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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