Current-assisted Raman activation of the Higgs mode in superconductors

Abstract

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.

Source-reported materials — not catalogue approval

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