Thermal transport in CuCr2X4 (X= S, Se, Te): Experiment and ab initio calculations

We present a combined experimental and theoretical study of the longitudinal and transverse thermoelectric properties of ferrimagnetic spinels CuCr2X4 (X = S, Se, Te). The thermoelectric power of all studied phases is positive and consists of two contributions; an almost linear diffusive thermopower observed at high temperature, and a strong enhancement below T-C attributed to magnon drag. The diffusive part of thermopower and resistivity decreases from X = S to Te, whereas the magnon drag enhancement is the biggest for X = Se, which has also the highest T-C. The thermopower was calculated by DFT method using GGA, GGA U, GGA+oeeHyb, and mBJ potentials. A good agreement with the experimental thermopower was achieved using GGA potential for X = S and with mBJ potential for X = Te. The mBJ potential, which was designed for sp-type semiconductors, better describes the valence bands of CuCr2X4 that exhibit stronger sp-orbitals character than sulfide and selenide. The anomalous Nernst effect (ANE) is negative at room temperature for all phases, the highest absolute value similar to 1.5 mu V/K is observed for X = Te and Se around room temperature, whereas ANE for X = S is much smaller. A sign change of ANE to positive is observed at 285 K for X = S and at 65 K for X = Se. The trend of anomalous Nernst conductivity is reproduced by Berry phase calculations if the renormalization of the bands due to the strong correlation effects is taken into account.