Discovery of superconductivity in La2IOs2with 5d Honeycomb Lattice
Kindo Group
5d transition metal compounds have gained considerable interest in the condensed matter community in the last decade. The combination of strong spin-orbit coupling and Coulomb interactions may stabilize a Mott insulating state with nontrivial ground states such as a quantum spin liquid on the honeycomb lattice. In metallic compounds, the spin-orbit coupling and electron-electron interactions may cause a Fermi surface instability that gives rise to various electronic orders and exotic superconductivity.
We focus on the intermetallic compound La2IOs2 with Gd2IFe2-type, which is known as the intermediate between cluster compounds and intermetallic phases. The crystal structure (Fig. 1) features a two-dimensional slab made of a transition-metal-centered trigonal prism of lanthanum, where the Os form the honeycomb network. We have prepared the pure polycrystalline and ~100 µm size single crystal samples of La2IOs2 and investigate its physical properties by the resistivity, torque, and specific heat measurements.
We have demonstrated the bulk superconductivity at Tc = 12 K in La2IOs2 by the physical property measurements. Despite the heavy constituent elements, which are generally unfavorable for a phonon mediated mechanism, Tc is the highest among lanthanoid iodides made of lighter elements such as La2IRu2 with Tc = 4.8 K. Moreover, electronic anomalies are observed at 60 K and 30 K, which are similar to those observed in isostructural La2IRu2 at 140 K and 85 K, pointing to the presence of the common electronic instability inherent to the Gd2IFe2-type crystal structure. We consider the fluctuations relevant to the electronic instability enhances the superconductivity in La2IOs2, while this point should be clarified in the future work. Furthermore, we observed that the superconductivity is robust against the magnetic field especially when the magnetic field is applied parallel to the honeycomb layer. We observed zero resistivity at least up to 12 T and the estimated upper critical field is around 40 T.
Our discovery indicates that La2IOs2 is a layered 5d electron system providing a platform to investigate the interplay between the electronic anomaly, superconductivity, and strong magnetic field. Further investigations to understand the superconducting pairing mechanism and the origin of the unusual properties are ongoing. Besides the interest from the physical perspective, our first-principles electronic structure calculations reveal the effective valence of Os is -1, indicating the Os is anionic. Examples of transition metal anions in solids are limited to a few intermetallic compounds including platinum or gold. Exploring the d-electron physical properties of relative compounds with anionic transition metals are also of interest.
References
- [1] H. Ishikawa, T. Yajima, D. Nishio-Hamane, S. Imajo, K. Kindo, and M. Kawamura, Phys. Rev. Materials 7, 054804 (2023)