Light-field induced anomalous Hall effect in graphene*

Optical driving has been proposed as a means of engineering topological properties in topologically trivial systems. One proposal for such a “Floquet topological insulator” is based on breaking time-reversal symmetry in graphene through a coherent interaction with circularly polarized light [1]. This was predicted to lift the degeneracy of the Dirac point, opening a topological band gap in the resulting photon-dressed band structure accompanied by dressed chiral edge states [2]. While quantum simulation experiments have validated aspects of this proposal [3,4], and Floquet–Bloch bands have been observed in a topological insulator [5], the transport properties of such a light-induced topological state have remained elusive in a real material.

In this talk, I will report on our recent observation of a light-induced anomalous Hall effect in graphene driven by a femtosecond pulse of circularly polarized light [6]. Electrical transport was probed using an ultrafast device architecture based on photoconductive switches. The dependence of the anomalous Hall effect on a gate potential used to tune the equilibrium Fermi level revealed multiple features that reflect a Floquet-engineered topological band structure, similar to the band structure originally proposed by Haldane [7]. This included an approximately 60 meV wide conductance plateau centered at the Dirac point, where a gap of equal magnitude was predicted to open. We found that when the Fermi level was tuned within this plateau, the estimated anomalous Hall conductance saturated around 1.8+/-0.4 e^2/h.

[1] T. Oka & H. Aoki. Phys. Rev. B 79, 081406 (2009)
[2] T. Kitagawa et al. Phys. Rev. B 84, 235108 (2011)
[3] M.C. Rechtsman et al., Nature 496, 196 (2013)
[4] G. Jotzu et al., Nature 515, 237 (2014)
[5] Y.H. Wang et al., Science 342, 453 (2013)
[6] J.W. McIver et al. Nat. Phys. 16, 38 (2020)
[7] F.D.M. Haldane, Phys. Rev. Lett. 61, 2015 (1988)