Electronic Phase Transition of IrTe_2 Probed by Second Harmonic Generation

We have utilized second harmonic generation (SHG) to disentangle the coupled first-order charge order/structural transition at T_\rm c\sim281 K in the transition-metal dichalocogenide IrTe_2, an exceptional layered material with 3D properties. The data from SHG shows extremely sharp transition in both the cooling and warming processes with less than 0.2 K transition window. Surface electronic symmetries of C_3v and S_2 are observed in the high temperature and low temperature phases, respectively. Compared to neutron diffraction data for the structural transition (Phys. Rev. B 88 (2013) 115122) and to the electrical resistivity for the microscopic transition (Phys. Rev. B 95 (2017) 035148), our data indicates the electronic transition at the surface is the precursor to the structural transition.