Dirac fermions in an antiferromagnetic semimetal

Abstract

Analogues of the elementary particles have been extensively searched for in condensed-matter systems for both scientific interest and technological applications [1][2][3] . Recently, massless Dirac fermions were found to emerge as low-energy excitations in materials now known as Dirac semimetals [4][5][6] . All of the currently known Dirac semimetals are non-magnetic with both time-reversal symmetry T and inversion symmetry P 7-9 . Here we show that Dirac fermions can exist in one type of antiferromagnetic system, where both T and P are broken but their combination PT is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyse the robustness of the Dirac points under symmetry protections and demonstrate its distinctive bulk dispersions, as well as the corresponding surface states, by ab initio calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism.The great success in the field of topological insulators 1,2 since last decade inspired the study of topological features of metals. Topological metals have non-trivial surface states and their bulk Fermi surfaces can be topologically characterized 3 . Among them, Dirac semimetals 4-6 have received special attention, because they host relativistic particles, the massless Dirac fermions, in a nonrelativistic set-up. In such Dirac materials, two doubly degenerate bands contact at discrete momentum points called Dirac points, and disperse linearly along all directions around these points. The four-fold degenerate Dirac points are unstable by themselves; hence, symmetry protection is necessary 7 . Following this guideline, several three-dimensional Dirac semimetals have been theoretically proposed, and some of them have been experimentally verified recently 8,9 . All of these materials have time-reversal symmetry T , inversion symmetry P, and certain crystalline rotation symmetry.If some of the symmetries are broken, massless Dirac fermions can in general be destroyed. For instance, when either T or P is broken, each doubly degenerate band is lifted, so that the Dirac cones can split into multiple Weyl cones 10 . This gives birth to Weyl semimetals [11][12][13][14][15][16][17] , and the chiral-anomaly-related transport phenomena can be observed as a signature 18,19 . However, the result of both T and P breaking remains obscure until now. It is thus natural to ask whether Dirac fermions can still exist in the absence of both T and P.In this letter, we answer the question in the affirmative, and provide a concrete example of such a Dirac semimetallic phase. We consider three-dimensional systems with the antiferromagnetic (AFM) order that breaks both T and P but respects their combination PT . The low-energy physics can be explicitly captured by the following four-band effective modelwhere d i (k), i = 0, 1, . . . , 5 are real functions of momentum k, and τ x,y,z (σ x,y,z ) are Pauli matrices for orbital (spin-related AFM) basis (see Supplementary Section 3 for details). The anti-unitary PT ...

Massive fermions with low mobility in antiferromagnet orthorhombic CuMnAs single crystals

“…The existence of Weyl points near the Fermi energy level (E F ) will lead to interesting spectroscopic and transport phenomena, such as Fermi arcs 4,16,21,22,29 , ultrahigh carrier mobility 34,35 , and chiral anomaly [36][37][38] . Interestingly, recent theoretical studies predict that even when the P and T symmetries are broken the combined P T symmetry can be preserved in the antiferromagnetic (AFM) orthorhombic CuMn(As/P) (oCuMn(As/P)) 39,40 . When the magnetic moments of Mn are aligned along the z axis and screw rotation symmetry S 2z survives, the Dirac points can still exist with considering spin-orbital coupling (SOC).…”

“…Furthermore, when the magnetic moments of Mn are along other directions, such as (111) and (101), the S 2z symmetry will be broken and the Dirac points will be gapped. In that case, oCuMnAs will become an AFM semiconductor 39,40 . Thus, o-CuMnAs provides a good platform to investigate the interplay between Dirac fermions and AFM state as well as the topological metal-insulator transition (MIT) driven by the Néel vector reorientation 39,40 .…”

“…In that case, oCuMnAs will become an AFM semiconductor 39,40 . Thus, o-CuMnAs provides a good platform to investigate the interplay between Dirac fermions and AFM state as well as the topological metal-insulator transition (MIT) driven by the Néel vector reorientation 39,40 . In contrast to theoretical studies, the experimental results of o-CuMnAs is relatively rare.…”

Two-Dimensional Massless Dirac Fermions in Antiferromagnetic AFe2As2 Chen1, Wang2, Song3 et al. 2017Phys. Rev. Lett.286220View full textAdd to dashboardCite

“…However, the formation of magnetic order is always accompanied with time-reversal symmetry breaking and sometimes followed by crystalline symmetry lowering. Therefore, it is a challenge to achieve MDF in magnetic ground states of solid-state electronic systems [11,12]. Moreover, two-dimensional (2D) MDF, which were realized in 2D materials and on the surfaces of threedimensional (3D) topological insulators [1][2][3][13][14][15][16][17], have rarely been observed in the bulk of 3D crystals [11].…”

Topological Hopf and Chain Link Semimetal States and Their Application to Co2MnGa

“…Their unique topological properties are predicted to give rise to a wide range of exotic transport and optical phenomena [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. By considering various rotational and mirror symmetries in both symmorphic and non-symmorphic contexts, researchers have predicted nodal-line semimetals [38], higher charge double Weyl states [39,40], eightfolddegenerate double Dirac fermions [41], non-symmorphic nodal-chain metals [42], the single threefold-and doubled sixfold-degenerate spin-1 Weyl points [43], nexus fermions [44][45][46], Kramers Weyl fermions [47], and magnetic Dirac semimetals [48][49][50]. Despite this diversity, topological semimetals can be further classified and characterized by the dimensionality of their band crossings in the bulk Brillouin zone (BZ).…”