Distinct Ranges of Superconducting Fluctuations and Pseudogap in YBa$_2$Cu$_3$O$_{6+x}$

The contribution of superconducting fluctuations (SCF) to the ab-plane conductivity has been determined accurately in a series of YBa$_2$Cu$_3$O$_{6+x}$ single crystals using high magnetic fields to restore the normal state behavior [1]. This allows us to determine within the same set of transport measurements both the field $H^{\prime}_c(T)$ and the temperature $T^{\prime}_c$ above which the SCFs are fully suppressed, and the pseudogap temperature $T^{\star}$. A careful investigation near optimal doping shows that $T^{\star}$ becomes smaller than $T^{\prime}_c$, which unambiguously evidences that the pseudogap cannot be assigned to preformed pairs [2]. In the nearly optimally doped samples, the SCF contribution to conductivity can be accounted for by Gaussian Aslamazov-Larkin fluctuations in the Ginzburg-Landau approach [3]. A phase fluctuation contribution might be invoked in the most underdoped sample in a $T$ range which increases when controlled disorder is introduced by electron irradiation. The analysis of the fluctuation magnetoconductance allows us to determine the critical fields $H_{c2}(0)$ which are found to be very similar to $H^{\prime}_c(0)$ and to increase with hole doping. These two depairing fields which are directly connected to the magnitude of the superconducting gap therefore follow the evolution of $T_c$, which is at odds with the sharp decrease of the pseudogap with increasing hole doping. \\[4pt] [1] F. Rullier-Albenque, H. Alloul, Cyril Proust, P. Lejay, A. Forget, and D. Colson, Phys. Rev. Lett. \textbf{99}, 027003 (2007). \\[0pt] [2] H. Alloul, F. Rullier-Albenque, B. Vignolle, D. Colson, A. Forget, Europhys. Lett. \textbf{91}, 37005 (2010). \\[0pt] [2] F. Rullier-Albenque, H. Alloul, G. Rikken, Phys. Rev. B \textbf{84}, 014522 (2011).