2020, Bastien, Gaël, Rubrecht, Bastian, Haeussler, Ellen, Schlender, Philipp, Zangeneh, Ziba, Avdoshenko, Stanislav, Sarkar, Rajib, Alfonsov, Alexey, Luther, Sven, Onykiienko, Yevhen A., Walker, Helen C., Kühne, Hannes, Grinenko, Vadim, Guguchia, Zurab, Kataev, Vladislav, Klauss, Hans-Henning, Hozoi, Liviu, van den Brink, Jeroen, Inosov, Dmytro S., Büchner, Bernd, Wolter, Anja U.B., Doert, Thomas

Recently, several putative quantum spin liquid (QSL) states were discovered in ~S=1/2 rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. A way to clarify the origin of the QSL state in these systems is to identify ways to tune them from the putative QSL state towards long-range magnetic order. Here, we introduce the Ce-based TLAF KCeS2 and show via low-temperature specific heat and μSR investigations that it yields magnetic order below TN=0.38 K despite the same delafossite structure. We identify a well separated ~S=1/2 ground state for KCeS2 from inelastic neutron scattering and embedded-cluster quantum chemical calculations. Magnetization and electron spin resonance measurements on single crystals indicate a strong easy-plane g~factor anisotropy, in agreement with the ab initio calculations. Finally, our specific-heat studies reveal an in-plane anisotropy of the magnetic field-temperature phase diagram which may indicate anisotropic magnetic interactions in KCeS2.