Studied by Optical Second Harmonic Generation*

Optical second harmonic generation (SHG) is a powerful and versatile technique used to study the electronic symmetry of surfaces and interfaces. Transition metal compounds, such as IrTe₂, are ideal candidates for broken symmetry studies. X-ray diffraction measurements of IrTe₂ reveal that when cooling below T_c,bulk ~ 280 K a first order structural bulk phase transition from 1x1x1 (at high temperatures) to 5x1x5 (at low temperatures) occurs. On the surface electron diffraction shows 1x1 to 5x1 phase change at T_c,surface ~ 270 K. To understand the nature of this transition, SHG coupled with azimuthal sample rotation is used to probe the phase change across T_c,surface. We observe the hysteresis loop of SHG intensity with a ΔT of ~ 3K, in agreement with the previous electron diffraction measurement. Four polarization combinations are utilized allowing sensitivity to both the in-plane (Te-Te bonds) and out-of-plane (Ir-Te bonds) surface responses. In addition, rotational anisotropy SHG measurements above and below T_c,surface yield distinct surface symmetry patterns. Comparing the results of SHG to bulk properties, key insight into the nature of the phase transition is obtained.