Merits of heavy-heavy diatomic molecules for electron electric-dipole-moment searches

The electric dipole moment of the electron ($e\mathrm{EDM}$) and the scalar-pseudoscalar (S-PS) interaction are probes of new physics beyond the standard model of elementary particles, but experiments to observe them using atoms and molecules are still in progress. Molecules that have a large effective electric field ($E_{\mathrm{eff}}$), S-PS coefficient ($W_s$), and permanent electric dipole moment (PDM) are in principle favorable candidates for such experiments, and hence, it is necessary to analyze these properties. In this work, we calculate $E_{\mathrm{eff}}$, $W_s$, and PDM for Ra systems: RaF, $\mathrm{Ra}X$ ($X=\mathrm{Cl}$, Br, I, and At), and $\mathrm{Ra}Z$ ($Z=\mathrm{Cu}$, Ag, and Au), using the Dirac-Fock and the relativistic coupled-cluster methods. The size of $E_{\mathrm{eff}}$ and $W_s$, and Ra of RaF, $\mathrm{Ra}X$, and $\mathrm{Ra}Z$ are overall similar, but the values of $\mathrm{Ra}X$ and $\mathrm{Ra}Z$ (except for RaAu) are larger than those of RaF, up to about $40\%$. We explain this finding by taking into consideration the large $s\text{−}p$ mixing for $\mathrm{Ra}X$ and $\mathrm{Ra}Z$, similar to what we had done in our previous work using hydrides and fluorides [Sunaga et al., Phys. Rev. A 95, 012502 (2017)]. We also discuss the suitability of $\mathrm{Ra}X$ and $\mathrm{Ra}Z$ molecules for $e\mathrm{EDM}$ experiments from the viewpoint of their large PDM and small polarizing electric field ($E_{\mathrm{pol}}$).