Double magnon-roton excitations in a spin supersolid candidate Rb2Co(SeO3)2

Supersolidity, an intriguing quantum state that shows characteristics of both superfluids and solids, has been recently observed in two triangular-lattice Co-based easy-axis antiferromagnets Na2BaCo(PO4)2 and K2Co(SeO3)2 (KCSO). This exotic quantum state of matter attracts much attention recently, as the strong quantum spin fluctuations can lead to a giant magnetocaloric effect and be useful in magnetic refrigeration. Spectroscopically, both a gapless Goldstone mode and a gapped pseudo-Goldstone mode, associated with transverse excitations of in-plane spin superfluid order and out-of-plane spin solid order, respectively, are expected to be observed at the K points, and two branches with separated roton minimum coexist at the M points. So far, solid evidences for the pseudo-Goldstone mode as well as the separated two roton minimum are still lacking, due to the limited energy resolution and the small amounts of samples used. As an isostructural compound of KCSO, Rb2Co(SeO3)2 (RCSO) shows strikingly similar magnetic and thermodynamical properties and behaves as another easy-axis antiferromagnet. RCSO has a better two dimensionality and a better proximity to the 2D easy-axis XXZ model, and can be easily grown up to a few grams, unlike the case of KCSO, making the INS experiments easier. We propose to perform high-resolution INS experiments at IRIS on single-crystal RCSO samples to probe the underlying double magnon-roton excitations.