Spectroscopies of superconducting collective modes: new advances and open questions

In the last few years, a number of experiments carried out with very intense THz fields, either in transmission or in pump-probe configuration, have shown the possibility to excite superconducting collective modes via light pulses. The signatures of the superconducting excitations manifest either in third-harmonic generation, in the case of narrowband pulses, or in well-defined oscillations of the transmitted field as a function of the pump-probe delay, in the case of broadband pulses. In the latter case, the phenomenon is completely analogous to the well-known excitations of Raman-active phonons in insulating and metallic systems. Despite the clear interest in the fundamental and applicative aspects of these techniques, a clear theoretical paradigm for the description of these experiments is still lacking. In this talk I will present a general scheme we recently developed [1,2] to describe step-by-step the processes behind the pump-probe detection of collective excitations, that can be equally well applied to ordinary phonons as well as electronic collective modes. I will then discuss a direct application of our interpretative scheme to the light-induced excitation of the so-called superconducting phase Leggett mode in the multi band MgB2 superconductor[2], and its connection to the phenomenon of the third-harmonic generation in superconductors[3,4]. Finally, I will present some recent results [5] on the role of the superconducting phase mode for the non-linear optical response in unconventional curate superconductors.


[1] M. Udina, T. Cea and L. Benfatto, Phys. Rev. B 100, 165131 (2019)
[2] F. Giorgianni, T. Cea, C. Vicario, C. P. Hauri, W. K. Withanage, X. Xi, and L. Benfatto, Nat. Phys. 15, 341 (2019).
[3]T. Cea, C. Castellani, L. Benfatto, Phys. Rev. B 93, 180507(R) (2016)
[4] T. Cea, P. Barone, C. Castellani, L. Benfatto, Phys. Rev. B 97, 094516 (2018)
[5] F. Gabriele and L. Benfatto, preprint 2020.