Two Pseudogaps in the Cuprates: Meingast et al. Reply

TL;DR

This paper discusses the separation of Cooper-pair formation and phase coherence in high-temperature cuprate superconductors, proposing that pairing occurs above the superconducting transition temperature, as indicated by thermal expansion data.

Contribution

It provides evidence that Cooper pairs form at temperatures well above Tc, supporting the pseudogap opening at T* and the phase-ordering transition at Tc in cuprates.

Findings

Pairing occurs above Tc, indicated by thermal expansion data.

The pseudogap opens at T*, above the superconducting transition.

Tc corresponds to the phase-ordering temperature, not pairing onset.

Abstract

In classical superconductors Cooper-pair formation and phase coherence occur simultaneously as the temperature is lowered below Tc. In high-temperature superconductors (HTSC), on the other hand, the small superfluid density and low associated phase stiffness of the superconducting condensate are expected to lead to a separation of the Cooper-pair formation and the phase-coherence temperatures, especially in underdoped materials [3]. The only real phase transition in this scenario is the 3d-XY phase-ordering transition at Tc [3,4]. In our Letter [2] we showed that Tc in underdoped and optimally doped YBCO is just such a phase-ordering temperature, and then the question naturally arises - where do the Cooper pairs form? The observed 3d-XY scaling of our thermal expansion data over a wide temperature range [2] suggests that pairing occurs at temperatures considerably above Tc, and it thus appeared quite natural for us to associate the opening of the pseudogap at T*.