Spin Coherence Time T2 in Metallic P-doped Si at Very Low Temperature

We report NMR measurements of transverse relaxation rate 1/T2 of the 31P nuclear spins in metallic Si:P (concentration of dopant P, n=18×1018 and 56×1018 cm−3) at temperatures between 45 mK and 5 K in a magnetic field of 7 T. Above 1.4 K, 1/T2 is constant independent of temperature as well as concentration and is determined by magnetic dipolar interaction between the 31P and 29Si nuclear spins. As temperature decreases below 1.4 K, 1/T2 increases over the dipolar-determined value by an order of magnitude and levels off around 0.6 K. The concentration dependence of 1/T2 at low temperatures suggests that 1/T2 below 1.4 K is determined by the coupling between the 31P nuclear spins. We understand 1/T2 at low temperatures originates from the RKKY interaction. We explain the temperature dependence of 1/T2 between 0.6 K and 1.4 K by the motional-narrowing expression of 1/T2 with a temperature-dependent correlation time of the fluctuating local field due to the RKKY interaction.