K3C60 belongs to the class of alkali-doped fullerides, organic superconductors made of C60 molecules intercalated by alkali atoms (e.g. K, Rb, Cs). These compounds, in analogy with high-Tc cuprates, are characterized by strong electron correlation and display insulator-to-metal transitions, as well high-temperature superconductivity (up to 40 K in Cs3C60).

However, at variance with copper oxides, in fullerides the crystal structure is not layered and superconductivity is known to be more conventional, being mediated by intramolecular vibrations and displaying s-wave symmetry.

Figure 1: (Left) Crystal structure of K3C60, where blue bonds link the carbon atoms in each C60 molecule and red spheres represent K atoms. (Right) C60 molecular distortion stimulated by the mid-infrared pulses.

In our experiment, we have resonantly excited local molecular vibrational modes in K3C60 using femtosecond laser pulses at mid-infrared wavelengths. The transient optical response was then probed with time-resolved THz spectroscopy, revealing a non-equilibrium state with the optical properties of a superconductor [1]. A gap in the real part of the optical conductivity of the photo-excited material, as well as a low-frequency divergence in the imaginary part, were detected for temperatures far in excess of the equilibrium Tc = 20 K, up to at least 150 K.

Figure 2: Light-induced superconductivity in K3C60 was investigated at high pressures in a Diamond Anvil Cell

More recently, we have managed to tune this effect with hydrostatic pressure and found that the superconducting-like features gradually disappear at around 0.3 GPa [2]. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K3C60, in which a larger electronic bandwidth induced by pressure is also detrimental for pairing. Crucially, this observation excludes alternative interpretations based on a high-mobility metallic phase. The pressure dependence also suggests that transient, incipient superconductivity occurs far above the 150 K hypothesized previously, and rather extends all the way to room temperature.

Different scenarios for optically-driven superconductivity in K3C60 have been suggested. While some of them assume a nonlinear coupling between different molecular modes, others point toward a vibrationally-driven, time dependent modulation of the local Coulomb potential. Regardless of the specific mechanisms, our data are indicative of striking emergent physics out of equilibrium, and provide new opportunities and challenges for both theory and experiments.

Related Methods & Experiments

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Possible light-induced superconductivity in K3C60 at high temperature
M. Mitrano, A. Cantaluppi, D. Nicoletti, S. Kaiser, A. Perucchi, S. Lupi, P. Di Pietro, D. Pontiroli, M. Riccò, S. R. Clark, D. Jaksch, and A. Cavalleri
Nature, 530, 461–464 (2016)

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Pressure tuning of light-induced superconductivity in K3C60
A. Cantaluppi, M. Buzzi, G. Jotzu, D. Nicoletti, M. Mitrano, D. Pontiroli, M. Riccò, A. Perucchi, P. Di Pietro, A. Cavalleri
Nature Physics, 14, 837–841 (2018)