High Field determination of superconducting fluctuations in high-Tc cuprates

F Rullier-Albenque,D Colson,A Forget,H Alloul

doi:10.1088/1742-6596/449/1/012010

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 YBa2Cu3O6+x single crystals. The fluctuation conductivity is found to vanish nearly exponentially with temperature, allowing us to determine precisely the field H'c(T) and the temperature T'c above which the SCFs are fully suppressed. T'c is always found much smaller than the pseudogap temperature A careful investigation near optimal doping shows that T'c is higher than the pseudogap T*, which indicates that pseudogap cannot be assigned to preformed pairs. For nearly optimally doped samples, the fluctuation conductivity can be accounted for by gaussian fluctuations following the Ginzburg-Landau scheme. 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 quite similar to H'c(0) and to increase with hole doping. Studies of the incidence of disorder on both T'c and T* enable us to propose a three dimensional phase diagram including a disorder axis, which allows to explain most observations done in other cuprate families.

Highlights

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
Field variation of the superconducting fluctuations (SCF) conductivity: Hc and upper critical field Hc2 From the data reported in Fig.2, we can extract the magnetic field Hc(T ) above which the MR recovers a H2 dependence, which we take as the sign that the normal state is completely restored
We have presented here a condensed report of our quantitative study of the superconducting fluctuation conductivity in YBCO that was extensively detailed in previous publications [6]-[8]

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. If we were able to improve our sensitivity by an order of magnitude, the values of Tc could only be increased by ∼ 15K and would correspond to an extremely low SCF contributions to the conductivity, about four orders of magnitude lower than at measured at Tc. In the case of the optimally doped compound, our value of Tc is in very good agreement with the onset temperature determined by magnetic susceptibility [12]. This has to be contrasted with determinations from Nernst measurements [4] or high field magnetoresistance measurements comparable to ours [15] which give much larger onset temperatures This clearly indicates that these different types of experiments do not detect the SCFs with a similar sensitivity as ours, or that they probe different types of SCFs (namely phase versus amplitude). This has been treated by inserting a cutoff phenomenologically, implying that the density of fluctuating pairs vanishes at Tc as detailed in ref.[7]

Field variation of the SCF conductivity

Conclusion