(CD₃ND₃)₂NaRuCl₆

Chemical formula:

(CD₃ND₃)₂NaRuCl₆

Lattice type:

Trigonal, space group P3̄

How to grow:

Hydrothermal method

Magnetic model:

Heisenberg antiferromagnet on a triangular lattice with Kitaev interactions

Why is it cool:

The first material realization of the ℤ₂ vortex crystal

(CD₃ND₃)₂NaRuCl₆ is the first member of a new family of triangular lattice antiferromagnets based on heavy 4d and 5d transition metal ions. Strong spin-orbit coupling entangles the spin and orbital degrees of freedom of Ru³⁺ into j_eff = 1/2 pseudospins and adds bond-dependent "Kitaev" exchange on top of the geometrically frustrated Heisenberg interactions. For this model, theory predicts that even a small Kitaev term condenses ℤ₂ vortices, the topological point defects of the triangular antiferromagnet, directly into the ground state, where they order into a periodic "vortex crystal". Our neutron diffraction experiments find precisely the fingerprint of that long-sought state below T_N ≈ 0.23 K: an incommensurate multi-q structure with three symmetry-equivalent Bragg peaks [1]. On top of that, a magnetic field applied along the c axis induces a cascade of no fewer than seven distinct ordered phases.

Magnetic phase diagram of (CD₃ND₃)₂NaRuCl₆ for the H || c field direction. Color indicates the measured magnetostriction coefficient.

• [1] J. Nagl, K. Yu. Povarov, B. Duncan, C. Näppi, D. Khalyavin, P. Manuel, F. Orlandi, J. Sourd, B. V. Schwarze, F. Husstedt, S. A. Zvyagin, O. Zaharko, P. Steffens, A. Hiess, D. R. Allan, S. A. Barnett, Z. Yan, S. Gvasaliya, A. Zheludev, ℤ₂ Vortex Crystal Candidate in the Triangular S = 1/2 Quantum Antiferromagnet , npj Quantum Mater. (2026); arXiv:2512.01793.