Polaronic transport and thermoelectricity in Mn 3 Si 2 Te 6 single crystals

We carried out a comprehensive study of the structural, electrical transport, thermal, and thermodynamic properties in ferrimagnetic ${\mathrm{Mn}}_{3}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ single crystals. Mn and Te $K$-edge x-ray absorption spectroscopy and synchrotron powder x-ray diffraction were measured to provide information on the local atomic environment and the average crystal structure. The dc and ac magnetic susceptibility measurements indicate a second-order paramagnetic to ferrimagnetic transition at ${T}_{c}\ensuremath{\sim}74$ K, which is further confirmed by the specific heat measurement. ${\mathrm{Mn}}_{3}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ exhibits semiconducting behavior along with a large negative magnetoresistance of $\ensuremath{-}87%$ at ${T}_{c}$ and a relatively high value of thermopower up to $\ensuremath{\sim}10$ mV/K at 5 K. Besides the rapidly increasing resistivity $\ensuremath{\rho}(T)$ and thermopower $S(T)$ below 20 K, the large discrepancy between the activation energy for resistivity ${E}_{\ensuremath{\rho}}$ and thermopower ${E}_{S}$ above 20 K indicates the polaronic transport mechanism. Furthermore, the thermal conductivity $\ensuremath{\kappa}(T)$ of ${\mathrm{Mn}}_{3}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ is notably rather low, comparable to ${\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$, and is strongly suppressed in the magnetic field across ${T}_{c}$, indicating the presence of strong spin-lattice coupling, also similar with ${\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$.