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The crystalline electric field (CEF) of Ce 3+ in trigonal symmetry has recently become of some relevance, for instance, in the search of frustrated magnetic systems. Fortunately, it is one of the CEF case in which a manageable analytic solution can be obtained. Here, we present this solution for the general case, and use this result to determine the CEF scheme of the new compound CeIr3Ge7 with the help of T -dependent susceptibility and isothermal magnetization measurements. The resulting CEF parameters B 0 2 = 34.4 K, B 0 4 = 0.82 K and B 3 4 = 67.3 K correspond to an exceptional large CEF splittings of the first and second excited levels, 374 K and 1398 K, and a large mixing between the ± 5 2 and the ∓ 1 2 states. This indicates a very strong easy plane anisotropy with an unusual small c-axis moment. Using the same general expressions, we show that the properties of the recently reported system CeCd3As3 can also be described by a similar CEF scheme, providing a much simpler explanation for its magnetic properties than the initial proposal. Moreover, a similar strong easy plane anisotropy has also been reported for the two compounds CeAuSn and CePdAl4Ge2, indicating that the CEF scheme elaborated here for CeIr3Ge7 corresponds to an exemplary case for Ce 3+ in trigonal symmetry.

We show that Ce-and Yb-based Kondo-lattice ferromagnets order mainly along the magnetically hard direction of the ground state Kramers doublet determined by crystalline electric field (CEF). Here we argue that this peculiar phenomenon, that was believed to be rare, is instead the standard case. Moreover, it seems to be independent on the Curie temperature TC, crystalline structure, size of the ordered moment and type of ground state wave function. On the other hand, all these systems show the Kondo coherence maximum in the temperature dependence of the resistivity just above TC which indicates a Kondo temperature of a few Kelvin. An important role of fluctuations is indicated by the non-mean-field like transition in specific heat measurements as well as by the suppression of this effect by a strong Ising-like anisotropy. We discuss possible theoretical scenarios.Kondo-lattice (KL) systems are typically intermetallic compounds based on trivalent Ce or Yb atoms and are characterized by the Kondo effect at low temperatures and subsequent Kondo coherence at even lower temperatures. The degenerate ground state multiplet (J = 5/2 for Ce and J = 7/2 for Yb) is split by the crystalline electric field (CEF), making Kramers doublets the prevalent ground state. Only in cubic structures the ground state can be a quartet, which is prone to multipolar order [1]. The first excited state is usually located at several tens of Kelvins above the ground state and does not participate in the magnetic ordering. In fact, depending on the strength of the Kondo and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, transition temperatures are usually in the order of a few Kelvin, often between 2 and 12 K, or below 1 K in systems with a very large distance (> 6Å) between the Ce atoms, like in Ce 4 Pt 12 Sn 25 [2], or strong Kondo effect, like in YbRh 2 Si 2 [3].

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We study the correlated quantum magnet, YbCl3, with neutron scattering, magnetic susceptibility, and heat capacity measurements. The crystal field Hamiltonian is determined through simultaneous refinements of the inelastic neutron scattering and magnetization data. The ground state doublet is well isolated from the other crystal field levels and results in an effective spin-1/2 system with local easy plane anisotropy at low temperature. Cold neutron spectroscopy shows low energy excitations that are consistent with nearest neighbor antiferromagnetic correlations of reduced dimensionality.The Quantum Spin Liquid (QSL) is a state of matter hosting exotic fractionalized excitations and long range entanglement between spins with potential applications for quantum computing 1-4 . Since QSL physics relies on quantum fluctuations that are enhanced by low spin and low dimensionality, spin-1/2 systems on two-dimensional lattices provide a natural experimental platform for realizing a QSL phase. It has also been shown that an effective spin-1/2 system can be generated even in compounds with high-angular-momentum ions like Yb 3+ and Er 3+ , where the combination of crystal-field effects and strong spin-orbit coupling lead to highly anisotropic interactions between effective spin-1/2 degrees of freedom 5 .Magnetic frustration plays a central role in stabilizing QSL phases 6 . While QSLs were traditionally associated with geometrically frustrated systems (e.g., triangular and kagome lattices), it has recently become well appreciated that exchange frustration due to highly anisotropic spin interactions can also stabilize QSL phases, even on bipartite lattices 7,8 . Most famously, bond-dependent spin interactions on the honeycomb lattice give rise to the Kitaev model, an exactly solvable model with a gapless QSL ground state 9 . A number of honeycomb materials, primarily containing 4d or 5d transition metals such as Ir or Ru have been put forth as realizations of the Kitaev model 10,11 . Prominent examples in- * This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paidup, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).FIG. 1: Monoclinic crystal structure of YbCl3 with a = 6.7291(3)Å, b = 11.6141(9)Å, c = 6.3129(3)Å and β = 110.5997(7) obtained at 10 K. Refined structure parameters are further described in SI 35 . (a) YbCl3 structure consisting of alternating planes of Yb 3+ cations (red spheres) forming a honeycomb lattice in the ab plane, with Cl − anions (green spheres) separatin...