Possible light-induced superconductivity in K3C60 at high temperature
Recent experiments on high-Tc cuprates demonstrated that strong mid-infrared pulses, resonantly tuned to specific phonon modes of the crystal lattice, can induce transient superconducting transport between the copper-oxygen planes at temperatures far above the equilibrium Tc.
Femtosecond X-ray diffraction performed under the same excitation conditions could give some hints on the possible mechanism responsible for these striking observations. However, a clear picture is still missing, mainly due to the lack of understanding of the physics of high-Tc cuprates even in their equilibrium ground state.
In a following experiment, we applied the same approach of resonant vibrational excitation to a completely different material: K-doped C60.
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 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.
In our experiment, we 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. A gap in the real part of the optical conductivity of the photo-excited material, as well as a low-frequency divergence of the imaginary part, were detected for temperatures far in excess of the equilibrium Tc = 20 K, up to at least 100 K.
These findings may be explained in terms of a nonlinear coupling between different vibrational modes which would displace the molecular structure (in analogy with nonlinear phononics), or, alternatively, by considering the effect of a time-dependent modulation of the onsite Coulomb repulsion.