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Very fast growth of the c-facet of a 4 He crystal was induced by acoustic waves. The growth velocity was larger at lower temperatures and saturated below about 400 mK. The velocity was proportional to the acoustic wave power. This fast growth cannot be explained by the spiral growth mechanism for the known value of the step mobility. We developed a step multiplication model for high-power acoustic waves and found reasonable agreement with the observed temperature and power dependence of the growth velocity.KEYWORDS: quantum solids, superfluid, acoustic radiation pressure, crystal growth, step DOI: 10.1143/JPSJ.75.023601A fundamental problem in crystal growth physics is how fast a facet can grow. In ordinary materials, the facet grows with a spiral growth mechanism well below the roughening transition temperature. 1) Step motion is limited by the diffusion of atoms and/or by the transport of latent heat. When two steps with opposite sign collide, they disappear without any reflection or transmission.Step density cannot increase indefinitely but reaches a steady-state value which is determined by the driving force for the crystallization. Recently, Parshin and Tsymbalenko have proposed a new type of collision of steps in the case of high mobility of steps at a large driving force.2) When high speed-steps collide, the steps pass through each other making another atomic layer on the facet due to the inertia of the liquid accompanying the step motion. This kinematical multiplication of steps makes the step density higher and the growth of the facet faster. Mobility of steps for a 4 He crystal in the superfluid liquid is not limited by the diffusion and can be very high at very low temperatures.3) We observed anomalously fast growth of the c-facet of a 4 He crystal induced by acoustic waves. The fast growth could not be explained by the spiral growth mechanisms and we possibly observed the multiplication of steps.We reported that acoustic waves induce crystallization of 4 He crystals at low temperatures. 4,5) We interpreted that this interface motion was induced by the acoustic radiation pressure. For the small displacement of the interface by a short acoustic wave pulse (1 ms), the acoustic radiation pressure model can explain the growth velocity of the rough and vicinal surfaces reasonably well at low temperatures by taking into account the orientation dependence of the growth coefficients. In this paper, we report the effect of the longer pulse (50 ms) applied to the vicinal surface from the crystal side. The displacement of the interface was much larger and the clear c-facet soon appeared on the top of the upheaval because the growth velocity of the c-facet was smaller than the vicinal surface. This growth velocity of the c-facet was much larger than the value expected from the spiral growth model with known step mobility. Since the pressure oscillation of the applied acoustic wave was large enough to accelerate steps to the velocity of sound, kinematical step multiplication was likely to occur. We constructed a g...

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