Zero- to Ultralow-field NMR

This article presents the basic principles and methodology used in modern implementations of zero-to ultralow-field NMR (ZULF NMR), with emphasis on the case where spin evolution is detected directly in the ZULF environment. In contrast to conventional high-field NMR, ZULF NMR allows for measurement of spin-spin interactions 'in their natural environment' free of truncation by dominant coupling to applied magnetic fields. However, the absence of a large applied magnetic field means that spin precession frequencies and equilibrium spin polarization-related to the sensitivity of inductive detection and to the magnitude of the measurable magnetization, respectively-are dramatically lower than in the high-field case. ZULF NMR thus requires the use of alternative detectors such as atomic magnetometers, along with the production of nonequilibrium spin polarization as prepared by, for example, prepolarization in a permanent magnet or parahydrogen-induced polarization. Nevertheless, ZULF NMR permits particularly high-resolution measurement of spin-spin couplings due to the high absolute magnetic field homogeneity and the absence of certain relaxation pathways such as chemical shift anisotropy or susceptibility-induced gradients. Furthermore, ZULF NMR is capable of directly detecting spin interactions that do not commute with the Zeeman Hamiltonian and are thus unobservable with high-field NMR.