A Charge Density Wave Transition in Na_xMnBi_y magnetic semimetal

The I-Mn-V antiferromagnet, NaMnBi, was recently reported to develop a very large magnetoresistance (MR) up to 10,000 % at 2 K under a magnetic field of 9 T. A strong positive MR is present in crystals showing a semiconductor-to-metal transition (SMT) while, in the absence of an SMT, a modest (20 %) MR is achieved. Here, we show that upon cooling below the magnetic transition, a spatial modulation appears due to charge and defect ordering in a checkerboard pattern, with two kinds of displacement vectors q₁=(2/3, 0, 1) and q₂=(2/3, 1/3, 1/2). This couples to a superlattice transition (T_s) that lowers the symmetry from centrosymmetric P4/nmm to non-centrosymmetric P-4m2. In crystals with a large MR a close to room temperature T_s is observed with both q₁ and q₂ present. In crystals with low MR however, T_s is lower and only q₁ is observed. T_s breaks spatial inversion symmetry and marks the onset of a charge density wave instability.