On the theory of supermodulation of the superconducting order parameter by supermodulation of the apex distance in optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (2506.15726v2)

Abstract: Recently using Scanning Josephson Tunneling Microscopy (SJTM) in the group of S\'eamus Davis a super-modulation of the superconducting order parameter induced by super-modulation of the distance $\delta$ between planar Cu and apical O was observed in [O'Mahony et al, On the electron pairing mechanism of copper-oxide high temperature superconductivity, PNAS Vol. 119(37), e2207449119 (2022)]. The authors conclude: "concurrence of prediction from strong correlation theory... with these observations indicates that... super-exchange is the electron pairing mechanism of Bi$2$Sr$_2$CaCu$_2$O${8+x}$." Additionally, the charge transfer energy $\mathcal{E}$, probably between O2$p_z$ and Cu$3d_{x^2-y^2}$ levels was studied by SJTM, too. In our current theoretical study we use LCAO approximation and Hilbert space spanned on 5 atomic orbitals: Cu$4s$, Cu$3d_{x^2-y^2}$, O2$p_x$, O2$p_y$, O2$p_z$. For the only super-exchange amplitude $J_{sd}$ we use the Kondo double electron exchange between Cu$4s$ and Cu$3d_{x^2-y^2}$ orbitals and its anti-ferromagnetic sign is determined by adjacent to the copper ion in-plane oxygen orbitals. Within this approximations we have calculated: "Measured dependence of... electron-pair density $n_p$ on the displacement $\delta$ of the apical O atoms from the planar Cu atoms" on [O'Mahony et al., Fig. 5C] and obtained an excellent accuracy. We discuss that the correlation between the shape of Fermi contour and the critical temperature of optimally hole-doped cuprates can be considered as an analogue of the isotope effect of phonon superconductors. The analyzed SJTM experiment is one of the best confirmations of the [J. R\"ohler, Plane dimpling and Cu4s hybridization in YBa$2$Cu$_3$O${7-x}$, Physica B: Cond. Matt. Vol. 284-288, 1041 (2000)] idea that the hybridization of Cu$4s$ with conduction band leads to increasing of $T_c$.