Pressure Driven Fractionalization of Ionic Spins Results in Cupratelike High-$T_c$ Superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$

Beyond 14 GPa of pressure, bilayered ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ was recently found to develop strong superconductivity above the liquid nitrogen boiling temperature. An immediate essential question is the pressure-induced qualitative change of electronic structure that enables the exciting high-temperature superconductivity. We investigate this timely question via a numerical multiscale derivation of effective many-body physics. At the atomic scale, we first clarify that the system has a strong charge transfer nature with itinerant carriers residing mainly in the in-plane oxygen between spin-1 ${\mathrm{Ni}}^{2+}$ ions. We then elucidate in electron-volt scale and sub-electron-volt scale the key physical effect of the applied pressure: it induces a cupratelike electronic structure via fractionalizing the Ni ionic spin from 1 to $1/2$. This suggests a high-temperature superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ with microscopic mechanism and ($d$-wave) symmetry similar to that in the cuprates.

Figure 1

Location of itinerant carriers and the importance of correlation with local moments. Panels (a) and (b) show the one-body spectral functions of the Curie-paramagnetic state in the low-pressure phase and high-pressure phase, respectively, containing a large number of random noncollinear Ni-spin directions. Contributions from orbitals of Ni, La, ${\mathrm{O}}^{\perp }$ (apical oxygen), and ${\mathrm{O}}^{\parallel }$ (in-plane oxygen) are represented in different colors. Notice that only one dominant charge carrier ${\mathrm{O}}^{\parallel }$ band crosses the Fermi level at zero that strongly scatters against local moments of Ni ions (thus its fuzziness).

Figure 2

Illustration of the multiscale emergence of pressure-induced fractionalization of ${\mathrm{Ni}}^{2+}$ ionic spin in the local bilayer $\mathrm{Ni}─\mathrm{O}─\mathrm{Ni}$ component. At Hartree scale, the dominant energy is the charge-transfer energy to Ni-$d^9$ configuration. At electron-volt scale, the charge freedom of Ni-$d^9$ is therefore frozen and the fully occupied ${\mathrm{O}}^{\perp }$ orbital becomes inactive (shadowed), leaving only spins dynamics of Ni orbitals active. At low pressure, the stronger screened Hund’s coupling ${\tilde{J}}_{\mathrm{H}}$ dictates spin-1 Ni ions at sub-electron-volt scale with large interlayer exchange energy $E_{\mathrm{x}}^{\perp }\sim 3J^{\perp }\sim 250\mathrm{meV}$. In contrast, at high-pressure, an unusually strong interlayer superexchange $J_{ZZ}$ emerges and causes an inactive (shadowed) spin-0 singlet of $d_{3z^2-r^2}$ orbitals. The sub-electron-volt description therefore resembles that of the high-$T_{\mathrm{c}}$ cuprates, with a “fractionalized” effective $1/2$ spin of ${\mathrm{Ni}}^{2+}$ ions.