Exploration of the Hg-based cuprate superconductors by Raman spectroscopy under hydrostatic pressure

TL;DR

This study uses Raman spectroscopy under pressure to investigate Hg-based cuprate superconductors, revealing how pressure affects their superconducting properties and the relationship between pairing energy and critical temperature.

Contribution

It provides new insights into the pressure dependence of superconducting gap and charge transfer mechanisms in Hg-based cuprates, highlighting differences between Hg-1201 and Hg-1223.

Findings

$T_c$ increases slower in Hg-1223 due to charge imbalance.

The apical oxygen mode energy variation is consistent across compounds.

Binding energy of Cooper pairs decreases with pressure, not following $T_c$.

Abstract

The superconducting phase of the $\mathrm{HgBa}_2\mathrm{CuO}_{4+\delta}$ (Hg-1201) and $\mathrm{HgBa}_2\mathrm{Ca}_2\mathrm{Cu}_3\mathrm{O}_{8+\delta}$ (Hg-1223) cuprates has been investigated by Raman spectroscopy under hydrostatic pressure. Our analysis reveals that the increase of $T_c$ 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 $\mathrm{CuO}_2$ 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 $\mathrm{CuO}_2$ 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 $T_c$ with pressure. It decreases while $T_c$ increases. In the particular case of Hg-1201, the binding energy collapses from 10 to 2 $K_B T_c$ 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 $T_c$ either with doping, raises the question of its link with the pseudogap energy scale which follows the same trend with doping.