| Abstract: In many physical systems with spontaneously broken symmetry, collective excitations of the amplitude and phase of an order parameter emerge. In the vacuum of space-time, we are familiar with the Higgs boson, which is the amplitude mode of the Higgs field. The Higgs field gives mass to many particles via the Higgs mechanism in the standard model. In the case of superconductors, the phase mode of the superconducting order parameter, i.e., the Nambu-Goldstone mode, is absorbed by the gauge field due to the Anderson-Higgs mechanism, while the order parameter’s amplitude mode, namely the Higgs mode, remains intact. Observing the Higgs mode in superconductors has been challenging because it does not couple to electromagnetic fields linearly. Recent developments in generating intense electromagnetic pulses in the terahertz (THz) frequency range have enabled the observation of the Higgs mode in a conventional s-wave superconductor through its nonlinear coupling to the electromagnetic field [1]. Its extension to unconventional d-wave superconductors is intriguing, whereas it has been nontrivial whether the Higgs mode in d-wave superconductors is observable or not.
In this colloquium, I will highlight the study of the Higgs mode in d-wave cuprate superconductors. First, the observation of the Higgs mode in cuprate superconductors using ultrafast THz pump-probe spectroscopy will be presented [2]. I will also introduce the novel applications of collective excitations, including the Higgs mode, to investigate the enigmatic superconducting order parameter of high-temperature cuprate superconductors in equilibrium [3] and out of equilibrium [4].
Finally, I will discuss future research in collective excitations in various quantum materials.
[1] R. Shimano and N. Tsuji, Annu. Rev. Condens. Matter Phys. 11, 103 (2020)
[2] K. Katsumi et al., Phys. Rev. Lett. 120, 117001 (2018)
[3] K. Katsumi et al., Phys. Rev. B 102, 054510 (2020)
[4] K. Katsumi et al., arXiv:2209.01633 (2022) |