Disentangling Cooper-pair formation above the transition temperature from the pseudogap state in the cuprates

The discovery of the pseudogap in the cuprates(1-3) created significant excitement as it was believed to be a signature of pairing(4), in some cases above room temperature. Indeed, a number of experiments detected phase-fluctuating superconductivity above the transition temperature T-c (refs 5-9). However, several recent experiments reported that the pseudogap and superconducting state are characterized by different energy scales(10-14), and probably compete with each other(15,16), leaving open the question of whether the pseudogap is caused by pair formation. Here we report the discovery of a spectroscopic signature of pair formation and demonstrate that in a region commonly referred to as the pseudogap, two distinct states coexist: one that is due to pair formation and persists to an intermediate temperature T-pair < T* and a second-the 'proper' pseudogap-characterized by the loss of spectral weight and anomalies in transport properties that extends up to T*. T-pair has a value around 120-150 K even for materials with very different T-c values and it probably sets a limit on the highest attainable T-c in the cuprates.