High-field-stabilized reentrant superconductivity in infinite-layer nickelate thin films

This paper reports the discovery of high-field-stabilized reentrant superconductivity in infinite-layer nickelate thin films with transition temperatures up to 40 K, where both low- and high-field superconducting states are attributed to a Jaccarino-Peter-like compensation mechanism that significantly enhances the upper critical field.

The Gist

The Big Idea: Superconductors That Love Strong Magnets

Usually, if you take a superconductor (a material that conducts electricity with zero resistance) and put it near a powerful magnet, the magnet acts like a bully. It pushes the superconducting electrons apart, killing the superconductivity. It's like trying to hold hands in a crowd of people pushing you apart; eventually, you let go.

However, this paper reports a rare and surprising discovery: the researchers found a material where a strong magnet actually helps the superconductivity come back to life after it has been killed by a weaker magnet. They call this "reentrant superconductivity."

The Material: A Special "Nickel Cake"

The material they studied is a thin film of a special type of nickel compound (called an infinite-layer nickelate). Think of this material as a very thin, delicate cake made of layers of nickel and oxygen.

• The Goal: They wanted to see if this "cake" could stay superconducting in extremely strong magnetic fields, which is usually impossible.
• The Setup: They made these films very thin (about 4 to 7 nanometers thick—thinner than a strand of DNA) and placed them on a special base.

The Experiment: The "Push and Pull" Game

Imagine the electrons in the material are trying to dance together in pairs (this is what makes them superconduct).

1. The Magnet's Push: When the researchers turned on a magnetic field, it tried to push the electron pairs apart. At a low field (about 1 Tesla), the dance stopped, and the material became a normal resistor again.
2. The Surprise Dip: As they increased the magnetic field even more, something weird happened. The resistance didn't just stay high; it dropped slightly.
3. The Big Comeback: When they cranked the magnetic field up to massive levels (around 20 to 65 Tesla—stronger than most hospital MRI machines), the resistance dropped all the way to zero again. The electrons started dancing in pairs once more, even though the magnet was stronger than ever.

The Secret Weapon: The "Internal Bodyguard"

Why did this happen? The paper explains it using a concept called the Jaccarino–Peter effect.

Imagine the magnetic field is a giant wind trying to blow the dancers apart. Usually, this wind wins. But in this specific material, there are special atoms (Europium) acting like internal bodyguards.

• These bodyguards have their own tiny magnetic fields pointing in the opposite direction of the giant wind.
• When the giant wind (the external magnet) gets strong enough, it forces these bodyguards to stand up and point their shields directly against the wind.
• At a certain strength, the bodyguards' shields perfectly cancel out the wind. The dancers are suddenly safe again, and the superconductivity returns.

The researchers found that about two-thirds of the Europium atoms in their material were in the right "state" to act as these bodyguards.

The Results: Breaking the Limits

The team tested several versions of this material with different temperatures and thicknesses.

• Low-Temperature Samples: They saw the superconductivity die at low fields, then come back at high fields (around 20–30 Tesla).
• High-Temperature Samples: In samples that were already superconducting at higher temperatures (up to 31.7 Kelvin), the superconductivity survived even more extreme magnetic fields, lasting up to 65 Tesla.

This is a huge deal because standard physics says superconductivity should be impossible at these field strengths. The "internal bodyguards" (the Europium atoms) allowed the material to survive where others would fail.

Why It Matters (According to the Paper)

The paper concludes that this isn't just a weird trick; it proves that we can engineer materials to handle magnetic fields that are normally destructive.

• They compared this to a previous discovery in a different type of material (Chevrel-phase compounds), but those materials only worked at very cold, low temperatures.
• This new nickel material works at much higher temperatures (up to 40 Kelvin in some cases), making it a much more promising candidate for future technologies that need to operate in super-strong magnetic environments.

In short: The researchers found a way to use the "bad guys" (magnetic atoms inside the material) to fight the "big bad wolf" (the external magnet), allowing the superconducting dance to continue even in the strongest winds imaginable.

Technical Summary

Technical Summary: High-field-stabilized reentrant superconductivity in infinite-layer nickelate thin films

Problem Statement
Magnetic fields typically suppress superconductivity through two primary mechanisms: Pauli pair breaking, where the Zeeman interaction aligns electron spins and disrupts spin-singlet Cooper pairs, and orbital pair breaking, where magnetic flux penetrates the superconductor as vortices, destroying phase coherence. While rare instances of magnetic-field-induced superconductivity (reentrant superconductivity) have been observed in Chevrel-phase compounds, organic conductors, uranium-based heavy-fermion systems, and moiré graphene, these materials are limited by intrinsically low superconducting transition temperatures ($T_c$). A significant challenge in the field is achieving a superconducting state that remains robust in high magnetic fields, particularly within high-temperature superconductors where $T_c$ values are substantially higher.

Methodology
The study investigates infinite-layer nickelate thin films with the composition $(Sm_{1-x-y-z}Eu_xCa_ySr_z)NiO_2$ (referred to as SECNO). The samples were synthesized on $NdGaO_3$ (110) substrates using a topotactic reduction process, with in-situ resistivity monitoring to ensure optimal reduction from the perovskite precursor to the infinite-layer phase. The films, ranging from 4 to 7 nm in thickness, were doped with rare-earth elements (Sm, Eu, Ca, Sr) to tune the transition temperature.

Experimental measurements were conducted at the National High Magnetic Field Laboratory using both DC and pulsed-field facilities. Key techniques included:

• Electrical Transport: Four-point resistance measurements as a function of magnetic field ($H$) and temperature ($T$), with the field applied along the crystalline $c$-axis and rotated to various tilt angles ($\theta$).
• Inductive Measurements: Radio-frequency (≈20 MHz) tank circuit measurements using a proximity detector to detect diamagnetic shifts associated with screening currents, confirming bulk superconductivity in the high-field regime.
• Spectroscopy: X-ray absorption spectroscopy (XAS) at the Singapore Synchrotron Light Source to determine the valence state of Europium (Eu).
• Magnetization: Measurements of the $NdGaO_3$ substrate and anomalous Hall effect in the films to characterize magnetic moments.
• Theoretical Modeling: The experimental phase diagrams were analyzed using a modified Werthamer–Helfand–Hohenberg (WHH) model that incorporates an internal exchange field ($H_J$) and the Tinkham model for anisotropy in two-dimensional superconductors.

Key Contributions and Results
The authors demonstrate the existence of high-field-stabilized reentrant superconductivity in SECNO films, a material class capable of $T_c$ values up to 40 K. The primary findings include:

1. Reentrant Superconductivity: In samples with lower $T_c$ (9.6 K and 11.7 K), resistance curves show a sharp transition to the normal state at low fields ($H \approx 1$ T), followed by a reentrant superconducting state at high fields. This high-field state is characterized by a broad resistivity minimum (centered around 15–20 T) and a subsequent drop in resistivity by several orders of magnitude, accompanied by a diamagnetic shift indicative of bulk superconductivity.
2. Phase Diagrams: The $T-H$ phase diagrams reveal two distinct superconducting phases: a low-field superconducting state (LFS) and a high-field-induced reentrant state (HFS). These phases merge as the magnetic field is rotated toward the in-plane $a$-axis or as the $T_c$ is increased via chemical substitution. In higher $T_c$ samples (up to 31.7 K), the transition between LFS and HFS manifests as a "kink" in the phase boundary.
3. Extreme Critical Fields: The high-field superconducting state persists to extraordinarily high magnetic fields, exceeding 60 T in some configurations. For samples with higher $T_c$, superconductivity is observed well beyond the Pauli paramagnetic limit (estimated at $\approx 20-39$ T depending on the sample), with $T_c$ suppressed by only $\sim 1$ K even at 42 T.
4. Mechanism Identification: The data is consistent with the Jaccarino–Peter effect, a field-compensation mechanism where an internal exchange field ($H_J$) generated by localized magnetic moments opposes the applied external field.
   • XAS measurements confirm that approximately 67% of Eu sites are in the $Eu^{2+}$ configuration ($J=7/2$).
   • The anomalous Hall effect matches the magnetization calculated from the Brillouin function for $m_J = \pm 7/2$, supporting the role of $Eu^{2+}$ moments.
   • The modified WHH model successfully reproduces the experimental phase boundaries using a large internal exchange field ($H_J \approx -59$ T to $-71$ T).
   • The angular dependence of the critical field follows the Tinkham model for a 2D superconductor, suggesting a very short interlayer coherence length.

Significance
The paper claims that these findings establish infinite-layer nickelates as a second system (after Chevrel-phase compounds) describable by the WHH model modified for an internal exchange field, but with the distinct advantage of high-temperature superconductivity. The observation of reentrant superconductivity in a material with $T_c$ comparable to cuprates (up to 40 K) suggests a pathway toward superconductors that operate at tens of tesla. The authors posit that the participation of rare-earth 5d electrons in the conduction bands provides a plausible exchange path ($4f \leftrightarrow 5d \leftrightarrow 3d_{Ni}$) linking transition-metal conduction electrons and rare-earth 4f moments, a mechanism likely absent in cuprates or iron-based superconductors.

The work suggests that engineered rare-earth magnetism in high-$T_c$ nickelates could serve as a realistic platform for ultra-high-field superconducting magnets, sensors, and quantum devices. Furthermore, the successful application of the WHH model (based on BCS theory) to these data provides clues that the pairing symmetry in nickelates is even (non-triplet), offering a starting point for understanding the pairing mechanism in these materials, though questions regarding unconventional pairing symmetries or non-phonon-mediated mechanisms remain open.