High-field studies of superconducting fluctuations in high-Tccuprates: Evidence for a small gap distinct from the large pseudogap

Abstract

We have used large pulsed magnetic fields up to 60 T to suppress the contribution of superconducting fluctuations (SCFs) to the $ab$-plane conductivity above ${T}_{c}$ in a series of YBa${}_{2}$Cu${}_{3}$O${}_{6+x}$ from the deep pseudogapped state to slight overdoping. Accurate determinations of the SCF contribution to the conductivity versus temperature and magnetic field have been achieved. Their joint quantitative analyses with respect to Nernst data allow us to establish that thermal fluctuations following the Ginzburg-Landau scheme are dominant for nearly optimally doped samples. The deduced coherence length $\ensuremath{\xi}(T)$ is in perfect agreement with a Gaussian (Aslamazov-Larkin) contribution for $1.01{T}_{c}\ensuremath{\lesssim}T\ensuremath{\lesssim}1.2{T}_{c}$. 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. For all dopings we evidence that the fluctuations are highly damped when increasing $T$ or $H$. This behavior does not follow the Ginzburg-Landau approach, which should be independent of the microscopic specificities of the superconducting state. The data permits us to define a field ${H}_{c}^{\ensuremath{'}}(T)$ and a temperature ${T}_{c}^{\ensuremath{'}}$ above which the SCFs are fully suppressed. The analysis of the fluctuation magnetoconductance in the Ginzburg-Landau approach allows us to determine the critical field ${H}_{c2}(0)$. The actual values of ${H}_{c}^{\ensuremath{'}}(0)$ and ${H}_{c2}(0)$ are found to be quite similar and both increase with hole doping. These depairing fields, which are directly connected to the magnitude of the superconducting gap, do therefore follow the ${T}_{c}$ variation which is at odds with the sharp decrease of the pseudogap ${T}^{*}$ with increasing hole doping. This is on line with our previous evidence that ${T}^{*}$ is not the onset of pairing. So the large gap seen by spectroscopic experiments in the underdoped regime has to be associated with the pseudogap. We finally propose here a three-dimensional phase diagram including a disorder axis, which makes it possible to explain most peculiar observations done so far on the diverse cuprate families.