Current-assisted Raman activation of the Higgs mode in superconductors

• Published in: Physical review. B 2020-06, Vol.101 (22), p.1, Article 220507
• Main Authors: Puviani, Matteo, Schwarz, Lukas, Zhang, Xiao-Xiao, Kaiser, Stefan, Manske, Dirk
• Format: Article
• Language: English
• Subjects: Activation; Dipole moments; Excitation; Harmonic generations; Higher harmonics; Magnetic dipoles; Photons; Resonance
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.101.220507

The Higgs mode in superconductors is a scalar mode without an electric or magnetic dipole moment. Thus, it is commonly believed that its excitation is restricted to a nonlinear two-photon Raman process. However, recent efforts have shown that a linear excitation in the presence of a supercurrent is possible, resulting in a resonant enhancement at Ω = 2Δ with the driving light frequency Ω and the energy of the Higgs mode 2Δ . This is in contrast to the usual 2Ω = 2Δ resonance condition found in nonlinear third-harmonic generation experiments. In this Rapid Communication, we show that such a linear excitation can still be described as an effective Raman two-photon process, with one photon at ω = 2Δ and one virtual photon at ω = 0 which represents the dc supercurrent. At the same time we demonstrate that a straightforward infrared activation with a single-photon excitation is negligible. Moreover, we give a general context to our theory, providing an explanation for how the excitation of the Higgs mode in both THz quench and drive experiments can be understood within a conventional difference-frequency generation or sum-frequency generation process, respectively. In such a picture, the observed resonance condition Ω = 2Δ is just a special case. With the same approach, we further discuss another recent experiment, where we find a suppression of odd-order higher harmonics in the presence of a dc supercurrent.