The U(1) gauge invariant version of the superconducting BCS state as formulated by P.W. Anderson is characterized by the Higgs mode as the elementary excitation of the superconductor in the two particle channel [1]. This mechanism inspired Y. Nambu and in particular P. Higgs to formulate the Higgs mechanism for elementary particle physics leading subsequentially to the discovery of the Higgs particle.[2-3] Varma pointed out early that the Higgs particle is a Raman active excitation. [4] Indeed experiments by Sooryakumar and Klein showed the first measurement of the Higgs mode in superconductors already in 1980 – a finding later confirmed in 2014. [5,6] Due to its weak coupling to light the Higgs mode remained quite elusive to experiments, despite the case in NbSe2 where it gained strength in the Raman cross section by coupling to a CDW. However, over the past 20 years there was a continuous build up of experimental evidence for the Higgs mode in Raman scattering of HTCs. Already in 2005 Budelmann et al. noticed by using resonance Raman spectroscopy a distinct in-gap quasiparticle excitation as part of the overall gap feature seen in HTCs.[7] In 2009 Saichu et al. obseverd and in gap feature reacting on a pump on a different time scale as compared to the expected pair breaking peak suggesting the presence of a distinct in-gap excitation.[8] Since then many studies by different techniques in particular THz measurements have provided a growing body of evidence for the presence of the Higgs mode.[9]
However, Raman scattering is susceptible to a combination of pair breaking excitations and superconducting quasiparticle excitations. The development of non-equilibrium Raman scattering (NEARS) allows to discriminate the different contributions by comparing the Anti-Stokes and Stokes (energy gain / energy loss) spectra in order to identify modes that get populated in a superconductor after a quench of the Mexican hat potential (see Fig. 1 – left) . We will discuss the detailed measurement procedure and will outline the presence of a new in-gap mode in the superconducting state. Its symmetry dependent behavior is consistent with the Higgs mode in the superconductor and incompatible with other excitations. From the fits it is then possible to determine the excitations landscape of a superconductor (see Fig. 1 – right). We will also outline future plans to further support our assignment and what needs to be done to systematically evaluate the presence of the Higgs mode in HTCs.
[1] Nambu, Y. Quasi-particles and gauge invariance in the theory of superconductivity. Phys. Rev. 117, 648 – 663 (1960). URL https://doi.org/10.1103/PhysRev.117. 648.
[2] Anderson, P. Coherent excited states in the theory of superconductivity: gauge invariance and the Meiss- ner effect. Phys. Rev. 110, 827 – 835 (1958). URL https://doi.org/10.1103/PhysRev.110.827.
[3] Higgs, P. Prehistory of the Higgs boson. Comptes Rendus Physique 8, 970 – 972 (2007). URL https://doi.org/ 10.1016/j.crhy.2006.12.006.
[4] Varma, C. M. Higgs boson in superconductors. Journal of Low Temperature Physics 126, 901 – 909 (2002). URL https://doi.org/10.1023/A:1013890507658.
[5] Sooryakumar, R. & Klein, M. V. Raman scattering by superconducting-gap excitations and their coupling to charge-density waves. Phys. Rev. Lett. 45, 660 – 662 (1980). URL https://doi.org/10.1103/PhysRevLett. 45.660.
[6] M ́easson, M.-A. et al. Amplitude Higgs mode in the 2H- NbSe2 superconductor. Phys. Rev. B 89, 060503 (2014).
[7] Budelmann, D. et al. Gaplike Excitations in the Super- conducting State of Bi2 Sr2 CaCu2 O8 Studied by Reso- nant Raman Scattering. Phys. Rev. Lett. 95, 057003 (2005). URL https://doi.org/10.1103/PhysRevLett. 95.057003.
[8] Saichu, R. P. et al. Two-Component Dynamics of the Or- der Parameter of High Temperature Bi2Sr2CaCu2O8+δ Superconductors Revealed by Time-Resolved Raman Scattering. Phys. Rev. Lett. 102, 177004 (2009). URL https://doi.org/10.1103/PhysRevLett.102.177004.
[9] Shimano, R. & Tsuji, N. Higgs mode in superconductors. Annu. Rev. Condens. Matter Phys. 11, 103 – 124 (2020). URL https://doi.org/10.1146/annurev-conmatphys-031119-050813.
[10] Tomke E. Glier and Mika Rerrer and Lea Westphal and Garret Lüllau and Liwen Feng and Sida Tian and Jakob Dolgner and Rafael Haenel and Marta Zonno and Hiroshi Eisaki and Martin Greven and Andrea Damascelli and Stefan Kaiser and Dirk Manske and Michael Rübhausen, Superconducting Higgs particle observed by non-equilibrium Raman scattering}, arXiv 2310.08162, 2024. https://doi.org/10.48550/arXiv.2310.08162