THz-field-induced multiferroicity in quantum paraelectrics - a review

Stanislav Kamba ¹ Christelle Kadlec ¹ Filip Kadlec ¹ Dalibor Repček ¹ André Maia ¹ Sergey Kovalev ² Jan-Christoph Deinert ² Igor Ilyakov ² Martina Basini ³ Stefano Bonetti ^3,4

¹Department of Dielectrics, Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic
²Free Electron Lasers FELBE, Helmholz Zentrum Dresden-Rossendorf, Dresden-Rossendorf, Germany
³Ultrafast Dynamics in Condensed Matter, Stockholm University, Stockholm, Sweden
⁴Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Venice, Italy

Recently, it has been demonstrated that THz pumping of optical phonons can transiently change the crystal structure, opening the possibility to flip the ferroelectric polarization or magnetization, or even induce transient superconductivity near room temperature. These phenomena can be explained by a nonlinear coupling between the resonantly driven polar phonon and another Raman-active mode. Here we will mainly focus on our recent experiments demonstrating a transiently induced ferroelectric and magnetic state in quantum paraelectric crystals KTaO₃, SrTiO₃ and (Eu_0.3Ba_0.1Sr_0.6)TiO₃.

Juraschek and Spaldin[1] predicted theoretically that circularly polarized phonons should carry an orbital magnetic moment even in diamagnetic or paramagnetic materials. The measurable orbital magnetic moment should be a consequence of exciting two perpendicularly polarized degenerate polar phonons using a circularly polarized THz radiation, and the highest orbital magnetic moment is expected to be linked with soft phonons in some perovskites.

In the quantum paraelectric KTaO₃, we pumped its ferroelectric soft mode using intense pulses generated by a free-electron laser (0.7 THz), and we measured the magnetization using the Faraday effect (800 nm probing wavelength). Below 100 K, we detected a transient magnetic moment on the picosecond time scale. Simultaneously, we detected a THz-field-induced second harmonic signal, demonstrating the breaking of inversion symmetry typical of ferroelectrics, and implying that suitable THz pumping brings KTaO₃ into a transient multiferroic state.

A similar result was obtained recently in SrTiO₃ at 300 K[2], and our experiments of this kind on (Eu_0.3Ba_0.1Sr_0.6)TiO₃ are in progress. In summary, we demonstrated that a transient multiferroicity can be induced by an excitation of polar phonons using intense pulses of THz radiation.

[1] D.M. Juraschek, N.A. Spaldin, Phys. Rev. Mat., 3, 064405 (2019)
[2] M. Basini et al. arXiv:2210.01690