标题： YbNi$_4$Mg：超重费米子，具有增强的威尔逊比和磁卡效应 显示英文标题

标题： YbNi$_4$Mg: Superheavy fermion with enhanced Wilson ratio and magnetocaloric effect

摘要： 对YbNi$_4$Mg的低温性质的全面研究揭示了超重费米子态的证据，其特征是大的电子比热系数$\gamma_0$ $\approx$ 5.65 J mol$^{-1}$K$^{-2}$和升高的威尔逊比$R_W$ = 32.1. 没有观察到磁序直到70 mK；然而，在$T^*$ = 0.3 K时，比热出现一个宽峰，同时在磁化率的导数 d$\chi$/d$T$和电阻率的导数 d$\rho$/d$T$中出现肩峰。这些特征表明一种协同但短程的磁性与超重费米液体交织在一起。大的威尔逊比值，在其他缺乏长程序的超重费米子化合物中也检测到，可能是剩余自旋涨落的标志。施加一个弱磁场$\sim$0.1 T会引起类似反铁磁的转变，如准绝热去磁测量所示，在温度-磁场图中出现一个宽谷。 这里，观察到一种增强的磁热冷却效应，源于对磁场敏感的超重费米子态，其效果可与已确立的绝缘磁性冷却剂如稀土石榴石Gd$_3$Ga$_5$O$_{12}$相媲美。 显示英文摘要

摘要： A comprehensive study of the low-temperature properties of YbNi$_4$Mg has revealed evidence of a superheavy-fermion state, characterized by a large electronic specific-heat coefficient $\gamma_0$ $\approx$ 5.65 J mol$^{-1}$ K$^{-2}$ and an elevated Wilson ratio $R_W$ = 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 d$\chi$/d$T$ and resistivity d$\rho$/d$T$. 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 $\sim$0.1 T induces a metamagnetic-like crossover, as demonstrated by the quasi-adiabatic demagnetization measurements showing a broad minimum in the temperature-field trace. Here, an enhanced magnetocaloric cooling effect stemming from the field-sensitive superheavy-fermion state is observed, rivaling that of the well-established insulating magnetic coolants like the rare-earth garnet Gd$_3$Ga$_5$O$_{12}$.