Phonon echoes in Si:P at very low temperature

We observed phonon echoes in P-doped Si (Si:P) at very low temperatures. We applied two radio-frequency pulses separated by a time delay of τ on Si:P and observed echo signal at t = 2τ in both insulating and metallic samples with varying dopant concentrations and of different sample forms of powders and bulk plates at temperature between 45 mK and 4 K. The echoes were much more pronounced in insulating powder samples than in metallic ones and in bulk ones. The echo intensity for a fixed τ increased very strongly as temperature was lowered but the echoes disappeared toward the superfluid-to-normal transition temperature of helium mixture in which the samples were immersed. We observed no appreciable change in the echo intensities as external magnetic field was varied up to 8 T. The echoes are interpreted to be dynamical polarization phonon echoes in piezoelectric powders of insulating Si:P with a dopant concentration n = 6 x 10¹⁷ cm^-3.