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Abstract:

Optically driven quantum materials exhibit a variety of non-equilibrium functional phenomena [1-11], 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 [12-16]. 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 [17-23]. 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

Abstract:

Photo-excitation at terahertz and mid-infrared frequencies has emerged as an effective way to manipulate functionalities in quantum materials, in some cases creating non-equilibrium phases that have no equilibrium analogue. In K3C60, a metastable zero-resistance phase was observed that has optical properties, nonlinear electrical transport and pressure dependencies compatible with non-equilibrium high-temperature superconductivity. Here we demonstrate a two-orders-of-magnitude increase in photo-susceptibility near 10 THz excitation frequency. At these drive frequencies, a metastable superconducting-like phase is observed up to room temperature. The discovery of a dominant frequency scale sheds light on the microscopic mechanism underlying photo-induced superconductivity. It also indicates a path towards steady-state operation, limited at present by the availability of a suitable high-repetition-rate optical source at these frequencies

Abstract:

In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases1,2,3,4,5,6,7. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic and chemical constraints8. Here we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO3, a material that shows only partial orbital polarization, an unsaturated low-temperature magnetic moment and a suppressed Curie temperature, Tc = 27 K (refs. 9,10,11,12,13). The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to Tneq > 80 K, nearly three times the thermodynamic transition temperature. We interpret these effects as a consequence of the light-induced dynamical changes to the quasi-degenerate Ti t2g orbitals, which affect the magnetic phase competition and fluctuations found in the equilibrium state14,15,16,17,18,19,20. Notably, the light-induced high-temperature ferromagnetism discovered in our work is metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities.