THz-field-induced transient magnetization and electric polarization in quantum paraelectric diamagnet KTaO₃

Juraschek [1] predicted a transient magnetization induced in non-magnetic systems by pumping two degenerate perpendicularly polarized optical phonons with a circularly polarized THz beam. Juraschek [1] and Dunnet [2] also proposed it should be possible to induce a transient multiferroicity in materials close to their ferroelectric quantum critical points, since fluctuating electric dipoles linked to the soft phonon could induce magnetization, whereas THz pumping of the soft mode would break the inversion symmetry, inducing a transient ferroelectric polarization.

No experimental confirmation of the predicted transient magnetization has been published so far. Recently, at the TELBE beamline, we conducted THz-pump, optical probe experiments with the quantum paraelectric KTaO₃, which revealed a transient magnetization and ferroelectricity.

The discovered magneto-optic Faraday effect is much stronger than expected from the theory. Moreover, the THz pump-induced transient birefringence was detected not only using a circularly polarized THz beam pump, but also upon pumping with a linearly polarized beam. We explain this by the THz field-induced electro-optic Kerr effect. Since the excited phonons were polarized in the sample plane, the induced magnetization should be perpendicular to the sample plane. Then, in theory, the magnitude of the transient birefringence should be the same after pumping using clockwise and anticlockwise circularly polarized THz radiation, which is not the case. We explain our observations by a combination of Faraday and Kerr effects, together with the directional dichroism which is allowed in non-centrosymmetric diamagnetic systems in the presence of magnetic field [3]. The observed effects are most prominent at temperatures where the pump pulse is resonant with the soft phonon frequency.

[1] D. M. Juraschek et al., Phys. Rev. Mat. 1, 014401 (2017).

[2] K. Dunnett et al. Phys. Rev. Lett. 122, 057208 (2019).

[3] A. I. Nikitchenko and R. V. Pisarev, Phys. Rev. B 104 (2021).