Ultra spin liquid in $\rm Lu_3Cu_2Sb_3O_{14}$

We analyze measurements of $C_{\rm mag}/T$, specific heat $C_{\rm mag}$ divided by temperature $T$, of the recently observed ultra spin liquid. The measurements are carried out in magnetic fields on the triangular lattice compound $\rm Lu_3Cu_2Sb_3O_{14}$. We show that the obtained heat capacity $C_{\rm mag}/T$ formed by ultra spin liquid as a function of temperature $T$ versus magnetic field $B$ behaves very similar to the electronic specific heat $C_{el}/T$ of the heavy fermion (HF) metal $\rm YbRh_2Si_2$ and that of the quantum magnet $\rm ZnCu_3(OH)_6Cl_2$. We further demonstrate that the spinon effective mass $M^*\propto C_{\rm mag}/T$ exhibits the universal scaling coinciding with that observed in HF metals and in $\rm ZnCu_3(OH)_6Cl_2$. Based on these observations we conclude that a strongly correlated spin liquid determines the thermodynamic properties of the ultra spin liquid of $\rm Lu_3Cu_2Sb_3O_{14}$.