Abstract:
In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature Tc. Here we show via a combination of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that signatures of such a state can be obtained via three independent observables in an underdoped sample of NdBa2 Cu 3 O 6 + δ . The pseudogap correlations were disentangled from the response of laser-broken pairs by clocking their characteristic time scales. The onset of a superconducting precursor state was found at a temperature TONS>T C, consistent with the temperature scale identified via static optical spectroscopy. Furthermore, the temperature evolution of the coherent vibration of the Ba ion, strongly renormalized by the onset of superconductivity, revealed a pronounced anomaly at the same temperature T ONS . The microscopic nature of such a precursor state is discussed in terms of preformed pairs and enhanced bilayer coherence.