Ultra spin liquid in Lu

We analyze recent measurements of C/T, specific heat C divided by temperature T, of a 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 $C_{\rm mag}$ formed by the ultra spin liquid as a function of temperature T vs. magnetic field B behaves very similar to the electronic specific heat $C_{\rm el}$ of the HF metal ${\rm YbRh_2Si_2}$ and the quantum magnet ${\rm ZnCu_3(OH)_6Cl_2}$ . We demonstrate that the effective mass of spinon $M^*\propto C_{\rm mag}/T$ exhibits the universal scaling coinciding with scaling observed in heavy-fermion (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 ${\rm Lu_3Cu_2Sb_3O_{14}}$