Floquet-Engineered Topological Transport in Quantum Materials

Quantum materials exhibit remarkable phenomena when driven by the strong fields in femtosecond pulses of light. Examples range from the emergence of metastable hidden phases in complex oxides [1] and dichalcogenides [2] to signatures of light-induced superconductivity in cuprates [3] and fullerides [4]. Recent years have seen a surge of interest in using long-wavelength laser pulses to create photon-dressed “Floquet-Bloch” states in quantum materials [5]. Much of this excitement is driven by the predictive power of Floquet theory, which has been used, for example, to correctly predict the formation of topological edge states in periodically-driven systems that exhibit no topological properties in equilibrium [6-7]. Many of these proposals have been verified in quantum simulation settings [8-9], but are only just beginning to be explored in quantum materials [10].<br/>In this talk, I will present results on the electrical transport properties of quantum materials driven by ultrafast pulses of mid-infrared light. These results were achieved using a device architecture based on laser-triggered photoconductive switches and waveguides, which allows transport measurements on microstructured quantum materials to be performed on femtosecond timescales. In monolayer graphene, we observe an anomalous Hall effect induced by circularly polarized light in the absence of an applied magnetic field [11]. The dependence of the effect on a gate potential used to tune the Fermi level reveals multiple features that reflect a Floquet-engineered topological band structure. The results are a critical first step towards realizing and controlling light-induced topological edge states in quantum materials.<br/>In the second part of the talk, I will discuss our recent results on the Weyl semimetal MoTe₂. We observe a rectified, circular dichroic photocurrent response that scales linearly (as opposed to quadratically) with the applied laser field, which is consistent with the formation of Floquet-Bloch states in this material. Furthermore, we find that when pumping the material strongly enough, the interlayer shear phonon mode couples to the transport properties of the out-of-equilibrium states. Pumped above a certain threshold, this phononic motion induces a permanent structural phase transition that changes the band structure topology, which we directly probe in the nonlinear transport sector. Our results demonstrate multiple ways in which ultrafast light-matter interaction can manipulate the topological responses of quantum materials via the breaking of symmetry.<br/> <br/><b>References</b><br/> <br/>[1] V. Kiryukhin <i>et al., Nature</i> <b>386</b>, 813-815 (1997)<br/>[2] L. Stojchevska <i>et al., Science</i> <b>344</b>, 177 (2014)<br/>[3] D. Fausti et al. <i>Science</i> <b>331</b>, 189-191 (2011)<br/>[4] M. Mitrano <i>et al.,</i> <i>Nature</i> <b>530</b>, 461-464 (2016)<br/>[5] T. Oka & S. Kitamura, <i>Annual Review of Cond. Matt. Phys.</i> <b>10</b>, 387-408 (2019)<br/>[6] T. Oka & H. Aoki. <i>Phys. Rev. B</i> <b>79</b>, 081406 (2009)<br/>[7] T. Kitagawa <i>et al., Phys. Rev. B </i><b>84</b>, 235108 (2011)<br/>[8] G. Jotzu <i>et al.,</i> <i>Nature</i> <b>515, </b>237–240 (2014)<br/>[9] M. Rechtsman <i>et al., Nature </i><b>496, </b>196–200 (2013)<br/>[10] Y.H. Wang <i>et al.,</i> Science <b>342</b>, 453 (2013)<br/>[11] J.W. McIver <i>et al., Nature Physics </i><b>16</b>, 38-41 (2020)