Nonequilibrium quantum phenomena: A gilded age of driving fields and monitoring capabilities

Light at THz frequencies is extremely well matched to the excitations and dynamics characteristic of many cooperative systems. Strong THz fields can be used for control as well as nonlinear spectroscopy of the collective responses of primary interest in quantum materials. Recent results involving collective electronic, vibrational, and spin degrees of freedom will be presented. A THz-induced transition into a topological insulator phase of the transition metal dichalcogenide MoTe₂ will be discussed. A single THz pulse drives the phase transition electronically, after which the new phase persists indefinitely [1]. A persistent optically-induced change in TaS₂ will also be discussed briefly. The transition is monitored on a single-shot basis with both optical and THz probe light [2]. Strong THz fields drive a “soft” lattice vibrational mode to induce a transition from the quantum paraelectric phase of SrTiO₃ to a transient ferroelectric phase [3]. Recent ultrafast x-ray diffraction measurements provide information beyond what could be determined through optical probes. Finally, THz-induced nonlinear responses of collective spin waves (magnons) have revealed coupling that is inherent in canted antiferromagnetic materials [4]. Two-dimensional THz spectroscopy, conducted using single-shot readout of the time-dependent signal field, reveals the coupled spin responses.

Many quantum (and other crystalline) phase transitions involve collective changes in the lattice parameters. We have developed a method for non-destructive generation of large-amplitude acoustic waves [5] that can be used to drive phase transitions by themselves or in conjunction with THz or optical excitation. The combination of multimodal excitation and control with probing from THz to x-ray spectral ranges and with real-time single-shot measurements can seem like a gilded age for fundamental study and potential practical applications of collective dynamics.



[1] J. Shi, et al., arXiv:1901.13609 (2019); Nat. Commun., in press (2022).

[2] F. Y. Gao, et al., Sci. Adv. 8, eabp9076 (2022).

[3] X. Li, et al., Science 364, 1079 (2019)

[4] Z. Zhang, et al., arXiv:2207.07103 (2022).

[5] J. Deschamps, et al., arXiv:2209.13897 (2022).