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<i>X</i>Br₃(<i>X</i>= Pd, Pt) possessed the quantized anomalous Hall conductivity (QAHC) with and without carrier doping. Due to unique QAHC, their transverse thermoelectric properties of<i>X</i>Br₃(<i>X</i>= 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<i>X</i>Br₃monolayers 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)<i>µ</i>V K^-1atT=300 K upon doping with 1.21 × 10¹⁴(5.64 × 10¹³) holes cm^-2. The large ANCs of the<i>X</i>Br₃monolayers 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<i>X</i>Br₃monolayers have significant potential for application in thermoelectric devices.