Evidence for precursor superconducting pairing above Tc in underdoped cuprates from an analysis of the in-plane infrared response

Abstract

We performed calculations of the in-plane infrared response of underdoped cuprate superconductors to clarify the origin of a characteristic dip feature which occurs in the published experimental spectra of the real part of the in-plane conductivity below an onset temperature ${{T}^{{\rm ons}}}$ considerably higher than ${{T}_{{\rm c}}}$ . We provide several arguments, based on a detailed comparison of our results with the published experimental data, confirming that the dip feature and the related features of the memory function $M(\omega )={{M}_{1}}(\omega )+{\rm i}{{M}_{2}}(\omega )$ (a peak in M _1 and a kink in M _2 ) are due to superconducting pairing correlations that develop below ${{T}^{{\rm ons}}}$ . In particular, we show that (i) the dip feature, the peak and the kink of the low-temperature experimental data can be almost quantitatively reproduced by calculations based on a model of a d -wave superconductor. The formation of the dip feature in the experimental data below ${{T}^{{\rm ons}}}$ is shown to be analogous to the one occurring in the model spetra below ${{T}_{{\rm c}}}$ . (ii) Calculations based on simple models, for which the dip in the temperature range from ${{T}_{{\rm c}}}$ to ${{T}^{{\rm ons}}}$ is unrelated to superconducting pairing, predict a shift of the onset of the dip at the high-energy side upon entering the superconducting state, that is not observed in the experimental data; (iii) the conductivity data in conjunction with the recent photoemission data (Reber et al 2012 Nat. Phys. http://dx.doi.org/10.1038/nphys2352 8 http://dx.doi.org/10.1038/nphys2352 , Reber et al 2013 Phys. Rev. B http://dx.doi.org/10.1103/PhysRevB.87.060506 87 http://dx.doi.org/10.1103/PhysRevB.87.060506 ) imply the persistence of the coherence factor characteristic of superconducting pairing correlations in a range of temperatures above ${{T}_{{\rm c}}}$ .

Keywords

• cuprate superconductors
• infrared response
• precursor superconducting pairing above Tc
• 74.25.Gz
• 74.72.-h