Anomalies in the NMR of silicon: Unexpected spin echoes in a dilute dipolar solid

NMR spin echo measurements of ${}^{29}\mathrm{Si}$ in silicon powders have uncovered a variety of surprising phenomena that appear to be independent of doping. These surprises include long tails and even-odd asymmetry in Carr-Purcell-Meiboom-Gill (CPMG) echo trains, and anomalous stimulated echoes with several peculiar characteristics. Given the simplicity of this spin system, these results, which to date defy explanation, present an interesting puzzle in solid state NMR.