At equilibrium, transport properties are key in revealing fingerprints of electronic interactions and long range orders in quantum materials. Conventionally, these are measured in the vicinity of zero frequency. We have developed a platform to extend these measurements to the ultrafast time scale. To this end, a microstructured chip composed of THz-waveguides and photo-conductive switches is used to generate and transport sub-picosecond current pulses. These pulses propagate through the material under study and are sampled, after transmission, yielding the transport properties from DC to 1 THz frequency. As this technique operates in the time domain, it also allows probing of transient states with sub-picosecond resolution, such as light-induced topological phases and non-equilibrium superconductivity. Additionally, probe current pulses with peak amplitudes in excess of tens of GA/m2 can be generated, giving access to nonlinear transport features such as critical currents in superconductors.
Light-induced anomalous Hall effect in graphene
J. W. McIver, B. Schulte, F.-U. Stein, T. Matsuyama, G. Jotzu, G. Meier and A. Cavalleri
Nature Physics, 16, 38–41( 2020)
Many non-equilibrium phenomena have been discovered or predicted in optically driven quantum solids. Examples include light-induced superconductivity and Floquet-engineered topological phases. These are short-lived effects that should lead to measurable changes in electrical transport, which can be characterized using an ultrafast device architecture based on photoconductive switches. Here, we report the observation of a light-induced anomalous Hall effect in monolayer graphene driven by a femtosecond pulse of circularly polarized light. 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. similar to the band structure originally proposed by Haldane. This includes an approximately 60 meV wide conductance plateau centred at the Dirac point, where a gap of equal magnitude is predicted to open. We find that when the Fermi level lies within this plateau the estimated anomalous Hall conductance saturates around 1.8 ± 0.4 e2/h.
Nonlinear transport in a photo-induced superconductor
E. Wang, J. Adelinia, M. Chavez Cervantes, T. Matsuyama, M. Fechner, M. Buzzi, G. Meier, A. Cavalleri
Optically driven quantum materials exhibit a variety of non-equilibrium functional phenomena, which are potentially associated with unique transport properties. However, these transient electrical responses have remained largely unexplored, primarily because of the challenges associated with integrating quantum materials into ultrafast electrical devices. Here, thin films of K3C60 grown by Molecular Beam Epitaxy were connected by coplanar terahertz waveguides to a series of photo-conductive switches. This geometry enabled ultrafast transport measurements at high current densities, providing new information on the photo-induced phase created in the high temperature metal by mid-infrared excitation. Nonlinearities in the current-voltage charactersitics of the transient state validate the assignment of transient superconductivity, and point to an inhomogeneous phase in which superconducting regions of the sample are connected by resistive weak links. This work opens up the possibility of systematic transport measurements in driven quantum materials, both to probe their properties and to integrate them into ultrafast optoelectronic platforms.