Generalization of the theory of three-dimensional quantum Hall effect of Fermi arcs in Weyl semimetal

The quantum Hall effect(QHE),which is usually observed in two-dimensional systems,was predicted theoretically and observed experimentally in three-dimensional(3 D)topological semimetal.However,there are some inconsistencies between the theory and the experiments showing the theory is imperfect.Here,we generalize the theory of the 3 D QHE of Fermi arcs in Weyl semimetal.Through calculating the sheet Hall conductivity of a Weyl semimetal slab,we show that the 3 D QHE of Fermi arcs can occur in a large energy range and the thickness dependences of the QHE in different Fermi energies are distinct.When the Fermi energy is near the Weyl nodes,the Fermi arcs give rise to the QHE which is independent of the thickness of the slab.When the Fermi energy is not near the Weyl nodes,the two Fermi arcs form a complete Fermi loop with the assistance of bulk states giving rise to the QHE which is dependent on the sample thickness.We also demonstrate how the band anisotropic terms influence the QHE of Fermi arcs.Our theory complements the imperfections of the present theory of 3 D QHE of Fermi arcs.

Observation of Fermi Arcs in Non-Centrosymmetric Weyl Semi-Metal Candidate NbP

We report the surface electronic structure of niobium phosphide NbP single crystal on （001） surface by vacuum ultraviolet angle-resolved photoemission spectroscopy. Combining with our first principle calculations, we identify the existence of the Fermi arcs originated from topological surface states. Furthermore, the surface states exhibit circular dichroism pattern, which may correlate with its non-trivial spin texture. Our results provide critical evidence for the existence of the Weyl Fermions in NbP, which lays the foundation for further research.

A semiclassical approach to surface Fermi arcs in Weyl semimetals

We present a semiclassical explanation for the morphology of the surface Fermi arcs of Weyl semimetals.Viewing the surface states as a two-dimensional Fermi gas subject to band bending and Berry curvatures,we show that it is the non-parallelism between the velocity and the momentum that gives rise to the spiral structure of Fermi arcs.We map out the Fermi arcs from the velocity field for a single Weyl point and a lattice with two Weyl points.We also investigate the surface magnetoplasma of Dirac semimetals in a magnetic field,and find that the drift motion,the chiral magnetic effect and the Imbert-Fedorov shift are all involved in the formation of surface Fermi arcs.Our work not only provides an insightful perspective on the surface Fermi arcs and a practical way to find the surface dispersion,but also paves the way for the study of other physical properties of the surface states of topological semimetals,such as transport properties and orbital magnetization,using semiclassical methods.

Consecutive topological transitions of helical Fermi arcs at saddle points in CoSi

The CoSi family hosts unconventional topological nodes with nonzero Chern numbers.The nontrivial topology is manifested by conspicuous surface Fermi arcs connecting surface projections of the nodes.Here,using angle-resolved photoemission spectroscopy,we have systematically investigated the(001)surface states of pristine and Ni-doped Co Si.The surface states form saddle-like band structures at/near the time-reversal invariant point near the Fermi level.The Fermi arcs undergo consecutive Lifshitz transitions at the saddle points X,leading to changes of the Fermi arc configuration.As the density of states has a van Hove singularity at the saddle points,exotic many-body physical phenomena may emerge accompanied by the topological transitions of surface Fermi arcs.

Chiral symmetry protected topological nodal superconducting phase and Majorana Fermi arc

With an external in-plane magnetic field, we show the emergence of a topological nodal superconducting phase of the two-dimensional topological surface states. This nodal superconducting phase is protected by the chiral symmetry with a non-zero magnetic field, and there are corresponding Majorana Fermi arcs(also known as flat band Andreev bound states) connecting the two Majorana nodes along the edges, similar to the case of Weyl semimetal. The topological nodal superconductor is an intermediate phase between two different chiral superconductors, and is stable against the effects of substrates. The two-dimensional effective theory of the nodal superconducting phase also captures the low energy behavior of a three-dimensional lattice model which describes the iron-based superconductor with a thin film geometry. The localizations of the Majorana nodes can be manipulated through external in-plane magnetic fields, which may introduce a non-trivial topological Berry phase between them.

Coexistence of Dirac and Weyl points in non-centrosymmetric semimetal NbIrTe_(4)

Using angle-resolved photoemission spectroscopy and density functional theory calculations methods,we investigate the electronic structures and topological properties of ternary tellurides NbIrTe_(4),a candidate for type-II Weyl semimetal.We demonstrate the presence of several Fermi arcs connecting their corresponding Weyl points on both termination surfaces of the topological material.Our analysis reveals the existence of Dirac points,in addition to Weyl points,giving both theoretical and experimental evidences of the coexistence of Dirac and Weyl points in a single material.These findings not only confirm NbIrTe_(4) as a unique topological semimetal but also open avenues for exploring novel electronic devices based on its coexisting Dirac and Weyl fermions.