Pressure-induced trans-proximate correlation in La$_4$Ni$_3$O$_{10}$ and possible routes to enhance its superconductivity

Abstract

We report an unexpected trans-proximate interlayer correlation (stronger correlation between disjoint layers than the adjacent ones) in the high-pressure phase of the recently discovered La$_4$Ni$_3$O$_{10}$ superconductors. Accompanied by an unusual pressure-induced fractionalization of Ni$^{2+}$ ionic spin from the standard spin-1 to spin-$\frac{1}{2}$, this trans-proximate correlation results from the emergence of a cross-layer trimer in our multi-energy-scale derivation of the electron dynamics. The resulting low-energy effective description resembles that of the cuprates and suggests a universal superconducting mechanism in all existing nickelate and cuprate superconductors. The rare trans-proximate correlation not only explains the weaker superconductivity in comparison with the related La$_3$Ni$_2$O$_7$ samples, but it also indicates a viable strategy to improve superconductivity in this trilayer nickelate by lowering layer symmetry. Such pressure-induced trans-proximate correlation is expected in many materials and examplifies the engineering of rich uncharted quantum states of matter through pressure.

Figures (3)

• Figure 1: Itinerant carriers and their correlation with ionic moments. One-body spectral functions of Curie-paramagnetic La$_4$Ni$_3$O$_{10}$ in the (a) low-pressure and (b) high-pressure phases. In both phases, charge carriers are holes on the in-plane oxygens (O$^\parallel$) that strongly scatter with adjacent Ni ionic spins, leading to the observed spread.
• Figure 2: Multi-scale illustration of the pressure-induced fractionalization of ionic spins and emergence of cross-layer trimer composite spins in La$_4$Ni$_3$O$_{10}$. Starting from Hartree scale, upon absorbing rapid charge fluctuations of Ni$^{2+}$ ions in the local Ni-O-Ni-O-Ni trilayer component, the fully occupied effective O$^{(\text{m})}$-$p_z$ orbital becomes inert (shadowed), leaving only spins dynamics of Ni orbitals active at eV scale. At low-pressure, the stronger effective Hund's coupling $\Tilde{J}_\mathrm{H}$ dictates a typical spin-1 Ni$^{2+}$ ions at sub-eV scale with large interlayer exchange energy. In contrast, due to the unusually strong interlayer super-exchange $J_{ZZ}$ at high-pressure, a cross-layer trimer composite $\frac{1}{2}$-spin (purple ellipsoid) emerges from spins in the $d_{3z^2-r^2}$ orbitals and effectively fractionalizes the Ni$^{2+}$ ionic spin from 1 to $\frac{1}{2}$ at sub-eV scale. To enhance the supercondcuting phase stiffness through relieving the trimer-induced interlayer correlation, a possible scenario is to promote singlet dimerization via lowering trilayer symmetry, for example with a O vacancy.
• Figure 3: Illustration of the unusual 'trans-proximate' correlation and possible enhancement of superconductivity with lowered local layer symmetry (a) Unusual trans-proximate interlayer correlations includes a strong correlation between disjoint outer layers that nonetheless correlate weakly with the adjacent middle layer. (b) Possible enhancement of superconductivity (SC) via lowered local layer symmetry that allow more freely fluctuating $\frac{1}{2}$-spins connected throughout the sample.