TlYbSe₂ as a member of the J=₂ ¹ triangular-lattice Yb delafossite family: From spin liquid to field-induced magnetic order

TlYbSe₂ is a new member of the Yb delafossite family, which differs strongly from the previously investigated alkali-metal Yb delafossites. It is characterized by a much larger saturation field (about 20 T) and a higher transition temperature (about 1.8 K) to the field-induced ordered state. As for the other Yb delafossites, long-range magnetic order is absent down to low temperatures at zero field. We report detailed macroscopic and microscopic measurements as a function of magnetic field and temperature to explore the H − T phase diagram of polycrystalline TlYbSe₂. Besides bulk methods, such as magnetization and specific heat, spectroscopic methods, such as electron spin resonance (ESR) and nuclear magnetic resonance (NMR), are applied. The magnetic anisotropy can be determined by means of these microscopic probes; in particular, Yb-ESR and Se-NMR are powerful tools here. The magnetization, ESR, and NMR data evidence a large anisotropy factor of about 10 for the ratio of the magnetic moment in the Yb-Se triangular-lattice plane and perpendicular to it. Such a strong spin anisotropy was only found in CsYbSe₂, which identifies both systems as prototype two-dimensional triangular-lattice spin liquids. Even though CsYbSe₂ shares the same spin anisotropy, it has a much smaller magnetic exchange coupling between the Yb moments than TlYbSe₂, which leads to a field-induced ordering at much lower temperatures (1 K) and a smaller saturation field of only 10 T. We speculate that this originates from the use of thallium as a monovalent cation (Tl⁺¹ with [Xe]4 f¹⁴5d¹⁰6s²) instead of the intensively studied alkali metals (Na⁺¹, K⁺¹, Rb⁺¹, Cs⁺¹) with completely filled (Ne, Ar, Kr, Xe) shells. In particular, the emergence of a Tl⁺¹-based lone-pair 6s²-Se-4p polarization might have a strong impact on the strength and anisotropy of the magnetic exchange among the Yb ions in the TlYbSe₂ lattice. In contrast to that, the spin anisotropy of the pseudo-spin-1/2 Yb ions remains unaltered by the presence of Tl 6s² lone pairs.