Topological states of quantum matter have attracted great attention in condensed matter physics and materials science. The study of time-reversal-invariant topological states in quantum materials has made tremendous progress. However, the study of magnetic topological states falls much behind due to the complex magnetic structures. Here, we predict the tetradymite-type compound MnBi2Te4 and its related materials host topologically nontrivial magnetic states. The magnetic ground state of MnBi2Te4 is an antiferromagetic topological insulator state with a large topologically non-trivial energy gap (∼ 0.2 eV). It presents the axion state, which has gapped bulk and surface states, and the quantized topological magnetoelectric effect. The ferromagnetic phase of MnBi2Te4 might lead to a minimal ideal Weyl semimetal.

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼107) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

Using ultracold atoms trapped in an optical lattice, we form a line-centered-square lattice in the condensedmatter physics, where a crossover from massive to massless Dirac fermion behavior can be easily achieved by tuning the laser intensities. The present Dirac fermions satisfy a three-component quantum equation for pseudospin-1 fermions, resulting in a single Dirac cone in the energy spectrum, a flat band touching at the Dirac point, and a vanishing Berry's phase. Interestingly, the massless Dirac fermions here may exhibit an all-angle Klein tunneling; i.e., the barrier is completely transparent for all incident angles.

Quantum spin Hall (QSH) state of matter is usually considered to be protected by time-reversal (TR) symmetry. We investigate the fate of the QSH effect in the presence of the Rashba spin-orbit coupling and an exchange field, which break both inversion and TR symmetries. It is found that the QSH state characterized by nonzero spin Chern numbers C± = ±1 persists when the TR symmetry is broken. A topological phase transition from the TR symmetry-broken QSH phase to a quantum anomalous Hall phase occurs at a critical exchange field, where the bulk band gap just closes. It is also shown that the transition from the TR-symmetry-broken QSH phase to an ordinary insulator state can not happen without closing the band gap. The quantum spin Hall (QSH) effect is a new topologically ordered electronic state, which occurs in insulators without a magnetic field.[1] A QSH state of matter has a bulk energy gap separating the valence and conduction bands, and a pair of gapless spin filtered edge states on the boundary. The currents carried by the edge states are dissipationless due to the protection of time reversal (TR) symmetry and immune to nonmagnetic scattering. The QSH effect was first predicted in two-dimensional (2D) models [2,3]. It was experimentally confirmed soon after, not in graphene sheets [2] but in mercury telluride (HgTe) quantum wells [3,4].Graphene hosts an interesting electronic system. Its conduction and valence bands meet at two inequivalent Dirac points. Kane and Mele proposed that the intrinsic spin-orbit coupling (SOC) would open a small band gap in the bulk and also establish spin filtered edge states that cross inside the band gap, giving rise to the QSH effect [2]. The gapless edge states in the QSH systems persist even when the electron spinŝ z conservation is destroyed in the system, e.g., by the Rashba SOC, and are robust against weak electron-electron interactions and disorder [2,5]. While the SOC strength may be too weak in pure graphene system, the Kane and Mele model captures the essential physics of a class of insulators with nontrivial band topology [6,7]. A central issue relating to the QSH effect is how to describe the topological nature of the systems. A Z 2 topological index was introduced to classify TR invariant systems [8], and a spin Chern number was also suggested to characterize the topological order [5]. The spin Chern number was originally introduced in finite-size systems by imposing spin-dependent boundary conditions [5]. Recently, based upon the noncommutative theory of Chern number [9], Prodan [10] redefined the spin Chern number in the thermodynamic limit through band projection without using any boundary conditions. It has been shown that the Z 2 invariant and spin Chern number yield equivalent description for TR invariant systems [10][11][12].The QSH effect is considered to be closely related to the TR symmetry that provides a protection for the edge states and the Z 2 invariant. An open question is whether or not we can have QSH-like phase in a system where the TR symmet...

van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless band structure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 10(11) Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications.

2D transition metal dichalcogenides are emerging with tremendous potential in many optoelectronic applications due to their strong light-matter interactions. To fully explore their potential in photoconductive detectors, high responsivity is required. Here, high responsivity phototransistors based on few-layer rhenium disulfi de (ReS 2 ) are presented. Depending on the back gate voltage, source drain bias and incident optical light intensity, the maximum attainable photoresponsivity can reach as high as 88 600 A W −1 , which is a record value compared to other individual 2D materials with similar device structures and two orders of magnitude higher than that of monolayer MoS 2 . Such high photoresponsivity is attributed to the increased light absorption as well as the gain enhancement due to the existence of trap states in the fewlayer ReS 2 fl akes. It further enables the detection of weak signals, as successfully demonstrated with weak light sources including a lighter and limited fl uorescent lighting. Our studies underscore ReS 2 as a promising material for future sensitive optoelectronic applications.

Ideal Weyl semimetals with all Weyl nodes exactly at the Fermi level and no coexisting trivial Fermi surfaces in the bulk, similar to graphene, could feature deep physics such as exotic transport phenomena induced by the chiral anomaly. Here, we show that HgTe and half-Heusler compounds, under a broad range of in-plane compressive strain, could be materials in nature realizing ideal Weyl semimetals with four pairs of Weyl nodes and topological surface Fermi arcs. Generically, we find that the HgTe-class materials with nontrivial band inversion and noncentrosymmetry provide a promising arena to realize ideal Weyl semimetals. Such ideal Weyl semimetals could further provide a unique platform to study emergent phenomena such as the interplay between ideal Weyl fermions and superconductivity in the half-Heusler compound LaPtBi.

The progress in exploiting new electronic materials has been a major driving force in solid-state physics. As a new state of matter, a Weyl semimetal (WSM), in particular a type-II WSM, hosts Weyl fermions as emergent quasiparticles and may harbour novel electrical transport properties. Nevertheless, such a type-II WSM material has not been experimentally observed. In this work, by performing systematic magneto-transport studies on thin films of a predicted material candidate WTe2, we observe notable negative longitudinal magnetoresistance, which can be attributed to the chiral anomaly in WSM. This phenomenon also exhibits strong planar orientation dependence with the absence along the tungsten chains, consistent with the distinctive feature of a type-II WSM. By applying a gate voltage, we demonstrate that the Fermi energy can be in-situ tuned through the Weyl points via the electric field effect. Our results may open opportunities for implementing new electronic applications, such as field-effect chiral devices.