YbNi$_4$Mg：具有增强Wilson比和磁热效应的超重费米子材料
YbNi$_4$Mg: Superheavy fermion with enhanced Wilson ratio and magnetocaloric effect
AI总结：
本研究发现YbNi$_4$Mg存在超重费米子态，具有大电子比热系数和高Wilson比，短程磁性与费米液体交织，弱磁场可诱发类变磁交叉，且其磁热冷却效应可媲美传统绝缘磁冷却剂。
摘要：
对YbNi$_4$Mg低温特性的综合研究揭示了超重费米子态的存在证据，该态的特征为大的电子比热系数$γ_0$≈5.65 J mol$^{-1}$ K$^{-2}$以及升高的Wilson比$R_W$=32.1。在低至70 mK的温度下未观测到磁有序；但在$T^*$=0.3 K处，比热出现宽峰，同时磁化率导数d$χ$/d$T$和电阻率导数d$ρ$/d$T$出现肩状特征。这些特征表明，一种合作性但短程的磁性与超重费米液体相互交织。这一较大的Wilson比在其他缺乏长程有序的超重费米子化合物中也被观测到，可能是残余自旋涨落的标志。施加约0.1 T的弱磁场会诱发类变磁交叉，准绝热退磁测量证实了这一点——温度-磁场轨迹中出现宽极小值。研究还观测到由对磁场敏感的超重费米子态产生的增强磁热冷却效应，其效果可与成熟的绝缘磁冷却剂（如稀土石榴石Gd$_3$Ga$_5$O$_{12}$）相媲美。
英文摘要：
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 $γ_0$ $\approx$ 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$χ$/d$T$ and resistivity d$ρ$/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}$.
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