Controlling the Band Filling and the Band Width in Nickelate Superconductors

Abstract: The new family of superconducting nickelates centered around La$<em>{3}$Ni$</em>{2}$O$_{7}$ possesses attractive features, such as the high transition temperature and the presence of an antiferromagnetic ground state at ambient pressure, suggesting an unconventional pairing mechanism. In the nonsuperconducting state, the possibility of different density-wave orders with opposite pressure dependencies is discussed, whose relationships and microscopic origins are largely unknown. However, sample-quality issues, such as impurity-phase formation or oxygen vacancies, impede the progress in the field. Here, we employ high-pressure synthesis and hydrostatic high-pressure transport techniques to investigate bilayer nickelates with controlled band width and filling, and perform a systematic study on their impact on the superconductivity and other characteristic properties. While increasing the tilting of the NiO$_6$ octahedra shifts the superconducting phase to higher pressure, simultaneous hole doping reverts this trend. We also observe up to three distinct anomalies in the nonsuperconducting state which are possibly related to density-wave formation.

• The paper demonstrates that independent tuning of band filling and bandwidth significantly affects superconducting onset and density-wave instabilities in bilayer nickelates.
• It employs high-resolution STEM, EELS mapping, and pressure-dependent transport measurements to systematically correlate structural distortions with electronic phase evolution.
• Results show that dopant-induced modulation of lattice distortion optimizes superconductivity, providing new design pathways for correlated oxide superconductors.

Control of Band Filling and Band Width in Nickelate Superconductors

Background and Motivation

The recent surge of interest in bilayer nickelate superconductors, particularly La$_3$Ni$_2$O$_7$ (3), has prompted a thorough investigation of their electronic structure and unusual superconducting properties. These compounds exhibit ambient-pressure antiferromagnetic states and high superconducting onset temperatures under pressure, suggesting unconventional pairing mechanisms possibly distinct from cuprates. The Ruddlesden-Popper (RP) structure enables independent tuning of electronic band width (via isovalent substitution) and band filling (via aliovalent doping), which provides a robust platform for systematic exploration of the interplay between lattice structure, charge carrier concentration, and emergent electronic phases.

Structural and Electronic Configuration

The crystal structure comprises perovskite-like bilayers of tilted NiO$_6$ octahedra separated by rock-salt-like LaO blocks, featuring two crystallographically distinct La positions (R(1), R(2)). This architectural motif is central to the observed electronic phenomena. The electronic structure, delineated via a bilayer model, shows pronounced bonding-antibonding splitting of the 3z$^2$-r$^2$ orbitals and smaller splitting for x$^2$-y$^2$, both with significant dispersion as hybridization increases.

Figure 1: Crystal and electronic structures, showing orthorhombic RP bilayers, La positions, and $e_g$ orbital splitting.

Atomic-resolution STEM and EELS mapping confirm phase purity and reveal Nd preferentially substituting at the R(2) site, facilitating systematic modulation of lattice distortion and band parameters. Such structural control is critical, as the alignment and tilting of NiO$_6$ octahedra correlate directly with superconducting phase emergence.

Transport under Pressure and Phase Evolution

Temperature-dependent resistivity measurements across pressures up to 20 GPa for 3, 2, and 1.9 compositions (La$_2$0Ni$_2$1O$_2$2, (La$_2$3Nd$_2$4)Ni$_2$5O$_2$6, (La$_2$7Nd$_2$8)Ni$_2$9O$_7$0) unveil systematic changes:

• For 3, metallic behavior persists to 10 GPa, beyond which a semiconductor upturn appears at low temperature, transitioning into superconductivity at pressures $_7$116 GPa with zero resistance at $_7$2 K at 20 GPa.
• 2 exhibits higher resistivity and semiconducting behavior up to 20 GPa where onset of superconductivity is barely reached, indicating band narrowing due to enhanced octahedral tilting.
• 1.9 undergoes a pressure-driven semiconductor-to-metal transition between 12 and 14 GPa, achieving zero resistance at $_7$3 K at 20 GPa.

Figure 2: Pressure dependence of temperature-resistivity for La-, Nd-, and Sr-doped nickelates across various GPa regimes.

Distinct anomalies are detected in the nonsuperconducting regime, interpreted as signatures of density-wave instabilities (CDW, SDW). Three characteristic anomalies ($_7$4, $_7$5, $_7$6) are quantified: $_7$7 (low-$_7$8 anomaly, suppressed with pressure, likely CDW), $_7$9 (high-$_6$0 anomaly, enhanced with pressure, candidate SDW), and $_6$1 (resistivity minima, origin unclear).

Pressure-Temperature Phase Diagrams

Comprehensive $_6$2-$_6$3 phase diagrams for each composition substantiate the interplay between band filling/band width and electronic phases:

Figure 3: $_6$4-$_6$5 phase diagrams outlining superconducting regions, resistivity contour maps, and anomalies in La$_6$6Ni$_6$7O$_6$8, (La$_6$9Nd$^2$0)Ni$^2$1O$^2$2, (La$^2$3Nd$^2$4)Ni$^2$5O$^2$6.

• In 3, superconductivity emerges promptly after suppression of CDW anomaly ($^2$7).
• Nd-substitution (2) increases critical pressure for superconductivity, confirming band narrowing.
• Sr doping in 2-x compositions counteracts band narrowing, lowers critical pressure for superconductivity, and sequentially suppresses $^2$8 anomaly. The superconducting regime is limited to a narrow band-filling range (optimal Sr/La/Nd ratios).

Composition Dependence of Physical Properties

Ambient-pressure resistivity, anomaly temperatures, and critical pressure for zero resistance are mapped against Sr concentration:

Figure 4: Sr concentration dependence of ambient resistivity, anomaly temperature $^2$9, and critical pressure for superconductivity in 3-x and 2-x nickelates.

• Nd$^2$0 substitution enhances $^2$1 anomaly and increases resistivity, attributed to band narrowing via increased octahedral distortion.
• Sr$^2$2 doping introduces holes, restores conductivity, reduces critical pressure for superconductivity, and suppresses DW anomalies.
• The solubility limit for Sr is evident at $^2$3, beyond which impurity phases and localization effects manifest.

Implications and Future Directions

These findings provide direct evidence for tunability of electronic phases in bilayer nickelates via independent control of band width and band filling. The phase diagrams reveal decoupling of CDW and SDW features, in contrast to cuprate systems where DW instabilities are typically intertwined, indicating a fundamental difference in ground state competition.

The results underscore the sensitivity of superconductivity to lattice distortion, carrier concentration, and impurity phase control. The high-pressure synthesis protocol significantly mitigates sample-quality-related discrepancies prevalent in prior studies. The observed narrow optimal doping window and bifurcation of DW anomalies may inform future theoretical models and high-T$^2$4 design strategies.

Experimental approaches combining high-resolution structural analysis, hydrostatic pressure, and fine compositional tuning establish a framework for dissecting correlated electron systems beyond the nickelate paradigm.

Conclusion

This work demonstrates systematic engineering of superconducting and density-wave phases in RP bilayer nickelates via controlled manipulation of band parameters. The pressure and dopant-dependent phase evolution clarifies the role of structural distortion and hole doping in governing superconducting onset and DW instabilities. The decoupling of DW anomalies, critical pressure trends, and optimized superconducting windows provide a quantitative roadmap for further exploration of correlated oxide superconductors and underscore the distinct electronic physics of the nickelate family relative to cuprates (2604.13875).