Comment on 'Temperature range of superconducting fluctuations above $T_c$ in YBa$_2$Cu$_3$O$_{7-δ}$ single crystals'

Contrary to the starting assumption of Grbić et al. (Phys. Rev. B 83, 144508 (2011)), here we will first argue that a 16 T magnetic field is not enough to quench all superconducting fluctuations above Tc in YBa$_2$Cu$_3$O$_{7-δ}$. We then conclude that through their measurements of microwave absorption these authors actually determine the ac-fluctuation magnetoconductivity at 16 T, instead of the zero-field ac paraconductivity as they pretend. So, the temperature proposed by Grbić et al. for the onset of the superconducting fluctuations, T', will correspond to the one at which the finite-field effects at 16 T become measurable in their experiments, and the actual fluctuations onset will be located well above T'. These conclusions, which also concern influential recent publications on that issue, are then confirmed by analyzing some of the Grbić et al., data on the grounds of the Gaussian Ginzburg-Landau (GGL) approach for the finite-field (or Prange) fluctuation regime.

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