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We reveal an interplay between temperature and radical concentrationnecessary to establish thermal mixing (TM) as an efficient dynamic nuclearpolarization (DNP) mechanism. We conducted DNP experiments by hyperpolarizingwidely used DNP samples, i.e., sodium pyruvate-1-$^{13}$C in water/glycerolmixtures at varying nitroxide radical (TEMPOL) concentrations and microwaveirradiation frequencies, measuring proton and carbon-13 spin temperatures.Using a cryogen consumption-free prototype-DNP apparatus, we could probecryogenic temperatures between 1.5 and 6.5 K, i.e., below and above the flashpoint of liquid helium. We identify two mechanisms for the breakdown of TM: (i)Anderson type of quantum localization for low radical concentration, or (ii)quantum Zeno localization occurring at high temperature. This observationallowed us to reconcile the recent diverging observations regarding therelevance of TM as a DNP mechanism by proposing a unifying picture and,consequently, to find a trade-off between radical concentration and electronrelaxation times, which offers a pathway to improve experimental DNPperformance based on TM.

Unified understanding of the breakdown of thermal mixing dynamic nuclear polarization: the role of temperature and radical concentration