Nova, Tobia

Date:   Wednesday, Mar. 20, 2019
Time:   11:00
Place:   ETH Zurich, Hönggerberg, HPF G 6
Host:    Ataç İmamoğlu

Nonequilibrium control of ferroic phases by optically-driven phonons

Tobia Nova
Max Planck Society, Germany

Intense light fields at terahertz and mid-infrared frequencies couple resonantly to the vibrational modes of solids, driving them into a regime of large amplitudes and nonlinear responses. Under such conditions, mode-selective excitation has proven to be an efficient route to control materials with strongly correlated electrons, capable of stimulating insulator-metal transitions and enhancing superconductivity.
In this talk, I will show how these principles can also be applied to control the ferroic properties of insulating oxides.
First, I will report on an experiment in which phonon excitation is used to induce and stabilize a dormant ferroelectric phase which is otherwise thermally inaccessible. Strontium Titanate (SrTiO3) is a quantum paraelectric where dipolar correlations grow with decreasing temperatures, albeit a long-range ferroelectric order never sets in. However, the nonlinear excitation of lattice vibrations will be shown to break inversion symmetry and induce a metastable ferroelectric phase in SrTiO3, as evidenced by the appearance of a large second-order optical nonlinearity, a soft-mode-like behavior and a closed-circuit electrical response.
I will then show that circularly polarized phononic fields can directly couple to the magnetization of a canted antiferromagnet. By simultaneously exciting pairs of non-degenerate orthogonal phonons, we drive rotations of the crystal-field ions. The time-reversal-breaking atomic motion mimics the application of a magnetic field pulse that is manifested in the excitation of coherent spin precession in the orthoferrite ErFeO3.