Special LASSP & AEP Seminar: Matt Day (Max Planck Institute)

Optical Control over Topological Transport in Quantum Materials

Driving with strong, periodic electromagnetic fields, known as Floquet engineering, is a powerful tool to manipulate the properties of quantum materials. Using circularly polarized light, artificial magnetic fields can be created in the photon-dressed Floquet-Bloch states that form. This mechanism, when applied to 3D Dirac and Weyl systems, is predicted to lead to photon-dressed movement of Weyl nodes which should be detectable in the transport sector. Using ultrafast transport methodology, we find direct evidence of transport from Floquet-Bloch states in the type-II Weyl semimetal Td-MoTe2. In our measurements we observed injection currents, and a helicity-dependent anomalous Hall effect whose scaling with laser field strongly deviate from the perturbative expectation of nonlinear optics. We show using Floquet theory that this discovery corresponds to the formation of a magnetic Floquet-Weyl semimetal state. Numerical ab initio simulations support this interpretation, indicating that the light-induced motion of the Weyl nodes contributes significantly to the measured transport signals. Moreover, from the amplitude of the light-induced anomalous Hall effect, we deduce that these photon-dressed states produce a large effective non-Maxwellian field (>30 T) during the drive.

In addition to modifying the topological transport properties during the field, we also discovered that mid-infrared light can efficiently excite coherent interlayer shear phonons which we can capture directly in the transport sector. We measure coherence between the phonon oscillation and the nonlinear Hall effect, which we interpret as an oscillation of the Weyl node positions in the Brillouin zone. Although this work is ongoing, it demonstrates the potential of using light to impose synthetic time-domain symmetries on quantum materials. Future pathways to optically create and control dissipation-free edge states will be discussed.

Bio:
Matt Day is a Humboldt Postdoctoral Fellow at the Max Planck Institute for the Structure and Dynamics of Matter. He specializes in the nonlinear optical and transport properties of quantum materials. He completed his undergraduate at Boulder and then went on to pursue a Ph.D. at the University of Michigan. There, with Steve Cundiff’s group, he developed battery-powered micro-frequency combs for precision spectroscopy. Toward the end of his Ph.D., he used light to control interactions between color centers in diamond, characterized using 2D spectroscopy. Through this, he became interested in the optical control over quantum materials, and subsequently moved to MPSD with James McIver’s group. There he helped develop on-chip THz spectroscopy and is currently studying the topological transport properties of Floquet-Bloch states in van der Waals materials.