High-temperature superconducting dome mapped in nickelate thin films

Physicists hunting for new superconductors often look for a very specific shape hidden in their data—a dome. 

In many of the world’s most intriguing superconducting materials, superconductivity appears only within a curved region of a phase diagram, rising to a peak before fading again. 

Spotting this superconducting dome is often a sign that researchers have stumbled onto the right ingredients for high-temperature superconductivity. 

Now, a new study reports this telltale feature in thin films of a nickel-based material, La₃Ni₂O₇, offering an important clue about how superconductivity might emerge in a new family of compounds.

“Our work was inspired by recent pioneering breakthroughs in the nickelate community, particularly the discovery of high-pressure superconductivity in bulk and strained thin films of La3Ni2O7,” Yuefeng Nie, one of the study authors and a professor at Nanjing University, told Phys.org.

Building an ultrathin nickelate with atomic precision

The team focused on a compound called La₃Ni₂O₇, a nickelate made of nickel and oxygen atoms arranged in layered structures. 

Nickelates have recently drawn intense interest because they resemble cuprates, the copper-oxide materials that hold the record for high-temperature superconductivity. 

However, despite this similarity, scientists still lack a clear map of how different electronic states appear in nickelates as conditions change. This phase diagram, a chart showing how a material behaves as variables such as doping or temperature change, is crucial for understanding how superconductivity emerges. 

“A key piece of the puzzle was still missing: the phase diagram. We wanted to see if this bilayer system has a ‘superconducting dome’—the classic hallmark of unconventional high-Tc superconductors,” Yuefeng Nie said. 

Creating the material itself was a major challenge. Nickelate thin films must be grown with atomic-level precision, otherwise their delicate electronic properties disappear. To accomplish this, the researchers used a technique called reactive molecular beam epitaxy (MBE). 

In simple terms, MBE allows scientists to build a crystal layer by layer, almost like assembling a structure with atomic-scale LEGO blocks. 

The researchers grew thin films of La₃Ni₂O₇ on specially chosen substrates that slightly compressed the crystal lattice, a process known as strain engineering.

Tuning the material to reveal superconductivity

Once the films were created, the researchers needed ways to carefully adjust the number of charge carriers—particles that carry electrical current inside the material. They used two main tuning knobs.

First, they replaced some lanthanum (La) atoms in the crystal with strontium (Sr) atoms. This process, called doping, changes how many charge carriers exist in the material. 

Second, they modified the amount of oxygen in the film through in-situ vacuum annealing, which can create small oxygen vacancies that further influence the material’s electronic behavior.

By combining strontium doping and oxygen tuning, the team produced many slightly different versions of the material. “Both methods serve as effective ways to tune the carrier concentration and modulate superconductivity, much like in cuprates,” Nie added.

They then measured their electrical properties and tracked a quantity known as the Hall coefficient, which reveals whether the dominant charge carriers behave like positively charged holes or negatively charged electrons. 

Since the electronic structure of nickelates involves several energy bands, determining the exact carrier density is difficult, so the Hall coefficient served as a practical way to map how the system evolves.

After compiling all the measurements, the researchers constructed a full phase diagram of the material. The result revealed a surprising pattern—superconductivity appeared and strengthened over a specific range of conditions, forming a curved region known as a superconducting dome. 

The peak of this dome occurred when the Hall coefficient changed sign, indicating that the dominant charge carriers switched from holes to electrons.

The significance of the superconducting dome

The appearance of a superconducting dome is significant because “these features look very similar to what we see in electron-doped copper-based superconductors.

It implies that superconductivity here may be closely related to a Fermi surface reconstruction and electronic symmetry, just like in the cuprates,” Nie added.

In short, the arrangement of electronic states inside the material changes as the dominant charge carriers switch.

The phase diagram provides an important roadmap for researchers studying nickelates and could help guide the design of new materials that superconduct at higher temperatures or without high pressure. 

However, the current results mainly capture the material’s overall behavior. 

So to understand the microscopic mechanisms behind the superconductivity, the study authors plan to use angle-resolved photoemission spectroscopy (ARPES) to directly observe how the electronic structure evolves during the carrier crossover.

The study is published in the journal Physical Review Letters.