Phase Transition Diagnostic in Iron Telluride by Nonlinear Optical Experiments

We have applied non-linear optical method (second harmonic generation (SHG) and third harmonic generation (THG)) to monitoring the phase transformation in the iron telluride compounds. The experiment was performed in the reflected geometry in which Temperature dependences of the SHG and THG have unambiguously shown that both methods are sensitive to the occurrence of the anomalies in the SHG and THG features near the phase transitions. However, the temperature positions of maxima are different, which may reflect a fact that in these two techniques the different multi-excited states interact differently with the critical bosons near the phase transitions. The role of the surface is crucial here because the surfaces are very sensitive to the presence of oxygen.