Toward Multimodal Control Over Quantum Phenomena

Quantum phases and phase transitions are exquisitely sensitive to coupled degrees of freedom that mediate key properties and dynamical behavior. Control over the relevant modes through applied pulsed fields can provide fundamental mechanistic insights and offer intriguing prospects for applications involving either transient or persistent induced responses. Electromagnetic and strain fields, applied through optical and acoustic pulses respectively, can exert profound effects on quantum materials.<br/><br/>Terahertz light fields are extremely well matched to the excitations and dynamics characteristic of many cooperative systems. Strong THz fields can induce responses that may reveal important mode-mode couplings, hidden quantum phases, and other phenomena of interest. 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.<br/><br/>Many quantum (and other crystalline) phase transitions involve changes in the lattice parameters. In such cases, strain can play a key role in controlling which phase is present. We have developed a facile method for laser generation of compressional or surface acoustic wave (SAW) shocks [5] that focus in the plane of a sample to reach stresses sufficient to cause material fracture, chemical decomposition, and other effects [6-8]. We have demonstrated a SAW-induced insulator-to-metal persistent phase transition in V₂O₃, extending prior study of a reversible transition induced by quasi-static high pressure [9]. In very recent work, we have developed a method for non-destructive generation of large-amplitude acoustic waves [10]. The approach permits repeated shocks to be delivered to the same sample region, and has been used to observe cumulative effects (i.e. fatigue) caused by hundreds or thousands of shocks. The shocks also may 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 measurement capabilities offers fascinating prospects for fundamental study and potential practical applications of collective quantum dynamics.<br/><br/>[1] J. Shi, et al., <i>arXiv</i>:1901.13609 (2019); <i>Nat. Commun</i>., in press (2022).<br/>[2] F. Y. Gao, et al., <i>Sci. Adv. </i><b>8</b>, eabp9076 (2022).<br/>[3] X. Li, T. Qiu, J. Zhang, E. Baldini, J. Lu, A. M. Rappe, and K. A. Nelson, <i>Science</i> <b>364</b>, 1079-1082 (2019).<br/>[4] Z. Zhang, et al., <i>arXiv</i>:2207.07103 (2022).<br/>[5] T. Pezeril, G. Saini, D. Veysset, S. Kooi, R. Radovitzky, and K.A. Nelson, <i>Phys. Rev. Lett</i>. <b>106</b>, 214503 (2011).<br/>[6] D. Veysset, T. Pezeril, S. Kooi, A. Bulou, and K.A. Nelson, <i>Appl. Phys. Lett</i>. <b>106</b>, 161902 (2015).<br/>[7] D. Veysset, et al., <i>Scr. Mater.</i> <b>158</b>, 42-45 (2019).<br/>[8] L. Dresselhaus-Cooper, et al., <i>J. Phys. Chem. A</i> <b>124</b>, 3301-3313 (2020).<br/>[9] D. Babich, et al., <i>arXiv</i>:2105.05093 (2021).<br/>[10] J. Deschamps, et al., <i>arXiv</i>:2209.13897 (2022).