Superconducting Higgs mode in cuprate thin films

Cuprate high-Tc superconductor hosts multiple competing orders such as the pseudogap, incommensurate magnetism, charge order, etc. These diverse phases were discovered via spectroscopy tools that probe the charge-/spin-excitations of an ordered state. Yet, despite the immense knowledge about cuprates learnt from these experiments, a coherent understanding of the microscopic mechanism behind high-Tc superconductivity is still lacking. A potential answer to this question may come from investigating the collective excitation of the superconducting order parameter itself, which is charge- and spin-neutral and difficult to access by conventional techniques. We accomplish this goal by using an undulator-based high-field THz pulse, which nonlinearly couples to the superconducting condensate and leads to characteristic third harmonic generation. Using this technique, we unveil the collective amplitude oscillation of the superconducting order parameter, the superconducting Higgs mode, in three archetypal families of cuprates for the first time. In addition, we show evidence for a hitherto unreported collective mode universally exhibited by optimally doped samples. A finite Higgs-like response above Tc is also observed in our experiment, suggesting a finite pairing amplitude above Tc.