High-temperature superconductivity in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ membranes under pressure

By incorporating freestanding infinite-layer Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ membranes into a diamond anvil cell, researchers demonstrated that hydrostatic pressure induces a strong, linear increase in superconducting transition temperature up to ~90 GPa without saturation, reaching near liquid nitrogen temperatures.

The Gist

Imagine you have a tiny, magical sheet of material that can conduct electricity with zero resistance (superconductivity) when it gets cold enough. Scientists have known about this "magic" in a specific type of material called nickelates for a few years, but it only works at temperatures around -250°C (17 Kelvin). That's cold, but not super cold.

The big question was: How much colder do we have to make it to get it to work at "room temperature" (or at least, liquid nitrogen temperatures)?

Usually, scientists try to squeeze these materials to make them work better. Think of it like squeezing a sponge: sometimes, squeezing changes how the sponge holds water. In the world of atoms, "squeezing" (applying pressure) changes how electrons move, which can boost the superconducting power.

The Problem: The "Sandwich" Trap

Previously, scientists tried to squeeze these nickelate sheets while they were stuck to a hard backing (like a piece of glass or ceramic).

• The Analogy: Imagine trying to squeeze a piece of wet clay that is glued to a brick. You can't squeeze it very hard because the brick stops you, or the clay cracks and breaks.
• The Limit: When they tried to apply high pressure, the "brick" (the substrate) broke the delicate nickelate film long before they could reach the really high pressures needed to see the full potential.

The Solution: The "Floating Sheet"

The team at Stanford and SLAC came up with a clever trick. They figured out how to make the nickelate film freestanding.

• The Analogy: Instead of the clay being glued to a brick, they dissolved the glue and the brick, leaving just the clay sheet floating in mid-air, supported only by a tiny bit of polymer (like a very thin, invisible plastic wrap).
• The Setup: They took this floating sheet and placed it inside a Diamond Anvil Cell (DAC). Think of this as a high-tech vice made of two tiny diamonds. You can squeeze these diamonds together with immense force—enough to crush a car into a cube, but on a microscopic scale.

The Experiment: Squeezing to the Limit

They put their floating nickelate sheet between the diamonds and started squeezing, going up to 90 Gigapascals (GPa).

• To put that in perspective: 90 GPa is about 900,000 times the air pressure at sea level. It's the kind of pressure found deep inside the Earth's core.

The Result: A Linear Super-Boost

Here is the amazing part. In most materials, when you squeeze them this hard, the superconductivity gets better for a while, hits a peak, and then gets worse (like a balloon that stretches too far and pops).

• The Discovery: This nickelate sheet didn't pop. It didn't even slow down.
• The Analogy: Imagine you are pushing a car up a hill. Usually, the car gets harder to push as you go higher, and eventually, it stops. But for this nickelate, every time they pushed the "gas pedal" (pressure), the car went faster and faster in a perfectly straight line.
• The Numbers: For every unit of pressure they added, the temperature at which the material became superconductive went up by 0.65 degrees.
  • It started at ~17°C (wait, no, 17 Kelvin, which is -256°C).
  • At the highest pressure, it jumped to ~74 Kelvin (-199°C).
  • That is a four-fold increase in performance!

Why This Matters

1. No "Ceiling" Yet: Unlike other superconductors (like copper-oxides or iron-based ones) that hit a wall and start getting worse under pressure, this nickelate is still climbing. It suggests that if we could squeeze it even harder (maybe with better diamonds), we might get it to work at even higher temperatures.
2. A New Playground: By making the material "freestanding," they opened a door to testing other 2D materials under extreme pressure without them breaking.
3. The Dream: The ultimate goal of superconductivity research is to find a material that works at room temperature. While 74 Kelvin isn't room temperature yet, this experiment proves that the "engine" of this material has a lot more power than we thought. It's like discovering a car engine that can go 200 mph when everyone thought it was capped at 50 mph.

In short: Scientists took a delicate, floating sheet of superconducting material, put it under the crushing weight of a diamond vice, and found that the harder they squeezed, the better it worked, with no sign of stopping. It's a major step toward understanding how to make superconductors that work in our everyday world.

Technical Summary

1. Problem Statement

Since the discovery of superconductivity in infinite-layer nickelates, researchers have sought to understand the intrinsic upper bound of their superconducting transition temperature ($T_c$). Previous efforts to enhance $T_c$ in thin films relied on:

• Compressive epitaxial strain: Using lattice-matched substrates.
• Chemical pressure: Substituting rare-earth ions (e.g., Samarium) to shorten the $c$-axis lattice parameter.

These methods successfully raised $T_c$ from ~15 K to ~40 K. However, further optimization was constrained by epitaxial stability limits (films cracking or degrading under high strain) and the inability to apply extreme hydrostatic pressure to substrate-bound thin films. Previous high-pressure studies on thin films were limited to ~12 GPa before the substrate-film interface failed, preventing the exploration of the material's behavior at higher pressures where $T_c$ might saturate or decline.

2. Methodology

To overcome the limitations of substrate-bound films, the authors developed a novel experimental platform utilizing freestanding infinite-layer nickelate membranes within a Diamond Anvil Cell (DAC).

• Sample Synthesis: Optimal doped $Nd_{0.85}Sr_{0.15}NiO_2$ thin films (~6.7 nm thick) were grown via pulsed laser deposition on a water-soluble sacrificial layer ($Sr_2Ca_1Al_2O_6$) with protective $SrTiO_3$ capping layers.
• Membrane Release: The sacrificial layer was dissolved in water, releasing a polymer-supported freestanding membrane.
• Integration: The membrane was transferred directly onto a diamond culet in a substrate-free geometry.
• Electrical Contacts: Using a patterned suspended silicon nitride stencil mask, the authors performed sequential ion milling and electron-beam evaporation to create $Ti/Au$ Ohmic contacts in a van der Pauw geometry.
• High-Pressure Setup: The sample was loaded into a non-magnetic DAC with metal gaskets insulated by cubic boron nitride (cBN). Silicone oil was used as the pressure-transmitting medium (PTM).
• Measurement: Electrical transport measurements were conducted under pressures ranging from ambient up to ~91.5 GPa. Magnetic field dependence (up to 13 T) was also measured to determine upper critical fields ($H_{c2}$).

3. Key Contributions

• Novel Platform: The successful development of a technique to integrate freestanding 2D nickelate membranes into a DAC, bypassing the mechanical failure limits of substrate-bound films.
• Extreme Pressure Access: Accessing the pressure regime up to ~90 GPa, far exceeding the ~12 GPa limit of previous thin-film studies.
• Discovery of Non-Saturating $T_c$ Enhancement: Providing the first evidence that infinite-layer nickelates do not exhibit the typical $T_c$ saturation or suppression seen in other high-$T_c$ families under extreme compression.

4. Key Results

• Linear $T_c$ Enhancement: The superconducting transition temperature ($T_c$) increased monotonically and linearly with pressure at a rate of 0.65 K GPa$^{-1}$.
• Record High $T_c$: At the maximum measured pressure of ~91.5 GPa, the onset $T_c$ reached ~74.2 K (with zero-resistance occurring near liquid nitrogen temperatures).
• Absence of Saturation: Unlike cuprates, bilayer nickelates ($La_3Ni_2O_7$), and iron selenides, which typically show a $T_c$ "dome" (rise followed by suppression due to overdoping or structural changes), the $Nd_{0.85}Sr_{0.15}NiO_2$ membranes showed no signs of saturation or downturn up to 90 GPa.
• Strengthening Superconducting State:
  • The in-plane coherence length ($\xi_{ab}$) decreased monotonically from ~2.39 nm (ambient) to ~1.54 nm (71.8 GPa), indicating a strengthening of the pairing interaction.
  • The upper critical field ($H_{c2}$) increased significantly with pressure.
  • The transition width narrowed at higher pressures (>60 GPa), suggesting that inhomogeneities became less limiting as pressure increased.

5. Significance

• Redefining the Upper Bound: The results suggest that the intrinsic pairing strength in infinite-layer nickelates can be raised to a surprisingly high scale, far exceeding the ~40 K achieved via chemical strain alone.
• Mechanistic Insight: The lack of $T_c$ suppression challenges theoretical models predicting that c-axis compression would lead to self-doping and an overdoped state (which usually suppresses $T_c$). Instead, the linear increase implies that the electronic structure remains favorable for superconductivity even at extreme compression.
• Broader Applicability: The technique of using freestanding membranes in a DAC is broadly applicable to other two-dimensional materials, opening new avenues for exploring high-pressure physics in 2D superconductors and quantum materials.
• Comparison to Cuprates: The behavior contrasts sharply with cuprates, where pressure often leads to overdoping and $T_c$ suppression. This suggests that infinite-layer nickelates may possess a unique mechanism for sustaining superconductivity under extreme lattice contraction.