Origin of the insulating state in the honeycomb iridate Cu2IrO3

Physical Review B American Physical Society (APS) 111:24 (2025) 245122

Authors:

Ying Li, Roger D Johnson, Yogesh Singh, Radu Coldea, Roser Valentí

From continuum excitations to sharp magnons via transverse magnetic field in the spin-1/2 Ising-like triangular lattice antiferromagnet Na2BaCo(PO4)2

(2025)

Authors:

Leonie Woodland, Ryutaro Okuma, J Ross Stewart, Christian Balz, Radu Coldea

Superconductivity in Hexagonal Zr6CoAl2-Type Zr6RuBi2 and Zr6FeBi2

Journal of the Physical Society of Japan 94:1 (2025)

Authors:

K Yuchi, D Nishio-Hamane, K Kojima, K Moriyama, R Okuma, Y Okamoto

Abstract:

We report the synthesis and electronic properties of polycrystalline samples of Zr6MBi2 (M = Ru and Fe) crystallizing in the hexagonal Zr6CoAl2-type structure. Based on their electrical resistivity, magnetization, and heat capacity data, Zr6RuBi2 and Zr6FeBi2 are found to exhibit bulk superconductivity below Tc = 4.9 and 1.4 K, respectively. Although Zr6RuBi2 is most likely a conventional superconductor, the considerably higher Tc for M = Ru compared with that for M = Fe differs from the trend in Tc for Zr6CoAl2-type superconductors reported to date. The superconductivity of an amorphous solid-solution phase, which may hinder elucidation of the superconducting properties of Zr6MBi2, is also discussed.

Topological Semimetal KAlGe with Novel Electronic Instability

Chemistry of Materials American Chemical Society (ACS) 37:1 (2025) 189-197

Authors:

Toshiya Ikenobe, Takahiro Yamada, Jun-ichi Yamaura, Tamio Oguchi, Ryutaro Okuma, Daigorou Hirai, Hajime Sagayama, Yoshihiko Okamoto, Zenji Hiroi

Compass-model physics on the hyperhoneycomb lattice in the extreme spin-orbit regime

Nature Communications Springer Nature 15:1 (2024) 10615

Authors:

Ryutaro Okuma, Kylie MacFarquharson, Roger D Johnson, David Voneshen, Pascal Manuel, Radu Coldea

Abstract:

The physics of spin-orbit entangled magnetic moments of 4d and 5d transition metal ions on a honeycomb lattice has been much explored in search for unconventional magnetic orders or quantum spin liquids expected for compass spin models, where different bonds in the lattice favour different orientations for the magnetic moments. Realizing such physics with rare-earth ions is a promising route to achieve exotic ground states in the extreme spin orbit limit, however this regime has remained experimentally largely unexplored due to major challenges in materials synthesis. Here we report successful synthesis of powders and single crystals of β-Na2PrO3, with Pr^{4+} j_eff = 1/2 magnetic moments arranged on a hyperhoneycomb lattice with the same threefold coordination as the planar honeycomb. We find a strongly noncollinear magnetic order with highly dispersive gapped excitations that we argue arise from frustration between bond-dependent, anisotropic off-diagonal exchanges, a compass quantum spin model not explored experimentally so far. Our results show that rare-earth ions on threefold coordinated lattices offer a platform for the exploration of quantum compass spin models in the extreme spin orbit regime, with qualitatively distinct physics from that of 4d and 5d Kitaev materials.