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Field evolution of quantum critical and heavy Fermi-liquid components in the magnetization of the mixed valence compound beta-YbAlB4

We present the high-precision magnetization data of the valence fluctuating heavy fermion superconductor $β$-YbAlB$_4$ in a wide temperature range from 0.02 K to 320 K spanning four orders of magnitude. We made detailed analyses of the $T/B$ scaling of the magnetization, and firmly confirmed the unconventional zero-field quantum criticality (QC) without tuning. We examined other possible scaling relationship such as $T/(B-B_c)^δ$ scaling, and confirmed that $δ= 1$ provides the best quality of the fit with an upper bound on the critical magnetic field $\vert B_c \vert <0.2$~mT. We further discuss the heavy Fermi-liquid component of the magnetization after subtracting the QC component estimated based on the $T/B$ scaling. The temperature dependence of the heavy Fermi-liquid component is found very similar to the magnetization of the polymorph $α$-YbAlB$_4$. In addition, the heavy Fermi-liquid component is suppressed in the magnetic field above $\sim$ 5 T as in $α$-YbAlB$_4$. This was also confirmed by the magnetization measurements up to $\sim 50$ T for both $α$- and $β$-YbAlB$_4$. Interestingly, the detailed analyses revealed that the only a part of $f$ electrons participates in the zero-field QC and the heavy fermion behavior. We also present a temperature - magnetic field phase diagram of \ybal to illustrate how the characteristic temperature and field scales evolves near the QC.