Influence of Ho$^{3+}$-doping on $^{13}$C dynamic nuclear polarization*

We report the effects of the addition of trace Ho-DOTA on the $^{13}$C dynamic nuclear polarization and the relaxation time of [1-$^{13}$C] acetate doped with trityl OX063 free radical. Our result shows that the polarization can be improved by a factor of 2.7 fold using Ho-DOTA doping, compared to the undoped samples, similar to the effect of Gd-HP-DO3A doping. In contrast to Gd-HP-DO3A doping, the $^{13}$C relaxation rate is only minimally affected by Ho-DOTA. The W-band electron spin resonance studies revealed a considerable reduction of the electron spin-lattice relaxation time $T_1$ of trityl OX063 at low temperatures, which corresponds to the $^{13}$C polarization enhancement, in agreement with the thermodynamic picture of DNP.