TY - GEN A1 - Cea, Tommaso A1 - Bucheli, Daniel A1 - Seibold, Götz A1 - Benfatto, Lara A1 - Lorenzana, José A1 - Castellani, Claudio T1 - Optical excitation of phase modes in strongly disordered superconductors T2 - Physical Review B N2 - According to the Goldstone theorem the breaking of a continuous U(1) symmetry comes along with the existence of low-energy collective modes. In the context of superconductivity these excitations are related to the phase of the superconducting (SC) order parameter and for clean systems are optically inactive; that is, single-mode excitations do not directly couple to light. Here we show that for strongly disordered superconductors phase modes acquire a dipole moment and appear as a subgap spectral feature in the optical conductivity. This finding is obtained with both a gauge-invariant random-phase approximation scheme based on a fermionic Bogoliubov–de Gennes state and a prototypical bosonic model for disordered superconductors. In the strongly disordered regime, where the system displays an effective granularity of the SC properties, the optically active dipoles are linked to the isolated SC islands, offering a new perspective for realizing microwave optical devices. Y1 - 2014 UR - http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.174506 U6 - https://doi.org/10.1103/PhysRevB.89.174506 SN - 2469-9969 VL - 89 SP - 174506 ER - TY - GEN A1 - Udina, Mattia A1 - Fioro, Jacopo A1 - Cea, Tommaso A1 - Castellani, Claudio A1 - Seibold, Götz A1 - Benfatto, Lara T1 - THz non-linear optical response in cuprates: predominance of the BCS response over the Higgs mode T2 - Faraday Discussions N2 - Recent experiments with strong THz fields in unconventional cuprate superconductors have clearly evidenced an increase of the non-linear optical response below the superconducting critical temperature Tc. As in the case of conventional superconductors, a theoretical estimate of the various effects contributing to the non-linear response is needed in order to interpret the experimental findings. Here, we report a detailed quantitative analysis of the non-linear THz optical kernel in cuprates within a realistic model, accounting for the band structure and disorder level appropriate for these systems. We show that the BCS quasiparticle response is the dominant contribution for cuprates, and its polarization dependence accounts very well for the third-harmonic generation measurements. On the other hand, the polarization dependence of the THz Kerr effect is only partly captured by our calculations, suggesting the presence of additional effects when the system is probed using light pulses with different central frequencies. Y1 - 2022 UR - https://pubs.rsc.org/en/content/articlelanding/2022/FD/D2FD00016D U6 - https://doi.org/10.1039/D2FD00016D SN - 1364-5498 VL - 237 SP - 168 EP - 185 ER -