Superconducting fluctuations and pseudogap in high-Tc cuprates

TL;DR

This study uses high magnetic fields to distinguish superconducting fluctuations from the pseudogap in cuprates, revealing that the pseudogap is not due to preformed pairs and mapping their phase diagram.

Contribution

It provides the first direct experimental separation of superconducting fluctuations and pseudogap effects, and introduces a three-dimensional phase diagram including disorder effects.

Findings

T'c exceeds T* indicating pseudogap is not from preformed pairs.

Superconducting fluctuations follow Ginzburg-Landau theory near optimal doping.

Disorder influences the temperature range of phase fluctuations and the phase diagram.

Abstract

Large pulsed magnetic fields up to 60 Tesla are used to suppress the contribution of superconducting fluctuations (SCF) to the ab-plane conductivity above Tc in a series of YBa2Cu3O(6+x). These experiments allow us to determine the field H'c(T) and the temperature T'c above which the SCFs are fully suppressed. A careful investigation near optimal doping shows that T'c is higher than the pseudogap temperature T*, which is an unambiguous evidence that the pseudogap cannot be assigned to preformed pairs. Accurate determinations of the SCF contribution to the conductivity versus temperature and magnetic field have been achieved. They can be accounted for by thermal fluctuations following the Ginzburg-Landau scheme for nearly optimally doped samples. A phase fluctuation contribution might be invoked for the most underdoped samples in a T range which increases when controlled disorder is introduced by electron irradiation. Quantitative analysis of the fluctuating magnetoconductance allows us to determine the critical field Hc2(0) which is found to be be quite similar to H'c(0) and to increase with hole doping. Studies of the incidence of disorder on both T'c and T* allow us to propose a three dimensional phase diagram including a disorder axis, which allows to explain most observations done in other cuprate families.