First discovery of two-band superconductivity in intercalated rhombohedral ZrNCl

Advance Snapshot:

Discovery of two-band superconductivity in the intercalated rhombohedral ZrNCl system, which was supported by the upper critical field and reversible magnetization experiments.

Research Motivation:

“Professor Yoshihiro Iwasa's group in Japan pioneered the study of the MNX (M=Ti, Zr, Hf; X=Cl, Br, I) system and its intercalation compounds. They proposed that upon electron doping, a single band originating from the M-atoms dominates near the Fermi level across these materials. However, their analysis did not account for the critical influence of different stacking polytypes – specifically, the hexagonal (H) versus rhombohedral (R) structures – on the conduction band electronic structure near the Fermi level. This specific point regarding the polytype dependence had been clearly demonstrated theoretically for H-ZrNCl, R-ZrNCl, and H-HfNCl in earlier work by Professor Claudia Felser and her group at the Max Planck Institute. The primary motivations for our present work were twofold: Firstly, to provide the first experimental confirmation of two-band superconductivity in the rhombohedral ZrNCl system, a phenomenon that had remained unreported prior to our study. This finding has universal applicability across the rhombohedral polytypes of the MNX family. Secondly, to establish this system as a new high-T_c two-band superconductor, distinct from the established examples of MgB₂ and the iron-based superconductors. Professor Iwasa expressed interest in our findings during discussions following my poster presentation at the International Conference on Novel Superconductors held at the Southern University of Science and Technology (SUSTech) in March.”

Two-band superconductors captivate condensed matter physicists with their multi-gap structures and complex interband interactions. Classic examples like MgB₂ and iron-based superconductors serve as ideal platforms for probing correlations between multi-band electronic structures and pairing mechanisms. The MNX family (M=Ti, Zr, Hf; X=Cl, Br, I) exhibits remarkably high superconducting transition temperatures (T_c) despite low carrier densities, alongside intriguing properties: record T_c ≈25 K [Nature 392, 580 (1998)], BEC-BCS crossover [Science 372, 190 (2021)], and proposed unconventional pairing schemes including spin-fluctuation-mediated pairing [Phys. Rev. Lett. 103, 077004 (2009)] and exciton-driven triplet pairing [PNAS 119, e2117735119 (2022)]. Despite these advances, the interplay between pairing symmetry and interband coupling remains unresolved, demanding deeper experimental scrutiny.

Recently, the Zhang Liyuan team at SUSTech, collaborating with the China Spallation Neutron Source, synthesized a novel organic-inorganic hybridized superlattice Na_0.1(1,3-DAP)_0.25ZrNCl single crystal via solvothermal intercalation.

Sodium ions and 1,3-diaminopropane molecules successfully penetrated the van der Waals gap of rhombohedral ZrNCl, expanding the interlayer spacing d from 9.2 Å (parent phase) to 13.9 Å. Organic molecules adopt a flat-lying configuration between ZrNCl layers. Electron donation from intercalants triggered an insulator-metal-superconductor transition, with resistivity measurements revealing a superconducting onset at 15.43 K, higher than previously reported intercalated ZrNCl systems.

Key evidence for two-band superconductivity emerged from the positive curvature of the upper critical field H_c2 near T_c, mirroring behavior in MgB₂. Crucially, the H_c2 (T) data was perfectly fitted using a two-band model, yielding zero-temperature in-plane and out-of-plane H_c2 values of 11.5 T and 2.9 T, respectively. The large diffusivity ratioη suggests weaker intraband scattering than in other two-band superconductors.

Magnetic characterization confirmed bulk superconductivity (90% diamagnetic shielding) and strong vortex pinning.

Most decisively, the experimental reversible magnetization curve showed stark deviation from single-band Ginzburg-Landau theory, ruling out anisotropic single-band scenarios.

Collectively, this work provides the first experimental verification of two-band superconductivity in rhombohedral ZrNCl. Intriguingly, the superconducting state exhibits two-fold rotational symmetry under in-plane magnetic fields, decoupled from the three-fold symmetric lattice – a possible signature of mixed-order-parameter pairing. These findings establish intercalated rhombohedral ZrNCl as a compelling quantum platform for multiband superconductivity research.

Conclusion and Future Perspectives:

The innovative intercalation strategy – inserting Na ions and organic molecules into rhombohedral ZrNCl – engineered a hybrid superlattice with expanded interlayer spacing and electron doping, inducing robust superconductivity at 15 K. The hallmark positive curvature of H_c2(T) and magnetization data conclusively demonstrate multiband behavior. This breakthrough not only fills a critical gap in ZrNCl superconductivity research but also highlights interlayer engineering as a powerful tool for electronic state control. Future work could optimize intercalants or hybrid architectures to design higher-T_c multiband superconductors. Moreover, the decoupling of two-fold superconducting symmetry from the three-fold lattice may offer new pathways to topological superconductivity or exotic pairing mechanisms.