Bulk quantum Hall effect of spin-valley coupled Dirac fermions in the polar antiferromagnet BaMnSb2

Abstract

Unconventional features of relativistic Dirac/Weyl quasi-particles in topological materials are most evidently manifested in the 2D quantum Hall effect (QHE), whose variety is further enriched by their spin and/or valley polarization. Although its extension to three dimensions has been long-sought and inspired theoretical proposals, material candidates have been lacking. Here we have discovered valley-contrasting spin-polarized Dirac fermions in a multilayer form in bulk antiferromagnet BaMnSb 2 , where the out-of-plane Zeeman-type spin splitting is induced by the in-plane inversion symmetry breaking and spin-orbit coupling (SOC) in the distorted Sb square net. Furthermore, we have observed well-defined quantized Hall plateaus together with vanishing interlayer conductivity at low temperatures as a hallmark of the half-integer QHE in a bulk form.The Hall conductance of each layer is found to be nearly quantized to 2(N+1/2)e 2 /h with N being the Landau index, which is consistent with two spin-polarized Dirac valleys protected by the strong spin-valley coupling. arXiv:2001.08683v1 [cond-mat.str-el] 23 Jan 2020 Researches of topological materials have currently been one of the central topics of the condensed matter physics. Their topologically non-trivial electronic structure leads to the relativistic quasiparticles, Dirac/Weyl fermions, whose most prominent feature is seen in QHE in 2D systems[1], such as the relativistic QHE in graphene[2, 3] and topological insulator films[4]. The half-integer quantization of the Hall plateaus and the zero-energy Landau level forming at the charge neutral Dirac point were experimentally clarified, which are associated with the Berry phase of Dirac fermions and hence have no analog in conventional 2D systems. More recently, the variety of QHE in topological materials has been further expanded by utilizing the spin and/or valley polarization in the system[5-19K21851, JP19H05173) and the Asahi Glass Foundation. The synchrotron radiation experiments were performed at the BL25SU of SPring-8 with the approval of the Japan Synchrotron

Tunable spin-valley coupling in layered polar Dirac metals

“…Therefore, it is plausible that the valence bands along the Γ-M line do not cross ϵ F in reality. Thus, the obtained Dirac valleys (α and β) for BaMnBi 2 are totally different from those for BaMnSb 2 ; the latter hosts only two equivalent valleys near the Y point along each M-Y line 19,20 . This suggests that the spin-valley coupling of the Dirac fermions in BaMnX 2 is highly sensitive to the lattice distortion and the SOC.…”

“…However, in BaMnSb 2 , the Sb − layer is slightly distorted to an orthorhombic one with zig-zag chains, leading to lattice polarization along the plane. Together with the SOC of Sb, this results in the formation of two Dirac valleys with valley-contrasting out-of-plane spin polarizations around the Y point, as was revealed by first-principles calculation and angle-resolved photoemission spectroscopy 19 .…”

“…1a) was determined by single-crystal X-ray structural analysis (for details see Methods, Supplementary Methods, and Supplementary Table 1). The obtained result shows that BaMnBi 2 has a polar orthorhombic structure with the same space group as BaMnSb 2 (Imm2), whereas the orthorhombicity (c − a)/a is estimated to be~0.15%, which is only 1/10 that of BaMnSb 2 (~1.3%) 19 . Despite such a small orthorhombicity, twin domains are clearly observed with a polarized microscope at room temperature (>290 K), as shown in Fig.…”

“…In the inset to Fig. 3d, we plot the in-plane conductivity σ xx ¼ ρ xx =ðρ 2 xx þ ρ 2 yx Þ and interlayer conductivity σ zz = 1/ρ zz as a function of B F /B, where B F is the SdH frequency and B F /B corresponds to the filling factor normalized by the spin-valley degeneracy factor 19,36 . Importantly, the SdH oscillations of σ xx and σ zz resemble each other well, including the fine structures at high fields (B F /B < 0.5).…”

“…Recently, the layered Dirac material BaMnSb 2 was found to have a polar structure with broken in-plane inversion symmetry in the bulk form 19,20 . BaMnSb 2 belongs to the AMnX 2 (A: alkaline earth, X: Bi, Sb) series of compounds [21][22][23][24][25][26][27] , in which each compound consists of a A 2+ -Mn 2+ -X 3− blocking layer and a X − Dirac fermion layer (see Fig.…”