Nuclear dipolar relaxation induced by interacting ground state electron spins

Abstract

In samples used for dynamic nuclear polarization (DNP), spin–lattice relaxation times are usefully increased by going to high magnetic field and low temperature, typically several tesla and below 1 K. But the relaxation times for dipolar components of the nuclear spin energy remain stubbornly shorter than those for the Zeeman energy: dipolar order decays faster than the polarization itself by a huge factor—up to four orders of magnitude or more in the materials studied thus far. Such fast nuclear dipolar relaxation poses experimental challenges, for instance, when transferring polarization between different nuclear spin species via intermediate nuclear order: a proven technique to polarize rare nuclear spins. The origin of this fast nuclear dipolar relaxation remained a mystery for a long time—existing theories of nuclear spin–lattice relaxation could at best predict about one order of magnitude difference between the nuclear dipolar and nuclear Zeeman relaxation rates—until it was recently discovered to be due to conversion of nuclear dipolar energy into super-hyperfine energy induced by nuclear flip-flop transitions. A previous article showed that the inclusion of this relaxation path enables a quantitative explanation of nuclear dipolar relaxation induced by photo-excited triplet states. This article extends the theory to nuclear dipolar relaxation induced by ground state electron spins and demonstrates that this new mechanism enables a precise quantitative prediction from first principles of the nuclear dipolar relaxation rate for Ca(OH)2 doped with O2- centres—in which DNP is caused by the solid effect (SE)—and LiF doped with F-centres—in which DNP is caused by thermal mixing (TM)—both at 5.5 T and 0.4 K. It is noticed that the proposed mechanism extends to other spin systems which has implications for e.g. TM and spectral diffusion.