Higgs and Goldstone modes are possible collective modes of an order parameter upon spontaneously breaking a continuous symmetry. Whereas the low-energy Goldstone (phase) mode is always stable, additional symmetries are required to prevent the Higgs (amplitude) mode from rapidly decaying into low-energy excitations. In high-energy physics, where the Higgs boson 1 has been found after a decades-long search, the stability is ensured by Lorentz invariance. In the realm of condensed-matter physics, particle-hole symmetry can play this role 2 and a Higgs mode has been observed in weakly-interacting superconductors 3-5 . However, whether the Higgs mode is also stable for strongly-correlated superconductors in which particle-hole symmetry is not precisely fulfilled or whether this mode becomes overdamped has been subject of numerous discussions 6-11 . Experimental evidence is still lacking, in particular owing to the difficulty to excite the Higgs mode directly. Here, we observe the Higgs mode in a strongly-interacting superfluid Fermi gas. By inducing a periodic modulation of the amplitude of the superconducting order parameter ∆, we observe an excitation resonance 1 arXiv:1912.01867v1 [cond-mat.quant-gas]
We have investigated the 4s 2 S 1/2 → 5p 2 P 1/2 transition (D 1 line) of the potassium isotopes 39 K, 40 K, and 41 K using Doppler-free laser saturation spectroscopy. Our measurements reveal the hyperfine splitting of the 5p 2 P 1/2 state of 40 K, and we have determined the specific mass shift and the nuclear field shift constants for the blue (405 nm) D 1 line.
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