Overall, the project was a great success. Here I will include an overview of some of our most significant/impactful results.
First, in collaboration with partners at the Technion Institute of Technology and the Weizmann Institute of Science, we discovered a new regime of universal dynamics in periodically-driven many-body systems. One of the biggest challenges that arises when strong laser or microwave driving fields are used to control quantum systems is that these control fields tend to heat up the system and destroy all of its fragile quantum mechanical characteristics or behaviors. In our work, we flipped this conventional wisdom on its head, and showed that, under appropriate conditions, the heating that naturally accompanies driving can actually be used as a resource, which pushes the system into a novel regime where new robustly quantized quantum transport phenomena can be observed. This work is published in Physical Review X.
Another one of the major goals of this project was to elucidate the role of electron-electron interactions in schemes where optical fields are used to modify a system's electronic properties. As a prototypical example, graphene has received wide attention for its wide range of outstanding electrical, mechanical, thermal, and optical properties. Previously, in a simplified setting where the (naturally strong) interaction between electrons in the material is ignored, it was shown that circularly polarized laser light could be used to dynamically induce a band gap in graphene, opening the potential for greater functionality. In a paper published in Physical Review Letters, we presented the first study of the effects of electron-electron interactions on light-induced gap opening in graphene and graphene-like systems. We identified promising parameter regimes where optically-induced dynamical gap opening can be observed experimentally, and provided a detailed characterization of the competing processes and timescales that must be considered to successfully implement this approach in experiments. To facilitate this study, we developed a new theoretical approach which formed the basis for an extensive set of numerical simulations, and will enable the community to undertake future studies of dynamics in driven electronic systems.
Interacting many-body systems support collective modes of excitation, which may have properties utterly unlike those of the microscopic constituent particles. When such collective modes are excited, the system may host strong oscillating internal fields, associated with the restoring force that sustains the collective oscillation. This property is used extensively in the field of nanoplasmonics, where collective charge density oscillations are routinely used to compress and enhance electric fields by many orders of magnitude over the values that can be applied externally. In a work with collaborator Justin Song at NTU in Singapore which was published in Nature Physics, we showed that such internal fields can indeed modify the electronic structure of a metallic system, thus altering its response characteristics to external driving fields. This feedback gives rise to nonlinear collective mode dynamics, which we showed can lead to novel types of non-equilibrium phase transitions and spontaneous symmetry breaking. In this work we made detailed estimates and showed that the phenomena that we describe should be observable using present day high quality graphene devices and laser sources.
While the topology of non-interacting driven systems is now reasonably well understood, a crucial outstanding question remains: under what conditions does a driven many-body system occupy its dynamically-induced "Floquet states" in such a way that robust band topology is reflected in observables? To address this question we undertook several wide-ranging studies of the steady states of driven semiconductors, identifying key parameter regimes, observables, and novel phases/phenomena that may emerge.
