Schottky Anomaly Observed in NMR of Metallic Si:P

We studied the $^{31}\mathrm{P}$ nuclear magnetic relaxation of metallic Si:P with doping concentration of $\sim6 \times10^{19}\mathrm{cm}^{-3}$ at very low temperatures from 3.5 K down to 45 mK and magnetic field of 7.4 T. Below 1 K, Nuclear spin-lattice relaxation studied by inversion recovery method showed two-step recovery in magnetization due to the effects of finite heat capacity of conducting electrons. Under given experimental conditions, the heat capacity of nuclear spins becomes comparable to that of conducting electrons, so the conducting electrons cannot be considered as a heat reservoir. Still, however, the initial magnetization recovery or the corresponding time constant $\mathrm{T}_1$ followed the Korringa’s law very well. Nuclear spin-spin relaxation time $\mathrm{T}_2$ above 1.5 K was ~14 msec independent of temperature and explained by dipolar fields from $^{31} \mathrm{P}$ and $^{29}\mathrm{Si}$ nuclear spins. As temperature decreased below 1.5 K, however, $\mathrm{T}_2$ started falling and again became constant of $\sim1.3$ msec below 600 mK.