Ultra spin liquid in $\mathrm{L}{\mathrm{u}}_3\mathrm{C}{\mathrm{u}}_2\mathrm{S}{\mathrm{b}}_3{\mathrm{O}}_{14}$

We analyze measurements of $C_{\mathrm{m}\mathrm{a}\mathrm{g}}\mathrm{/}T$, specific heat $C_{\mathrm{m}\mathrm{a}\mathrm{g}}$ divided by temperature $T$, of the recently observed ultra spin liquid. The measurements are carried out in magnetic fields on the triangular lattice compound $\mathrm{L}{\mathrm{u}}_3\mathrm{C}{\mathrm{u}}_2\mathrm{S}{\mathrm{b}}_3{\mathrm{O}}_{14}$. We show that the obtained heat capacity $C_{\mathrm{m}\mathrm{a}\mathrm{g}}\mathrm{/}T$ formed by ultra spin liquid as a function of temperature $T$ versus magnetic field $B$ behaves very similar to the electronic specific heat $C_{el}\mathrm{/}T$ of the heavy fermion (HF) metal $\mathrm{Y}\mathrm{b}\mathrm{R}{\mathrm{h}}_2\mathrm{S}{\mathrm{i}}_2$ and that of the quantum magnet $\mathrm{Z}\mathrm{n}\mathrm{C}{\mathrm{u}}_3(\mathrm{O}\mathrm{H}{)}_6\mathrm{C}{\mathrm{l}}_2$. We further demonstrate that the spinon effective mass $M^{\ast }\propto C_{\mathrm{m}\mathrm{a}\mathrm{g}}\mathrm{/}T$ exhibits the universal scaling coinciding with that observed in HF metals and in $\mathrm{Z}\mathrm{n}\mathrm{C}{\mathrm{u}}_3(\mathrm{O}\mathrm{H}{)}_6\mathrm{C}{\mathrm{l}}_2$. Based on these observations we conclude that a strongly correlated spin liquid determines the thermodynamic properties of the ultra spin liquid of $\rm Lu_3Cu_2Sb_3O_{14}$.