Nontrivial Berry phase in magnetic BaMnSb ₂ semimetal

Abstract

The subject of topological materials has attracted immense attention in condensed-matter physics because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although both Dirac and Weyl fermions have massless nature with the nontrivial Berry phase, the formation of Weyl fermions in 3D semimetals require either time-reversal or inversion symmetry breaking to lift degeneracy at Dirac points. Here we demonstrate experimentally that canted antiferromagnetic BaMnSb 2 is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov-de Hass oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the nontrivial Berry phase. The ordered magnetic arrangement (ferromagnetic ordering in the ab plane and antiferromagnetic ordering along the c axis below 286 K) breaks the time-reversal symmetry, thus offering us an ideal platform to study magnetic Weyl fermions in a centrosymmetric material.topological material | nontrivial Berry phase | 3D semimetal T he observation of the quantum Hall effect has led to the discovery of new phases associated with topological ordering. In the past decade, topological materials have emerged as a new frontier of condensed matter physics due to new physical concepts and potential applications. Theory has played a major role in predicting new topological materials, which have been realized experimentally. Topological insulators, which are metallic at the surface but insulating in bulk (for a review, see ref. 1), have been extensively investigated. Surface states in these materials can be described by the Dirac equation with the Fermi surface formed by Dirac points. Dirac fermions are effectively massless because their dispersion is linear in energy. If the bulk is superconducting, the surface hosts Majorana fermions (for a review, see ref.2). More recently, a new class of topological materials, namely, Dirac or Weyl semimetals, has appeared. Such topological semimetals are characterized by the presence of electron and hole pockets touching with either degeneracy (Dirac) or nondegeneracy (Weyl). If both time reversal and inversion symmetries are preserved, the system is a Dirac semimetal (3). If either time-reversal or inversion symmetry is broken, the Dirac points split and turn into Weyl points, making the system a Weyl semimetal (3). If both timereversal and inversion symmetries are broken, the system may become a Weyl superconductor (4).The above framework makes the search of topological semimetals possible. For example, Dirac semimetals should have centrosymmetry to preserve the inversion symmetry. However, a noncentrosymmetric crystal structure would favor Weyl semimetal configuration (5). If a system is centrosymmetric and magnetically ordered, whether time-reversal sy...

Crystal growth, microstructure, and physical properties of SrMnSb2

“…To estimate the carrier concentration in SrMnSb 2 , we measured Hall resistivity ρ xy on Batch A crystals. Figures 13(a) and (b) show the field dependence of ρ xx and ρ xy at T = 2, 5, and 10 K. One can see that ρ xy oscillates above 6 T. Most importantly, the slopes of the curves are positive and persist in this manner to a maximum temperature of 340 K. The positive slope indicates that the dominant charge carriers responsible for transport are holes, similar to reports for BaMnSb 2 [18] and SrMnSb 2 [20]. A linear fit of ρ xy yields the Hall coefficient R H through the relation ρ xy = R H H, and Figure 13(c) shows the temperature dependence of R H .…”

“…And first-principles calculations of AMnSb 2 (A = Sr, Ba; replacing Bi with Sb) have indicated Dirac fermionic behavior could be realized in BaMnSb 2 [17]. Subsequent transport and magnetization measurements and relativistic first-principles calculations have suggested that BaMnSb 2 is a 3D WSM by virtue of weak ferromagnetism due to canted Mn moments in the antiferromagnetic (AFM) structure [18]. The reported canting leads to a ferromagnetic (FM) component which breaks the time-reversal symmetry in the crystal and establishes WSM behavior.…”

Spin dynamics in antiferromagnetic oxypnictides and fluoropnictides: LaMnAsO, LaMnSbO, and BaMnAsF

“…Manganese (Mn) based pnictide compounds with MnP n (P n = P, As, Sb, and Bi) layers have been in the spotlight by virtue of their intriguing magnetic properties, most notably the recently discovered Dirac semimetals AMnP n 2 (A = Ca, Sr, and Ba) [1][2][3]. The quasi two-dimensional (2D) AMnP n 2 have been recognized as the three-dimensional (3D) analogs of the 2D graphene with linearly dispersing bands that cross at the Fermi energy [2].…”

Influence of magnetism on Dirac semimetallic behavior in nonstoichiometric Sr1−yMn1−zSb2(yZhang1, Okamoto2, Stone3 et al. 2019Phys. Rev. B135170View full textAdd to dashboardCite

“…It is therefore challenging to explore whether there are indeed Dirac/Weyl points in proximity to the Fermi level and what drives their formation in Sr 1−y Mn 1−z Sb 2 . Furthermore, SrMnSb 2 offers a wonderful opportunity to address an important question whether there is a close correlation between magnetic order and band topology in 3D Dirac compounds.In addition to SrMnSb 2 , other Alkaline earth ternary AMnC 2 "112" compounds (A =Sr, Ca, Ba, C= Bi or Sb) [8][9][10][11][12] were reported to be Dirac semimetal candidates with the coexistence of AFM order. An interplay between magnetic order and electronic transport properties was found in CaMnBi 2 [13] due to coupling of the interlayer ferromagnetic component to the planar Bi electrons.…”