High-Pressure Breakthrough: Reaching 74 K in Infinite-Layer Nickelate Membranes

Executive Summary

TL;DR: Researchers from Stanford and SLAC have shattered the previous temperature limits for infinite-layer nickelate superconductivity by applying extreme pressure (up to 91.5 GPa) to freestanding membranes. By removing the "handcuffs" of the growth substrate, they achieved a $T_c$ of 74.2 K, moving from the liquid helium to the liquid nitrogen temperature regime.

Background Positioning: This work represents a massive leap in the nickelate field. It moves the study from "epitaxial strain tuning" (limited to ~40 K) to "extreme volumetric compression," revealing a unique linear scaling of $T_c$ that distinguishes nickelates from the well-known "dome" behavior of cuprates.

The "Substrate Bottleneck"

Since the 2019 discovery of superconductivity in $NdNi{O}_2$, the community has known that "squeezing" the lattice (lattice contraction) increases $T_c$. However, experts faced a wall:

1. Epitaxial Limits: Thin films are grown on substrates. If you squeeze too hard, the film cracks or the substrate dictates the structural response.
2. High Pressure Failures: Previous attempts to compress films on their substrates led to sample deterioration at relatively low pressures (~12 GPa).

The authors' insight was simple but technically daunting: Delete the substrate. By using a water-soluble sacrificial layer, they created a freestanding "nickelate leaf" that could be placed directly into a Diamond Anvil Cell (DAC).

Methodology: The Freestanding Advantage

The core of the experiment involves a complex transfer process (Fig. 1). The membrane is only about 6.7 nm thick, protected by $SrTi{O}_3$ capping layers. Once freed, it behaves like a 2D material flake.

Fig 1: Development of the DAC platform for freestanding membranes. Note the van der Pauw geometry for electrical contacts directly on the diamond culet.

By using silicone oil as a pressure-transmitting medium, the researchers subjected the $N{d}_{0.85}S{r}_{0.15}Ni{O}_2$ membrane to nearly hydrostatic pressure. Unlike previous studies, the membrane survived up to the mechanical limits of the diamonds (~90 GPa).

Results: Breaking the "Dome"

The most striking finding is the Linearity. In almost all unconventional superconductors (Cuprates, Fe-based, etc.), $T_c$ eventually hits a peak and falls (the "superconducting dome") because of over-doping or structural changes.

In $N{d}_{0.85}S{r}_{0.15}Ni{O}_2$, the $T_c$ just keeps going.

Fig 2: Comparison of Tc(P) across different materials. The nickelate membrane (solid symbols) shows a persistent linear rise of ~0.65 K/GPa with zero sign of saturation, unlike the bilayer nickelate (La3Ni2O7) or Hg-based cuprates.

Key Metrics:

• Maximum $T_c$: 74.2 K (at 91.5 GPa).
• Scaling: 0.65 K per GPa.
• Coherence Length (${\xi }_{ab}$): Reduced from 2.39 nm to 1.54 nm, indicating that the Cooper pairs are more tightly bound as pressure increases.

Deep Insight: Why Doesn't it Overdope?

Theoretically, pressure should cause "self-doping" by compressing the rare-earth $5d$ electron pockets, which usually suppresses superconductivity at high levels. The fact that the $T_c$ hasn't peaked suggests:

1. The pairing mechanism in infinite-layer nickelates is incredibly robust against self-doping.
2. The lack of buckling (structural simplicity) in the infinite-layer phase allows the electronic structure to evolve cleanly without the "structural noise" that plagues more complex cuprates.

Conclusion & Future Outlook

This work demonstrates that the "speed limit" for $T_c$ in nickelates is much higher than previously thought. If the linear trend continues, 100 K+ might be achievable if diamond technology allows for higher pressures.

Limitations: The transition width at high pressure is relatively broad, likely due to the non-hydrostatic nature of solidified silicone oil above 3 GPa. Future experiments using Helium as a pressure medium might yield even cleaner results.

Takeaway: The "freestanding membrane + high pressure" workflow is a new gold standard for exploring the limits of 2D materials and unconventional superconductors.