Strain derivatives of T c in HgBa₂CuO₄+ δ: the CuO₂ plane alone is not enough

The strain derivatives of Tc along the a and c axes have been determined for HgBa₂CuO₄+δ (Hg1201), the simplest monolayer cuprate with the highest Tc of all monolayer cuprates (Tc = 97 K at optimal doping). The underdoped compound with the initial Tc of 65 K has been studied as a function of pressure up to 20 GPa by magnetic susceptibility and x-ray diffraction. The observed linear increase in Tc with pressure is the same as previously found for the optimally doped compound. The above results have enabled an investigation of the origins of the significantly different Tc values of optimally doped Hg1201 and the well-studied compound La₂- xSrxCuO₄ (LSCO), which has a maximal Tc of 40 K, or only 40% of that of Hg1201. Hg1201 can have almost identical CuO₆ octahedra as LSCO if specifically strained. When the apical and in-plane CuO₂ distances are the same for the two compounds, a large discrepancy in their Tc remains. Differences in crystal structures and interactions involving the Hg-O charge reservoir layers of Hg1201 may be responsible for the different Tc values exhibited by the two compounds.