Polaronic transport and thermoelectricity in Mn${}_3$Si${}_2$Te${}_6$ single crystals

Abstract

We carried out a comprehensive study of the structural, electrical transport, thermal and thermodynamic properties in ferrimagnetic Mn${}_3$Si${}_2$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 $T_c$ $\sim$ 74 K, which is further confirmed by the specific heat measurement. Mn${}_3$Si${}_2$Te${}_6$ exhibits semiconducting behavior along with a large negative magnetoresistance of -87\% at $T_c$ and relatively high value of thermopower up to $\sim$ 10 mV/K at 5 K. Besides the rapidly increasing resistivity $\ddot{I}(T)$ and thermopower $S(T)$ below 20 K, the large discrepancy between activation energy for resistivity $E_{\ddot{I}}$ and thermopower $E_S$ above 20 K indicates the polaronic transport mechanism. Furthermore, the thermal conductivity $\hat{I}º(T)$ of Mn${}_3$Si${}_2$Te${}_6$ is notably rather low, comparable to Cr${}_2$Si${}_2$Te${}_6$, and is strongly suppressed in magnetic field across $T_c$, indicating the presence of strong spin-lattice coupling, also similar with Cr${}_2$Si${}_2$Te${}_6$.