Exploration of the Hg-based cuprate superconductors by Raman spectroscopy under hydrostatic pressure
Abstract
The superconducting phase of the HgBa2CuO4+δ (Hg-1201) and HgBa2Ca2Cu3O8+δ (Hg-1223) cuprates has been investigated by Raman spectroscopy under hydrostatic pressure. Our analysis reveals that the increase of Tc with pressure is slower in Hg-1223 cuprate compared to the Hg-1201 due to a charge carrier concentration imbalance (accentuated by pressure) between the CuO2 layers of Hg-1223. We find that the energy variation under pressure of the apical oxygen mode from which the charge carriers are transferred to the CuO2 layers, is the same for both the Hg-1223 and Hg-1223 cuprates and it is controlled by the inter-layer compressibility. At last, we show that the binding energy of the Cooper pairs related to the maximum amplitude of the d- wave superconducting gap at the anti-nodes, does not follow Tc with pressure. It decreases while Tc increases. In the particular case of Hg-1201, the binding energy collapses from 10 to 2 KB Tc as the pressure increases up to 10 GPa. These direct spectroscopic observations joined to the fact that the binding energy of the Cooper pairs at the anti-nodes does not follow Tc either with doping, raises the question of its link with the pseudogap energy scale which follows the same trend with doping.