Chirped amplitude mode in photo-doped superconductors

D. Golež,1,2 T. Blommel,3 J. Kaye,4 Y. Murakami5

1JSI, Jamova cesta 23, SI-1000 Ljubljana, Slovenia

2University of Ljubljana, Kongresni trg 12, 1000 Ljubljana

3Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA

4Center for Computational Quantum Physics, Flatiron Institute, New York, New York, 10010, USA

5Center for Emergent Matter Science, RIKEN, Wako, Saitama 351-0198, Japan

When a continuous symmetry is spontaneously broken, two types of collective modes emerge: phase and amplitude mode fluctuations of the order parameter. The latter is a massive excitation living at the edge of the gap. Recently, signatures of the amplitude mode were observed in photo-doped superconductors using third-harmonics generation in the terahertz regime, which opened the field of nonlinear light-amplitude mode coupling.

We will show that a weak photo-excitation in superconductors leads to a long-lived prethermal phase with a reduced order parameter and superimposed amplitude oscillations. As we increase the intensity of the pump pulse, the amplitude mode oscillations exhibit chirping; namely, the frequency slows down as a function of time. The chirping gets amplified as we approach the nonthermal critical point – an excitation at which the intensity of the light is large enough to destroy the superconducting order. We identify signatures of the chirped amplitude mode in photo-induced current after a monocycle or Gaussian pulse and propose a transmission line circuit experiment to detect the phenomena. In the last part, we will present recent theoretical advances based on the compressed representation of quantum propagators, which allow the analysis of time scales relevant for the collective mode dynamics.