Magnetism and symmetry-lowering phase transition in topological Weyl semimetal candidates AMnX$_2$ (A=Ca, Yb; X=Bi,Sb)

We report polarized neutron diffraction studies on single crystals of Dirac semimetals and Weyl semimetal candidates CaMnBi2, YbMnSb2, and YbMnBi2. ARPES investigations postulated the presence of the canted antiferromagnetic (AFM) state in YbMnBi2, which breaks the time reversal symmetry (TRS) and induces type-II Weyl state (WS). However, neutron diffraction does not find any indication of the proposed spin canting thereby questioning the existence of WS in the bulk. In a relative Dirac semimetal, CaMnBi2, spin-canted AFM state with broken TRS leading to WS was inferred at low temperature, T<Ts, from the studies of the optical properties and magnetic torque. The proposal was aimed to explain the gapping of the Fermi surface below Ts observed in optical reflectivity, which accompanies the resistivity anomaly in the same temperature range, below Ts ≈ 46 K. In our neutron diffraction work we find no evidence for the proposed spin canting. Instead, we uncover a combined structural and magnetic transition at Ts, where the structure of CaMnBi2 changes from high-temperature tetragonal P4/nmm with C-type magnetic ordering described by P4/n′m′m space group to orthorhombic Pnma with magnetic ordering of space group Pn′m′a′. The transition is manifested by the appearance, at T < Ts, of the superlattice Bragg reflections of (H,0,L + 0.5) type (H ̸= 0 and L are integers), which have similarly-sized nuclear and magnetic components. A similar structural distortion was recently reported in YbMnSb2, where it persists to high temperatures above magnetic ordering, TN ≈ 350 K, which led authors to postulate that YbMnSb2 crystallizes in an orthorhombic Pnma structure and the resultant lower crystal symmetry governs the electronic properties of the material. On the contrary, in CaMnBi2 we find that lowering of the crystal and magnetic symmetry happens at low temperature, in the magnetically ordered phase, and therefore belongs to the realm of low-energy physics governed by electronic interactions. The refined atomic displacements and the magnitude of the distortion parameter in CaMnBi2 comparable to that in iron-based superconductors are suggestive of electronic origin of the observed phase transition, such as orbital ordering or electronic nematicity.