Ab-initio Prediction of Gigantic Anomalous Nernst Effect in Ferromagnetic Monolayer Transition Metal Trihalides.

The anomalous Hall conductivity of all transition metal trihalides was explored using first-principles calculations. Employing the Fukui-Hatsugai-Suzuki method, we found that ferromagnetic monolayers XBr3 (X = Pd, Pt) possessed the quantized anomalous Hall conductivity (QAHC) with and without carrier doping. Due to unique QAHC, their transverse thermoelectric properties of XBr3 (X= Pd, Pt) were investigated. Employing the semi-classical Boltzmann transport theory, the transverse thermoelectric coefficient of each monolayer was analyzed. Anomalous Nernst coefficients (ANCs) of the XBr3 monolayers were prominent both at and near
the Fermi level. Under an assumed relaxation time of 10 fs, the maximum ANCs for the PdBr3 (PtBr3) monolayer reached -54.1 (-23.3) μV/K at T = 300 K upon doping with 1.21 × 1014 (5.64 × 1013) holes/cm2. The large ANCs of the XBr3 monolayers
were attributed to the opening of a narrow bandgap generated by spin-orbit coupling both at and near the Fermi level, which led to a large Seebeck-induced charge current and large anomalous Nernst conductivity. These results suggest that ferromagnetic XBr3 monolayers have significant potential for application in thermoelectric devices.