Thermoelectric and Thermal Conductivity Studies on Mn(Bi_1-xSb_x)₂Te₄

MnBi₂Te₄ is the first established intrinsic antiferromagnetic topological insulator [1], which can support various topological quantum states such as quantum anomalous Hall insulator and axion insulator in 2D thin layers. Theory predicts it can also host an ideal type-II Weyl state in its ferromagnetic (FM) phase [2,3]. While the FM phase of MBT is accessible through the spin flop/flip transition under magnetic fields, the predicted Weyl state is not observed in pristine MBT. Recently, Lee et al. [4] found the ideal Weyl state can be achieved by tuning the Sb concentration in Mn(Bi_1-xSb_x)₂Te₄. In this talk, we will report thermopower and thermal conductivity studies on the lightly hole-doped samples which display the Weyl state. We find the thermopower under the magnetic fields above the spin flip transition field (H_c2) is clearly suppressed when the material undergoes the paramagnetic-to-FM transition, consistent with the expected electronic structure transition driven by the magnetic fields. Furthermore, our experiments also showed the linear thermal conductivity enhancement above H_c2 is much more significant for H//c than for H//ab, which is in line with the prediction that the Weyl state is present for H//c but not for H//ab.