Electronic Raman scattering on the underdoped HgBa2Ca2Cu3O8+delta high-T-c superconductor: 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 models, we have studied strongly underdoped HgBa2Ca2Cu3O8+partial derivative With well prepared surface conditions. This choice was made such as to (i) minimize the overstoichiometry of oxygen partial derivative, (ii) avoid the need of phonon background subtraction from the raw data, and (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 1 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 cubiclike dependence (n = 3.02 +/- 0.1). The B-2g and B-1g spectra display a single well defined maximum at 5.6k(B)T(C) and 9K(B)T(c), respectively. In mixed Alg channels an intense electronic peak centered around 6.4k(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(x2-y2) 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(x2-y2) model. The sharper and more intense Al, 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 underdoped Hg-1223 crystals with the d(x2-y2) model, provided that the oxygen doping delta is not too strong. This surprisingly strong sensitivity of the electronic Raman spectra to the low lying excitations induced by oxygen doping in the superconducting state is emphasized here and remains an open question.