Electronic Raman scattering on the underdoped HgBa2Ca2Cu3O8+δ high-Tc superconductor: The symmetry of the order parameter

Abstract

In order to obtain high quality, reliable electronic Raman spectra of a high-Tc superconductor compound, comparable with existing models, we have studied strongly underdoped HgBa2Ca2Cu3O8+δ with well prepared surface conditions. This choice was made such as to (i) minimize the overstoichiometry of oxygen δ, (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 B2g, B1g, A1g+B2g, and A1g+B1g channels. The B2g spectrum exhibits a linear frequency dependence (exponent n=1.12±0.03) at low energy whereas the B1g one shows a cubiclike dependence (n=3.02±0.1). The B2g and B1g spectra display a single well defined maximum at 5.6kBTc and 9kBTc, respectively. In mixed A1g channels an intense electronic peak centered around 6.4kBTc is observed. The low-energy parts of the spectra (below 350 cm−1) correspond to the electronic response expected for a pure dx2−y2 gap symmetry (and can be fitted up to the gap energy for the B1g channel). However, in the upper parts (above 350 cm−1), the relative position of the B1g and B2g peaks needs expanding the B2g Raman vertex to second-order Fermi surface harmonics to fit the data with the dx2−y2 model. The sharper and more intense A1g 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 dx2−y2 model, provided that the oxygen doping δ 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.

Source-reported materials — not catalogue approval

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