Crystal-surface polarity enables nonreciprocal transport in a centrosymmetric kagome magnet

Overview & Synopsis

Every crystal surface breaks inversion symmetry even when the bulk is centrosymmetric, creating an intrinsic boundary polarity ($\mathbf{\mathit{P}}$). Here we show that this polarity enables zero-field nonreciprocal transport in the centrosymmetric kagome magnets TbMn$_6$Sn$_{6-x}$In$_x$. In symmetric TbMn$_6$Sn$_{4.4}$In$_{1.6}$ microdevices, opposing side surfaces exhibit second-harmonic longitudinal resistances of comparable magnitude and opposite sign. Reversing the spontaneous magnetization ($\mathbf{\mathit{M}}$) reverses both signals, whereas rotating $\mathbf{\mathit{M}}$ so that the toroidal moment $\mathbf{\mathit{P}} \times \mathbf{\mathit{M}}$ has no component along the current suppresses the response. In pristine TbMn$_6$Sn$_6$, the signal switches off across the spin-reorientation transition, providing a chemically ordered control of the same symmetry relation. Spatial potential mapping indicates that the nonlinear boundary contribution is distinct from the bulk linear conduction. Spin-orbit-coupled surface states associated with topological bands may enhance the effect. Crystal-surface polarity therefore offers a broadly applicable route to zero-field rectification in centrosymmetric magnetic conductors around room temperature.