Raman scattering on under - doped Hg − 1223 high - Tc superconductors : investigations on the symmetry of the order parameter

In order to obtain high quality, reliable electronic Raman spectra of a high-T c superconductor compound, comparable with existing model, we have studied strongly under-doped HgBa 2 Ca 2 Cu 3 O 8+δ. This choice was made such as to i) minimize oxygen disorder in the Hg−plane generated by oxygen doping ii) avoid the need of phonon background subtraction from the raw data iii)eliminate traces of parasitic phases identified and monitored on the crystal surface. Under these experimental conditions we are able to present the pure electronic Raman response function in the B 2g , B 1g , A 1g + B 2g and A 1g + B 1g channels. The B 2g spectrum exhibits a linear frequency dependence (exponent n = 1.12 ± 0.03) at low energy whereas the B 1g one shows a cubic-like dependence (n = 3.02 ± 0.1). The B 2g and B 1g spectra display two well defined maxima at 5.6 k B T c and 9 k B T c respectively. In mixed A 1g channels an intense electronic peak centered around 6.4 k B T c is observed. The low energy parts of the spectra (below 350 cm −1) correspond to the electronic response expected for a pure d x 2 −y 2 gap symmetry (and can be fitted up to the gap energy for the B 1g channel). However, in the upper parts (above 350 cm −1), the relative position of the B 1g and B 2g peaks needs expanding the B 2g Raman vertex to second order Fermi surface harmonics to fit the data with the d x 2 −y 2 model. The sharper and more intense A 1g peak appears to challenge the Coulomb screening efficiency present for this channel. As compared to previous data on more optimally doped, less stoichiometric Hg−1223 compounds, this work reconciles the electronic Raman spectra of under-doped Hg − 1223 crystals with the d x 2 −y 2 model, provided that the oxygen doping is not too strong. This apparent extreme sensitivity of the electronic Ra-man spectra to the low lying excitations induced by oxygen doping in the superconducting state is emphasized here and remains an open question.