Electronic raman scattering on under-doped mercurate high-Tc superconductors: the symmetry of the order parameter

Abstract In order to obtain electronic Raman spectra of a high- Tc superconductor compound, comparable with existing model, we have studied polished, under-doped HgBa 2 CuO 6+ δ and HgBa 2 Ca 2 Cu 3 O 8+ δ . This choice was made such as to i) minimize the over- stoichiometry of oxygen δ ii) avoid the need of phonon background subtraction from the raw data iii) eliminate traces of parasitic phases from the crystal surface. Under these experimental conditions we present the pure electronic Raman response function in the B 2 g and B 1 g channels. The B 2 g spectrum exhibits a linear frequency dependence (exponent n = 1.12 ± 0.03) at low energy whereas the B 1 g one shows a cubic-like dependence ( n = 3.02 ± 0.1). 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 δ is not too strong. This surprisingly strong sensitivity of the electronic Raman spectra to oxygen doping in the superconducting state in emphasized here and remains an open question.