Wenckebach, W.Th. “Dynamic Nuclear Polarization via the Cross Effect and Thermal Mixing: B. Energy Transport.” Journal of Magnetic Resonance 299 (February 2019): 151–67.
The fundamental process of dynamic nuclear polarization (DNP) via the cross effect (CE) and thermal mixing (TM) is a triple spin ﬂip, in which two interacting electron spins and a nuclear spin interacting with one of these electron spins ﬂip together. In the previous article (Wenckebach, 2018) these triple spin ﬂips were treated by ﬁrst determining the eigenstates of the two interacting electron spins exactly and next investigating transitions involving these exact eigenstates and the nuclear spin states. It was found that two previously developed approaches—the scrambled states approach and the ﬂuctuating ﬁeld approach—are just two distinct limiting cases of this more general approach. It was also shown that triple spin ﬂips constitute a single process causing two ﬂows of energy: a ﬂow originating in the electron Zeeman energy and a ﬂow originating in the mutual interactions between the electron spins. In order to render their deﬁnitions more precise, the former ﬂow was denoted as the CE and the latter as TM.