High nuclear polarization in a LaAlO3 crystal doped with dilute Nd3 + ions for frozen–spin 139La targets in searches for time–reversal invariance violation

Single crystals of LaAlO3 doped with Nd3 + ions are candidate materials for frozen–spin 139La targets in time–reversal invariance violation searches via compound nuclear resonances. In previous dynamic nuclear polarization (DNP) experiments, 0.03 mol% of Nd concentration has shown the highest 139La polarization. However, a high saturation polarization and a long spin–lattice relaxation time T1 are essential requirements for frozen–spin targets. Although a lower concentration leads to a longer T1, it also reduces the DNP efficiency. For this reason, we performed the DNP experiments with the 0.01 mol% crystal under conditions comparable to previous experiments, specifically at 2.33 T and 1.3 K. As a result, the 139La polarization reached 26 ± 1% before saturation, which is the highest value reported under comparable conditions. Analysis of the buildup curve, calibrated by numerical calculations, estimated the saturation polarization to be 52 ± 3% and the buildup time to be 111.2 ± 8.7 minutes, indicating that T1 is longer than 110 minutes. These results show that the LaAlO3 crystal doped with 0.01 mol% Nd is more suitable than previously examined crystals for use as frozen–spin 139La targets.