Theoretical designing of multiband Nickelate and Palladate superconductors with $d^{8+\delta}$ configuration

TL;DR

This study explores the potential for high-temperature superconductivity in multiband nickelate and palladate materials with specific electron configurations, focusing on the effects of pressure, doping, and orbital energy offsets.

Contribution

It extends previous theoretical models to predict superconductivity in synthesized and hypothetical materials by analyzing orbital energy offsets and the effects of pressure and doping.

Findings

Large energy level offsets favor superconductivity.

Pressure application can enhance orbital offsets.

Electron doping may further promote superconductivity.

Abstract

In a previous study, we proposed a possibility of high $T_c$ superconductivity in mixed-anion nickelates with $d^{8+\delta}$ electron configuration. The theory was based on the fact that the two-orbital Hubbard model, when all the intra- and interorbital interactions have the same magnitude, is equivalent to the bilayer Hubbard model, which has been suggested to exhibit high $T_c$ superconductivity. The energy level offset $\Delta E$ in the two-orbital model is transformed to twice the interlayer hopping in the bilayer model, and hence appropriately large $\Delta E$ is favorable for superconductivity in the former. Extending this idea to multiorbital systems, we previously suggested materials with large energy level offset between $d_{x^2-y^2}$ and other $d$ orbitals, such as Ca$_2$NiO$_2$Cl$_2$, to be good candidates for high $T_c$ superconductivity, but such materials have not been synthesized to our knowledge. In the present study, we first focus on Sr$_2$NiO$_2$Cl$_2$, which has been synthesized in the past but has small $\Delta E$, and study the effect of applying pressure, which enhances $\Delta E$. We also study a 4d analogue of Sr$_2$NiO$_2$Cl$_2$, namely, Sr$_2$PdO$_2$X$_2$ ($X=$ Cl, F, H) , in which $\Delta E$ turns out to be large. The analysis using fluctuation exchange approximation suggests possibility of superconductivity in these systems with large $\Delta E$. We also study the effect of electron doping of these material, which is expected to enhance superconductivity, within the virtual crystal approximation.