Spin Nematic Liquid Crystal and Scalar Spin Chirality in Tetragonal Lattice YbMnBi 2

Abstract

A spin nematic order, analogous to a nematic liquid crystal, characterizes the spontaneous breaking of spin-space rotational symmetry while preserving time-reversal ( T ) symmetry. On the other hand, the order parameter characterizing the T -symmetry breaking is the composite three-spin order, i.e., the scalar spin chirality (SSC) χ i j k = ⟨ S i · ( S j × S k ) ⟩ , where S i , S j , S k are spins at neighboring sites i , j , k , respectively, and nonzero SSC is known to induce anomalous Hall effect (AHE). Although a spin nematic phase has been suggested in the frustrated magnets and the square-lattice iridate, how a spin nematic phase might affect magneto-transport properties is unknown. Here we use polarized neutron scattering to show that tetragonal lattice A MnBi 2 ( A = Ca , Yb) is a strictly c -axis-aligned collinear antiferromagnet ( C -type) with T N ≈ 270 and 290 K. On cooling from 450 K to T N , low-energy spin excitations in YbMnBi 2 spontaneously change from isotropic to anisotropic in spin space within the tetragonal plane, forming a dynamic spin nematic phase around 400 K due to heavy Yb-induced spin-orbit coupling, before gapping out below T N . Similar polarized neutron scattering measurements on CaMnBi 2 reveal isotropic paramagnetic scattering without a spin nematic phase above T N . Under an in-plane external magnetic field, the Yb 3 + moments may interact with the dynamic spin nematic phase to induce nonzero SSC, giving rise to AHE and anomalous Nernst effect (ANE) in YbMnBi 2 that is absent in CaMnBi 2 above T N . Theoretical analysis of Ginzburg-Landau theory based on the symmetry indicates that the coupling terms between the nematic order and SSC of 5 order in the spin operators are allowed under an external magnetic field B . This could explain the rapid increase of AHE as a function of B in YbMnBi 2 . Our results, therefore, provide compelling evidence for dynamic SSC-induced AHE and ANE in the paramagnetic phase of a compensated collinear antiferromagnet, opening a new avenue for the physics of composite orders of multiple spins for room-temperature spintronics without magnetic order.