Research: Long-Range magnetic order in the S = 1/2 triangular-lattice antoferromagnet KCES2

Research on magnetic frustration has attracted considerable attention as it can lead to the competition between different states of broken magnetic symmetry and the possible realization of quantum spin liquid (QSL) states. QSL host long-range entanglement with possible topological order and ground state degeneracy. Due to e.g. geometric constraints of the crystal lattice, spins cannot all "freeze" together in a ground state and instead fluctuate even at temperatures close to absolute zero. These properties lead to unusual fractionalized excitations, which offer crucial new elements in quantum information processing.

Recently, several Ce-based and Yb-based triangular-lattice antiferromagnets with the delafossite structure were reported and the realization of a QSL state was proposed. However, the origin of these states remains under debate: Anisotropic magnetic interactions induced by the strong spin-orbit coupling as well as next-nearest neighbor magnetic interactions are candidates. A way to clarify the origin of the QSL state in these systems is to identify ways to tune them from the putative QSL towards long-range magnetic order.

In our work we introduce the new Ce-based triangular-lattice antiferromagnet KCeS₂ and show that it yields magnetic order below T_N = 0.38 K despite the same delafossite structure. The formation of a well separated S = ½ ground state of KCeS₂ is theoretically predicted by our quantum chemical calculations and experimentally observed by inelastic neutron scattering experiments. We also report and quantify a strong easy-plane g-factor anisotropy together with a six-fold in-plane anisotropy of the specific heat, possibly indicating anisotropic magnetic interactions in this material. To our knowledge, KCeS₂ is the first triangular-lattice antiferromagnet, where such an in-plane anisotropy is observed.

G. Bastien, B. Rubrecht, E. Haeussler, P. Schlender, Z. Zangeneh, S. Avdoshenko, R. Sarkar, A.  Alfonso, S. Luther, Y. A. Onykiienko, H. C. Walker, H. Kühne, V. Grinenko, Z. Guguchia, V. Kataev, H.-H. Klauss, L. Hozoi, J. van den Brink, D. S. Inosov, B. Büchner, A. U. B. Wolter, T. Doert,
Long-range magnetic order in the S = 1/2 triangular lattice antiferromagnet KCeS₂,
SciPost Physics 9, 041 (2020) (arXiv)