Higgs mode in a strongly interacting fermionic superfluid
Higgs mode in a strongly interacting fermionic superfluid
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DOI：
10.1038/s41567-018-0128-6
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发表时间：
2018-08-01
期刊：
影响因子：
19.6
通讯作者：
Koehl, M.
中科院分区：
文献类型：
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作者：
Behrle, A.;Harrison, T.;Koehl, M.
希格斯模式和戈德斯通模式是自发破缺连续对称性时有序参数的可能集体模式。尽管低能戈德斯通（相位）模式始终稳定，但需要额外的对称性来防止希格斯（振幅）模式快速衰减为低能激发。在高能物理学中，希格斯玻色子(1)经过数十年的探索才被发现，其稳定性是由洛伦兹不变性保证的。在凝聚态物理领域，粒子空穴对称性可以发挥这一作用(2)，并且在弱相互作用超导体中观察到希格斯模式(3-5)。然而，对于粒子-空穴对称性未精确满足的强相关超导体，希格斯模式是否也稳定，或者该模式是否变得过阻尼，一直是众多讨论的主题(6-11)。实验证据仍然缺乏，特别是由于直接激发希格斯模式的困难。在这里，我们观察到强相互作用的超流体费米气体中的希格斯模式。通过引入超导有序参数 Delta 振幅的周期性调制，我们观察到频率为 2 Delta/h 的激发谐振。对于强耦合，当库珀对变成紧密结合的二聚体时，峰宽度变宽，最终该模式消失，这表明希格斯模式最终不稳定。
Higgs and Goldstone modes are possible collective modes of an order parameter on 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 the subject of numerous discussions(6-11). Experimental evidence is still lacking, in particular owing to the difficulty of exciting 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 Delta, we observe an excitation resonance at the frequency 2 Delta/h. For strong coupling, the peak width broadens and eventually the mode disappears when the Cooper pairs turn into tightly bound dimers signalling the eventual instability of the Higgs mode.