Low-temperature ultrasonic measurements were performed in order to investigate the vacancy in single-crystal silicon. The longitudinal elastic constants of non-doped and boron-doped silicon, grown by a floating-zone method, exhibited appreciable softening with decreasing temperature down to 20mK. The softening of boron-doped silicon was easily suppressed by applied magnetic fields of up to 2T, while the softening of non-doped silicon was robust in fields; even up to 16T. The softening of the elastic constants in high-purity crystalline silicon was attributed to a coupling of the elastic strains of the ultrasonic waves to electric quadrupoles of the vacancy orbital.

Ultrasonic Study of Vacancy in Single Crystal Silicon at Low Temperatures. M.Akatsu, T.Goto, H.Y.Kaneta, H.Watanabe, Y.Nemoto, K.Mitsumoto, S.Baba, Y.Nagai, S.Nakamura: Journal of Physics - Conference Series, 2009, 150[4], 042002