Superconducting fluctuations and pseudogap in high-Tccuprates

F Rullier-Albenque,H Alloul

doi:10.1051/epjconf/20122300016

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.

Highlights

A careful investigation near optimal doping shows that Tc′ is higher than the pseudogap temperature T ⋆, which is an unambiguous evidence that the pseudogap cannot be assigned to preformed pairs
One of the most puzzling feature of the high-Tc cuprates is the existence of the so-called pseudogap phase in the underdoped region of their phase diagram
We want to emphasize here that specific effects induced by disorder are probably at the origin of many confusions in the study of high-Tc cuprates. This leads us to propose in fig.10 a 3D phase diagram where the effect of disorder has been introduced as a third axis

Summary

Introduction

One of the most puzzling feature of the high-Tc cuprates is the existence of the so-called pseudogap phase in the underdoped region of their phase diagram. Either T ∗ is found to merge with the superconducting dome in the overdoped part of the phase diagram, or to cross it near optimal doping These different representations are associated with different lines of thought. It has been proposed that the pseudogap could be ascribed to the formation of superconducting pairs with strong phase fluctuations [3] This scenario has been supported by the observation of a large Nernst effect and of diamagnetism above Tc, which delineates another line Tν below which strong superconducting fluctuations and/or vortices persist in the normal state [4]. The effect of disorder introduced by electron irradiation at low temperature will be presented

Experimental

Pseudogap and onset of superconducting fluctuations

Quantitative analysis of the paraconductivity in the Ginzburg-Landau approach

Field variation of the SCF conductivity

Influence of disorder

Conclusion