A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

We show that the pseudorelativistic physics of graphene near the Fermi level can be extended to three dimensional (3D) materials. Unlike in phase transitions from inversion symmetric topological to normal insulators, we show that particular space groups also allow 3D Dirac points as symmetry protected degeneracies. We provide criteria necessary to identify these groups and, as an example, present ab initio calculations of β-cristobalite BiO(2) which exhibits three Dirac points at the Fermi level. We find that β-cristobalite BiO(2) is metastable, so it can be physically realized as a 3D analog to graphene.

Dirac Semimetal in Three Dimensions

A 3D Graphene? Discoveries of materials with exciting electronic properties have propelled condensed matter physics over the past decade. Two of the best-known examples, graphene and topological insulators, have something in common: a linear energy-momentum relationship—the Dirac dispersion—in their two-dimensional (2D) electronic states. Topological insulators also have a more mundane aspect of their electronic structure, characterized by a band gap. Another class of materials, topological Dirac semimetals, has been proposed that has a linear dispersion along all three momentum directions—a bulk Dirac cone; these materials are predicted to have intriguing electronic properties and to be related to other exotic states through quantum phase transitions. Liu et al. (p. 864, published online 16 January) detected such a state in the compound Na3Bi by using photoemission spectroscopy. Angle-resolved photoemission spectroscopy is used to detect bulk Dirac cones in a three-dimensional analog of graphene. Three-dimensional (3D) topological Dirac semimetals (TDSs) represent an unusual state of quantum matter that can be viewed as “3D graphene.” In contrast to 2D Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. By investigating the electronic structure of Na3Bi with angle-resolved photoemission spectroscopy, we detected 3D Dirac fermions with linear dispersions along all momentum directions. Furthermore, we demonstrated the robustness of 3D Dirac fermions in Na3Bi against in situ surface doping. Our results establish Na3Bi as a model system for 3D TDSs, which can serve as an ideal platform for the systematic study of quantum phase transitions between rich topological quantum states.

Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi

The quantum Hall effect is usually observed in 2D systems. We show that the Fermi arcs can give rise to a distinctive 3D quantum Hall effect in topological semimetals. Because of the topological constraint, the Fermi arc at a single surface has an open Fermi surface, which cannot host the quantum Hall effect. Via a ``wormhole'' tunneling assisted by the Weyl nodes, the Fermi arcs at opposite surfaces can form a complete Fermi loop and support the quantum Hall effect. The edge states of the Fermi arcs show a unique 3D distribution, giving an example of ($d\ensuremath{-}2$)-dimensional boundary states. This is distinctly different from the surface-state quantum Hall effect from a single surface of topological insulator. As the Fermi energy sweeps through the Weyl nodes, the sheet Hall conductivity evolves from the $1/B$ dependence to quantized plateaus at the Weyl nodes. This behavior can be realized by tuning gate voltages in a slab of topological semimetal, such as the TaAs family, ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$, or ${\mathrm{Na}}_{3}\mathrm{Bi}$. This work will be instructive not only for searching transport signatures of the Fermi arcs but also for exploring novel electron gases in other topological phases of matter.

3D quantum Hall effect of Fermi arcs in topological semimetals

Fermions--elementary particles such as electrons--are classified as Dirac, Majorana or Weyl. Majorana and Weyl fermions had not been observed experimentally until the recent discovery of condensed matter systems such as topological superconductors and semimetals, in which they arise as low-energy excitations. Here we propose the existence of a previously overlooked type of Weyl fermion that emerges at the boundary between electron and hole pockets in a new phase of matter. This particle was missed by Weyl because it breaks the stringent Lorentz symmetry in high-energy physics. Lorentz invariance, however, is not present in condensed matter physics, and by generalizing the Dirac equation, we find the new type of Weyl fermion. In particular, whereas Weyl semimetals--materials hosting Weyl fermions--were previously thought to have standard Weyl points with a point-like Fermi surface (which we refer to as type-I), we discover a type-II Weyl point, which is still a protected crossing, but appears at the contact of electron and hole pockets in type-II Weyl semimetals. We predict that WTe2 is an example of a topological semimetal hosting the new particle as a low-energy excitation around such a type-II Weyl point. The existence of type-II Weyl points in WTe2 means that many of its physical properties are very different to those of standard Weyl semimetals with point-like Fermi surfaces.

/pdf/type-ii-weyl-semimetals-3b84bm463g.pdf

Type-II Weyl semimetals

In this work, we propose four novel holey carbon nitride monolayer C7N3, C10N3, C13N3, and C19N3 stoichiometries intriguingly behaving as ideal 2D Dirac semimetals whose Dirac cones are rightly loc...

Dirac Semimetals in Homogeneous Holey Carbon Nitride Monolayers

Coexistence of Weyl and Type‐II Triply Degenerate Fermions in a Ternary Topological Semimetal YPtP

Topological Phase Transition in 2D 1T′‐WSTe

It is generally believed that conductivity plateaus can only exist in the insulators with topological nontrivial bulk occupied states. Such rule exhibits in two-dimensional quantum (anomalous) Hall effect, quantum spin Hall effect, and three-dimensional topological insulators. In this work, we propose spin Hall conductivity (SHC) plateau can be formed in three-dimensional metallic materials with triply degenerate points around the Fermi level. With the help of a four-band k ⋅ p model, we prove that SHC plateaus can be formed between | 3 2 , ± 3 2 〉 and | 1 2 , ± 1 2 〉 states of a metallic systems. Our further ab initio calculations reveal that a nearly ideal SHC plateau exhibits in an experimentally synthesized TaN. The plateau width reaches up to 0.55 eV, hoping to work at high temperatures. The electrical conductivity tensors of TaN indicates that its spin Hall angle reaches -0.57, which is larger than many previous reported materials and makes it an excellent candidate to produce stable spin current. • Spin Hall conductivity plateau can form in a semimetal with triply degenerate points. • TaN can achieve such SHC plateau. • This work paves a significance way for the application of SHE in real devices. 

Intrinsic spin Hall conductivity plateau in topological semimetals with triply degenerate points

A Novel Carbon-Nitrogen Dirac Semimetal C5n4 Monolayer

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Dirac Semimetals in Homogeneous Holey Carbon Nitride Monolayers

Coexistence of Weyl and Type‐II Triply Degenerate Fermions in a Ternary Topological Semimetal YPtP

Topological Phase Transition in 2D 1T′‐WSTe

Intrinsic spin Hall conductivity plateau in topological semimetals with triply degenerate points

A Novel Carbon-Nitrogen Dirac Semimetal C5n4 Monolayer