Clocking the onset of bilayer coherence in a high-Tc cuprate

In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature T-c. 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 NdBa2Cu3O6+delta. 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 T-ONS > T-c, consistent with the temperature scale identified via static optical spectroscopy. Furthermore, the temperature evolution of the coherent vibration of the B a 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.