🔬 condensed matter

Bulk Superconductivity in Rocksalt LaN $_{1-x}$

This paper reports the synthesis of bulk rocksalt-structured LaN $_{0.94}$ via high-pressure laser floating-zone processing, establishing it as the first superconducting rare-earth mononitride with a transition temperature of 5.95 K.

Original authors: Caeli Benyacko, Lin-Ding Yuan, Josiah A. Turner, Laila Reimanis, Siyuan Ji, Cheng Li, Erick A. Lawrence, Hanna Z. Porter, James M. Rondinelli, Stephen D. Wilson

Published 2026-09-22

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CC BY 4.0

Original authors: Caeli Benyacko, Lin-Ding Yuan, Josiah A. Turner, Laila Reimanis, Siyuan Ji, Cheng Li, Erick A. Lawrence, Hanna Z. Porter, James M. Rondinelli, Stephen D. Wilson

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of materials science, scientists often look for substances that can conduct electricity without any resistance at all. When this happens, the material becomes a superconductor, a state that allows for incredibly efficient energy transfer and powerful magnetic fields. For decades, researchers have studied a family of compounds made from rare earth metals combined with nitrogen. These materials usually form a specific, tightly packed crystal structure, but they have been known for strange and varied behaviors, ranging from unusual magnetic states to acting as semiconductors. One member of this family, lanthanum nitride, has been a subject of long debate. While some early experiments hinted at a tiny, partial ability to superconduct, others suggested it was simply a metal or a semiconductor. The difficulty in studying it lies in its extreme sensitivity to air and moisture, which can ruin the sample before it is even measured, and the fact that its atomic structure looks very similar to that of pure lanthanum metal, making them hard to tell apart with standard tools.

A team of researchers has now settled this debate by creating a pure, bulk sample of lanthanum nitride and proving that it is indeed a superconductor. To achieve this, they avoided the traditional methods that often leave the material impure or damaged. Instead, they used a specialized furnace that melts the material using high-powered lasers while it is surrounded by nitrogen gas at a pressure three thousand times greater than the air we breathe. This high-pressure environment allowed them to react the metal with the gas and then melt the resulting compound into a uniform liquid, which was then cooled to form a solid. This process gave them a sample that was chemically consistent and free from the contamination that had plagued previous attempts.

When the team examined this new material, they found it had a specific chemical makeup, containing slightly less nitrogen than a perfect ratio would suggest. This small deficiency was actually crucial. Computer simulations had previously predicted that a perfectly balanced version of this crystal would be unstable and would likely collapse or change shape. The slight lack of nitrogen atoms, however, acted like a stabilizer, allowing the crystal to hold its shape and remain stable. Once they confirmed the structure was solid and pure, they began testing its physical properties. They measured how electricity flowed through the material as they cooled it down, observing that it behaved like a metal at higher temperatures but suddenly lost all electrical resistance when the temperature dropped to just below six degrees above absolute zero.

Further tests confirmed that this was not just a surface effect or a tiny, isolated patch of superconductivity. By measuring the material's ability to repel magnetic fields and its heat capacity, the researchers determined that the entire bulk of the sample had transitioned into a superconducting state. They also found that this material could withstand a magnetic field more than ten times stronger than what pure lanthanum metal can handle before losing its superconducting ability. This distinction is vital because it proves the superconductivity comes from the nitride compound itself, not from any leftover metal. The findings suggest that the specific arrangement of atoms in this crystal, particularly the way the lanthanum atoms are spaced, plays a key role in enabling this state. While the exact mechanism is still being explored, the discovery establishes lanthanum nitride as the first confirmed superconductor in its family of rare earth compounds, opening the door for scientists to explore similar materials and potentially uncover new ways to manipulate electricity and magnetism.

Technical Summary: Bulk Superconductivity in Rocksalt LaN $_{1-x}$

Problem Statement
The rare earth mononitride (LnN) family, particularly lanthanide nitrides, is known for exhibiting unusual electronic properties, yet the existence of bulk superconductivity within this series remains an open question. While nonmagnetic rocksalt transition metal nitrides are well-known superconductors, the LnN series has historically lacked confirmed bulk superconducting members. Specifically, the intrinsic electronic ground state of lanthanum nitride (LaN) has been a subject of debate, with first-principles simulations predicting it to be either a semiconductor or a semimetal, while experimental reports have been ambiguous due to unquantified nitrogen vacancies and the extreme air sensitivity of the material. Previous reports of superconductivity in LaN were limited to partial volume fractions or filamentary states at very low temperatures ( $T \approx 1.3$ K), and distinguishing LaN from the face-centered cubic (FCC) polymorph of elemental lanthanum (La) is challenging using conventional X-ray diffraction due to similar d-spacings.

Methodology
To address synthesis challenges and phase purity issues, the authors employed a high-pressure, laser-based floating-zone (HPLFZ) furnace.

Synthesis: Polycrystalline LaN was synthesized by reacting lanthanum metal rods with high-pressure nitrogen gas (3000 psi) within the furnace. This method provided direct visual feedback and allowed for the melting of nitrides at high temperatures ( $\approx 2450^\circ$ C), a significant increase over the melting point of elemental La ( $\approx 920^\circ$ C). The process involved rigorous purification of the gas environment and "hot-purging" to remove surface oxides.
Characterization: The resulting samples were characterized using combined synchrotron X-ray and neutron powder diffraction to verify crystal structure and stoichiometry. Neutron diffraction was crucial for distinguishing the nitrogen sublattice from the lanthanum sublattice.
Physical Property Measurements: Electrical transport, heat capacity, and magnetization measurements were conducted on polycrystalline samples. Strict protocols were implemented to minimize air exposure, including loading samples in argon gloveboxes and using protective coatings (GE varnish, Parafilm) during transfer.
Computational Modeling: Density Functional Theory (DFT) calculations were performed to model the electronic band structure and phonon dispersions for both stoichiometric LaN and nitrogen-deficient LaN $_{1-x}$ .

Key Results

Structural Verification: Combined diffraction data confirmed the formation of a phase-pure rocksalt structure. While synchrotron X-ray data showed a trace amount of hexagonal La, neutron diffraction revealed a stoichiometry of LaN $_{0.94}$ . The refined lattice parameter was $a = 5.2990(2)$ Å.
Dynamic Stability: First-principles calculations indicated that stoichiometric rocksalt LaN is dynamically unstable, exhibiting negative phonon branches. However, simulations of the nitrogen-deficient LaN $_{0.963}$ structure (corresponding to a La $_{27}$ N $_{26}$ model with $x \approx 0.04$ ) showed a stable phonon dispersion with no imaginary branches. This confirms that a dilute nitrogen deficiency, consistent with the experimentally refined stoichiometry of LaN $_{0.94}$ ( $x \approx 0.06$ ), is sufficient to stabilize the cubic rocksalt structure.
Superconductivity:
- Transport: Electrical resistivity measurements showed metallic behavior down to 2 K, with a superconducting transition occurring at $T_c = 5.95$ K.
- Heat Capacity: Specific heat measurements ( $C_p/T$ vs. $T^2$ ) confirmed a bulk superconducting transition at $T_c$ , yielding a Sommerfeld coefficient $\gamma_e = 3.86 \pm 0.05$ mJ mol $^{-1}$ K $^{-2}$ . The jump in heat capacity was noted as small, potentially indicating an unconventional state or sample degradation.
- Magnetization: Zero-field cooled (ZFC) magnetization data revealed a full superconducting volume fraction. Field-cooled (FC) data showed a paramagnetic Meissner effect, which the authors attribute to flux pinning in an inhomogeneous superconductor or surface contamination, rather than intrinsic unconventional pairing.
- Upper Critical Field: Measurements under varying magnetic fields allowed for the extrapolation of the upper critical field ( $H_{c2}$ ). Using a Ginzburg-Landau model, $H_{c2}(0)$ was estimated at 1.15 T, significantly higher than that of type-I FCC La ( $\approx 0.11$ T), confirming LaN $_{0.94}$ as a type-II superconductor.

Significance and Claims
The paper establishes LaN $_{0.94}$ as the first bulk superconducting member of the rare earth mononitride family. The authors claim that the superconductivity is intrinsic to the bulk material, distinguishing it from previous reports of filamentary or partial superconductivity. The study highlights that the unique processing space afforded by high-pressure optical reactors is essential for synthesizing phase-pure, nearly stoichiometric LaN, overcoming the limitations of traditional diffusion-limited synthesis routes.

The authors propose that the superconductivity in LaN $_{0.94}$ is likely driven by the structural similarity of its FCC La sublattice to that of the metastable FCC polymorph of La (which has a similar $T_c$ ) and the binary oxide LaO. They suggest that the lattice anharmonicity endemic to the FCC La sublattice plays a key role in electron pairing, supporting a conventional BCS scenario rather than an exotic mechanism. The work motivates future exploration of the LnN series, including magnetic variants and alloys, to search for unconventional superconductivity, while noting that the specific role of nitrogen vacancies in stabilizing the structure and influencing the superconducting state requires further theoretical and experimental quantification.

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