Strong Anisotropy of Dirac Cones in SrMnBi2 and CaMnBi2 Revealed by Angle-Resolved Photoemission Spectroscopy

Abstract

The Dirac materials, such as graphene and three-dimensional topological insulators, have attracted much attention because they exhibit novel quantum phenomena with their low energy electrons governed by the relativistic Dirac equations. One particular interest is to generate Dirac cone anisotropy so that the electrons can propagate differently from one direction to the other, creating an additional tunability for new properties and applications. While various theoretical approaches have been proposed to make the isotropic Dirac cones of graphene into anisotropic ones, it has not yet been met with success. There are also some theoretical predictions and/or experimental indications of anisotropic Dirac cone in novel topological insulators and AMnBi2 (A = Sr and Ca) but more experimental investigations are needed. Here we report systematic high resolution angle-resolved photoemission measurements that have provided direct evidence on the existence of strongly anisotropic Dirac cones in SrMnBi2 and CaMnBi2. Distinct behaviors of the Dirac cones between SrMnBi2 and CaMnBi2 are also observed. These results have provided important information on the strong anisotropy of the Dirac cones in AMnBi2 system that can be governed by the spin-orbital coupling and the local environment surrounding the Bi square net.

Revealing the ultrafast light-to-matter energy conversion before heat diffusion in a layered Dirac semimetal

“…A crescent-like distribution is observed in the map as reported in previous ARPES studies [34,40,41]. Panels in Fig.…”

“…1(a)]. The Bi square net has two Bi sites in its unit cell and hosts four anisotropic Dirac cones in the Brillouin zone [34,40,41]. The charge dynamics around E F is governed by the Bi electrons in the square net and is virtually decoupled from the Mn moments in the MnBi layers [34,37].…”

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

“…[6], where it was pointed out that this structure would support metallic spacer layers which could aid in elucidating the mechanism for high temperature superconductivity. Attempts to synthesize iron pnictide compounds in this structure were not originally successful, but new Mn based materials in this structure were found [7,8] and it was observed theoretically [7] and experimentally [8][9][10][11] that the spacer layers exhibit Dirac cones [12]. Recently, Fe superconductors in the 112 structure were synthesized, (Ca,Pr)FeAs 2 [13], and Ca 1−x La x FeAs 2 [14].…”

Unusual Dirac Fermions on the Surface of a Noncentrosymmetric α -BiPd Superconductor

“…The states reported in Ref. 23 for the case of Sr/CaMnBi 2 cannot be classified into one of the 1D, 2D or 3D type of Dirac cone because, it is in the arc shape rather than cone shape and moreover the Fermi velocity varies between Γ − M and Γ − M . The Dirac states in Ru 2 Sn 3 are of quasi-1D [22], while the states in β-Bi 4 I 4 are of 1D but dispersing in the out-ofplane [24] in contrast to the in-plane dispersing asymmetric Dirac states of α-BiPd.…”

“…On the other hand, there are theoretical predictions [19][20][21] suggesting anisotropic Dirac states with massless fermions in one direction and massive fermions on the other direction. Till date, only a few experimental papers are reporting the asymmetric Dirac states in Ru 2 Sn 3 [22], Sr(Ca)MnBi 2 [23], and β-Bi 4 I 4 [24]. Unusual band structure of this material, surface state is gapped and asymmetric, make this a unique compound which will attract tremendous future research interests.…”