Possible bipolar effect inducing anomalous transport behavior in the magnetic topological insulator Mn( Bi 1 − x Sb x ) 2 Te 4

${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ has attracted tremendous interest as the first discovered intrinsic magnetic topological insulator. Due to its heavily $n$-doped nature, hole doping is required to isolate the effects of the topological states from the bulk, which is necessary for further electronic applications. Here, we systematically measure the resistivity, Seebeck coefficient, and thermal conductivity of $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}\phantom{\rule{0.16em}{0ex}}(0\phantom{\rule{0.16em}{0ex}}\ensuremath{\le}x\ensuremath{\le}0.51)$ single crystals. We find that the carrier concentrations at room temperature can be continuously tuned from $\ensuremath{-}9.47\ifmmode\times\else\texttimes\fi{}{10}^{19}$ to $5.21\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ while varying $x$ from 0 to 0.51. In the crystals with the Fermi level located close to the charge neutral point in the bulk band gap, drastic changes in the resistivity, Seebeck coefficient, and thermal conductivity are observed around a certain temperature ${T}^{*}$. Our results suggest that the bipolar effect possibly plays an important role in determining the transport properties in narrow bulk band topological insulators when the Fermi level is located near the charge neutral point inside the bulk gap.