Probing Low-energy Magnetic Excitations in Triangular-Lattice Phosphates Rb₃Nd(PO₄)₂ and Rb₃Yb(PO₄)₂

The recently discovered triangular-lattice phosphates Rb₃RE(PO₄)₂, where RE represents a rare-earth element, exhibit strong frustration effects arising from the ideal triangular lattice formed by RE³⁺ ions. Among them, Rb₃Yb(PO₄)₂ (RYP) and Rb₃Nd(PO₄)₂ (RNP) stand out for their distinct magnetic behaviors. RYP, with a trigonal structure, does not exhibit long-range magnetic ordering even down to 50 mK, making it an ideal candidate for exploring quantum spin liquid states. In contrast, RNP, which adopts a monoclinic structure, also lacks long-range order above 1.8 K but shows distinct magnetic anisotropy. Despite these differences, both materials display remarkable magnetocaloric effects in the sub-Kelvin regime, indicating strong low-energy quantum fluctuations.We propose to study the low-energy magnetic excitations of RYP and RNP using cold-neutron backscattering measurements at the IRIS spectrometer. These compounds, despite their structural differences, provide a unique opportunity to explore how lattice distortions and magnetic anisotropy influence their quantum behavior. By mapping the low-energy spin dynamics, we aim to extract their spin Hamiltonians and gain deeper insights into the collective magnetism of rare-earth triangular-lattice antiferromagnets, especially focusing on the nature of their quantum excitations and frustration-driven phenomena.