Circular photogalvanic effect in inversion symmetric crystal: the case of ferro-rotational order in Ta₂NiSe₅

Ta₂NiSe₅, an excitonic insulator candidate, exists in a novel broken symmetry phase below 327K. Strong electron-phonon interaction possibly due to an excitonic instability leads to a structural transition in this material into a ferro-rotational ordering of dipoles. Such an ordering, which preserves both spatial inversion and time-reversal symmetries, has been difficult to measure due to low coupling to dipolar fields. The second harmonic (SHG) signal arising from higher-order couplings with the optical field for such a system remains low. The effect of such an order on the optoelectronic transport properties of the system is investigated in our work. We use circular photogalvanic effect (CPGE), and by eliminating any lower-order contributions using symmetries of the system, show that ferro-rotational order can be directly probed. A phenomenological model will be discussed to understand the CPGE response along with the implications of the ferro-rotational order.