No 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 ons considerably higher than T 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(a peak in M 1 and a kink in M 2 ) are due to superconducting pairing correlations that develop below T 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 ons is shown to be analogous to the one occurring in the model spetra below T c . (ii) Calculations based on simple models, for which the dip in the temperature range from T c to T 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. 8 606, Reber et al 2013 Phys. Rev. B 87 060506) imply the persistence of the coherence factor characteristic of superconducting pairing correlations in a range of temperatures above T c .− 7 (Y-123) [6].More recently, the interpretation of the T ons scale in terms of a precursor superconductivity has been supported by results obtained by Uykur and coworkers [7]. They address the persistence of the superfluid density above T c and the impact of Zn doping on T ons (T p in the notation of [7]). This interpretation, however, has not yet been commonly accepted. Two reasons are: (i) the c-axis response of Y-123 is a fairly complex quantity due to the specific bilayer structure of this compound. Its interpretation therefore requires a detailed understanding of the c-axis electrodynamics of the bilayer compounds [16][17][18][19][20][21][22][23]. (ii) UD cuprates are known to exhibit ordered phases distinct from superconductivity, in particular, charge modulations have been reported [24,25], that set in at temperatures comparable to T ons . This has fueled speculations that the T ons scale is determined by an order OPEN ACCESS RECEIVED

We extend Gor'kov theory to address superconducting pairing at high magnetic fields and general temperatures with arbitrary attractive interaction strength. This analysis begins with a new interpretation of the high-field Gor'kov gap equation which we associate with an instability in a generalized particle-particle ladder series. Importantly, this interpretation of the non-linear gap equation enables a treatment of pairing which is distinct from condensation. We also show how to consolidate two distinct fermionic pairing schemes in real and momentum space, both corresponding to an Abrikosov lattice. Numerical results for the fermionic local density of states demonstrate that gapless structure in a field is robust and presumably relevant to quantum oscillation experiments. We find that despite their differences, both pairing schemes contain very similar physics. Our formalism is designed to explore a variety of magnetic field effects in the so-called pseudogap phase and throughout the BCS-BEC crossover.arXiv:1112.1112v3 [cond-mat.supr-con]