Fermi-liquid transport beyond the upper critical field in superconducting La2PrNi2O7 thin films

Why this ultrathin superconductor matters

Superconductors carry electricity with zero resistance, promising ultra-efficient power lines, powerful magnets, and faster electronics. A new family of nickel-based superconductors has recently stunned physicists with critical temperatures approaching those of the best copper-oxide materials. This study focuses on an especially promising member, an ultrathin film of La2PrNi2O7, and asks a basic but crucial question: what kind of “normal” metal lies underneath its superconducting state when superconductivity is switched off by a strong magnetic field?

Peeling back the superconducting layer

In many unconventional superconductors, the normal state just above the transition temperature behaves in a highly unusual way: its electrical resistance often increases in direct proportion to temperature, a hallmark of so‑called "strange metals." In contrast, ordinary metals follow a more familiar rule where resistance grows with the square of temperature. To find out which scenario applies to La2PrNi2O7 thin films, the researchers used extremely strong pulsed magnetic fields, up to 64 tesla, to suppress superconductivity and expose the underlying metallic behavior over temperatures from just 1.5 kelvin up to room temperature.

A surprisingly conventional metal underneath

The measurements show that once superconductivity is quenched, the film behaves like a classic Fermi liquid—a metal where well-defined electron-like quasiparticles carry current and scatter off one another in a predictable way. The electrical resistivity follows a nearly perfect temperature‑squared law as the temperature approaches absolute zero. The same quadratic trend appears in the “Hall angle,” a measure of how charge carriers bend sideways in a magnetic field. In addition, the magnetoresistance—the change in resistance with magnetic field—grows with the square of field strength and neatly collapses onto a single curve when plotted in a standard way known as Kohler scaling. Together, these signatures signal a highly coherent, strongly interacting, yet fundamentally conventional metallic state.

Unusual heaviness and directional behavior

Even though the underlying metal is Fermi-liquid-like, it is far from ordinary. By combining their transport data with an empirical relationship known as the Kadowaki–Woods ratio, the authors infer that the charge carriers in La2PrNi2O7 behave as if they are about ten times heavier than free electrons. This “heaviness” reflects strong electronic correlations, meaning electrons strongly influence each other’s motion. The team also tracks how the upper critical magnetic field—the field strength needed to destroy superconductivity—depends on temperature and field orientation. They find that the film can withstand more than twice as much field when the field lies in the plane of the layers than when it points perpendicular to them, revealing a pronounced two‑dimensional character similar to that of well-known high‑temperature copper‑oxide superconductors.

A common yardstick across many quantum materials

Using estimates of the carrier density and the inferred effective mass, the researchers calculate an effective Fermi temperature, a measure of the energy scale of the underlying electrons. They then compare the ratio of the superconducting transition temperature to this Fermi temperature with values from a wide range of exotic superconductors, including cuprates, iron-based materials, heavy-fermion compounds, organic superconductors, and magic-angle graphene. La2PrNi2O7 falls right on the same empirical line, where the transition temperature is about five percent of the Fermi temperature. This reinforces the view that, despite their microscopic differences, many strongly correlated superconductors share a common organizing principle that sets the scale of their transition temperatures.

What this means for future superconductors

For non-specialists, the key message is that this nickelate thin film hosts an unusually robust superconducting state emerging from an equally unusual yet orderly metallic background. Rather than a chaotic strange metal, the normal state behaves as a heavy, strongly interacting but well-disciplined Fermi liquid, with electrons scattering off each other so strongly that they overshadow vibrations of the crystal lattice even up to room temperature. By firmly establishing this starting point and placing La2PrNi2O7 on the same universal scale as other unconventional superconductors, the work provides a solid foundation for understanding how high-temperature superconductivity arises in this new family—and suggests that carefully tuning strain or doping could push their performance even higher.

Citation: Hsu, YT., Liu, Y., Kohama, Y. et al. Fermi-liquid transport beyond the upper critical field in superconducting La₂PrNi₂O₇ thin films. Nat Commun 17, 3760 (2026). https://doi.org/10.1038/s41467-026-70250-4

Keywords: nickelate superconductors, Fermi liquid, thin films, high magnetic fields, strongly correlated electrons