The concept of mass generation by means of the Higgs mechanism was strongly inspired by earlier works on the Meissner–Ochsenfeld eect 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 2 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 2 and the threshold energy for electromagnetic absorption, which vanishes critically towards the superconductor–insulator transition. We identify the excess absorption below 2 as strong evidence of the Higgs mode in two-dimensional quantum critical superconductors.
The study of the properties of disordered superconductors is a subject of ongoing intense activity, mostly because it is viewed as being one of the few physical systems that can be tuned through a two-dimensional quantum critical point, which is not mean-field-like. The softening of the Higgs mode is direct proof that the SIT transition is a quantum critical point in which a diverging timescale is detected. Evidently, the vicinity to the QPT offers a unique opportunity to study the nature of the low-energy collective excitations in superconductors. Going beyond disordered superconductors, our findings can play a role in tracing collective excitations in other quantum critical condensedmatter systems and might influence related fields such as Bose-condensed ultracold atoms, quantum statistical mechanics and high-energy physics.