type-I Weyl Points Research Articles

Weyl semimetallic phase in high pressure CrSb2 and structural compression studies of its high pressure polymorphs

In this study, high pressure synchrotron powder X-ray diffraction is used to investigate the compression of two high pressure polymorphs of CrSb2. The first is the CuAl2-type polymorph with an eight-fold coordinated Cr, which can be quenched to ambient conditions from high-pressure high-temperature conditions. The second is the recently discovered MoP2-type polymorph, which is induced by compression at room temperature, with a seven-fold coordinated Cr. Here, the assigned structure is unambiguously confirmed by solving it from single-crystal X-ray diffraction. Furthermore, the electrical properties of the MoP2-type polymorph were investigated theoretically and the resistance calculations under pressure were accompanied by resistance measurements under high pressure on a single crystal of CrSb2. The calculated electronic band structure for the MoP2-type phase is discussed and we show that the polymorph is semimetallic and possesses type-I Weyl points. No further phase transitions were observed for the CuAl2-type structure up to 50 GPa and 40 GPa for the MoP2-type structure. Even though the CuAl2-phase has the highest coordination number of Cr, it was found to be less compressible than the MoP2-phase having a seven-fold coordinated Cr, which was attributed to the longer Cr-Sb distance in the CuAl2-type phase. The discovery of a type-I Weyl semimetallic phase in CrSb2 opens up for discovering other Weyl semimetals in the transition metal di-pnictides under high pressure.

Type-I Weyl points induced by negative coupling in photonic crystal

Type-I Weyl points induced by negative coupling in photonic crystal

Photonic topological Weyl degeneracies and ideal type-I Weyl points in the gyromagnetic metamaterials

As a new topological phase in three-dimensional momentum space, Weyl semimetals extend the repertoire of exotic topological states. In this paper, we find the existence of photonic Weyl points in electromagnetic continuous gyromagnetic metamaterials. The dispersion of the longitudinal mode of one of the two bands forming the Weyl point is flat; thus, the Weyl degeneracy point of the system is exactly at the critical transition between the type-I and type-II Weyl points. The Fermi arc connects the projections of the Weyl points with opposite chirality at the interface between the gyromagnetic metamaterial and vacuum. Full-wave simulation results show that surface waves can bypass sharp defects and achieve robust, unidirectional optical transmission without reflection. Furthermore, taking the spatial nonlocal effects into account, the ideal type-I Weyl points can exist in this class of metamaterials. This paper sheds light on fundamental understanding of topological physics, and the study of Weyl points in metamaterials will greatly enrich the field of topological photonics.

Two-dimensional Weyl semimetal with coexisting fully spin-polarized type-I and type-II Weyl points

As novel topological states of matter, Weyl semimetals (WSM) have been well studied in three-dimensional (3D) system. However, two-dimensional (2D) WSMs, especially type-II ones, are rarely reported. Here, we propose that monolayer Cr2C is a 2D WSM with coexisting fully spin-polarized type-I and type-II Weyl points. The material is ferromagnetic and has a high Curie temperature of 562 K. It is metallic in the spin-up channel but insulating in the spin-down channel, showing a half metal band structure. The low-energy bands in the spin-up channel form one pair type-I Weyl points and three pairs of type-II Weyl points, which are protected by the rotation symmetry in high-symmetry path. The Weyl points can survive under spin-orbit coupling with magnetization along y direction. The Weyl points show fully spin-polarized Fermi arc of edge states within the projected Weyl points. Most interestingly, we find the 2D type-II Weyl points show unusual Magnetoresponse with complete Landau levels (LLs) collapse independent with the magnetic-field. Such LLs collapse is different with type-II Weyl points in 3D system. The material provides a promising platform to study the fundamental physics of 2D type-I and type-II Weyl points, as well potential entanglements between them.

Ideal Type-II Weyl Phase and Topological Transition in Phononic Crystals.

Ideal Weyl points, which are related by symmetry and thus reside at the same frequency, could offer further insight into the Weyl physics. The ideal type-I Weyl points have been observed in photonic crystals, but the ideal type-II Weyl points with tilted conelike band dispersions are still not realized. Here we present the observation of the ideal type-II Weyl points of the minimal number in three-dimensional phononic crystals and, in the meantime, the topological phase transition from the Weyl semimetal to the valley insulators of two distinct types. The Fermi-arc surface states are shown to exist on the surfaces of the Weyl phase, and the Fermi-circle surface states are also observed, but on the interface of the two distinct valley phases. Intriguing wave partition of the Fermi-circle surface states is demonstrated.

Hexagonal wurtzite MnO in ferromagnetic state: A magnetic topological spin-gapless Weyl semimetal

Magnetic topological materials have attracted increasingly attentions in recent years due to their exotic electronic behaviors emerging from the couplings of topological, magnetic, and crystalline symmetries. In this work, based on the first-principles calculations, we propose that hexagonal wurtzite MnO is a magnetic topological spin-gapless semi-half-metal with two pairs of type-I Weyl fermions near the Fermi level in ferromagnetic state, which is a promising candidate material in spintronic and piezoelectric applications. In the absence of spin-orbit coupling (SOC), it hosts one triple degeneracy point (TP) in the irreducible Brillouin zone. Owing to weak SOC, the TP splits into two type-I Weyl points that are very close to each other. The Fermi arc surface states connecting the projected Weyl points with opposite chirality are observed. Our results therefore provide a wonderful platform to study the interplay of magnetism and topology.

Generating controllable type-II Weyl points via periodic driving

Type-II Weyl semimetals are a novel gapless topological phase of matter discovered recently in 2015. Similar to normal (type-I) Weyl semimetals, type-II Weyl semimetals consist of isolated band touching points. However, unlike type-I Weyl semimetals which have a linear energy dispersion around the band touching points forming a three dimensional (3D) Dirac cone, type-II Weyl semimetals have a tilted cone-like structure around the band touching points. This leads to various novel physical properties that are different from type-I Weyl semimetals. In order to study further the properties of type-II Weyl semimetals and perhaps realize them for future applications, generating controllable type-II Weyl semimetals is desirable. In this paper, we propose a way to generate a type-II Weyl semimetal via a generalized Harper model interacting with a harmonic driving field. When the field is treated classically, we find that only type-I Weyl points emerge. However, by treating the field quantum mechanically, some of these type-I Weyl points may turn into type-II Weyl points. Moreover, by tuning the coupling strength, it is possible to control the tilt of the Weyl points and the energy difference between two Weyl points, which makes it possible to generate a pair of mixed Weyl points of type-I and type-II. We also discuss how to physically distinguish these two types of Weyl points in the framework of our model via the Landau level structures in the presence of an artificial magnetic field. The results are of general interest to quantum optics as well as ongoing studies of Floquet topological phases.