Dynamic Nuclear Polarization in Solids: The Birth and Development of the Many-Particle Concept

Abstract

A brief review is presented on historical aspects of the dynamic nuclear polarization (DNP) research in 1960-1970s of the last century. Much attention is given to development of the many-particle approach, from pioneering studies of the 3-spin electron-nuclear cross effect to development of spin-temperature concept and thermal mixing mechanism of DNP in solids. Applicability of different models to modern DNP data is briefly discussed.This brief review is focused primarily on historical aspects: we will try to recollect the atmosphere of research in the fields of dynamic nuclear polarization (DNP) and solid-state spin dynamics, typical for the 1960s and 1970s of the last century. The paper does not claim to be exhaustive; in particular, the original references are often replaced by books and reviews. Much attention is given to studies performed in the Soviet Union; some of them are little-known because of the former Iron Curtain and the language barrier (the publications were in Russian). We hope to improve this situation and apologize in advance for a possible bias.At that time, the DNP technique was already well known and successfully applied, mostly in polarized proton targets. The physical interpretation of the DNP in non-conducting solids was restricted to the simplest two-particle picture including an electron spin S i of paramagnetic impurity coupled with a nuclear spin I j of the host lattice by magnetic dipole-dipole interaction. Following the ''solid effect'' (SE) mechanism suggested by Abragam and Proctor [1] (see also the

Mechanisms of dynamic nuclear polarization in insulating solids

“…Indeed, preparation of polarized targets remains an area of active interest in experimental particle physics, most recently using pulsed DNP methods ( vide infra ). For the interested reader there are excellent reviews of these early experiments [14–17]…”

Simultaneous DNP enhancements of 1H and 13C nuclei: theory and experiments

“…2 Samples containing radicals with broad EPR spectra that can simultaneously polarize several types of nuclei (with Larmor frequencies narrower than the EPR spectral width) generally show two DNP features: (i) the MW frequency swept DNP spectra of different nuclei are similar, 2,19,20,[37][38][39][40][41]56,57 and (ii) a polarization exchange exists between the different nuclei even without MW irradiation. 2,24,36 The DNP spectra obtained with narrow-line radicals, sometimes exhibit enhancements of low gamma nuclei for MW irradiation frequencies outside the EPR spectrum. This formalism predicts that all nuclei in the sample reach the same nuclear spin temperature, which also means that their enhancements at each MW frequency are equal and thus their DNP spectra become the same.…”

A versatile and modular quasi optics-based 200 GHz dual dynamic nuclear polarization and electron paramagnetic resonance instrument

“…In recent years, it has become apparent that the commonly accepted theoretical framework of thermal mixing (TM) [26–31] or explaining DNP under non-MAS and low temperatures (< 150 K) may not be consistent with experimental ssDNP observations, in particular the lineshapes of DNP spectra and the underlying saturation profile of the EPR spectrum. Consequently, several attempts were made to improve and expand beyond the TM formalism [32–37], notably by the Vega group [38–42], Tycko et al [43–45], and Köckenberger et al [46–48] who relied explicitly on the quantum mechanical, rather than thermodynamic, description of ssDNP.…”