Low-temperature quantum magnetism of the frustrated magnet LiYbSe₂

Yb³⁺ is a good source of anisotropic effective spin-1/2 moments resulting from crystal field splitting and spin-orbit coupling. Many triangular-lattice Yb³⁺ based systems have recently been discovered and studied in an attempt to remove the intrinsic disorder inherent to the first material of that class, YbMgGaO₄. This includes compounds such as NaYbO₂ and KYbS₂. Here, we report on LiYbSe₂, a Yb³⁺ compound that crystalizes in the pyrochlore structure. The magnetic susceptibility displays a Curie-Weiss behavior at low temperatures with a negative Weiss constant, indicating that LiYbSe₂ is an antiferromagnet. Our heat capacity measurements down to 70 mK at several magnetic fields up to 14 T show evident magnetic fluctuations below 10 K. A peak is visible from 0.5 T onwards, suggesting the modification of the low-energy magnetic spectral weight by the magnetic field. This 0 T response is the hallmark of gapless correlated magnetic fluctuations, but the entropy change does not quite reach Rln2 expected for fully developed correlations of S_eff = 1/2 moments. It indicates that (1) no higher energy crystal-field level participates in the low-temperature magnetism and that (2) a missing entropy is possible in this system.