The concept of mass generation by means of the Higgs mechanism was strongly inspired by earlier works on the Meissner–Ochsenfeld effect in superconductors. In quantum field theory, the excitations of longitudinal components of the Higgs field manifest as massive Higgs bosons. The analogous Higgs mode in superconductors has not yet been observed owing to its rapid decay into particle–hole pairs. According to recent theories, however, the Higgs mode should decrease below the superconducting pairing gap 2Δ and become visible in two-dimensional systems close to the superconductor–insulator transition. For experimental verification, we measured the complex terahertz transmission and tunnelling density of states of various thin films of superconducting NbN and InO close to criticality. Comparing both techniques reveals a growing discrepancy between the finite 2Δ and the threshold energy for electromagnetic absorption, which vanishes critically towards the superconductor–insulator transition. We identify the excess absorption below 2Δ as strong evidence of the Higgs mode in two-dimensional quantum critical superconductors.
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We are grateful for useful discussions with D. Arovas, L. Benfatto, S. Gazit, D. Podolsky and E. Shimshoni. We acknowledge support from the the GIF foundation grant I-1250-303.10/2014 and from the Deutsche Forschungsgemeinschaft. U.S.P. acknowledges financial support from the Studienstiftung des deutschen Volkes. B.G. acknowledges support from the Russian Ministry of Education and Science (Program 5 top 100) and A.A. acknowledges support from the ISF and BSF foundations. M.S. acknowledges support from the NSF Graduate Research Fellowship and N.T. acknowledges support from grant DOE DE-FG02-07ER46423 (N.T.) and computational support from the Ohio Supercomputing Center.
The authors declare no competing financial interests.
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Sherman, D., Pracht, U., Gorshunov, B. et al. The Higgs mode in disordered superconductors close to a quantum phase transition. Nature Phys 11, 188–192 (2015). https://doi.org/10.1038/nphys3227
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