Crystal field excitations in rare-earth triangular-lattice phosphates Rb3Ln(PO4)2 (Ln = Nd, Tm, and Yb)

The recently discovered triangular-lattice phosphates Rb₃Ln(PO₄)₂ with Ln being an rare-earth element shows very strong frustration effects associated with a perfect triangular lattice of the Ln3+ ions, but only receives very limited investigations so far. Magnetization measurements indicate the absence of long-range magnetic ordering or spin freezing in this family above 2 K, and Rb₃Yb(PO₄)₂ does not order even down to 50 mK, thus providing a suitable platform for examining the possible quantum spin liquid physics and studying the effect of geometrical frustrations. We propose to measure the CEF excitations in Rb₃Nd(PO₄)₂ (monoclinic structure with Kramers ions), Rb₃Tm(PO₄)₂ (trigonal structure with non-Kramers ions) and Rb₃Yb(PO₄)₂ (trigonal structure with Kramers ions), by performing INS experiments at MARI on single-crystal samples of these compounds with distinct magnetic properties.
As the collective magnetism in these rare-earth triangular-lattice antiferromagnets strongly depends on their magnetic anisotropy, which is largely correlated with their CEF Hamiltonian and single-ion magnetism of Ln3+ ions, an inelastic neutron scattering (INS) study on their CEF excitations will be crucial for decoding their CEF Hamiltonian on the single-ion level and further understanding the collective magnetism of the rare-earth ions in these compounds.