electronic structures can realize the quantum spin Hall effect (QSHE) by exploiting adiabatic continuity and the direct calculation of the Z(2) topological invariant. We predict that the QSHE can be observed in an experimentally accessible low temperature regime in silicene with the spin-orbit band gap of 1.55 meV, much higher than that of graphene. Furthermore, we find that the gap will increase to 2.9 meV under certain pressure strain. Finally, we also study germanium with a similar low-buckled stable structure, and predict that spin-orbit coupling opens a band gap of 23.9 meV, much higher than the liquid nitrogen temperature.

Topological aspect and quantum magnetoresistance of ß-Ag2Te.

To explain the unusual nonsaturating linear magnetoresistance observed in silver chalcogenides, the quantum scenario has been proposed based on the assumption of gapless linear energy spectrum. Here we show, by first principles calculations, that ß-Ag2Te with distorted antifluorite structure is in fact a topological insulator with gapless Dirac-type surface states. The characteristic feature of this new binary topological insulator is the highly anisotropic Dirac cone, in contrast with known examples, such as Bi2Te3 and Bi2Se3. The Fermi velocity varies an order of magnitude by rotating the crystal axis.

Three-dimensional topological insulators in I-III-VI2 and II-IV-V2 chalcopyrite semiconductors.

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.

Half-Heusler compounds as a new class of three-dimensional topological insulators.

Using first-principles calculations within density functional theory, we explore the feasibility of converting ternary half-Heusler compounds into a new class of three-dimensional topological insulators (3DTI). 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 [001] 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.

Order-disorder phase transition and dissociation of hydrogen sulfide under high pressure: ab initio molecular dynamics study.

The structural and dynamical properties of phase IV and V of hydrogen sulfide were investigated by means of extensive ab initio molecular dynamics simulations. Starting from an experimental proposal for the structure of phase IV, an Ibca symmetry with a stable hydrogen bonding network is found at 15 GPa and 100 K. Molecular dynamics simulations at increasing temperature and at the pressure of 15 GPa suggest that phase IV will transform to a proton disordered structure at 15 GPa and 350 K. The newfound structure has a hexagonal lattice of P63/mmc symmetry, which is believed to be the remaining crystalline structure of phase V. The high mobility of protons in phase V is believed to be the key point to the dissociation and decomposition of hydrogen sulfide.