A comprehensive study of the low-temperature properties of YbNi4Mg has revealed evidence of a superheavy-fermion state, characterized by a large electronic specific-heat coefficient γ0≈ 5.65 J mol−1 K−2 and an elevated Wilson ratio RW = 32.1. No magnetic ordering was observed down to 70 mK; however, a broad maximum appears in the specific heat at T∗ = 0.3 K, along with a shoulder in the derivative of susceptibility dχ/dT and resistivity dρ/dT. These features indicate a cooperative yet short-ranged magnetism entwined with the superheavy Fermi liquid. The large Wilson ratio, which is also detected in other superheavy-fermion compounds lacking long-range order, might be a signature of residual spin fluctuations. Applying a weak magnetic field of ∼0.1 T induces a metamagnetic-like crossover, as demonstrated by the quasi-adiabatic demagnetization measurements showing a broad minimum in the temperature-field trace. Here, an enhanced magnetocaloric cooling effect stemming from the field-sensitive superheavy-fermion state is observed, rivaling that of the well-established insulating magnetic coolants like the rare-earth garnet Gd3Ga5O12.
@article{arxiv.2412.08043,
title = {YbNi$_4$Mg: Superheavy fermion with enhanced Wilson ratio and magnetocaloric effect},
author = {Xiaoci Zhang and Te Zhang and Zhaotong Zhuang and Zixuan Leng and Zixuan Wei and Xinyang Liu and Junsen Xiang and Shuai Zhang and Peijie Sun},
journal= {arXiv preprint arXiv:2412.08043},
year = {2024}
}