Seminars Archive

Light-induced high-temperature superconductivity in K3C60

Abstract
Recent experiments on cuprates [1,2,3,4] indicated that strong midinfrared (MIR) pulses, resonantly tuned to lattice degrees of freedom, induce transient superconducting transport between the copper-oxygen planes even at temperatures far above the equilibrium Tc. In this talk, I will present instead recent MIR pump-THz probe experiments where the concept of resonant phonon excitation is applied to a BCS-like superconductor, K3C60. K3C60 belongs to the class of alkali fullerides (A3C60), a family of organic superconductors made of C60 molecules intercalated by alkali atoms (e.g. K, Rb, Cs) [5]. These compounds are close to a Mott insulator-to-metal transition [6] and exhibit superconductivity with high transition temperatures (up to 40K in Cs3C60) [7]. Superconductivity is here mediated by high-energy intramolecular vibrations (100-200 meV) with Jahn-Teller character that favor the creation of Cooper pairs and the establishment of a superconducting state with s-wave symmetry [5,6,8,9]. Here, we excited local molecular vibrations of K3C60 [10] using femtosecond laser pulses at mid-infrared wavelengths. By measuring the transient optical response of the photo-excited material at THz frequencies, we identified a non-equilibrium state with the optical properties of a superconductor. A transient gap in the real part of the optical conductivity and a low-frequency divergence of the imaginary part were detected for base temperatures far in excess of the equilibrium Tc = 20 K, up to about T’ = 200 K [10]. The present experiment is strongly suggestive of a new type of superconductivity that is directly stimulated [10] by the laser field and indicates highly novel emergent physics away from equilibrium. [1] S. Kaiser et al., Phys. Rev. B 89, 184516 (2014) [2] W. Hu et al. Nature Materials 13, 705 (2014) [3] R. Mankowsky et al., Nature 516, 71 (2014) [4] D. Fausti et al., Science 331, 189 (2011) [5] O. Gunnarsson, Rev. Mod. Phys. 69, 575–606 (1997). [6] M. Capone et al., Rev. Mod. Phys. 81, 943 (2009) [7] T. Palstra et al., Solid State Communications 93, 327–330 (1995). [8] L. Degiorgi et al., Nature 369, 541–542 (1994). [9] S. C. Erwin et al., Science 254, 842–845 (1991). [10] M. Mitrano et al., arXiv preprint arXiv:1505.04529 (2015)